Thermodynamic and Aerodynamic Analysis of an Air-Driven Fan System in Low-Cost High-Bypass-Ratio Turbofan Engine

energies

Article Thermodynamic and Aerodynamic Analysis of an Air-Driven Fan System in Low-Cost High-Bypass-Ratio Turbofan Engine

Weiyu Lu * , Guoping Huang, Xin Xiang, Jinchun Wang and Yuxuan Yang

College of Energy and Power Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China; [email protected] (G.H.); [email protected] (X.X.); [email protected] (J.W.); [email protected] (Y.Y.) * Correspondence: [email protected]

Received: 27 March 2019; Accepted: 16 May 2019; Published: 20 May 2019

Abstract: In some cases, the improvement of the bypass ratio (BPR) of turbofans is pursued for military or civilian purposes owing to economic, environmental, and performance reasons, among others. However, high-BPR turbofans suffer from incompatibility of spool speed, complex structure for manufacture, development difficulty, and substantially increasing costs, especially for those with small batch production. To deal with the issues, a novel low-cost concept of high-BPR turbofan with air-driven fan (ADTF) is presented in this research. First, the problems faced by high-BPR turbofans are discussed, and the difficulties of geared turbofan (GTF), which is developed as a solution to the problems, are analyzed. A novel turbofan with potential advantages is proposed, and its basic theory is interpreted. Second, high-BPR ADTF is analyzed at the top level, and the design principle and important primary parameters are discussed. Some important concepts and criteria are proposed, enabling the comparison between ADTF and GTF. Finally, an air-driven fan system, the core part of ADTF, is exploratorily designed, and numerical simulation is performed to demonstrate its feasibility.

Keywords: turbofan engine; high bypass ratio; low cost; air-driven fan

1. Introduction Airbreathing propulsion systems, such as turbofan engines, are widely used for civilian and military purposes. Since the 1990s, aviation companies have been suffering from a serious operating cost problem due to high fuel prices. Also, the future of this industry is affected by the growing environmental concerns. Restricting noise, limiting engine exhaust gas emissions, and reducing toxic wastes challenge the industry with the increasingly stringent environmental regulations [1]. To protect the environment, the Advisory Council for Aeronautics Research in Europe has set specific goals in the aviation industry to be achieved by year 2020 [2]. The engine requirements toward these goals include 80% NOx emission reduction, 20% fuel consumption (and CO2 emissions) reduction per passenger-kilometer, and 10 dB noise reduction per certification point, with the data in the year 2000 as the basis of comparison [3]. Therefore, decreasing fuel consumption is of great interest [2]. Aspects of fuel economy, emissions, and noise continue to compel the aircraft industry to search for fuel-efficient, low noise, and pollutant-free aircraft engines [1]. Although further improving the efficiency of the parts of turbofan engines is difficult, increasing propulsive efficiency by increasing bypass ratio (BPR) (equivalent to reducing exhaust velocity) decreases specific fuel consumption [2], which also contributes to the reduction of emissions. In addition, the reduction of exhaust velocity lowers noise level. Apart from turbofans, open rotors, propfans, and unducted fan engines can also be used in improving fuel consumption as their propulsive efficiency is better than that of turbofan

