Study and Numerical Simulation of Unconventional Engine Technology

Home , Pulsejet, Pulse detonation engine, Valveless pulsejet

STUDY AND NUMERICAL SIMULATION OF UNCONVENTIONAL ENGINE TECHNOLOGY

by ANJALI SHEKHAR B.E Aeronautical Engineering VTU, Karnataka, 2013

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science, Aerospace Engineering, College of Engineering and Applied Science, University of Cincinnati, Ohio 2018

Thesis Committee:

Chair: Ephraim Gutmark, Ph.D.

Member: Shaaban Abdallah, Ph.D.

Member: Mark Turner, Sc.D. An Abstract of Study and Numerical Simulation of Unconventional Engine Technology by Anjali Shekhar

Submitted to the Graduate Faculty as partial fulﬁllment of the requirements for the Master of Science Degree in Aerospace Engineering University of Cincinnati December 2018

The aim of this thesis is to understand the working of two unconventional aircraft propulsion systems and to setup a two-dimensional transient simulation to analyze its operational mechanism. The air traﬃc has nearly increased by about 40% in past three decades and calls

for alternative propulsion techniques to replace or support the current traditional propulsion

methodology. In the light of current demand, the thesis draws motivation from renewed inter-

est in two non-conventional propulsion techniques designed in the past and had not been given

due importance due to various ﬂaws/drawbacks associated. The thesis emphasizes on the work-

ing of Von Ohains thermal compression engine and pulsejet combustors. Computational Fluid

Dynamics is used in current study as it offers very high flexibility and can be modified easily to

incorporate the required changes. Thermal Compression engine is a design suggested by Von

Ohain in 1948. The engine works on the principle of pressure rise caused inside the engine

which completely depends on the temperature of working ﬂuid and independent of rotations

per minute. The design data has been provided for turbo-prop and turbo-jet engines, in the

current study the design data for turbo-prop engine is used and geometric optimization study

is conducted to obtain the required dimensions to setup a two-dimensional engine unit. The

suggested RPM value is used to calculate the time required to complete the process. The sim-

ulation is conducted and the results of simulation are veriﬁed using the basic thermodynamic

equations. The vital parameters of the engine such as the thermal eﬃciency, energy output and

thrust are calculated. A brief analysis of the performance is conducted to decide the applica-

tion of this engine in current scenario. The similarities of the engine operation is compared to

currently tested pressure gain devices.

ii In the next research topic, Pulsejet combustor simulation study is set up by considering three diﬀerent geometries. The dimensions of the combustion chamber are ﬁxed, the tailpipe being short, medium and long are considered for the study. The simulation is designed to capture the pressure, temperature and the location of auto ignition. The results obtained from the simulation in current study are compared with the experimental results obtained from a separate study.

The peak pressure, the pressure variation along then pulsejet, the location of auto ignition and the frequency of operation of each conﬁguration is compared with experiment results and the observation is presented. The simulation is used to study the reasons for auto ignition, which is in turn used to establish a standard mechanism of operation for pulsejets. The results are also used to study the behavior of pulsejets with variation in geometry.

Thesis Supervisor: Ephraim J Gutmark

Title: Distinguished Professor and Ohio Eminent Scholar

This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author. To my family and friends Acknowledgments

I would like to take this opportunity to express my sincere gratitude to my professor and advisor, Dr. Ephraim Gutmark. He has provided me continuous support and motivation during my graduate studies. He is a great source of inspiration to me, his lectures on aircraft propulsion and aeroacoustics have helped me gain great insight into the world of aerospace. I am thankful to him for his time and for helping my out in each step i have taken towards my thesis.

I would like to thank my committee members Dr Mark Turner and Dr Shaaban abdallah for taking time out of their busy schedules and for accepting to be part of my thesis committee.

I owe special thanks to Dr. Villalva Rodrigo and my lab mates Vijay Anand, Alexander

Zahn and William Stoddard for helping me start my research work and for continuous support during my tenure as graduate student at gas dynamics and propulsion laboratory at University of Cincinnati.

This could not be possible without the support of my near and dear ones. I would like to thank my father Chandra Shekhar and mother Rama Devi for believing in my dreams and helping me pursue it. My extended family for being a pillar of strength. A special note of thanks to my friends back in India for their unconditional support.My colleagues and management team at Exel Composites deserve a special mention for supporting me and guiding me in the right career path.

Finally, kudos to my friends and roommates at Cincinnati for putting up with me and un- conditionally supporting me through this journey. For being part of my ups and downs and making me feel at home :)

Anjali Shekhar

vi Contents

Abstract ii

Acknowledgments vi

Contents vii

List of Figures x

List of Abbreviations xiii

1 Introduction 1

1.1 Motivation ...... 1

1.2 Objectives ...... 2

1.3 Contributions ...... 3

1.4 Organization of Thesis ...... 4

2 Aircraft Propulsion 5

2.1 Evolution of Aircraft Propulsion ...... 6

2.2 Development of Thermal Compression Engine ...... 12

2.3 Pressure Gain Combustors ...... 12

2.3.1 Rotating detonation Engines ...... 13

2.3.2 Pulse detonation Engines ...... 14

2.3.3 Wave Rotor Engines ...... 15

2.3.4 Comparison of TCE with other know processes ...... 16

2.4 Pulsejets ...... 18

2.4.1 History of pulsejets ...... 18

2.4.2 Working Mechanism of Pulsejets ...... 20 vii 2.4.3 Pulsejet behaviour ...... 22

2.4.4 Numerical Simulation of Pulsejets ...... 23

3 Construction and Working of Thermal Compression Engine 26

3.1 Construction of Thermal Compression Engine ...... 26

3.2 Working of Thermal Compression Engine ...... 28

3.3 Thermodynamic Cycle Analysis of TCE ...... 30

3.4 Geometric Optimization of TCE ...... 32

3.4.1 Cell Characteristics ...... 32

3.4.2 Nozzle and Diﬀuser Sizing ...... 34

3.4.3 Sector Area ...... 35

3.4.4 Timings ...... 36

4 Simulation study and performance analysis of thermal compression engine 38

4.1 Computation as an analytically approach ...... 38

4.2 Numerical Simulation ...... 39

4.2.1 Geometry ...... 39

4.2.2 Simulation Setup and Reﬁnement Study ...... 40

4.2.3 Simulation Sequence ...... 41

4.2.4 Boundary Conditions and Sub-Process Simulation ...... 42

4.2.5 Simulation Results ...... 44

4.3 Performance Parameters ...... 50

4.4 Results and Discussion ...... 52

5 Design and Working of Pulsejet Combustors 55

5.1 Types of Pulsejets ...... 55

5.1.1 Valveless Pulsejets ...... 56

5.1.2 Valved Pulsejets ...... 58

5.1.3 Working of Pulsejet Combustor ...... 59

viii 6 Design, Numerical Computation and Analysis of Pulsejet Behaviour 62

6.1 Geometry ...... 63

6.2 Simulation Model ...... 64

6.3 Convergence Study ...... 65

6.4 Output Setup ...... 66

6.5 Simulation methodology ...... 67

6.6 Results and Discussions ...... 69

6.6.1 Comparison with Experimental Results ...... 72

6.6.2 Pulsejet behaviour as Helmholtz resonator or quarter wave tube . . . . 80

6.6.3 Comparing with Vortex tube ...... 80

6.6.4 Conclusion ...... 81

7 Discussion and Future works 82

7.1 Thermal Compression Engine - Discussion ...... 82

7.2 Pulsejet Engine - Discussion ...... 83

8 Curricular Practical training - Report 84

8.1 Introduction ...... 84

8.2 Pultrusion process: ...... 85

8.3 Research Focus: ...... 86

8.3.1 Resin Bath ...... 86

8.3.2 Heat Transfer ...... 87

8.3.3 Design Capability ...... 88

9 Appendix 89

ix List of Figures

2-1 Bent bow propulsion ...... 6

2-2 Rubber Band propulsion ...... 6

2-3 Steam Powered Airship by Giﬀard ...... 7

2-4 Engine from wright Brother’s 1903 Airplane ...... 8

2-5 Curtiss Water Cooled 8-Cylinder Engine ...... 8

2-6 Pratt & Whitney’s Wasp Engine ...... 9

2-7 Conventional Subsonic Engines ...... 11

2-8 Conventional Supersonic Engines ...... 11

2-9 Rotating detonation Engine ...... 13

2-10 Pulse detonation Engine - working ...... 14

2-11 Wave Rotor ...... 15

2-12 The Flying Bomb V-1 ...... 19

2-13 Wave diagram of quarter wave tube ...... 22

3-1 Cell Rotor ...... 27

3-2 Sectional view of TCE ...... 27

3-3 Sector area for sub processes ...... 28

3-4 Four sub-processes of TCE ...... 29

3-5 P-V diagram for the engine process ...... 30

4-1 Simpliﬁed 2D Geometry Created for transient simulation using ANSYS FLUENT . 40

4-2 Boundary conditions for four sub-processes ...... 41

4-3 Static pressure plot - First Time-step ...... 44

4-4 Static temperature plot - First Time-step ...... 44

4-5 Static pressure plot - Nineteenth Time-step ...... 45

x 4-6 Static temperature plot - Nineteenth Time-step ...... 45

4-7 Static pressure plot - Thirty ﬁfth Time-step ...... 46

4-8 Static temperature plot - Thirty ﬁfth Time-step ...... 46

4-9 Static pressure plot - Forty seventh Time-step ...... 47

4-10 Static temperature plot - Forty seventh Time-step ...... 47

4-11 Static pressure plot - Fifty fourth Time-step ...... 48

4-12 Static temperature plot - Fifty fourth Time-step ...... 48

4-13 Exit velocity captured at the end of ﬁrst cycle ...... 49

5-1 Eccopette - valveless pulsejet ...... 57

5-2 Ecrevisse - valveless pulsejet ...... 57

5-3 Valved Pulsejet construction ...... 59

5-4 Ignition in valved pulsejet ...... 60

5-5 Combustion in valved pulsejet ...... 60

5-6 Intake in valved pulsejet ...... 61

5-7 Compression in valved pulsejet ...... 61

6-1 Geometries of three pulsejet conﬁgurations ...... 63

6-2 Static Pressure Plots from Simulation SC-ST ...... 70

6-3 Static Temperature Plots from Simulation SC-ST ...... 71

6-4 Pressure plots obtained from experimentation ...... 72

6-5 Pressure plots obtained from experimentation - 10x ...... 73

6-6 Pressure plots obtained from Simulation for SC-ST ...... 74

6-7 Pressure plots obtained from Simulation for SC-MT ...... 74

6-8 Pressure plots obtained from Simulation for SC-LT ...... 75

6-9 Temperature plots obtained from Simulation for SC-ST ...... 76

6-10 Temperature plots obtained from Simulation for SC-MT ...... 76

6-11 Temperature plots obtained from Simulation for SC-LT ...... 77

6-12 Auto ignition location for 3 cases of simulation ...... 78

6-13 Physical testing results for location and propensity of combustion ...... 78

xi 8-1 Pultrusion Process Schematic Diagram ...... 85

8-2 Viscosity Curve - Polyester Sample ...... 87

xii List of Abbreviations

UAV ...... Unmanned Aerial Vehicle CFD ...... Computational Fluid dynamics TCE ...... Thermal compression engine RDE ...... Rotating detonation engines PDE ...... Pulse detonation engine SNECMA ...... Socit Nationale d’tude et de Construction de Moteurs d’Aviation PJ ...... Pulse jet 2D...... Two Dimensional 3D...... Three Dimensional SC...... Short Combustor SC-ST ...... Short Combustor Short Tailpipe SC-MT ...... CShort Combustor Medium Tailpipe SC-LT ...... Short Combustor Long Tailpipe RPM ...... Revolutions per Minute UAS ...... Unmanned Aerial System 6-DOF ...... Six Degrees of Freedom

xiii Chapter 1

Introduction

1.1 Motivation

Aviation industry has shown fast paced developments over the past two decades. Air travel has gained momentum and has resulted in various new designs of aircrafts with varying pay- loads for civilian and military applications. Propulsion system being the heart of the aircraft, has undergone tremendous improvement to cater the needs of current demand. In this process, various engines designed in the past and not given due importance have gained renewed interest. An unconventional engine with simplicity, low energy consumption and comparable performance to conventional subsonic engine is a promising field of research. Technological advancement has resulted in tools like computational fluid dynamics being used in design and testing of aircraft engines. They are economical and flexible option during the nascent stages of engine design and research. Obsolete engine technologies are designed and simulated using

CFD principles and modiﬁcations are made in construction and working to ﬁt in the current aviation market.

Thermal compression engine, a design concept by von Ohain in 1948 explains a special propulsion system which works on the principle of pressure rise caused due to increase in temperature of the working substance. The increase in the static pressure within the combustion element is independent of rotations per minute like the conventional steady, deflagrative engines. It is simple in construction and assures good controlabillity. The performance of the engine was theoretically proven to be good and couple of sample tests were performed in the early 1950s. von Ohain has stated that the engine can handle high temperatures due to good 1 internal cooling. The design data for turbojet and turbo-prop engine using the concept of thermal compression engine is available in von Ohains report. This engine offers a lot of flexibility for modification. Various sizing changes and geometric optimization can be carried out before

fixing the final design. In this engine the pressure rise can be controlled by the flow of hot gases, which in turn can be altered by the geometry. Hence the performance of the engine can be varied by small changes in geometry and based on the results obtained the engine will have multiple applications.

The simplicity of construction, low weight, easy integration and ease of operation has led the pulsejet combustor to be considered as a viable micro propulsion system. The micro- propulsion systems like pulsejets are increasingly used as industrial dries, helicopter accel- erators and to power unmanned aerial vehicles. Pulsejet engines are characterized by multiple pulses of combustion per second. They can operate even at static conditions and provide same thrust values. In spite of all these advantages the pulsejets have limited use because of unreliable operation and issues with starting of the engine. The pulsejets have also shown varying behavior with changes in geometry, changes in dimensions of combustion chamber and tailpipe and the presence of ﬂare at the rear end of tailpipe results in diﬀerent pulsejet behaviors. The er- ratic performance of pulsejets creates wide scope for research, in deciding the operating mechanism and reason for subsequent auto-ignitions in pulsejets with varying geometries. Deciding the exact mechanism resulting in pulsejet operation would help in stable functioning and the horizon of applications can be widened.

1.2 Objectives

The research primarily focuses on the two-dimension transient simulation of two unconventional aircraft engines. This thesis will address the following aspects in this research:

* Study the construction and working of thermal compression engine

* Geometric optimization and sizing using the available design data

* Simulation to mimic the working of the engine and to capture pressure and temperature

2 * Calculate the performance parameters of TCE and evaluate the applications

* Study the working of pulsejet combustor and theories explaining operational mechanism

* Design and simulation of three diﬀerent geometries of pulsejets to show the working and

to attain pressure and temperature values at seven diﬀerent port locations

* Validation by comparing the simulation results to the results obtained from physical testing

1.3 Contributions

The current work will serve as pipeline into further research in the ﬁeld of thermal compression engine and its applications. It is also one step towards establishing a standard operational mechanism of pulsejet combustors. The main contributions are:

* The entire geometry of thermal compression engine is ﬁxed and the time required to com-

plete four sub-processes leading to one complete cycle is calculated.

* A two-dimensional numerical simulation showing the working of the thermal compression

engine is setup. Pressure and temperature values are recorded.

* The simulation results are used to calculate the vital performance parameters of the engine

and a performance evaluation is presented.

* Three geometries of pulsejet each with a short combustion chamber and tailpipe being

short, medium and long are designed and a two-dimensional transient simulation is setup

to show the complete cycle of pulsejet operation. The temperature and pressure values are

obtained from seven measuring ports.

* The peak pressure, pressure variation, frequency of operation and location of subsequent

combustion are compared with physical testing values obtained from a separate study for

same geometries.

* The conference presentations resulting from the present work is provided in the appendix.

3 1.4 Organization of Thesis

This thesis consists of seven chapters and a final chapter containing the report of my Cur- ricular Practical Training. The first chapter is the introduction. Chapter 2 is the literature review of evolution of aircraft propulsion systems. It provides brief history of thermal compression engine and some similar engines in function. A overview of pulsejet combustors, their operation and simulation studies conducted are included. Chapter 3 deals with construction, geometric optimization and working details of thermal compression engine. Chapter 4 presents the simulation methodology and results. It also provides the complete performance evaluation of the engine Chapter 5 deals with the types of pulsejets and the working of pulsejet combustors, Chapter 6 presents the design of three different geometric configurations , the simulation methodology and simulation results for all three configurations along with the comparison between simulation and physical testing results . Chapter 7 presents the conclusive remarks, future works and summarizes this thesis by discussing the contributions and objectives achieved through this research work. Chapter 8 eight provides summary of my work during curricular practical training and Chapter 9 gives details of conference papers from my research.

4 Chapter 2

Aircraft Propulsion

The aircraft propulsion system, incorporating the engine is responsible for the generation of required mechanical power to sustain the flight [38]. The aircraft propulsion has evolved over years, starting from simple bent bow propulsion system in 1978 to todays ultra-complex aircraft engines producing supersonic thrust. The majority of the engine applications are limited to sub sonic transport aircraft, cargo aircraft and the engines specially designed for military applications. The main function of the subsonic passenger and cargo aircraft engine is provide sufficient thrust to balance the drag of the aircraft during cruise and provide excessive thrust to accelerate the aircraft during other maneuvers. In case of fighter aircraft and other supersonic aircraft, thrust required will be higher. Excessive thrust is required to overcome high drag produced as a results of increased speed. Engines for such high speed applications are designed with afterburner or high bypass fans.

In the time lapse of 40 years various new engine technologies and operating mechanisms have been experimented and a handful of them have sustained in the market. There are various areas of engine operation which still prove to be challenging. The researchers are actively involved in reducing the noise and emission with every new variant of the engines released.

The aim is to go greener and softer. Various engine technologies not utilized in commercial or military applications due to some short comings are used for secondary applications such as to power unmanned aerial vehicles, power helicopters and as industrial driers and other applications requiring low thrust values.

5 2.1 Evolution of Aircraft Propulsion

The early attempts of propulsion were recorded in 4th and 5th century in Chinese paintings.

The paintings showed the birds feathers used as counter rotating fans of model helicopter and were driven by wooden or whalebone bow. This was the first found evidence of heavier than air propulsion system[34]. The models inspired by these paintings were known as bent bow propulsion and were first flown by Launoy and Bienvenu in 1784. Later in 1792 Sir. George

Cayley, known as the Father of Aerial Navigation attempted to ﬂy these models. The bent bow propulsion model is shown in ﬁgure (2-1).

Figure 2-1: Bent bow propulsion

The basic bent bow propulsion model was later upgraded by the use of twisted rubber bands by Alphonse in 1870s. The evidence also claims that the Wright brothers attributed their interest in ﬂights to the toy helicopters which were powered by rubber bands. The upgraded propulsion model using rubber bands is shown in ﬁgure (2-2).

Figure 2-2: Rubber Band propulsion

6 In the middle of 18th century, attempts to use steam power to propel the aircrafts was slowly gaining momentum. Henri Giﬀard from Paris built a 3-hp steam plant which weighed about 159 kgs was used to propel the dirigible airship. The airship is shown in ﬁgure (2-3). Mozhaiski in

Russia in 1884 and Clement Ader in 1890 are documented to have built and completely tested the steam driven aircrafts. Although no technical details of the engine design are available

[34]. In 1884 two airships using electric batteries as propulsion system were tested but no much details about the performance are available. The most powerful engine by the end of

18th century was the two-cylinder compound engine constructed by Sir Harim Maxim. It was well advanced than the aircraft on which it was assembled.The engine was rated to produce

3636 hp and weighed 816 kgs. The power to weight ratio was exceptionally high at that time.

Figure 2-3: Steam Powered Airship by Giﬀard

In the beginning of 19th century the first successful flight by wright brothers started a whole new era in development of flying machines. The flight by wright brothers demonstrated the benefits of internal combustion engines and this led to ceased use of steam power. The engine used by them was a 4-cylinder water cooled horizontal design. It used spark timing to control the functioning of the engine. It produced about 16 hp for the first minute later switched to steady 12 hp power. The engine used by wright brothers is shown in figure (2-4)

7 Figure 2-4: Engine from wright Brother’s 1903 Airplane

The period from 1910 to 1920 saw rapid development in propulsion systems. The world war 1 happening during this period, led to many new engine designs and modiﬁcations to old engines. The two main engines which pioneered the change were Curtiss and the liberty engine.

Curtiss was a robust engine with aluminum crankshaft, cast iron cylinders and barrels enclosing water for cooling. It was used by both US army and navy. The engine had several leakages which caused emergency landings in multiple scenarios and thus had to be replaced by liberty engine. Figure (2-5) shows the Curtiss engine.

Figure 2-5: Curtiss Water Cooled 8-Cylinder Engine

8 The liberty engine was another powerful and most advanced engine before 1920’s. It had welded cylinders without a radial structure.It was designed under extraordinary circumstances immediately after US entered the world war. Initially produced engines had issues with crack- ing of cylinder heads and burning of exhaust valves. These problems slowly faded away with time and modern production techniques[11].

After 1920, the next two decades saw rapid developments in propulsion. The wright aviation merged with Lawrence to form Pratt and Whitney which turned out to be a major player and continues as top engine manufacturer till date. Pratt & Whitney built wasp, a large 18 cylinder, air cooled engine which included great detailed designing and also initiated the use of composites in engine. Wasp was the ﬁrst engine in united states to produce 425 hp at 1900 rpm and go into service. The later years until 1940 saw various 18- cylinder radial engines and also miniature vertical engines with two to four cylinders. The liquid cooled engines were gradually replaced by light weight air cooled engines owing to their large weight and bulky design.

This helped in reducing the frontal area of the aircraft and reducing the drag [19]. Figure (2-6) shows the Wasp engine.

Figure 2-6: Pratt & Whitney’s Wasp Engine

9 During the early 1930’s while the radial and vertical engines were serving as major propulsion systems, simultaneous eﬀorts were taking place all over the world to develop a fully functional jet engine[45]. Yuri and Igor in 1933 tested the ﬁrst ramjet engine using hydrogen as fuel.

The first supersonic flight using that engine was tested later that year. The continued efforts by von Ohain, Junkers and Heinkel using their ideas to create a successful jet engine was finally fruitful in 1939. Heinkel’s HeS3 was declared to be the first usable jet engine.

With the advent of jet engine in 1939 and successful flight in the next consecutive year, there has been no looking back. Various companies like BMW, Lockheed corporation and Northrop corporation started the production of jet engines. With each variant the engine showed better performance and robust structure. Once the functionality of jet engine was fixed the next area that captured the attention of scientific community was the fuel efficiency. In an attempt to decrease the fuel consumption, concept of turbofan was introduced. In 1950 the first turbofan engine was successfully tested. In 1958 Boeing entered the business and contributed to the development immensely. In 1968 General Electrical TF39 became the first high bypass fan engine to be successfully tested. The advancement in engine technology led to faster and fuel efficient engines.

The idea of supersonic jet airplane was first realized in 1975. Tupolev Tu-144 was the first supersonic flight and was used for mail service between Moscow and Alma-Ata, In the subsequent year the first supersonic passenger aircraft Concorde was put into operation. Although it was an iconic development the Concorde was discarded from service in 2003 due to various issues faced by passengers and pilots. In 2002 Hyshot, the first scramjet engine was ignited and operated. In 2004 Hyper X another scramjet was built and tested to maintain the altitude and attain mach 10. The propulsion has evolved a long way from bent bow model to today’s hypersonic engines. In the rush of development there were several unconventional engines designed to serve various purposes but went unnoticed due to some short comings or lack of proper analysis techniques[45]. Figure (2-7) shows the conventional subsonic engines currently in use and

ﬁgure (2-8) shows the supersonic and hypersonic engines.

10 Figure 2-7: Conventional Subsonic Engines

Figure 2-8: Conventional Supersonic Engines

11 2.2 Development of Thermal Compression Engine

Dr. Hans von Ohain, the co-inventor of the jet engine proposed various designs of en-

gines during his research term of 50 years[45]. Ohain with doctoral degree in physics and

aerodynamics has made huge contribution to the ﬁeld of propulsion. As a student, ohain was

convinced that the engine without a propeller was feasible. He worked in co-ordination with

Heinkel to design the ﬁrst operational jet engine in 1938. It was successfully integrated into

aircraft He 178, which had its ﬁrst ﬂight in 1939[39].

After the success of He 178, the engine was modiﬁed to be larger and provide better perfor-

mance. The compressor and the turbine were connected through longer shaft and combustion

chamber was placed in between them. Several companies in Germany had started with the

manufacturing of jet engines and the Heinkel design was not progressing well by 1942. In the

mean time Ohain had started working on an unconventional engine concept which did no in-

clude the compressor and turbine. Ohain completed the theoretical calculations and ﬁrst tested

the prototype in 1946 and the test report along with the working principle was ﬁrst published

in 1948. This report named as the ” Report on special gas turbine principle” is the basis for

thermal compression engine technology.

Pabst von Ohain conceptualized an engine without a compressor or a turbine. This was

totally against the principle of conventional subsonic engine. In this engine the pressure rise

is independent of the Rotations per minute (RPM). The engine operates on the principle of

unsteady pressure rise. The unsteady processes leading to pressure gain like in thermal com-

pression engine, has been observed in diverse devices and are tested currently.Such devices are

referred to as pressure gain combustors.

2.3 Pressure Gain Combustors

The pressure gain combustors are said to have better thermal eﬃciency and greater thrust production by simply modifying the thermodynamic cycle to include a unsteady pressure rise.

About 5 to 15 % savings in fuel can be obtained by creating thermodynamic unsteadiness based on diﬀerent applications. The currently tested pressure gain combustors include rotating

12 detonation engines[25], pulse detonation engines[21] and wave rotors[6].

2.3.1 Rotating detonation Engines

Rotating detonation engines (RDE) is a form of pressure gain combustor in which at-least one detonation wave continuously passes through the annular channel[4]. The construction of

RDE consists of a rotating annular combustion chamber as shown in Figure (2-9). The fresh fuel air mixture is allowed through one end of the annular chamber and a source of ignition is set up initially to start the detonation. Once the ignition occurs and the ﬁrst detonation wave is generated the process becomes self sustained. The detonation wave passes through the annular chamber and ignites the fresh air-fuel mixture and helps in producing the energy required to keep the process going. The exhaust gases expand through the chamber and are pushed out through the inlet ﬂow of fresh air-fuel. Unlike pulse detonation engine RDE is a continuous process. Although a fully functional RDE is not under production, the experiments and the numerical simulations have proved its potential[25].

Figure 2-9: Rotating detonation Engine

The functioning of RDE relies on the concept of unsteady pressure rise which helps in thrust production. The main advantage associated with RDE is the total elimination of ignition source after the start of engine. It has application in rockets and gas turbines. One major drawback is the periodic operation of the engine. 13 2.3.2 Pulse detonation Engines

Pulse detonation engine (PDE) is another form of pressure gain combustion system in which the detonation wave is used to produce thrust. PDE consists of an open chamber structure with valves on the front end to control air-fuel ﬂow. The hot exhaust gases expand through the rear end of the tube. Initially the air-fuel mixture is introduced through the inlet and the mixture is detonated. The detonation wave created passes through the rear end of the tube and results in combustion of unburnt gases in its path. Once the detonation wave has passed from the rear end the entire tube is purged with fresh air and the cycle repeats.The PDE in theory oﬀers various advantages over conventional turbojet engines. It produces the same thrust value as turbojet without any compressor or turbine, hence reducing the weight of the engine [10]. Figure (2-10) shows the systematic working of a PDE.

Figure 2-10: Pulse detonation Engine - working

PDE is in experimental stage and has not recorded a successful production till date[40].The main limitation of PDE is that the detonation results in temperatures up to 3500 F, hence the material properties limit the test time. Providing favorable conditions for subsequent detonations and creating conditions for complete burning of fuel- air are other limitations associated.

Although PDE’s are proven to be eﬃcient propulsion system theoretically, various modiﬁca- tions and research is required before materializing this technology.

14 2.3.3 Wave Rotor Engines

Wave rotor is an unsteady internal ﬂow device consisting of channels carved on a rotating

spindle [37]. They are used as pressure exchange devices, where two ﬂuids with diﬀerent pres-

sures are brought in contact [20]. As per the unsteady wave mechanism, when two ﬂuids with

varying pressure are in contact for a short period of time the pressure equalization occurs [14].

Wave rotors used as gas turbine topping devices have been proven to increase the eﬃciency

and reduce the fuel consumption [37]. They are usually placed between the compressor and the

turbine and help in extracting maximum energy from the combustion process[44]. The wave

rotor is shown in ﬁgure (2-11)

Figure 2-11: Wave Rotor

Although the construction and working of the thermal compression engine is similar to that

of wave rotor technology there are some key diﬀerences between the two. The wave rotor attempts to optimize the formation of compression and expansion waves during combustion

[20], but the thermal compression engine attempts to make the pressure waves negligible via geometry(channels, diﬀuser and valves). The thermal compression engine attempts to create two separate static processes, pressure equalization and pressure exchange out of one dynamic process occurring in the wave rotor.

15 2.3.4 Comparison of TCE with other know processes

Constant Volume Combustion Process

The thermal compression engine shares some similarity in operation with constant volume

combustion engine. The combustion occurs over a ﬁxed volume in both engines and the same

pressure temperature relation still holds good.The ratio of initial and ﬁnal pressure is same as

the ratio of initial and ﬁnal temperature. The scavenging stage and the and expansion stage

are also the same. The only variation is that in case of constant volume combustion, the air is

heated in the cells and hot gases are formed from the same fresh gases particles and no exchange

occurs as in case of the thermal compression engine. The thermal compression engine provides

better eﬃciency when compared to constant volume combustion engine based on the same

temperatures[35].

Constant Pressure Combustion Process

The thermal compression engine, when compared to normal constant pressure combustion

process shows higher beneﬁts. In case of constant pressure combustion process the turbine

power is used to run the compressor. The net output is therefor the diﬀerence between the

turbine power and the compressor power. Hence the eﬃciency of the constant pressure engine is

a function of both turbine efficiency and compressor efficiency.Slight decrease in the efficiency

of either the compressor or turbine will have adverse eﬀect on total eﬃciency. In contrast to

this, the eﬃciency of TCE is a product of ideal values multiplied by the factor including turbine

efficiency. Thus the dependency of overall efficiency is much flatter as compared to constant pressure combustion engine. In order to obtain equal thermal efficiency, the compressor and turbine efficiency of the constant pressure combustor must be 10% higher than the efficiency of turbine in TCE[35]. In case of turbine efficiency, the TCE has a lower hand. The turbine in

TCE should handle fluctuating pressure values due to rotation of the cell rotor. This inherent pressure values will limit the turbine efficiency between 65% to 70%. While the turbine handle constant pressure flows will have slightly high efficiency values.