Energies 2019, 12, 1917; doi:10.3390/en12101917 www.mdpi.com/journal/energies Energies 2019, 12, 1917 2 of 17 engines owing to their low exhaust velocities [4]. However, flight speed is the main limitation of these concepts. For a conventional propeller with straight blades, compressibility effects play a significant role and decreases propeller efficiency when the flight Mach number is above 0.6 [2]. Thus, this research focuses on turbofans to guarantee high-efficiency aircrafts at high subsonic flight speed. To summarize, the improvement of the BPR of turbofans may solve the main issues of fuel economy, emissions, and noise in the civil aviation industry. With respect to military uses, different missions have different demands on airbreathing propulsion systems. In recent years, small/mid-size low-cost unmanned air vehicles (UAVs), cruising missiles, and small aircrafts have been proven to be of great importance [5,6]. In this study, airbreathing propulsion systems used for UAVs and missiles have something in common with civil engines. Other types of engines, such as low-BPR turbofan used in fighter engines, are not included here. Although UAVs have military or civilian purposes, such as combat, data gathering, surveillance missions [6], hurricane science, and communication relay [7], this study focuses only on their military uses. High performance, which means long endurance, long range, and high speed, is important for small UAVs, especially for military uses, entailing more requirements on their propulsion systems. High-BPR turbofan is a good option for propulsion systems because of its advantage of low fuel consumption and high speed. The improved performance of small turbofans must be within the constraints of manufacturing costs and engine operational durability [8]. Therefore, the increase in fuel economy is realized through improvement of aerothermodynamic components or thermodynamic parameters, such as BPR, and the increase of BPR really matters for the application of UAVs in military use (some kinds of missiles are similar) to further increase mission range and endurance. Nevertheless, the improvement of the BPR of turbofans faces difficulties for civilian and military purposes. For civil turbofans, potential benefits, such as actual fuel saving, due to improvement of BPR are limited. Fan core speed incompatibility and low-cost contradiction rise as two major problems, which will be discussed in detail in the following passage. Due to the incompatibility of spool speed, the conventional turbofan design becomes unfavorable for BPR well above 10 [9]. To solve this problem, geared turbofan (GTF) configuration has been accepted as a promising candidate for the future engines with BPR above 10 [10]. Introducing a reduction gear between the fan and LPC/LPT system enables each low-pressure (LP) component to rotate at its optimum speed in terms of efficiency, stage loading, and noise [3]. However, the introduction of the gearbox also brings complicated structures and so-called low-cost contradiction. Thus, the optimal design of civil turbofans with high BPR is a tradeoff among specific fuel consumption, complexity, and cost. Similar problems also exist in turbofans for military purposes, such as for small UAVs. In designing turbofans for this purpose, several main aspects, such as performance (including range, endurance, and speed), reliability, and cost (including manufacturing cost, development costs, maintenance cost, and fuel consumption), are considered and balanced [11]. So, fan core speed incompatibility and low-cost contradiction must also be settled in some cases for military purposes. Thus, the main issues of fan core speed incompatibility and low-cost contradiction concerning the improvement of BPR of turbofans are discussed in detail below.

1.1. Fan Core Speed Incompatibility Problem Given a signiﬁcant increase in BPR, the turbofan must reduce its spool speed to maintain its fan tip speed as the radius increases for the same engine core. This, in turn, increases the number of stages for the LPC and LPT to maintain appropriate eﬃciencies and pressure ratios for these parts. In addition, reduced LP spool speed requires high torque of the LP shaft, and this will lead to big shaft diameters and increased core size. Furthermore, in order to run a bigger diameter fan, LP turbine diameter and its number of stages must increase [1,2,8,9]. In conclusion, the fan of high-BPR turbofans prefers low spool speed, but the LP turbine driving the fan prefers high spool speed. This contradiction is hard to solve, and we need a compromise to harmonize these two factors. Energies 2019, 12, 1917 3 of 17 Energies 2018, 11, x FOR PEER REVIEW 3 of 17

Three-rotor scheme and GTF aim to solve the incompatibility problem. In this section, GTF is Three-rotor scheme and GTF aim to solve the incompatibility problem. In this section, GTF is taken as an example to show collateral shortcomings and difficulties when solving the taken as an example to show collateral shortcomings and difficulties when solving the incompatibility incompatibility problem to some extent. To solve the fan core speed incompatibility problem, a problem to some extent. To solve the fan core speed incompatibility problem, a reduction gear system reduction gear system is introduced in GTF, to decouple the fan from the rest of the LP system (as is introduced in GTF, to decouple the fan from the rest of the LP system (as illustrated in Figure1a). As illustrated in Figure 1a). As a result, the fan and LP system can operate at their preferred optimum a result, the fan and LP system can operate at their preferred optimum speeds. With regards to the speeds. With regards to the relief of the incompatibility problem, the advantages of GTF, such as relief of the incompatibility problem, the advantages of GTF, such as high component efficiencies of high component efficiencies of LPT, intermediate pressure compressor and LPC, low fan tip speed LPT, intermediate pressure compressor and LPC, low fan tip speed that also results in low fan noises, that also results in low fan noises, and low diameter with less stages of LPT, are apparent when and low diameter with less stages of LPT, are apparent when BPR is high [1]. All disadvantages of BPR is high [1]. All disadvantages of GTF are involved with the introduction of gearbox and GTF are involved with the introduction of gearbox and gearbox efficiency, including a more complex gearbox efficiency, including a more complex structure with an attachment system, such as fuel structure with an attachment system, such as fuel cooling capacity, development and manufacture cooling capacity, development and manufacture difficulty, reliability, and durability problem, as difficulty, reliability, and durability problem, as BPR further increases [1]. BPR further increases [1].