16 TCE has a slightly higher end over other pressure gain combustors and other similar engines.The principle of operation of TCE is based on simple constant volume combustion process. It deals with deﬂagrative combustion unlike detonations in case of RDE and PDE, hence is more controllable. The pressure rise is completely dependent on temperature rise which en- hances the control. The construction of the TCE is simple and mainly consists of a cell rotor and combustion chamber connected through valves, which forms the basic assembly. The inlet fan and turbine are separate parts connected to the assembly, which can be eliminated for initial testing. TCE can handle high temperatures as the fresh air and hot air alternatively enters the rotor providing good internal cooling. The construction and operation of TCE is very similar to constant volume combustion engine. The TCE provides better eﬃciency as compared to constant volume combustion engine based on the same temperatures[35].

The design details provided by von ohain in his special report, was manufactured and couple of physical tests were setup in 1950’s. The test results reported unexpected behaviours and slight deviation from theoretical performance. Improper welding and unexpected leakage were common problems which were rectified in subsequent tests. The engine was able to drive itself and net power output was zero. The tests performed did not mention about problems with the starting of the engine or noise.The final test performed on TCE, disintegrated the turbine from the rotor. In this case some amount of excessive energy was available in the process. Although the test data’s are insufficient, they can serve as initial proof of operation of TCE.

The thermal compression engine still remains as an unexplored alternative engine technology and offers high flexibility in design and manufacturing. The changes in geometry is said to have huge impact on the performance of the engine. Taking advantage of this feature, numerical simulation technique can be used to mimic the working of the engine and to study the performance parameters. Design data for the turbo prop engine and turbojet engine are available in the special report to start off with the initial calculations.

The work done here mainly focuses on geometric optimization and numerical simulation of thermal compression engine for the turbo-prop design features given in the report. a brief study on performance parameters is also presented. Detailed explanation will be presented in chapter

3 and chapter 4.

17 2.4 Pulsejets

The idea of having an engine which can produce thrust without utilizing any moving parts is fascinating[3].The principle of pulsejet engine is based on the pressure gradient between the atmosphere and within the pulsejet. The pressure gradient helps in opening and shutting of the inlet valves, thus eliminating the need for inlet fan to suck fresh air. The compression eﬀect occurs when vacuum is created in combustion chamber and that creates a suction force which pulls the high pressure air from both inlet and exit towards the combustion chamber.

The pulsejet does not include any compressor or turbine for its operation. Pulsejets are very simple in construction, which qualify them as a viable micro propulsion system. In spite of the simple construction and reliable power output, pulsejets pose several disadvantages. Pulsejets are associated with ignition issues and unreliable operation[29]. Pulsejet has better application as a combustor in gas turbine than as engine by itself. It produces same power as constant pressure combustor with much less mechanical loss and low fuel consumption. The limitation associated is that the turbine can’t handle unsteady ﬂow eﬃciently[22]. Hence pulsejets are not used in commercial aircraft applications currently. owing to its simplicity and small size, pulsejets are extensively used for several secondary applications and are in research stage for primary aircraft applications.

2.4.1 History of pulsejets

The innovation of pulsejets dates back to 1864, when N. Teleshov a Russian inventor ﬁrst patented the design[43].the ﬁrst working pulsejet was later patented by a Russian engineer V.V.

Karavodin. The basic design of pulsejet underwent various modifications until 1934. In 1934, the German government granted funds to Schmidt to develop a bomb. Schmidt started working on a bomb using the technology of the pulsejet engine named V-1. The pulsejet used for V-1 used a single-use-spark-ignition to initiate the combustion process. the spark plug guarantied one ignition sufficient to produce the first pulse. The early prototypes of V-1 showed inferior performances. Various modifications were performed and later an unmanned bomb system incorporating the pulsejet technology was delivered at the final years of the war. This boosted

18 the interest in pulsejet technology. Figure (2-12) shows the V-1 Bomb

Figure 2-12: The Flying Bomb V-1

Although the V-1 served to be a good fit in the war, it failed to meet the German ministry specifications. The pulsejet designed had poor accuracy and insufficient range. the production cost was extremely high for the performance it delivered. That time also witnessed multiple

German companies working on Schmidt’s technology. Argus company oﬀered to work with

Scmidt, and this iconic deal resulted in betterment of pulsejet technology. the original design of pulsejet used in V-1 was perfected and later called as the As 109-014. As 014 was first tested in 1942. This engine was evaluated to be simple, cost effective and well performed[27]. It used single automotive spark plug for ignition. The engine was capable to operate with all grades of petroleum as fuel. The inlet was connected to a high pressure source to aid in the initiation of the pulsejet. Once the engine started and sufficient temperature was obtained the external hoses were disconnected. The pulsejet produced 2,200 N (490 lbf) of static thrust and approximately

3,300 N (740 lbf) in flight[27]. Although it generated insufficient thrust for take-off it could operate stationary while on launch ramp. The v-1 bomb integrated with this improvised engine was of much use in the warfare. the noise associated with it’s operation earned it a nickname

”buzz bomb”. It was extensively used for V-1 bombs in the remaining war period and is famous for bombing London.

Another application of pulsejets in war was Himmelsturmer flight pack. Himmelsturmer 19 was the use of pulsejets to jump over obstacles, rather than creating flight. The Nazi’s experimented with flight packs to provide food and urgent medical needs to soldiers. The Him- melsturmer consisted of a backpack with fuel, oxygen and a pulsejet. The oxygen was used to supplement the air drawn due to pressure difference between the combustion chamber and atmosphere. Himmelsturmer had very short flights due to fuel constrains and the operator had to expend fuel to slow down or before landing. Himmelsturmer was tested during the end of war and was never used in combat.

Post war, the interest in pulsejets slowly faded away and since then not many functional

ﬂights have used pulsejets. Boeing holds many patents for pulsejet designs and is continuously striving to set up pulsejet combustion in experimental aircraft’s and UAV’s. Pulsejets can be used as simple devices which convert fuel into heat energy, hence they are increasingly used as industrial drying and home heating equipment’s of late.

2.4.2 Working Mechanism of Pulsejets

The working of pulsejet is simple in theory. Various designs of pulsejet are manufactured and the combustion is sustained after the trial and error approach. Although the pulsejets are operational and are used in several secondary applications, the exact mechanism of operation remains unknown. There are various theories suggesting the operation of pulsejet but none of them are successfully established yet.

1. Hot wall Theory:

This theory suggests that the hot walls of the pulsejet will result in the sustained pulse

combustion[16][18]. If this theory was to be believed then, how the pulsejet sustains after

the ﬁrst spark ignition cannot be explained. This theory also fails to explain the starting

of pulsejet combustor with cold walls. There is a common agreement in the scientiﬁc

community that agrres that warming the walls of the pulsejet during the starting of ignition

will ease the ignition process. But this does not determine hot wall theory as the sole cause

for the sustained operation. The pulsejet can be started with cold walls and still have a

sustained combustion. This proves that the operation of pulsejet depends on various other

factors than hot walls. 20 2. Residual Flame Interaction Theory:

This theory suggests that the subsequent combustions in the pulsejet occurs due to the

turbulent ﬂame residue from the preceding combustion. If this is to be true then the pulse-

jet operation must be extremely sensitive to the fuel used, this is not observed in actual

scenario. In addition if residual ﬂame interaction was the cause for auto-ignition, then

the speciﬁc operation frequency of the pulsejet and frequency variation with reactants and

geometry cannot be explained[33].

3. Inertia Theory:

This theory suggests in pulsejet the ﬁrst combustion causes high pressure in the com-

bustion chamber. The compression wave passing through the outlet reﬂects as expansion

wave and decreases the pressure in the combustion chamber. The vacuum created will

result in suction force and the residual gases from tailpipe will be dragged back. The hot

gas residue coupled with pressure results in subsequent combustions[8]. This eﬀect of

pressure waves in gases is referred to as ’Kadenacy Eﬀect’[42]. This theory just assumes

that the pressure variation within the pulsejet causes the operation.

4. Acoustic Theory:

This theory suggests that, unlike inertia theory where the entire gas particles are displaced

small perturbations around the equilibrium results in pulsating combustion of the pulsejet

combustor. The acoustic theory states that the system of acoustic waves in the pulsejet

engine as the reason for working. As per the acoustics theory the perturbation propa-

gates through the entire length of the pulsejet tailpipe through the exchange of momentum

between small ﬂuid regions[8].

Although multiple theories have explained the concept of pulse combustion occurring in the pulsejet engine, none of them have been completely eﬀective in explaining all the phenomenon associated. hence the actual operating mechanism must be dependent on more than one theory.

The actual process is a combination of multiple physical process causing auto ignition. The exact mechanism is yet to be ascertained.

21 2.4.3 Pulsejet behaviour

The other common indecision among the aircraft community members is that, if the pulsejet works as a Helmholtz resonator[24][49] or as quarter wave tube[15][48]. Geometrically the main diﬀerence between the Helmholtz resonator and the quarter wave tube is that the quarter wave tube is assumed to have same cross section through its entire length. The Helmholtz resonator has an increased volume as compared to the neck it is attached to[31].

If the pulsejet was to behave as a quarter wave tube, each cycle of operation will include two expansion waves and two compression waves. The second compression wave will result in the fresh combustion event. The pulsejet combustor with valves closed at the inlet and open at the other end is theoretically assumed to behave as quarter wave tube. The combustion creates two pressure waves, one passing thought the inlet and other towards the rear exit. the wave hitting the closed inlet will reflect as compression wave while the wave moving towards the exit will reflect as strong expansion wave. The expansion wave now hitting the inlet will reflect as expansion wave and the compression wave will reflect back as expansion wave from the outlet. The expansion wave reflected from the inlet will reach the exit and strike back as a compression wave and result in auto-ignition of the fresh feed. Figure (2-13) shows the wave diagram of quarter wave tube.

Figure 2-13: Wave diagram of quarter wave tube

22 The Helmholtz resonator is a ﬁxed volume encompassing compressible ﬂuid, held within

rigid boundaries and having a small opening[23]. If the pulsejet was to behave as the Helmholtz

resonator the combustion chamber acts as the ﬂuid cavity and the tailpipe as the small opening.

In case of Helmholtz resonator, each cycle is composed of one compression wave and one ex-

pansion wave[26]. Combustion occurs every cycle and is prone to diﬀerent locations along the

length. The research in the area of establishing pulsejet behaviour continues, actual mechanism

of the pulsejet still remains unresolved.

The uncertainty in operation of the pulsejets have urged the researchers to continue to con-

duct extensive research to ﬁnd the exact mechanism leading to auto-ignition. The idea is to

come up with the working manual for pulsejets. In order to achieve this several geometries

of pulsejet have to be tested with diﬀerent fuel combinations to determine the optimal case.

The experimental analysis of plethora of pulsejet samples is time consuming and an expensive affair. An alternative approach to achieve these results is through numerical simulation. The advancement in the field of computational fluid dynamics has enabled in setting up numerical simulation to mimic almost all physical processes.

2.4.4 Numerical Simulation of Pulsejets

Numerical simulations offer high design versatility and flexibility as compared to physical testing. Considering the fact that the pulsjet technology requires various modifications in the geometry and boundary conditions, at this stage numerical simulation is a viable option. The commercially available numerical solvers have various codes to simulate an unsteady, viscous

flow. The numerical code can be modified to incorporate combustion physics and also acoustic effects. Utilizing this technology several numerical simulations have been performed in the past few decades. Most of the simulations setup in the past are designed to capture the pressure variation across the length of the pulsejet combustor and also the frequency of operation of the pulsejet. A team of researchers from North Carolina State university (NCSU) have conducted series of experimental testing and numerical simulations on different geometries of pulsejet engines in an attempt to establish a fixed operating mechanism. This research provides pipeline into future numerical simulations of pulsejet engines.

23 T. Geng and his research team from NCSU conducted research on hobby-scale pulsejet sample, a commercially available pulsejet in 2007. The study included the concepts of gas dynamics, acoustics and chemical kinetics to understand the complex process resulting in pulsejet operation[15]. The 50 cm long pulsejet was experimentally tested, chemiluminescence and laser Doppler velocitometry were used to capture the combustion timing and the exit velocity respectively. Numerical simulation was performed on the same geometry using CFX solver. The turbulent ﬂow was modelled using k-epsilon model. The WestbrookDryer single step combustion model was used to simulate the combustion physics. The simulation results were compared with the experimental results. The comparison was setup for pressure values and the total thrust obtained. A good match was demonstrated between the experimental and simulation results. the study concluded that the pulsejet can be designed as a 1/6 wave tube, computational and experimental veriﬁcation was presented.

In 2007, Geng with his team setup a new study on 8 cm valveless pulsejet. The overall length of the pulsejet engine was just 8 cm and was the smallest engine ever known to be tested as per the author[17]. This experiment took into consideration the effects of gas dynamics, acoustics and chemical kinetics. The pulsejet considered was valveless configuration. Hydro- gen was used as the fuel and two different geometries with forward facing inlet and rearward facing inlet were considered in experimentation. The numerical simulation was setup in CFX using mostly the same combustion model and viscocity model considered for the earlier simulation. The only change was that the wall heat transfer co-efficient was included in this simulation for inner and outer valves. In case of 8 cm pulsejet the surface area-volume ratio is very high hence the effect of heat transfer is predominant. The study concluded that good match was obtained between simulation and experimentation results for peak pressure and frequency values.

The experiment also demonstrated that the rearward facing inlet produced higher thrust values as compare to the forward facing inlet conﬁguration. The study stated that in case of valved pulsejet the frequency was just the function of the length of combustor in case on valveless conﬁguration, the frequency was a function of length raised to negative power of 0.22. Similar studies were conducted on 15 cm valveless pulsejet combustor and the results and conclusions were comparable.

24 In 2008, Geng and his team in co-ordination with the NASA Glenn Research Center, Cleve- land set up another experimental and numerical simulation study as a continued research in the direction of pulsejet operation. The earlier simulations conducted were mainly focused on the combustion and the exhaust process of the pulsejet, in the current study a simulation model was designed to mimic the valve motions. The experimental setup was arranged for the same model to measure the valve movements and inlet mass flow in addition to the peak pressure and frequency values. In addition the simulation is also used to track the external flow field dominated by vortex formations. This study provides a direct comparison between the experiments and simulation for mechanical valved pulsejets. As the sub model designed takes into consideration the exact inlet mass flow, fuel flow rate and valve movements the thrust value and the frequency obtained are accurate. The experimental results for combustion chamber pressure, exit flow velocity, pressure-velocity phase relationship, vortex ring location and vorticity level are compared to the simulation results obtained for the same. The compared results were in good agreement. In addition to comparison, the study suggested to use the external vortex formation results to understand the flow mixing at the outlet[18]. This study achieved higher mile stone in materializing the complete simulation of the pulsejet operation.

inspired by the series of simulations conducted on pulsejet combustors, the current study focuses on 2D transient simulation of pulsejet combustors. The study involves three diﬀerent geometries of pulsejets which are numerically simulated. The results of simulation are compared with the results of a separate experimental analysis conducted on same geometries. The peak pressure, pressure variation, the propensity of combustion, probable location of combustion and the operational frequency of pulsejet are compared and detailed study is presented in chapter 5 and chapter 6.

25 Chapter 3

Construction and Working of Thermal

Compression Engine

The basic concept of thermal compression engine is based on unsteady pressure rise which is brieﬂy explained in section 2.3. A detailed explanation of the construction of the engine including necessary sub-components and valve opening which results in the operation of thermal compression engine is presented here.

3.1 Construction of Thermal Compression Engine

The construction of thermal compression engine mainly consists of a cell rotor shown in

3-1 and a combustion chamber placed above it. The current design considered, consists of a cell rotor with 26 cells , placed within a metallic housing. For ease of manufacturing, the cells parallel to axis are preferred. The exterior of the cell rotor can be varied with applications. The housing consists front plate and back plate placed in the front end and the rear end of the cell rotor respectively. The plates are designed to partially cover the cell rotor on both the ends.

The front plate covers the sector of the cell rotor which is not involved in the scavenging fresh air and the back plate covers the area not contributing to the exhaust of hot gases.

As seen in ﬁgure 3-2, the combustion chamber is located eccentrically with respect to the rotor axis. The front end of the combustion chamber is connected to the cell rotor through the front channel and the rear end of the combustion chamber is connected to cell rotor through the rear channel. A supercharging fan is attached to the inlet of the cell rotor and is used to supply

26 pre-compressed air to the cell during scavenging. The supercharging fan is powered by the cell rotor. A independently operating turbine unit can be attached at the rear end of the cell rotor to streamline the exhaust ﬂow and also enhance the thrust of the engine. In the present study, for simplicity of operation the turbine is not considered.

Figure 3-1: Cell Rotor

Figure 3-2: Sectional view of TCE

27 3.2 Working of Thermal Compression Engine

The working of the thermal compression engine comprises of four main sub processes.

Over one cycle of operation each cell of the cell rotor undergoes four partial processes. The four partial processes are the scavenging, pressure equalization, pressure exchange and the exhaust. The detailed working of the engine is explained by considering a single cell.

Figure 3-3: Sector area for sub processes

Considering a cell placed in the region open to scavenging, shown as sector one in ﬁgure

3-3. The cell at this location is not covered by the front plate nor by the back plate. It is open on both the ends and is not connected to the combustion chamber through either of the channels.

At this stage the pre-compressed gas from the supercharging fan enters the cell and clears the residual exhaust gases present from the previous cycle. this completes the scavenging process.

As the cell rotates it enters the second sector shown as sector two in ﬁgure 3-3. At this location the the front plate and back plate will seal the cell from both ends and the front channel connecting the cell and combustion chamber is open. The hot gases from the combustion chamber enter the cell and increase the pressure of fresh gases in the cell. the pressure equalization process ideally occurs until the pressure in the cell reaches the pressure in the combustion chamber. Kinetic energy is developed due to pressure diﬀerence between the fresh scavenged gas in the cell and the hot gases from the combustion chamber. The kinetic energy produced is converted into mechanical energy and force is applied on the walls of the cell. This is the second sub-process called pressure equalization. As the cell continues to rotate, it enters the third sector.

The third sector is shown in ﬁgure 3-3, at this stage the front and back plate continues to

28 seal the cell on either ends and both channels connecting the combustion chamber and the cell are open. The third sub process of pressure exchange occurs when the compressed gas from the cell enters the combustion chamber and the hot gases from combustion chamber continues to enter the cell. Ideally, at the end of the pressure exchange process the combustion chamber is ﬁlled with fresh compressed gas and the cell contains the hot combustion products.

Finally, the cell rotates and reaches the fourth sector shown in 3-3, in which cell and combustion chamber are not connected anymore. The front plate continues to seal the cell entry and the back plate has uncovered the cell. The cell expels the hot gases through the exit and completes the ﬁnal sub process of exhaust. Energy is developed in the expansion sub process and is used to drive the cell rotor in subsequent cycles. The cell continues to rotate and repeat the same processes.

Figure 3-4 shows all the four sub-processes that will take place in the thermal compression engine over one cycle of operation.

Figure 3-4: Four sub-processes of TCE

29 3.3 Thermodynamic Cycle Analysis of TCE

The total process of the thermal compression engine is captured in the Pressure-Volume diagram (P-V diagram) shown in ﬁgure 3-5

Figure 3-5: P-V diagram for the engine process

The point 1 on the P-V diagram represents the state of air about to enter the cell rotor. At this point the velocity of air is zero. At the end of the scavenging process the air will reach point 5. At this state the velocity of air is equal to the velocity of the cell rotor. The total energy imparted to fresh air to bring it from state one to 4 is represented by the diagonally and horizontally shaded area between the points 1-4. the area between point 1 and 2 represents the energy required to increase the static pressure of the gases from point 1 to 2 during scavenging process in a rotor. The area between 2 and 3 and the area between 3 and 4 represents the kinetic energy of fresh air in rotor, due to circumferential velocity and the axial velocity respectively. 30 At the end of the scavenging process, both the front plate and the rear plate closes the cell and results in a compression wave moving in the direction opposite to that of the scavenging

ﬂow. This compression wave settles the scavenged air and is represented by state 5 on the P-V diagram. The area of triangle between points 2-5-13 represents the energy required to bring the in-ﬂowing fresh air to state 5. This area is said to be equal to the diagonally shaded area between points 2 and 3, which represents the kinetic energy due to axial velocity of gas at state 2. The gas at the stage 5, enters the second sector and undergoes pressure equalization.

During second process, the energy is attained is the kinetic energy of hot gases entering the cell. This total attainable energy is represented by the area of triangle 5-6-9. This energy is directly transmitted to the rotor with a certain eﬃciency. The eﬃciency value is a function of the circumferential velocity of the cell and the average circumferential velocity of the gases.

von Ohain in his report, has mentioned that the total area between the points 5-6-9 multiplied by the turbine eﬃciency must be equal to the total area between points 1-4 after accounting for the scavenging losses and the frictional losses. This is mentioned to be the condition for the auto-rotation of the cell rotor in thermal compression engine.

In the third sub process the compressed gas in the cell passes through a diﬀuser and reaches the combustion chamber, this is given by 5-6. The pressure of gases reaching the combustion chamber is greater than the pressure of gases in the cell. The total energy attained in the constant pressure exchange process, is give by the area covered by 6-7-8-9. In actual process the mixing of hot and cold gases and the diﬀuser losses makes this energy negligible and can be ignored.

The next sub-process in the exhaust process. The total attainable energy in the exhaust process is given by the area covered by the triangle between the points 9-10-11. The complete thermodynamic cycle assumes an ideal process[35].

31 3.4 Geometric Optimization of TCE

3.4.1 Cell Characteristics

The brief construction details are presented in the previous section. Considering the construction details, it is important to arrive at optimum geometry to help in proper working of the engine. The main sizing to be decided is the rotor dimensions. The length of the rotor, the inner and outer diameter are used to arrive at the detailed dimensions of each cell in the cell rotor.

The length, width and height of the cell are calculated and are used to setup a 2D simulation.

An optimum length is decided for the front and the rear channel connecting the cell rotor and the combustion chamber for the exchange of hot gases.

Von Ohain in his report on thermal compression engine has suggested two sets of construction data. one set corresponding to turbo jet design and the other set detailing the turbo-prop engine. In the current study the initial variables for turbo-prop are considered to set up complete geometric optimization of the engine. The initial design data is given in table 1. The

TCE geometry was partially designed from actual speciﬁcations in the Von Ohain document and partially from the targeted working ﬂuid characteristics for the engine.

Sr. no. Parameter Value Unit

1. Mass ﬂow (m) 10.5 lb/s

2. Velocity of scavenging air (v) 400 f t/s

3. RPM of the rotor 11000

4. Length of the cell rotor 7.8 inch

5. Outer diameter of the cell rotor 15.6 inch

6. Inner diameter of the cell rotor 9 inch

Table 3.1: Design data for turbo-prop engine

Using the inner diameter, outer diameter and the length of the cell rotor as starting point of the design, the complete geometry of the cell rotor is calculated using simple mathematical formulas. The volume between the outer cylinder and the inner cylinder comprises of the cells and the cell walls. The volume of the outer and inner cylinders are calculated using the cylinder 32 volume equation given in equation (3.1)

2 Vcylinder = (π ∗ r ∗ h) (3.1)

The volume of the outer cylinder is found to be 1490.85 inch3 and the volume of the inner cylinder is 496.21 inch3. The difference between the two volumes will give the total volume of the cells and the cell walls. The next step is to fix the cell wall thickness in order to find the dimensions of the cell. The cell wall thickness was determined to be a quarter inch based on initial guesses and is a free variable to be changed in later design revisions. The cell rotor is considered to have 26 cells, this results in total 26 cell walls. The thickness of each wall

Wcellwall being 0.25 inch. The cell wall height is calculated using the inner and outer diameter of cylinder. The height will be the radial diﬀerence between the outer and inner diameter with a small cell wall housing gap. The cell wall housing gap is set to two percent of the radial distance between the inner and outer diameter. The cell height Hcell is given by equation (3.2).

Hcell = 0.02 ∗ ((Douter/2) − (Dinner/2)) (3.2)

The height of the cell wall is calculated to be 3.23 inch. The total volume of the cell wall can now be calculated using the rotor length, cell wall width and the cell wall height. Equation

(3.3) shows the cell wall volume calculation.

Vcellwall = 26 ∗ Hcell ∗ Lcell ∗ Wcellwall (3.3)

The total volume of the cell wall is calculated to be 0.0985 inch3. this leaves a total of 0.56 inch3 of volume for 26 cells. From this calculation the volume of each cell can be obtained.

Once the total volume of individual cells are calculated, the next task is to ﬁnd all the dimensions of the cell. The cell length is the given design input and the cell radial height is calculated using equation (3.2). The missing dimension at this point is the cell width. The cell width can be easily calculated with the known values of volume, length and the height. Equation (3.4) shows the cell width calculation.

33 Vcellwall = Vcellwall/(Hcell ∗ Lcell) (3.4)

The complete summary of the cell characteristics of the thermal compression engine calculated using the basic design data is presented in SI units in table 3.2.

Sr. no. Cell characteristics Value Unit

1. Number of cells (n) 26

2. Outer Diameter of rotor (Douter) 0.39624 m

3. Inner Diameter of rotor (Douter) 0.2286 m

4. Axial length of rotor (Lcell) 0.19812 m 5. Cell wall thickness (t) 5.08x10−3 m

6. Cell wall- housing gap 3.35x10−3 m

7. Total cell wall volume 2.15x10−3 m3

8. Total volume of cells 1.42x10−2 m3

9. Single cell Volume 5.44x10−4 m3

10. Cell radial height 8.21x10−2 m

11. Cell width 3.34x10−2 m

Table 3.2: Cell characteristics of the rotor

3.4.2 Nozzle and Diﬀuser Sizing

The nozzle connects the combustion chamber to the cell and allows uni-directional ﬂow

of hot gases from the combustion chamber to cell during pressure equalization process. The

diﬀuser is another opening connecting the combustion chamber and the cell, this allows uni-

directional ﬂow of compressed gases from the cell to combustion chamber during the pressure

exchange sub-process. The length of these two openings have a high impact on the perfor-

mance of the engine and hence is a critical design parameter. The length of nozzle determines

the amount of hot gases entering the cell and thus the energy produced in the equalization

sub-process. The diﬀuser length determines the amount of hot gases entering the combustion

chamber for the next combustion process. 34 In the current design process, the engineering drawing given in von Ohain’s report is as-

sumed to be as per scale. The drawing depicting the cut section view shows same length of

opening for both nozzle and diﬀuser. The length of the openings are measured to be about 15%

of the total length of cell rotor. Considering that drawing is drawn to scale, the opening length

for the nozzle and diﬀuser are calculated to be 15% of the total cell length. The nozzle and diﬀuser opening in the 2D geometry will be 0.0297 m.

3.4.3 Sector Area

The forward looking aft and aft looking forward areas of the engine are the two circles which are split up into sectors corresponding to the engine process they serve. Figure 3-3 shows the aft face divided into four sectors. The scavenge area of the rotor engine was speci-

ﬁed to be 35-40% of the full frontal area by Von Ohain so for this engine the scavenge area was set to 30%. For the forward looking aft circle the only process happening at that plane was the scavenge process so a front plate was designed to be the full frontal area minus the scavenge area. The front plate covers the remaining 70% of the front area which is not involved in scavenging process. The pressure equalization area during which the hot air from the combustion chamber enters the cell and the exchange area where there is exchange of gases between the cell and combustion chamber, get a little bit more complicated.

The exchange sector area must overlap into the pressure equalization area as per the engine process. The nozzle connecting the combustion chamber to the cell remains open during pressure equalization as well as pressure exchange. Hence the the exchange area during which the diﬀuser connecting the cell to the combustion chamber opens, is part of the equalization area.

In the current design considered, the pressure equalization sector during which nozzle inlet is open, is sized to be 35% of the full frontal or aft area. The exchange section is sized to be 40% of the pressure equalization sector area. The expansion sector and the scavenge sector are the open sections on the aft looking forward circle and the expansion sector is set to be 35% of the total area. The back plate is supposed to cover the aft end of the cell rotor not contributing to scavenging or the exhaust process. The back plate is designed to be 35% of the total area.

This current distribution of the sector areas can be modiﬁed or reassigned after initial test-

35 ing/simulation to determine the optimal timing for the hot gas products to expand out of the cells. In order to arrive at the sector areas for each sub-process, the entire frontal area of the cell rotor is calculated using 3.5

2 A f rontal = π ∗ (Router) (3.5)

The outer radius of the cell rotor is used to calculate the full frontal area, A f rontal. Using the result of overall area the scavenging area, pressure equalization area and exhaust area are divided in the ratio of 6:7:7.

Sr. no. Parameter Value Unit

1. Outer radius of cell rotor 0.19812 m

2. Total frontal area 0.123 m2

1. Scavenging area 3.69 10−2 m2

2. Equalization Sector Area 4.32 10−2 m2

3. Exchange Sector Area 1.73 10−2 m2

4. Expansion Sector Area 4.32 10−2 m2

Table 3.3: Sector area Distribution

3.4.4 Timings

The desired rotor speed in rotations per minute was determined from the last tested thermal compression engine. According to the Von Ohain document, the last tested engine reached a maximum speed of 8000 rpm. The proposed engine is larger and would most likely not need to spin at the exact same rate. For the current design von Ohain has suggested a RPM of 11,000.

This value is considered to calculate the timings for each sub-process. The desired rotation was broken into rotations per second and then seconds per rotation was calculated.

The sector area ratios of each process was used to determine the amount of time a cell would spend in each process during one rotation. These timings will be used in the simulation to determine process start and end times. Since the sector area for pressure equalization and exhaust is the same the time required for both the processes are equal. The scavenging would 36 require slightly less time compared to pressure equalization and the exhaust sub-processes. The exchange process will have the least available time compared to other three sub-processes and is a part of pressure equalization.

Table 3.4 shows the complete time requirement for each sub-process in one cycle of operation of thermal compression engine.

Sr. no. Parameter Value Unit

1. Desired RPM 11000

2. Rotations per second 133.3330

3. Seconds per Rotation 54 ms

4. Scavenge Time 16 ms

5. Pressure Leveling Time 19 ms

6. Exchange Time 7 ms

7. Expansion Time 19 ms

Table 3.4: Sector area Distribution

37 Chapter 4

Simulation study and performance

analysis of thermal compression engine

In this chapter, the complete methodology of the numerical simulation, the results obtained

and the performance of the thermal compression engine is presented.