Figure 1. LayoutLayout of of geared geared turbofan (GTF) and air-driven turbofan (ADTF). (a) GTF; (b) ADTF. 1.2. Low-Cost Contradiction 1.2. Low-Cost Contradiction A bottleneck exists, whereby the so-called incompatibility of spool speed limits the increase of A bottleneck exists, whereby the so-called incompatibility of spool speed limits the increase of BPR. Solutions to this problem, such as the three-rotor scheme and GTF, result in increased structural BPR. Solutions to this problem, such as the three-rotor scheme and GTF, result in increased complexity and development and manufacture cost, especially for some cost-sensitive or small-batch structural complexity and development and manufacture cost, especially for some cost-sensitive or production situations, such as military and scientific uses. For example, shafting structures, bearing, small-batch production situations, such as military and scientific uses. For example, shafting and lubrication systems are complex in the three-rotor scheme. Furthermore, development is extremely structures, bearing, and lubrication systems are complex in the three-rotor scheme. Furthermore, difficult because only Rolls Royce can manufacture RB211, and the three-rotor scheme suffers from development is extremely difficult because only Rolls Royce can manufacture RB211, and the high maintenance cost. Moreover, the great difficulty of GTF lies in the design and manufacture of three-rotor scheme suffers from high maintenance cost. Moreover, the great difficulty of GTF lies in high-power gear box. Pratt and Whitney took nearly a decade to develop the first GTF, PW8000G [12], the design and manufacture of high-power gear box. Pratt and Whitney took nearly a decade to which is based on a prototype PW8000. Thus, under comprehensive consideration, the performance develop the first GTF, PW8000G [12], which is based on a prototype PW8000. Thus, under and cost of high-BPR turbofans contradict each other, and fine balance routinely contributes to the comprehensive consideration, the performance and cost of high-BPR turbofans contradict each optimum choice of BPR. other, and fine balance routinely contributes to the optimum choice of BPR.

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1.3. Thinking of A Solution While thinking about other ways to solve the fan core speed incompatibility and low-cost contradiction, we first noticed that whatever the specific structure is, turbofans share the same principle; that is, the higher the bypass mass flow rate to which the core transfers energy, the less specific fuel consumption (SFC) and high performance the turbofans have. GTF solves the fan core speed incompatibility problem. The gearbox has two functions, reducing the spool speed and transferring energy from LP turbine to fan. Thus, solutions that satisfy the two functions are also promising. However, the turbofan theory does not restrict the means of transferring energy. Any method that can transfer energy from core to bypass, if consistent with the physical theories and reduce the fan speed, may be used for high-BPR turbofan. This concept should use mature techniques and should not introduce many complex structures to limit the cost and relieve the low-cost contradiction. For example, tip turbine is used to solve this problem in some cases [13,14]. Thus, to solve the two main issues, a novel concept of high-BPR turbofan with air-driven fan (ADTF) is proposed to increase BPR. In this concept, energy is transferred in an aerodynamic way rather than mechanical way (such as in GTF). Compared with the three-rotor scheme and GTF, this concept has simpler structures and may be promising to overcome the difficulties and uncertainty, balancing the contradiction between performance and cost.

2. Concept of High-BPR ADTF In this section, a novel concept of high-BPR ADTF is introduced. For a traditional two-spool turbofan, the fan is directly driven by LPT. In addition to the traditional two-spool turbofan, GTF adds a reduction gear box to transfer energy from the core while reducing the LP spool speed, as illustrated in Figure1a. The new turbofan concept we propose is also derived from a traditional two-spool turbofan, but the gear box is replaced by an air turbine, which is surrounded by red dashed lines as illustrated in Figure1b. The new concept is named air-driven turbofan or ADTF. ADTF has five important components, namely, additional fan rotor (1), additional fan stator (2), air turbine rotor (3), air turbine stator (4), and rotational casing (5), as shown in Figure1b. Other parts in Figure1b share the same parts of the traditional two-spool turbofan or GTF shown in Figure1a. Therefore, ADTF can be easily developed from a prototype traditional two-spool turbofan and save much design cost. Such design has already been authorized as a Chinese invention patent [15]. To further interpret the basic theory of air-driven fan system, the velocity triangles of the fan and air turbine are shown in Figure2. The outer flow path includes additional fan rotor and its stator. At Section1, the air from the atmosphere only has axial velocity C 1. Meanwhile, its relative velocity is W1 in the relative coordinate system of additional fan. After flowing through the additional fan rotor, the air is pressurized with its circumferential velocity increases from 0 to Cu12. The stator before Section2 changes the absolute velocity of the air from C12 to C2 and the static pressure increases. Moreover, the inner flow passes by air turbine rotor and its stator. Air turbine is the main part that extracts energy from moderate pressure air. A natural and traditional idea is to introduce a prototype fan stator to transfer circumferential velocity into static pressure and an air turbine stator to accelerate the flow and produce circumferential velocity for the air turbine rotor. Inspired by a patent [16], this idea is replaced by a currently better scheme in which the air turbine stator is installed downstream from the air turbine rotor. As a result, the part of the prototype fan stator can be saved, and complexity and weight can be reduced. At Section3, the air has circumferential velocity components. The air turbine rotor decelerates the circumferential velocity while extracting energy. The air turbine stator is aimed to eliminate possible circumferential velocity. Meanwhile, in some cases, its circumferential velocity component is so small that the air turbine stator can be cancelled or just replaced as a support plate. EnergiesEnergies 2018, 201911, x, 12FOR, 1917 PEER REVIEW 5 of5 17 of 17

Figure 2. Velocity triangles of the rotors and stators in the ADTF. Figure 2. Velocity triangles of the rotors and stators in the ADTF. A new concept of high-BPR ADTF and its theory are outlined in this section. To design a high-BPR AADTF new with concept better performanceof high-BPR and ADTF understand and theits advantagestheory are and outlined disadvantages in this of section. ADTF, conceptual To design a high-BPRand thermodynamic ADTF with better analysis performance of high-BPR ADTFand unde is presentedrstand inthe the advantages next section. and disadvantages of ADTF, conceptual and thermodynamic analysis of high-BPR ADTF is presented in the next section. 3. Thermodynamic Analysis of High-BPR ADTF