Numerical computation is a method of representing any physical model on computer using

mathematical models[41]. Simulations are used in various ﬁelds such as astrophysics, weather

forecast, medical ﬁeld to gain new insight into technology. There are simulations which run

quickly and produce instant results and some take months to mimic the working of physical sys-

tem. The simulation results are stored and can be post-processed to give graphs, visualizations

and also numerical data. The numerical simulations have come hand in hand with technology

and is the current revolution in the ﬁeld of science.

4.1 Computation as an analytically approach

In the current study computational ﬂuid dynamics incorporating internal ﬂow is taken as

the analytically approach to ﬁnd the performance of the thermal compression engine. the nu-

merical simulation offers a high flexibility for design changes. Given that the TCE technology is not much explored and few test results are available it is good to have flexibility to changes.

In addition the availability of very little physical system data makes it impossible to set up experimentation without detailed analysis. Budgetary constraint also pose a hurdle to set up experimentation at this stage.

38 The general observation portrays CFD to be the best possible approach to deal with TCE at nascent stages of research. The thorough study reveals various pinch points associated with the numerical simulation. The simulation of unsteady ﬂow is complicated and time consuming. the solvers used to create numerical simulation has several limitations. It is important to select the appropriate solver which can give required results. The simulations are always associated with certain errors due to mesh sizing and time step sizing issues. Numerical computation is always a compromise between accuracy and computation cost. All physical phenomenons occurring in the actual process cannot be captured by simulations. Furthermore, physical data is required to validate the results obtained through simulations.

4.2 Numerical Simulation

Here, the geometry, methodology of simulation and the boundary conditions considered to simulate diﬀerent sub-processes are presented. After debating the pro’s and con’s of numerical simulation, a simple 2D transient simulation was decided to be the best ﬁt at this stage.

4.2.1 Geometry

The concept of the thermal compression engine is simpliﬁed to incorporate into a simple

2D geometry to set up a numerical simulation. The cross section of a single cell along with the combustion chamber is considered to be the basic model for simulation. The cell is modeled as a rectangular plane, the horizontal dimension is the length of the cell and the vertical dimension denotes the cell height. The combustion chamber is placed above the cell with dimensions being the same as shown in von Ohain’s drawing which is assumed to be to scale. The cell and combustion chamber are connected through two openings, the first opening is the nozzle in the front end from where the hot gases from combustion chamber enter the cell and the second opening is the rear diffuser which allows flow from cell to combustion chamber. The front and rear end of cell, nozzle and diffuser are designed to be opened and closed for different sub-processes of the simulation. Except the four openings all others sides are designated to be impermeable walls. Figure 4-1 shows the simplified geometry used for numerical simulation.

39 Figure 4-1: Simpliﬁed 2D Geometry Created for transient simulation using ANSYS FLUENT

The geometry was created using SoildWorks designer workbench. Three separate bodies are created, the cell, the combustion chamber and another separate piece continuing from the cell to track the interaction of the cell with the outer atmosphere. Each body is created into a

2D model by deﬁning a planar surface for the drawing. The nozzle and the diﬀuser openings are considered to be 0.0297 m each.

4.2.2 Simulation Setup and Reﬁnement Study

In the current study, the simulation is modelled as unsteady, two-dimensional, and axisymmetric viscous ﬂow. The energy model is activated to track the changes in energy level and viscosity is modelled using standard k-epsilon equations. After ﬁnalizing the geometry for the

2D simulation, a default mesh was initially generated. Later an extensive grid refinement study was conducted and the mesh size was fixed to be 1mm for the cell and the combustion chamber region and the mesh size gradually increases to 3mm as it reaches the atmosphere patch. The total number of elements are 0.42 million. Time step refinement study was conducted to fix the time step size to capture the process effectively. The Courant number was fixed to be unity and based on that the time step size was calculated to be 0.1 milliseconds. It was ensured that all the residuals are converging at required rate. Since the simulation is two-dimensional and the number of elements are not too large, the simulation converges in a reasonable time.

40 4.2.3 Simulation Sequence

The simulation of the engine process is setup using ANSYS ﬂuent. The simulation is in transient mode and each time step of the process is captured. The time required for each process was calculated using the total RPM and was summarized in the table 3.4. Those values are used to ﬁx the number of time-step for each sub-process.

The simulation is divided into four stages, each stage depicting one sub process of the cycle. The ﬁrst process to be simulated is the exhaust sub-process, it is followed by scavenging sub-process and later the pressure equalization sub-process is simulated and next comes the pressure exchange sub-process. Later the exhaust process repeats thus completing one full cycle of operation of the engine. The simulation is designed to capture the values of pressure and temperature in the cell and combustion chamber during each sub-process and to track the exit velocity of gases from the cell rear end during the exhaust process.

Figure 4-2: Boundary conditions for four sub-processes

41 4.2.4 Boundary Conditions and Sub-Process Simulation

1. Exhaust process:

The ﬁrst sub process considered to be simulated is the exhaust process. The expansion

stage exists over 19 ms as per the calculation shown in table 3.4. During the exhaust pro-

cess, the front end of cell remains closed and is modelled as a wall. The nozzle and diﬀuser

connecting cell and combustion chamber are closed and are again conditioned to be walls.

Impermeable wall boundary condition is used in the simulation setup for all the walls.

The data provided in the paper assumes considerable leakage, the simulation is designed

to rectify the losses caused due to leakage. The entire cell is patched to be ﬁlled with air at

1500 K temperature and 506626 N/m2 pressure before the start of exhaust process. This is

based on the assumption that the maximum possible temperature and pressure would not

exceed ﬁve times the ambient conditions. The rear end of the cell is opened to atmosphere

and the mesh intersection is created between the cell and combustion chamber, which al-

lows the ﬂow of hot gases from the cell into the atmosphere. At the end of 19 ms, the

exhaust process will come to an end.

2. Scavenging process:

The second sub-process to be simulated is the scavenging process. During this sub-

process, the front end of the cell is changed from impermeable wall to open entity and the

cell exit remains to be open to the atmosphere. The pre-compressed air from the super-

charging fan is allowed to enter the cell during scavenging sub-process. This is achieved in

the numerical simulation by creating a pressure inlet boundary condition at the cell entry.

The pre-compressed air is passed at a pressure of 124630 N/m2 through the inlet. Ideally,

the residual hot gases must be completely expelled through the open rear end at the end of

16 ms of scavenging. The exhaust and the scavenging process completes at total of 35 ms

in the simulation.

3. Pressure equalization process:

The third sub-process to be simulated is the Pressure equalization process. In this simu-

lation, both the front end and the rear end of the cell are selected to be walls. The noz- 42 zle connecting combustion chamber and cell for ﬂow of hot gases from the combustion

chamber, is kept open for the next 12 ms. Before starting the simulation, the combustion

chamber is patched to be ﬁlled with hot gases at 1500 K temperature and 506626 N/m2

pressure. Once the nozzle wall is open the simulation proceeds, the hot gases from com-

bustion chamber are pushed through the nozzle and enter the cell. The high pressure gases

entering the cell will cause increase in pressure and temperature within the cell. This pro-

cess produces energy required to sustain the rotation of the cell rotor. Ideally at the end of

third stage the pressure of pre-compressed air in the cell must be equal to the pressure of

the hot gases entering from combustion chamber. The simulation will complete 47 ms at

the end of pressure equalization process.

4. Pressure exchange process:

The ﬁnal process to be simulated is the pressure exchange sub-process. This occurs over

the last 7 ms. During this stage the nozzle and diﬀuser, both openings connecting the cell

and the combustion chamber are designed to be open. The fresh compressed gases in the

cell now ﬂows to the combustion chamber through the diﬀuser opening and hot gases from

the combustion chamber continue to rush into the cell through the nozzle. At the end of

this process, ideally the cell should be ﬁlled with hot gas from the combustion chamber

and combustion chamber should be ﬁlled with fresh compressed gases. This process end

after 7 ms and completes one complete cycle of operation of the thermal compression

engine over a time span of 54 ms.

After the end of 54 ms the boundary conditions of the simulation are changed to the that of the exhaust stage during ﬁrst step and the simulation is allowed to run for few more time-steps.

This is done in order to obtain the exit velocity value of the hot gases during the exhaust. The values obtained after the first cycle are more reliable compared to the values obtained in first step as the first step was simulated based on the assumption that the temperature and pressure of exhaust gases are five times ambient condition. The value of exit velocity obtained after first cycle is more realistic and accurate.

43 4.2.5 Simulation Results

Figure 4-3 and figure 4-4 represents the static pressure and static temperature of the thermal compression engine at the first time-step corresponding to first millisecond. The plots are obtained from fluent solver. The exhaust of hot gases has just started from the open rear end of the cell to the outer atmosphere. The pressure and temperature values will begins to decrease within the cell. This process occurs for 19 ms.

Figure 4-3: Static pressure plot - First Time-step

Figure 4-4: Static temperature plot - First Time-step

44 Figure 4-5 and ﬁgure 4-6 represents the static pressure and static temperature of the thermal compression engine at the nineteenth time-step corresponding to nineteenth millisecond. The pressure and temperature within the cell has fallen to almost 30-35% of the initial value by the end of exhaust phase. At this stage the boundary conditions are altered to start scavenging sub-process

Figure 4-5: Static pressure plot - Nineteenth Time-step

Figure 4-6: Static temperature plot - Nineteenth Time-step

45 Figure 4-7 and figure 4-8 represents the static pressure and static temperature of the thermal compression engine at the thirty fifth time-step corresponding to thirty fifth millisecond.At the end of scavenging, the cell would have reached ambient pressure and the temperature at the front end of the cell is ambient with slightly higher temperature at the cell exit. At the end of scavenging the pressure equalization begins.

Figure 4-7: Static pressure plot - Thirty ﬁfth Time-step

Figure 4-8: Static temperature plot - Thirty ﬁfth Time-step

46 Figure 4-9 and ﬁgure 4-10 represents the static pressure and static temperature of the thermal compression engine at the forty seventh time-step corresponding to forty seventh millisecond. At the end of pressure equalization the hot gases from combustion chamber enters the cell and increases the pressure and temperature of the fresh air in the cell. After this the ﬁnal step, pressure exchange takes place.

Figure 4-9: Static pressure plot - Forty seventh Time-step

Figure 4-10: Static temperature plot - Forty seventh Time-step

47 Figure 4-11 and figure 4-12 represents the static pressure and static temperature of the thermal compression engine at the fifty fourth time-step corresponding to fifty fourth millisecond.

At the end of pressure exchange, the cell is filled with hot gas from the combustion chamber and combustion chamber is filled with fresh compressed gases. Later the nozzle and diffuser are closed and expansion stage repeats to start next cycle.

Figure 4-11: Static pressure plot - Fifty fourth Time-step

Figure 4-12: Static temperature plot - Fifty fourth Time-step

48 Figure 4-13 shows the velocity plot at the end of 55 ms, the velocity across the cell exit is monitored for four to ﬁve iterations in the simulation by creating surfaces. The average value of velocity is calculated and the value obtained is used as the exit velocity in the performance and thrust calculation.

Figure 4-13: Exit velocity captured at the end of ﬁrst cycle

In case of expansion processes in the thermal compression engine there is a limiting factor which results in non-uniform exit velocities. The expansion primarily is caused due to pressure gradient between the hot gases and the ambient air outside the cell. As the gradient begins to drop, the exit velocity of hot gases also reduces and a state is reached where the gradient becomes zero and a part of hot gas residues tend to remain within the cell. Due to varying exit velocity from the cell there is no uniform thrust developed. This leaves the turbine with non-uniform exit velocity which has a bad eﬀect on the turbine eﬃciency.

von Ohain proposed an alternative of using expansion waves to aid in uniform expansion of exhaust gases. But the velocity of expansion waves depends on the temperature of gases in the cell, hence the expansion process timings may vary and affect the operation of the engine. In addition the exhaust is immediately followed by scavenging and the fresh air scavenged has diff rent velocity from that of hot gases and the expansion waves could create interference between the exhaust and scavenging process, which in turn affects the functioning of TCE [35].

49 4.3 Performance Parameters

To verify the accuracy of the simulation, it is necessary to compare it with physical testing.

At this stage, no physical test has been set up to verify the working of thermal compression

engine. The best possible way to verify the result at this stage is to verify the pressure and

temperature rise occurring in the cell during the pressure equalization process using thermody-

namic energy balance equation. The process of pressure equalization can be related to an open

system. The static temperature of hot gases in the cell after the pressure equalization process is

calculated using open system energy balance equation[28].

V2 gZ V1 gZ Q − W = m (U + P ∀ + 2 + 2 ) − m (U + P ∀ + 2 + 1 ) (4.1) s out out out out 2g 2 in in in in 2g 2

Since there is no out flow from the cell mout is zero and the first term vanishes. As there is no mechanical work done on the system Ws is zero. The elevation change can also be neglected and, using these changes in equation 4.1 the equation is simplified into

V1 mC (T − T ) = −m (U + P ∀ + 2 ) (4.2) p 2 1 in in in in 2g

In equation 4.2, m indicates the combined mass of gases in the cell and gases entering from combustion chamber. The right-hand side of the equation consists of initial condition of the hot gases entering from the combustion chamber. The physical condition of the gases entering the cell from combustion chamber are known values. Using these values, the temperature in the cell after pressure exchange, T2 is calculated. The temperature after pressure exchange is found to be 873.41 K.

The average static temperature of hot gases in the cell after the hot gases enter from the combustion chamber during pressure exchange process is measured from the simulation, and is found to be 974.36 K. Although there is observable diﬀerence between the calculated and

the simulated values, the simulation provides visual evidence of the process. And also the

simulation assumes surface average of temperature values while the theoretical study assumes

50 mass averaged temperature values. Which is one of the reasons for variation of temperature.

Therefore values obtained from simulation are used for further calculations.

Since the combustion chamber and cell form a hermetically sealed space during pressure

exchange process, the volume of gases within the cell and combustion chamber remains con-

stant, as per constant volume combustion process the change in pressure is proportional to

change in temperature.

P T = h (4.3) P0 T f

The pressure ratio is found from Equation 4.3. Using the temperature values before and

after pressure exchange sub-process, the energy developed in the pressure exchange process

and the exhaust process are calculated.

γ−1 Th γ Th γ E2 = RT f − 1 − − 1 (4.4) T f γ − 1 T f

The energy release in the partial process 2 - pressure equalization stage is calculated using equation 4.4. The energy obtained during this process is transferred to the cell rotor with certain eﬃciency. This energy is used to sustain the auto-rotation of the cell rotor and is also used to drive the supercharging fan.

γ−1 γ T f γ Th E4 = RTh 1 − − RT f − 1 (4.5) γ − 1 Th T f

The energy release in the sub process four expansion stage is found using Equation 4.5.

The total energy released in one complete cycle is the sum of energies obtained from pressure

equalization and exhaust process. The heat released per unit mass of working substance is

given by equation 4.6.

γ−1 P γ W = Cp Th − T f (4.6) P0

In the current study, the numerical simulation setup does not include the combustion model and the solver is not setup to capture heat release data. Hence calculation using equation 4.6

51 has no data for comparison. Since the equation is a function of known parameters theoretical calculation is carried out to ﬁnd the heat per unit mass of working substance.

The thrust of the thermal compression engine is calculated using the limited data available from simulation. The exit velocity of the residual gases from the cell rear end is obtained by tracking the velocity magnitude at the exit surface. The maximum thrust is calculated by using the average values of the exit velocity for four time-steps.

Thrust of an engine in simple terms is given by[36]

dm T = V (4.7) exit dt

dm Where Vexit is exit velocity and dt represents the mass ﬂow rate. Mass ﬂow rate can be expressed as

dm = ρAV (4.8) dt exit

Where, A is the surface area of cell exit, and ρ is density of gas exiting the cell. Equation

4.8 gives the thrust value for a single cell. Considering that the engine has total of twenty six cells and that about 35% of them participate in exhaust event at a given time the total thrust obtained is found considering nine cells.

4.4 Results and Discussion

The current study consists of a simpliﬁed and concise approach to demonstrate the working of thermal compression engine. To keep it simple, the combustion model and heat transfer model has not been been included in the current simulation. The working engine has been demonstrated by examining the temperature and pressure values during each sub-process. As there is no actual experimental data available for the current design, veriﬁcation of all the simulation results remains a challenge. The best possible way to verify is to consider temperature values obtained from the simulation at end of the pressure equalization process and compare it to theoretical values calculated for pressure equalization process by using open system energy balance equation. The temperature value calculated from the open system energy balance equa- 52 tion is compared with the average temperature in the cell after pressure equalization obtained

from the numerical simulation.

About 12% variation is seen in simulation value and calculation results. Simulations are

always associated with certain error due to mesh and time step reﬁnement issues and in current

case 2D transient simulation is considered in contrast to actual 3D process. Considering the

simpliﬁcations in the simulation, a 12% variation can be justiﬁed. The simulation results are

used to calculate various performance parameters of the thermal compression engine. The

results are tabulated in Table 4.1

Sr. no. Parameter Value Unit

1. Pressure Ratio 3.25

2. Energy released in partial process 2 72959.66 J

3. Energy released in partial process 4 86169.02 J

4. Total Energy released in one cycle 159126.67 J

5. Work done by working substance 555427 W

6. Thermal eﬃciency 28.57%

7. Exit velocity of hot gases 300 m/s

8. Thrust produced by singe cell 309.96 KN

9. Total Thrust of the engine 2789 N

Table 4.1: Performance parameters of thermal compression engine

The overall energy released in one complete cycle of operation is the sum of energy release during pressure equalization and exhaust process. The overall energy of about 160 KJ is released of which 73 KJ is during pressure equalization where kinetic energy is converted to mechanical and 87 KJ during the exhaust. The total energy achieved is used to operate the supercharging fan and to sustain the rotation of the cell rotor. The ideal thermal eﬃciency is

calculated to be 28.57%. von Ohains paper has suggested that the overall thermal eﬃciency

can be between 18-22% by maintaining the pressure ratio ranging between 4 and 5. In the cur-

rent study pressure ratio of 3.25 has been recorded and the eﬃciency calculated is ideal value.

Assuming frictional losses and energy losses occurring during transformation from one form 53 to another the thermal eﬃciency must fall into suggested range. The thrust obtained from the thermal compression engine designed using the data provided for a turbo-prop type is 2.78 KN.

The current research provides a pipeline to further research using 3D simulation and experimental testing of thermal compression engines. It has provided a brief demonstration of the working and some sample calculations to provide the estimate of the performance parameters for the design suggested by von Ohain. After analyzing the results obtained in current scenario it can be concluded that the simplicity of construction and good controllability of this engine can be used in applications involving average thrust values.

The thrust value is almost 50% of thrust obtained by currently used commercial turboprop engines [32]. This design of the engine was planned in 1948 and is completely different from current technology. The concept of turbine was not introduced and no nozzles were used to increase the exit velocity and in turn enhance the thrust. The maximum thrust value obtained is directly calculated from the exit velocity of hot gases from cell rotor. Several modifications should be made to fix the low thrust issue and variable thrust due to non-uniform exit velocity before incorporating this design into todays aviation technology.

54 Chapter 5

Design and Working of Pulsejet

Combustors

In this chapter, an introduction to pulsejet types and their working mechanism is presented.

Pulsejets are light weight, easily build-able, low cost propulsion systems which gained fame during the world war. They can be constructed with very few movable parts and an advantage associated is that they produce same thrust when at rest[5]. The pulsejets experience high levels of vibration and also high noise due to intermittent pulsating operation. Pulsejets require highly heat resistant material for their construction as the body becomes really hot during operation.

5.1 Types of Pulsejets

The pulsejets are mainly of two types. The valved pulsejets or the traditional type of pulsejets and the valveless pulsejets, technically referred to as the acoustic type or aerodynamically valved pulsejets . Both these types work on the same common principle of intermittent combustion and the combustion products expelled out which creates thrust in the opposite direction and the working is sustained through series of compression and expansion waves created as a result of combustion and exhaust throughout the process. The only variation in the two types of pulsejet engines is the mechanical design and arrangement to accommodate the inlet ﬂow of fuel and air and also the exhaust ﬂow of combustion products. A brief explanation of the design and working of both the types are presented. The current study deals with numerical simulation of valved type pulsejets.

55 5.1.1 Valveless Pulsejets

The ﬁrst valveless pulsejet was developed in 1909 by Georges Marconnet and a french

propulsion research group named SNECMA (Socit Nationale d’tude et de Construction de

Moteurs d’Aviation) experimentally tested the working [43]. The most successful application

of valveless pulsejet was in Dutch drone Aviolanda AT-21 [30].

The valveless pulsejets are very simple in construction without any moving parts associated.

The absence of mechanical valves makes it lighter than the valved pulsejets. The geometry is

designed and constructed to control the inlet and outﬂow through the engine openings. Once

constructed and optimized, the geometry requires no maintenance over years of operation. The

valveless pulsejets are manufactured in diﬀerent geometries and sizes owing to their propose

and application. In case of valveless pulsejets the main diﬀerence is that the exhaust of the

combustion products occurs through both inlet and the exhaust valve. Although the major

portion of exhaust takes place through the exit.

The working of the valveless pulsejet is similar to that of valved pulsejet except for the

exhaust flow direction which in turn affects the thrust generation. Initially the fuel particles are atomized and are mixed with the air, the air-fuel mixture is passed to the combustion chamber and the first combustion is initiated by using spark plug or any other alternative ignition source.

Once the combustion takes place, two pressure waves are generated as a result of high pressure resulting from the explosion. One of the pressure wave moves in the forward direction towards the exit of the pulsejet. The other wave moves in backward direction towards the inlet. By tuning the puslejet accurately, resonating combustion can be achieved. In design of valveless pulsejets the inlet is inverted to face the direction of the exit. This helps in enhancing the thrust value by the exhaust gases passing out through the inlet. Some valveless pulsejets designs have shown large fuel consumption, where as certain designs are fuel eﬃcient.

Figure 5-1 shows a model of valveless pulsejets initially constructed by SNECMA, Ec- copette Pulsejet. The first type called Escopette pulsejet, has a detachable tube placed in the left end of the pulsejet, during the inlet phase the detachable part is placed as shown in the figure and the air enters the tube through the sides. During the exhaust phase the tube is attached to the inlet forming the inverted tube which directs the exhaust flow in the right direction. This

56 model also creates variable length during operation. This model was used in 1950 to propel

ﬂying manned glider called Emouchet, proving the good reliability to the engine[8].

Figure 5-1: Eccopette - valveless pulsejet

Figure 5-2 shows another model of valveless pulsejets constructed by SNECMA, Ecrevisse

Pulsejet. This is a conventional valveless pulsejet with inverted inlet or the U-inlet. it is composed of a single part with a tube being attached to the inlet section. Fuel air mixture is supplied through the attached tube during the inlet phase. The spark plug is used to initiate the combustion, once the combustion occurs pressure waves generated passes each through the inlet and the exit. The exhaust gases are expelled through both openings with major part passing through the exit. The exit tube is longer compared to the inverted inlet.This engine was actually used to run the propulsion tests of the Coloptre, an aircraft during the 1950s. But due to excessive fuel consumption the use of this engine was suspended[8].

Figure 5-2: Ecrevisse - valveless pulsejet

57 5.1.2 Valved Pulsejets

In this section, the design details, construction and operation of valved pulsejet is provided.

The valved pulsejet are slightly more complicated in design as compared to valveless pulsejets.

They consist of few movable parts which increase the complexity of design. The valved pulse-

jets weigh more than the valveless types and are mostly fuel eﬀective compared to valveless

pulsejet. The valved pulsejets consists of a one way valve placed in the inlet section to allow

unidirectional ﬂow of the fuel-air mixture. The arrangement of the valves are made in such a

way that they are sensitive to pressure.

Once the combustion occurs and the pressure within the combustion chamber experiences

the peak, the valves shut down and thus form a closed inlet. This results in all the exhaust

gases passing through the outlet of the pulsejet engine. Once the exhaust is expelled out and

the pressure within the combustion chamber falls below ambient condition, vacuum is created

which opens the valves and allows the air-fuel mixture for the next cycle. Although the concept

of valved pulsejet is to stop the blow back of exhaust gases through the inlet valve there is some

percentage of leakage that occurs during the closing and opening of the valves.

The most common type of valves used are daisy type or the rectangular valve grid type.

The daisy type consist of fragile thin metal sheet like structures designed in the shape of daisy

petals which are used to cover the circular openings in events of pressure peak. These are easier

to construct on a small scale but are not eﬀective as compared to rectangular valve grid type.

The rectangular grid type consists of twelve to fourteen rectangular metallic strips connected to the rod. They are pressure sensitive and ﬂip to cover during high pressure and open up during vacuum conditions. The size and number of strips vary with each design. This type is more reliable and robust and are commonly used as reed valves.

In case of valved pulsejet combustors the frequency of operation is a function of length of the engine. The small pulsejet are said to have an operational frequency of about 250-300. As the size of the engine increases the frequency experiences a downfall. The frequency of the famous buzzing bomb was about 45 pulses per second. It was a valved pulsjet engine used in

V-1 ﬂying bomb. Figure 5-3 shows the details of valved pulsejet combustor.

58 Figure 5-3: Valved Pulsejet construction

5.1.3 Working of Pulsejet Combustor

One prominent reason for the pulsejets to remain a relatively under-explored area despite their advantages is that the exact mechanism of pulsejet operation is not well-established. There are multiple hypotheses explaining the working of pulsejet using quarter wave theory[16],

Helmholtz Resonator theory[33], and theory suggesting that the hot walls of the pulsejet help in sustaining the combustion process[18].

Pulsejet combustors are simple in construction and working. They are low cost propulsion systems with high noise levels. the pulsejets do not contain any external devices like compres- sors to compress the working ﬂuid they make use of resonance for its operation. The pulsejets undergo simple processes such as ignition, combustion, intake and compression in one complete cycle of operation.

1. Ignition:

The ﬁrst process that occurs in the pulsejet engine is the ignition, once the fuel-air mixture

is ﬁlled in the combustion chamber an external source of trigger is utilized to ignite the

mixture. Usually spark plugs are used as source of ignition. As a result of trigger the initial

explosion occurs around the source and results in sudden peak pressure and temperature.

59 Figure 5-4: Ignition in valved pulsejet

2. Combustion:

Once the fuel air mixture is triggered the combustion proceeds through the length of air-

fuel mixture until the major portion of it is burnt out. During this phase the hot products of

combustion expand and are expelled through the exit of the pulsejet. The hot gases exiting

with high speed creates an opposite force called thrust in the forward direction.

Figure 5-5: Combustion in valved pulsejet

3. Intake:

The hot gases exiting through the outlet continues to occur even after the pressure within

the chamber reaches the ambient condition. This might be due to elastic nature of air

which has a tendency to stretch or it may be due to expansion waves reﬂecting from the

outlet which reduces the pressure in combustion chamber. The eﬀect of this process results

in vacuum inside the combustion chamber. The tendency of the vacuum is to pull the

gases. As a result the fuel air mixture is sucked through the inlet and the gases from the

atmosphere are pulled inwards through the exit opening. This concludes the intake phase.

60 Figure 5-6: Intake in valved pulsejet

4. Compression:

Again assuming the elastic nature of the gas molecules, the gas being pulled inward from

the exit opening continues to occur until it is forced to end. The fuel-air mixture from

the inlet continues to enter as long as the pressure within the combustion chamber is less

than the ambient value. Once the pressure reaches the ambient condition the valves are

shut. The air ﬂow from the exit continues and compresses the incoming ﬂow and results in

pressure rise. The pressure rise combined with high temperature of walls and the exhaust

gases create suitable conditions for fresh combustion thus repeating the cycle.

Figure 5-7: Compression in valved pulsejet

The current study mainly focuses on the combustion and operation of valved pulsejets and the eﬀect of geometry on the performance parameters. Detailed description of the design, simulation setup and the operating parameters are deﬁned in the next chapter.

61 Chapter 6

Design, Numerical Computation and

Analysis of Pulsejet Behaviour

This chapter mainly focuses on the pulsejet geometry, simulation technique and methodology and the results obtained. The simulation results are compared with the experimental results obtained from separate experimental study[7] conducted on same geometries as used in the simulation.

The computational advancement has helped to simulate the complete working of pulsejet engine using advanced numerical tools. The simulation provides time and space resolved dynamics of pressure and combustion inside the pulsejet combustor and thus helps in better understanding of the mechanism[8]. Major advantages associated with simulation is that it oﬀers high ﬂexibility with geometry, the boundary condition can be easily varied in minimum time and the cost associated with simulation is very less compared to physical testing.

Chapter 2 has mentioned about various numerical simulations performed on the pulsejets in the past and the parameters study in those simulations. In current study a 2D transient simulation is setup using ANSYS and Fluent solver. The simulation is designed to capture multiple physical phenomenon occurring during one cycle in the pulsejet operation. The combustion model is used to simulate the combustion process, as a result of combustion the compression waves created are observed in the simulation through pressure variation and the same holds good for expansion waves. The opening and closing of inlet valve is simulated by using the pressure inlet boundary condition and the impermeable wall boundary condition at the intake zone respectively . 62 6.1 Geometry

The computation is conducted on three diﬀerent geometries, with each conﬁguration having

fixed dimensions of the combustion chamber and the length of the tailpipe being short, medium and long are considered respectively. All the configurations are designed with a divergent flare at the end of the tail pipe. The two dimensional geometries of the pulsejet combustors of three different configurations are created using SolidWorks design workbench. Every configuration of the pulsejet consists of combustion chamber and the tailpipe as major components.

In the beginning of the simulation in order to initiate the combustion process, a small block of the combustion chamber is separated from main geometry and is designed to work as the ignition zone as shown in ﬁgure 6-1. The tailpipe starts from the combustion chamber at one end and connects with the atmosphere on the other end. A patch of atmosphere is created in the geometry as shown in Figure 6-1, to track the expansion wave formation from the exit and to track the interaction of the pulsejet with the outer atmosphere. Axis-symmetric geometry is considered for simulation in the current case. This will reduce the computation time by half and also reduce computation cost. The dimensional details of the three geometries considered are given in detail in table 1.