3. ThermodynamicAfter the proposal Analysis of high-BPR of High-BPR ADTF, ADTF we must know the design criteria that direct us to design a well-performing ADTF and inform us how design parameters affect one another. For convenience Afterof analysis the proposal and comparison, of high-BPR some special ADTF, parameters, we must such know as effitheciency design of equivalent criteria that aerodynamic direct us to designreduction a well-performing gear box (EARGB) ADTF and eandffective inform BPR, areus definedhow design and discussed parameters using theaffect equivalent one another. method. For convenienceAt the end of of analysis this part, and armed comparison, with the design some criteria specia andl parameters, theoretical analysis, such as thermodynamic efficiency of analysisequivalent aerodynamicis performed reduction to evaluate gear the box eff ects(EARGB) that diff anderent effective main parameters BPR, are have defined on the and performance discussed of ADTFusing the equivalentbased onmethod. a prototype At the turbofan. end of Athis rough part, comparison armed with between the design ADTF criteria and GTF and is then theoretical made, further analysis, thermodynamicshowing the analysis characteristics is performed and application to evaluate scope th ofe ADTF. effects that different main parameters have on the performance3.1. Criteria for of Design ADTF Parameters based on a prototype turbofan. A rough comparison between ADTF and GTF is then made, further showing the characteristics and application scope of ADTF. For a prototype turbofan that will be adapted to an ADTF, three main design parameters are 3.1 Criteriaknown, for namely, Design mass Parameters flow rate in the bypass of prototype fan m f 0, total pressure ratio of prototype fan T π f 0, and total temperature at the outlet of prototype fan ∗f 0. The air-driven fan system (including the Foradditional a prototype fan and turbofan air turbine) that has will four be main adapted overall to design an ADTF, parameters: three Mass main flow design rate ofparameters additional are known,fan mnamely,f , total pressuremass flow ratio ofrate additional in the fanbypassπ f , mass of flowprototype rate of airfan turbine m f 0 , mtotala, and pressure total pressure ratio of drop ratio of air turbine π . Figure1b shows that m = m . Thus, only three design* parameters must prototype fan π , and totala temperature at the aoutletf 0of prototype fan T . The air-driven fan be determined.f 0 Now, we discuss the constraints for these design parameters. f 0 system (including the additional fan and air turbine) has four main overall design parameters: Mass (1) Power balance relation π flow rate of additional fan mf , total pressure ratio of additional fan f , mass flow rate of air Power balance means that the power that air turbineπ produce equals to that needed= by the turbine ma , and total pressure drop ratio of air turbine a . Figure 1b shows that mmaf0 . Thus, additional fan. Therefore, only three design parameters must be determined. Now, we discuss the constraints for these design   parameters.  1  k 1 m C T   = m C T ( −k ) a p ∗f 01 k 1 ηa f p 1∗ π f 1 /η f (1)  − −  − (1) Power balance relation πa k where T is the ambient total temperature, η is the isentropic efficiency of additional fan, η is the Power balance1∗ means that the power thatf air turbine produce equals to that neededa by the isentropic efficiency of air turbine, C is isobaric specific heat capacity of air, and k is ratio of specific additional fan. Therefore, p heats of air. 1 k −1 mCT**（1(1)/−=）η mCT πηk − apf01k −1 a f p f f (1) π k a

* η η where T1 is the ambient total temperature, f is the isentropic efficiency of additional fan, a is the isentropic efficiency of air turbine, Cp is isobaric specific heat capacity of air, and k is ratio of specific heats of air.

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(2) Thrust condition The thrust produced by the air-driven fan system is determined by its application scenario. Figure1b shows that the thrust of air-driven fan system consists of two parts, one produced by the additional fan and the other produced by the air turbine. According to the formula of nozzle thrust:

q r k 1 k 1 . − . − F = ma 2CpT (1 (πa/π ) k ) + m 2CpT (1 (1/π ) k ) (2) a∗ − f 0 f ∗f − f where F is the required thrust of the air-driven fan system, Ta∗ is total temperature at the outlet of air turbine, and T∗f is total temperature at the outlet of additional fan. According to the formula for power of turbomachinery, we deduce that

 k 1  T = T [1 (1 1/π −k )η ]  a∗ ∗f 0 a a − −k 1 . (3)  −  T = T [1 + (π k 1)/η ] ∗f 1∗ f − f We combine Equations (2) and (3) to obtain the following thrust condition: r r k 1 k 1 k 1 k 1 . −k − . −k − F = ma 2CpT [1 (1 1/π )ηa](1 (πa/π ) k ) + m 2CpT [1 + (π 1)/η ](1 (1/π ) k ). (4) ∗f 0 − − a − f 0 f 1∗ f − f − f

(3) Total pressure relation To design a high-performance high-BPR turbofan, potential losses must be minimized. For the newly proposed concept of high-BPR ADTF, some apparent aerodynamic losses can be easily reduced due to fine primary parameter design, but not until the stage of detailed aerodynamic design. When the flow exits the air turbine, it will mix with the bypass flow, thereby resulting in the loss of mixture. To avoid this flow loss as much as possible, the total pressure at the outlet of air turbine must be the same (or nearly the same) as that at the outlet of the additional fan, which is the total pressure relation shown as π f = π f 0/πa. (5)

In summary, three main independent design parameters m f , π f , and πa satisfy the relations described by Equations (1), (4), and (5). These design parameters are then determined because the number of unknown parameters is equal to that of the equations.