Figure 6-1: Geometries of three pulsejet conﬁgurations

63 In case of all three conﬁgurations the diameter of the combustion chamber (Dc) and the length of the combustion chamber (Lc) remains the same. The diameter of the tailpipe is also same for all three conﬁgurations. The only parameter varying is the length of the tailpipe

◦ (LT ). The divergent ﬂare at the rear end of the pulsejet is designed to be at angle of 15 for all conﬁgurations. The dimensions of the ignition zone are (0.01*0.005) m2. The dimensions of the atmospheric patch created is (0.12*0.24) m2.

Part Parameter Value Unit

Combustion Chamber Length 0.118 m

Diameter 0.061 m

Short Tailpipe Length 0.3 m

Diameter 0.032 m

Medium Tailpipe Length 0.45 m

Diameter 0.032 m

Long Tailpipe Length 0.6 m

Diameter 0.032 m

Table 6.1: Dimensions for pulsejet combustors

6.2 Simulation Model

In the current study, the simulation is modelled as unsteady two-dimensional, axisymmetric viscous ﬂow. The energy model is activated to track the change in energy throughout the cycle of operation. The viscosity is modelled using the standard k-epsilon equations. All the parameters in k-epsilon model are maintained to be default values. In wall boundary condition, all the standard wall functions are considered. The combustion in pulsejet is simulated using the species transport model, all the reactions are considered to be volumetric. Propane is used as the primary fuel for combustion and air is used as oxidizer, propane-air two step reaction is used as the mixture material for combustion. Turbulence chemistry reaction is calculated using laminar ﬁnite rate. First order implicit method is used as the primary solving technique to solve the transient simulation. 64 6.3 Convergence Study

It is very important to check that the numbers obtained from the simulation are accurate and reliable. The mesh size, time step size and number of iterations must be chosen eﬀectively.

In computation, mesh size is always a compromise between the accuracy and the computation time. Too coarse mesh will result in inaccurate results and convergence issues. Fine mesh will always require more computation time and result in high computational cost but will give better results compared to coarse mesh. The accuracy is inversely proportional to mesh size. It is necessary to come up with an optimum size which is a good compromise between accuracy and computational cost. Thus in case of any simulation, mesh sizing and time step reﬁnement study is a mandatory step.

An extensive grid refinement study was conducted with the mesh size starting from 1E-03, the mesh size was gradually decreased and the effect of mesh size on the convergence and the variable value was studied. After several iterations the mesh size was fixed to be 800µm within the pulsejet region and the mesh size is gradually increased to 1mm by the time it reaches the atmosphere patch at the end of the tailpipe region. The total number of elements obtained in the small, medium and large tailpipe configurations after using the selected mesh size are 0.12,

0.2 and 0.28 million elements respectively.

Once the mesh size was ﬁxed the next task is to decide the optimum value of the time step.

Time step refinement study was conducted starting from the initial value of one second. The time step size was gradually decreased to fix an accurate time step size to capture the process effectively. The Courant number was fixed to be unity, which is the most precise value for the simulation to be effective. Based on the Courant number and the velocity of the flow domain the time step size was calculated. The time step size was decided to be 0.1 milliseconds. After

ﬁxing the mesh size, time step size and the Courant number it was ensured that the residuals are converging at required rate. Since the simulation is two-dimensional and the number of elements are not too large, the simulation converges in less time.

65 6.4 Output Setup

The main objective of the current study is to setup a simulation to demonstrate the working of three diﬀerent geometries of pulsejet combustors and to compare the values of pressure, temperature, frequency and the probable location of auto ignition obtained from simulation with the physical testing results obtained from a separate study pertaining to same geometries as simulation.

In order to establish a right method to compare the results, in all three conﬁgurations of the pulsejet combustors, seven diﬀerent port locations are marked in the simulation. These marked port locations are activated to track the static pressure and static temperature values at each time step during one complete cycle of operation.

Of the seven ports, the first three ports are placed within the combustion chamber and the coordinates are identical for all three configurations of pulsejets, as the dimensions of the combustion chamber geometry is fixed in the current study. The fourth port is placed in the transition zone where the the tailpipe continues from the combustion chamber forming a ta- pering zone. The co-ordinates for the fourth port are also same for all three configurations of the pulsejets. The remaining three ports are distributed in the tailpipe across its length and the location of ports in the tailpipe varies depending on the length of the tailpipe and are distributed to track pressure and temperature till the exit of tailpipe.

The exact locations of the ports for short combustor short tailpipe (SC-ST), short combustor medium tailpipe (SC-MT) and short combustor long tailpipe (SC-LT) are shown in table 6.2.

P1, P2, P3, P4, P5, P6 and P7 represent the seven locations of ports along the length of the pulsejet combustor.

P1 P2 P3 P4 P5 P6 P7

SC-ST 0.19 0.39 0.106 0.166 0.217 0.319 0.39

SC-MT 0.19 0.39 0.106 0.166 0.268 0.421 0.523

SC-LT 0.19 0.39 0.106 0.166 0.37 0.523 0.676

Table 6.2: Port location for Pulsejet Combustors

66 6.5 Simulation methodology

The simulation in the current study is setup to demonstrate one complete cycle of pulsejet operation. In summary it is designed to capture multiple compression and expansion waves and the opening and closing of inlet valve. The pressure waves that are generated because of the combustion, expansion waves generated once the compression waves strike the outlet and the compression wave reflecting from the inlet, are seen in the simulation. Later the fresh air and fuel will flow through the inlet, and fresh air flows through the rear end once depression sets in the combustion chamber. The suction force creating compression will slow down the flow from inlet. The above conditions resulting in new combustion can be seen in the simulation. The fresh combustion resulting in next pressure peak, completes one cycle. The complete details of each process is explained in this section.

The complete cycle of operation of the pulsejet engine is divided into two main steps in the simulation. The ﬁrst half of the simulation mainly concentrates on the generation of compression wave due to combustion and how the compression wave is reﬂected as a strong expansion wave and creates low pressure in the combustion chamber. The second half of the simulation shows the consecutive intake of fuel and air through the inlet on the onset of vacuum, and how a series of events results in the auto-ignition of the fresh feed. The auto-ignition ensures an ongoing process of pulsations.

1. Step 01:

In case of ﬁrst step, before the start of the simulation, the entire burner is initialized with

stoichiometric ratio of propane and air mixture. The ignition zone separated out within the

burner is initialized with a temperature of 2250 K and acts as source of ignition. Once the

simulation is initiated the propane ﬁlled in the combustion chamber with air as oxidizer,

undergoes combustion due to high temperature in the ignition zone. The combustion is

initiated around the walls of the ignition zone and proceeds in all the directions. The com-

bustion results in high pressure and generates two pressure waves one travelling towards

the inlet and one towards the exit of tailpipe. The compression wave hitting the wall at

the inlet is reﬂected as a compression wave. The compression wave moving towards the

67 outlet reﬂects as strong expansion wave. the tendency of the expansion wave is due reduce

the pressure, as a result the reﬂected expansion wave reduces the pressure in the pulse-

jet combustor. Once the pressure falls below ambient pressure, vacuum is created and the

ﬁrst step of the simulation comes to an end at this stage.

2. Step 02:

In actual scenario once the pressure in the burner falls below ambient pressure the valves

at the inlet open and fresh air fuel mixture ﬂows through the combustion chamber until

the pressure value is restored and shuts the valves thus ending the inﬂow. The second step

of the simulation is modiﬁed to mimic the actual process, the inlet which was designed

to be an impermeable wall in ﬁrst step is changed to pressure inlet boundary condition.

In case of pressure inlet, the pressure of the incoming ﬂow must be mentioned this will

automatically calculate the velocity of the ﬂow. The propane and air mixture at ambient

pressure is allowed in stoichiometric ratio again through the inlet until the pressure in the

combustion chamber is restored to ambient condition. Later the inlet is again changed

to impermeable wall. The eﬀect of expansion wave is that the pressure in the tailpipe

would have fallen below ambient condition at the rear end of the tailpipe. As a result

a compression wave would have started from back end. This compression waves slows

down the incoming air fuel and the second stage of simulation continues until the fresh

mixture undergoes auto ignition which is assumed to be due to to the combined eﬀect of

compression wave and high temperature.

This common simulation methodology is used for all there configurations considered. Ev- ery configuration varies in the time taken to create vacuum and subsequently have the auto- ignition process. The end of first step and the initiation of the second step is decided individu- ally for each configuration after a series of trial and error technique. The SC-LT configuration takes the maximum time to complete a cycle and SC-ST takes the least time. The details of the simulation and the results are discussed in the next section.

68 6.6 Results and Discussions

The simulation results for all the three different geometries of pulsejets are summarized using static pressure and static temperature plots at different time steps during one complete cycle of operation. The pressure plot is used to track the onset of compression waves and expansion waves. The temperature plot is used to track the progress of combustion and importantly to find the point of auto-ignition at the end of second step.

Figure 6-2 shows the static pressure plots for SC-MT and 6-3 shows the static temperature plots for SC-MT Configuration. All three configurations follow similar pattern of operation, except that the time required to complete one cycle and location of auto-ignition are different.

First plot is at the beginning of the simulation. The pulsejet is in an ambient pressure condition and the temperature is standard room temperature except for ignition zone which is patched to be 2500 K to initiate the combustion, as shown in the temperature plot a. Plot b has captured the combustion occurrence. The pressure waves generated due to the combustion are marked in the pressure plot b. Plot c shows the expansion waves generated once the compression waves strike the outlet. In plot d, the pressure in the combustion chamber has fallen below ambient pressure, the depression opens the valves and this marks the end of the first part of the simulation. Plot e shows the flow of fresh air fuel mixture from inlet initiating the second step of simulation. Plot f shows the expansion wave reflecting from the open end as a compression wave which slows the flow from inlet. In plot g, the compression wave and high temperature resulting in fresh combustion due to auto-ignition is shown. The location of fresh combustion is different for all three configurations. Plot h shows the pressure rise caused due to next combustion and the cycle repeats.

The computational results of the three configurations with varying tail pipe length in the present study are compared against the experimental results for same geometries obtained from a separate study[7]. Pressure data from seven ports, frequency of operation and the probable location of combustion of the three different geometric configurations are compared.

69 Figure 6-2: Static Pressure Plots from Simulation SC-ST

70 Figure 6-3: Static Temperature Plots from Simulation SC-ST

71 6.6.1 Comparison with Experimental Results

In a separate study, experimental analysis was conducted on twelve different geometries of the pulsejet combustor[7]. The main aim of the experimental study was to analyze the behaviour of the pulsejet. The configurations varied from each other in the length of the tailpipe, length of the combustion chamber and by adding flare at the aft-end. Piezoelectric pressure sensors are used to measure the pressure and ionization probe to measure the temperature.

The sensors are placed at seven locations along the length of the pulsejet. The location of sensors for the three conﬁgurations with varying lengths of tailpipe are exactly same as the locations that are marked in the simulation. The study measured the peak pressure, frequency of operation and the combustion activity in the pulsejets. An other interesting observation from the experimental study is that the probable location of combustion can be determined by identifying the ionization probe capturing the combustion data[7]. The study concluded by proposing an operational mechanism for valved pulsejet operation.

The pressure, frequency and the combustion data for short combustor, short medium and long tail conﬁgurations of pulsejet are used to validate the simulation results obtained. The data collected about the propensity of combustion and the location of combustion obtained from the ionization probe is used to verify the location of auto-ignition obtained from the simulation.

Figure 6-4: Pressure plots obtained from experimentation

72 Figure 6-4 shows the pressure plots obtained from the experimental study by placing pressure sensors at seven ports. The pressure readings are obtained over a period of few seconds and the data is not clearly visible to analyze. In order to make better comparison the pressure value is magniﬁed to 10x and only two cycles are considered for comparison. In case of the simulation the data is available until the auto-ignition of fresh feed which initiates the second cycle. Hence with the data available for two cycles in experimentation, the peak pressure value and frequency of operation of the pulsejet can be easily compared.

Figure 6-5: Pressure plots obtained from experimentation - 10x

Figure 6-5 shows the pressure plots for ﬁrst two cycles. The pressure plot shows that the pressure variation for the ﬁrst three ports follow the same pattern and are almost overlapping.

The fourth port to seventh port which are dispersed in the tailpipe, show little variation in the pattern with each port. It can be noticed that there is in-phase pressure increase within the first three ports in the combustion chamber and blip in pressure values in case of tailpipe. The peak pressure observed for the physical test is up to 0.4 bar for the SC-MT configuration. The data provided above is for SC-MT configuration but the experimental data is similar for all stable operation of pulsejets hence the same graph is used as standard for comparing all three cases.

73 Figure 6-6: Pressure plots obtained from Simulation for SC-ST

Figure 6-7: Pressure plots obtained from Simulation for SC-MT

74 Figure 6-8: Pressure plots obtained from Simulation for SC-LT

Figure 6-6 shows the pressure plots obtained from the simulation for short combustor short tailpipe. Figure 6-7 shows the pressure plots obtained from the simulation for short combustor medium tailpipe. Figure 6-8 shows the pressure plots obtained from the simulation for short combustor long tailpipe. The pressure values along the ﬁrst three ports almost have same values, as the distance of port increases from the starting point the pressure values tend to disperse more from the trend. This is observed from the physical testing results for pressure along the seven ports too. The in-phase pressure increase within the combustion chamber and blip in tailpipe pressure values for all three cases are in good agreement with physical test results obtained. A peak pressure of 0.14 bar is obtained for SC-ST, peak pressure of 0.2 for

SC-MT and peak pressure of 0.17 bar for SC-LT from the simulation. The peak value obtained from physical testing is around 3.5 to 4.0 bar the diﬀerence in value can be attributed to propane being used in simulation against gasoline for experimentation. The simulation completes one cycle of operation. In order to establish the periodic nature of pulsejets several cycles must be completed and the pattern of pressure waves must be observed.

75 Figure 6-9: Temperature plots obtained from Simulation for SC-ST

Figure 6-10: Temperature plots obtained from Simulation for SC-MT

76 Figure 6-11: Temperature plots obtained from Simulation for SC-LT

Figure 6-9 shows the temperature plots obtained from the simulation for short combustor short tailpipe. Figure 6-10 shows the temperature plots obtained from the simulation for short combustor medium tailpipe. Figure 6-11 shows the temperature plots obtained from the simulation for short combustor long tailpipe. It is again seen that the first three ports have almost same temperature over entire cycle of operation. The temperature rises from initial room temperature and reaches a peak value of about 2450K. The ports located near the initialization zone experience high temperatures earlier as compared to ports dispersed farther away. Once the expansion wave creates vacuum and fuel air flow starts through the inlet the ports near combustion chamber starts to cool down due to cool air inlet. The walls of the pulsejet away from inlet cools down but still retains temperature of about 500K. Since the simulation setup does not consider the heat transfer effect, it is unlikely that the auto-ignition of fresh feed is caused solely due to heat transfer from the hot walls of the combustion chamber. Auto-ignition seems to be a function of the mixture gradient inside the combustor. This explains the starting of pulsejet combustor with cold walls. The temperature plots for all three configurations follows the same pattern and attain almost comparable peak temperature.

The other interesting comparison to be made is the probable location of consequent combustions occurring in the pulsejet engine. The simulation results suggest that in small tail pipe

77 conﬁguration the fresh combustion occurs at the end of combustion chamber and the experimental results suggest propensity of combustion is high in the middle of combustion chamber.

In case of medium tailpipe conﬁguration, the auto-ignition of fresh feed has occurred in the transition zone between the combustion chamber and the tail pipe and the experimental results also suggest that the propensity of combustion is high at the transition zone. In case of long tailpipe, the combustion occurs in the end of combustion chamber for both simulation and experimental data. Table-3 shows the experimental results of the exact location of combustion propensity and percentage of occurrence for the three conﬁgurations considered. Figure 6-12 shows the simulation results in which the location of auto-ignition is marked. Figure 6-13 shows the simulation results in which the propensity of combustion and location is mentioned.

Figure 6-12: Auto ignition location for 3 cases of simulation

Figure 6-13: Physical testing results for location and propensity of combustion

78 The frequency of operation of pulsejet for short tailpipe (SC-ST) was observed to be 255Hz

in experimentation, the frequency value calculated for the simulation by using the time required

for one complete cycle is found to be 118. It can be observed that the value from simulation

has about 45% divergence from physical test frequency. The frequency of operation of pulsejet

for medium tailpipe (SC-MT) was found to be 215Hz in experimentation. It is calculated to

be 101 Hz from simulation. About 50% divergence is observed. The frequency of operation

of pulsejet for long tailpipe (SC-LT) was observed to be 190 Hz in experimentation; it is 72Hz

from simulation. A 60% divergence is observed in this case.

The simulation study currently performed is a two-dimensional transient analysis against

the three- dimensional actual scenario. The fuel used for physical testing was gasoline and

in case of simulation propane is considered. The diﬀerence in fuel used might result in con-

siderable change in value as density of propane and gasoline are 2.01 kg/m3 and 720 kg/m3 respectively. The burn rate for the two fuels are also diﬀerent, one ounce of gasoline is consumed in 4.18 minutes and one ounce of propane is consumed in 10.30 minutes[12]. Reaction

Rate Constant K, given by equation 6.1

−E K = Ae a (6.1) RT

Arrhenius Equation showed that the chemical rate/ time of reaction using propane is about

1.4 times that of gasoline. The ignition delay also plays an important role in determining

the operational frequency. Ignition delay depends on various factors, of which pressure is

vital factor. Since the peak pressure obtained from the simulation is less than experimental

value there is more delay in ignition which decreases the frequency of operation. Although

simulation tries to mimic the physical phenomenon completely, there are certain details that

cannot be captured. The interaction of the pulsejet with the atmosphere may not be captured in

the current simulation setup. The two-dimensional simulation setup will have large deviation

from actual three-dimensional experimental setup. A three-dimensional pulsejet combustor

designed with reed valves, fuel tube and ignitor can be numerically simulated to work as actual

engine. It will help in a realistic comparison and give more accurate values which can be

compared to physical testing results.

79 6.6.2 Pulsejet behaviour as Helmholtz resonator or quarter wave tube

The behaviour of pulsejet engine as a Helmholtz resonator or quarter wave tube has always

been questionable. Using the frequency values obtained from the simulation, we can make a

comparison with Helmholtz frequency calculated using equation 6.2[2]

r c S f = (6.2) 2Π VL

Where, f is the frequency of operation, C is speed of sound in air, S is surface area, v is the volume and L is the length. The above formula is used to ﬁnd the frequency of operation of Helmholtz tube with the given dimension details of the pulsejet combustor. This value can be compared to the frequency obtained from the simulation for the part when the inlet remains open. The frequency values calculated using the 6.2 for the pulsejet geometries having short, medium and long tailpipe are 286, 257 and 249 respectively. These values almost completely agree with experimental results and has same variation with respect to simulation as recorded in previous section. The frequency values calculated considering quarter wave tube are agreeable considering very low temperature of pulsejet operation. Which is not practically feasible. Hence it can be said that advanced simulation with acoustic model included should give frequencies matching the Helmholtz frequency values. Accurate comparison cannot be established considering current simulation frequencies.

6.6.3 Comparing with Vortex tube

By virtue of working mechanism and applications, pulsejet and the vortex tube seem to be completely diﬀerent from each other and do not form any basis for comparison. But closer

observation into their physics reveals that both of them have hot and cold ﬂow regions oper-

ating inside the same tube. In case of vortex tube, compressed gases are injected through the

tube opening in tangential direction. They are then accelerated thought the swirl tubes. The

peripheral gas passes through the other end and remains hot. The remaining ﬂuid is passed

through the inner swirl tubes with smaller radius, which results in increases speed. The energy

conservation principle ensures that the heat energy is converted into the kinetic energy thus

80 creating a cold flow. the hot and cold flow are separated [47]. In case of pulsejets the hot and cold region coexist due to their working mechanism, Combustion creates hot gases which enter the atmosphere though the outlet with small amount of gases passing out though inlet. During the second half of pulsejet operation, the vacuum created in the combustion chamber opens the valves and results in inflow of fresh fuel gas mixture. This divides the tube in hot-cold zone. Unlike vortex tube where the hot - cold regions are radially separated, pulsejets have axial hot-cold zones in contact. Separating the hot-cold zone in pulsejets will result in flame extinguishment and stop the operation [13].

6.6.4 Conclusion

The simulation and physical testing are giving comparable results for pressure variation, peak pressure and location of combustion onset, axially, in the pulsejet. The pattern of change in frequency of operation of the pulsejet combustors with the change in length of the tailpipe as obtained from the simulation is also in match with the frequency pattern obtained from experimentation. In both the cases the frequency decreases with increase in the length of tailpipe which is well justiﬁed. As the length of the tailpipe increases the expansion wave generated from the rear end will take time to reach the combustion chamber and decrease the pressure. In the same way the compression wave assumed to be causing the ignition of the fresh mixture resulting in new cycle will also be delayed with increase in the length of tailpipe. The magnitude of frequency of operation is the only parameter which is not in good agreement with the physical test results. It is possible that the simulation is not accounting for the acoustic frequency associated with the pulsejet operation, since acoustic solvers were not used in our simulation.

The auto ignition of the fresh feed is seemingly caused due to the combined effect of compression wave and high temperature. The expansion wave from the outlet reflects as a compression wave, which will slow down the incoming flow of the air fuel mixture. The high temperature inside the pulsejet engine also contributes in providing sufficient activation energy to the incoming fuel air mixture. Once required activation energy is achieved the auto-ignition of the fresh feed occurs. The location of auto ignition is different for each geometry and depends on the length of the tailpipe of the pulsejet combustor.

81 Chapter 7

Discussion and Future works

7.1 Thermal Compression Engine - Discussion

This chapter summarizes the previous studies conducted on the thermal compression engine and the pulsejet engine. It also suggests some research areas for future work. The current research provides a basic understanding of the propulsion technology used in TCE and pulse jet and the simulation helps in better understanding of their working and performance parameters.

In-depth learning is necessary to completely rediscover the technology and put it to implemen- tation in the future.

In case of TCE, a basic understanding of working of engine is established and simulation is setup to mimic the working. Using the design parameters from Von Ohain report the performance parameters are determined. Advanced studies and techniques to sustain combustion and achieving optimal thrust value, would result in a very simple propulsion system with no rotating parts. Some areas of future interest in case of TCE which require improvement are:

1. Modifications can be made in the basic simulation model to help in advanced studies, like using combustion module with different fuels, altering the geometries and also by varying the nozzle and diffuser throat size which connects the combustion chamber to the cell rotor and study each of its effect on the performance of the engine.

2. Since it is convenient to make changes to geometry in case of simulation, geometric optimization study can be performed by experimenting with diﬀerent valve lengths to decide the best design with optimal eﬃciency and thrust.

82 3. Once an optimized design is obtained, the next step would be to setup physical testing. It can be done by building up a basic unit with cell rotor, combustion chamber and connecting valves. For initial testing the fan and turbine can be eliminated. Depending on performance parameters obtained for diﬀerent cases the application of this concept can be concluded.

7.2 Pulsejet Engine - Discussion

Pulsejet Engines on the other hand have been an hot topic of research in the past decade.

Various research activity have been recorded in the ﬁeld of simulating the pulsejet and sustaining the combustion. The current study focuses on simulation with additional parameters like the static temperature, location of auto ignition compared to similar research studies. In addition the pressure data is also studied and compared to physical test results. Using the current simulation as the basic setup various advanced studies can be conducted to understand the behavior of pulsejet engines.

1. The variation in peak pressure and the location of combustion, when diﬀerent fuels are used in the pre-existing geometry is an interesting point to be studied.

2. The geometry of the combustion chamber can be varied and its eﬀect on pulsejet performance can be studied.

3. The flare designed at the end of the tailpipe in the current configuration can be eliminated and its effect on the operation can be studied. These studies would be helpful in finalizing an optimized design for the pulsejet engine with most efficient geometry and fuel combination.

83 Chapter 8

Curricular Practical training - Report

8.1 Introduction

This chapter provides a summary of my job role and responsibilities during my curricular practical training and how the skills gained through my research in Gas dynamics and propulsion laboratory has helped me develop deeper understanding of concepts in my job. I have been working as Product Development Engineer with Exel composites Starting from May 1st,

2018. The Product development engineer directly reports to the Director of engineering at Exel

Composites. The main goal of this position is to assist in full life cycle of product development starting from conceptualization till commercialization. The nature of work for Product development engineers would be to understand the customer requirement and suggest suitable design to match the requirements. Once the design is ﬁnalized analysis is conducted and the

final product, material and manufacturing process is fixed. Later manufacturing support is provided by creating drawings and bill of material. Once the final product is manufactured, quality analysis is conducted to assess the product. The Engineer is involved from cradle to grave for all products developed. Exel composites specializes in design, analysis and manufacturing of composite products for various applications including aerospace, automotive and mechanical parts mainly through pultrusion process. We at Exel work with various industries with prime focus being aerospace, wind energy and automobile. Few of our products include the blades of wind mills, aircraft structures, support rods for boats, automotive body components and brack- ets. Exel has also produced composite parts to construct aircraft wing, spar structures, T-C-I profiles and engine components. 84 8.2 Pultrusion process:

Pultrusion is a type of specialized composite manufacturing process mainly used to produce continuous lengths of reinforced polymers having constant cross-sectional area. Pultrusion consists of flexible textiles forming the reinforcement (example: carbon fiber and glass fiber) the reinforcement may also include fabrics such as carbon or glass mats. The matrix is provided by the resin bath which contains resin such as polyester, vinyl ester or epoxy. In addition to resin the bath also contains additives, catalysts and fillers. Additives are used to achieve required properties like color, UV resistance and fire resistance. Catalysts are used to speed the reaction and reduce the curing time and temperature. Fillers are mainly used in glass fiber pultrusion and help in filling the void space between matrix and reinforcement[46].

In the pultrusion process the fibers and fabrics are guided through guide plates with required tension to reach the resin bath, the reinforcements are wetted in the resin bath and the squeeze plates are used to remove the excess resin. The wet reinforcements are guided to enter the die maintained at appropriate temperature required for curing of the resin bath. The reinforcement and matrix begin to cure inside the die and solidified part emerges out of the die. The part is pulled out of the die using hydraulic pullers and the part is cut to required length. This provides a complete overview of the pultrusion process. The figure shows a schematic representation of pultrusion process[1].

Figure 8-1: Pultrusion Process Schematic Diagram

85 8.3 Research Focus:

I have spent the last four years gaining skills in the ﬁeld of CAD modeling, FEA and CFD simulations and product development. I am pursuing my masters in University of Cincinnati with focus on ﬂuids and gas dynamics. My masters thesis is mainly based on computational

fluid dynamic simulation of two engines involving the concepts of fluid flow, pressure, temperature, heat transfer, mixing and energy production. During my tenure as Product Development

Engineer at DSC, I have been involved in the design and manufacturing of tools and processes for pultrusion. The major steps involved in pultrusion are guiding the material through plate holes, resin bath dipping, forming through the die and ﬁnally curing to get the ﬁnished product. My focus area in the pultrusion process is related to resin bath preparation, curing and

ﬂuid mixture analysis. I have applied the principles and fundamentals of engineering acquired through my thesis and research into my role at DSC and helped in improving the process and design to create eﬃcient and robust products based on requirements and applications. The main engineering areas that related to my thesis are:

8.3.1 Resin Bath

1. Resin plays an important role in pultruded components. It holds the ﬁbers together provides a stable matrix for the reinforcements and determines the mechanical properties of

finished product. It is very important to study the resin properties and the mixing of resin along with additives and catalysts. The fundamental properties of fluids and its behavior under the effect of pressure and temperature has been part of my thesis. I have used the basic concepts of fluid dynamics and mixing of fluids to help with my research at Exel composites. The liquid bath containing resin, pigments, filler and catalyst at controlled pressure and temperature is studied by applying the principles of fluid dynamics.

2. The first study is to determine the viscosity of the bath and to do a detailed thermal analysis to understand the behavior of mixture. The increase in viscosity of resin mixture occurs above a certain temperature called transition temperature. Once that temperature has been achieved the viscosity of resin system exponentially increases and the fluid finally undergoes

86 curing and thus solidiﬁes. Various combinations of additives, ﬁllers and catalysts are tried and

optimum mixture with favorable curing temperature is decided. 3. Determining the curing

temperature and curing time is next study. Curing study of the resin mixture is a crucial step in

designing the pultrusion process, die temperature and die geometry. I have used the principles

of CFD to determine perfect resin system for multiple products[9]. The ﬁgure shows the vis-

cosity proﬁle of a polyester sample. At about 3800 F the resin starts to cure and instantly the

solidiﬁcation is triggered in the sample resin system.

Figure 8-2: Viscosity Curve - Polyester Sample

8.3.2 Heat Transfer

I have studied the principles and calculations for heat transfer and cooling in aircraft engines. Heat transfer and cooling will be an integral part of curing process in pultrusion. To produce quality products, it is important to ensure perfect curing of the reinforcement and matrix. This happens when the die is maintained at perfect temperature at the required location.

The heat transfer calculations are performed to come up with the temperature proﬁle along

87 the length of the die. Periodic thermostat analysis is also conducted by passing a thermostat through the die opening to verify the values of temperature proﬁle.

8.3.3 Design Capability

My background with CAD modeling and CFD simulation has helped me in designing the resin bath system and die opening using analysis and simulation softwares like using Solid

Works and Ansys to understand the eﬀect of pressure and temperature on ﬂuids and mixing.

Having worked on complicated mechanism such as propulsion with multiple interrelated processes and physics involved, I am conﬁdent to design tools and processes for pultrusion process to create products in more eﬃcient way.

Conclusively, I will be using the principles of fluid dynamics, effects of pressure and temperature on fluids, mixing, heat transfer and cooling alongside tools such as Solid Work and

Ansys to carry out my work at Exel. The Position at Exel has helped me get right industrial exposure and gain true sense of professionalism. It has sharpened my design and CFD skills which has in turn helped me improve my contribution to thesis. I ﬁrmly believe that my technical, interpersonal and presentation skills have been enhanced. At the end of my CPT (Curricular

Practical Training), I have dedicated this part of my thesis to the work and research carried out in designing an eﬃcient mechanism and work ﬂow for pultrusion at Exel Composites.

88 Chapter 9

Appendix

Conference Papers

• Shekhar, A., Zahn, A., Rodrigo, V & Gutmark, E.(2017, October). Theoritical study and

Numerical simulation of Von Ohain thermal compression engine. Presented in Internationl

Conference for jet wakes and separated ﬂows - ICJWSF 2017

• Shekhar, A., Anand, V., Stoddard, W & Gutmark, E.(2017, October). Numerical Sim-

ulation of Pulsejet Combustors. Presented in Internationl Conference for jet wakes and

separated ﬂows - ICJWSF 2017

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