3.2. Theoretical Analysis ADTF is an entirely new concept, which changes the thermodynamic cycle of the prototype turbofan and lacks mature primary parameters analysis method and model in commercial gas turbine performance simulation software. To quickly determine the parameter sensitivity of ADTF, the equivalent method is used, treating ADTF as a variant of GTF. Two new and unique parameters are then proposed to realize this equivalence. The efficiency of EARGB in ADTF is equivalent to the efficiency of a reduction gear box in GTF, whereas effective BPR in ADTF is equivalent to BPR in GTF. The two parameters are proposed and deduced in the following passage, and primary parameters are then analyzed using commercial gas turbine performance simulation software GasTurb [17]. (1) Analysis of power transfer efficiency In order to improve propulsion efficiency, turbofans do their best to transfer the available energy from core to bypass. When core engines of turbofans are the same, SFC is only dependent on BPR and power transfer efficiency, which is an important index that reflects how well the energy is transferred Energies 2019, 12, 1917 7 of 17 from core to bypass. For a normal turbofan or GTF, power transfer efficiency can be expressed simply as multipliers: ηt = ηmηgeη f 0ηo (6) where ηt is the power transfer efficiency (from core to bypass), ηm is the mechanical efficiency, ηg is the gear box efficiency, eη f 0 is the power transfer efficiency of the prototype fan, and ηo is the power transfer efficiency of bypass duct. Usually, eη f 0 , η f 0. The function of pressure ratio is defined as

k 1 A = π −k 1. (7) −

Then, the relation between eη f 0 and η f 0 is

. k 1 m C T ( −k ) f 0 p ∗f 0 1 1/π f 0 A f 0 + η f 0 = − = eη f 0 k 1 . (8) . −k A f 0 + 1 m CpT (π 1)/η f 0 1∗ f 0 − f 0

Thus, eη f 0 = η f 0 only when η f 0 = 1 or A f 0 = 0. Moreover, the relation between ηo and commonly used σo (total pressure recovery coeﬃcient of bypass duct) is:

. k 1 k 1 m C T ( ( ) −k ) −k f 0 p ∗f 0 1 1/ σoπ f 0 A f 0 1/σ + 1 = − = − o ηo k 1 . (9) . −k A f 0 m CpT (1 1/π ) f 0 ∗f 0 − f 0 For ADTF, the air-driven fan system uses a part of the available energy from the core turbofan, so the thermodynamic cycle parameters of the equivalent turbofan are changed. Imitating Equation (6), the power transfer eﬃciency of the equivalent turbofan can be expressed as

ηt = ηmηegeη f 0ηo (10) where ηeg refers to the efficiency due to the added parts, such as air turbine and additional fan of ADTF, which only depend on the design level of turbomachine; and ηeg corresponds to ηg in form, so it is named the efficiency of EARGB. (2) Efficiency of EARGB and effective BPR For ADTF, through energy transfer analysis, we know that

. k 1 ( + )( −k ) Cp m f T∗f maTa∗ 1 1/π f = − ηegeη f k 1 (11) −k maCpT (π 1)/η 1∗ f 0 − f 0 where eη f is the power transfer eﬃciency of the additional fan. According to Equations (1), (3), (5), and (7), we can deduce that A f 0 A f A f A f 0 + η f 0 ηeg = ( − η f ηa + ) . (12) A f 0(A f 0 + 1) A f 0 A f + η f

According to this equation, when η f 0 = η f = ηa = 1 and ηeg = 1, no loss occurs in the power transfer process; when η f = ηa = 1 and ηeg = (A f 0 + η f 0)/(A f 0 + 1), the loss in the power transfer process only comes from the fan of main engine. Moreover, the range of ηeg is determined as

A f 0 + η f 0 A f 0 + η f 0 eη f 0ηa = ηa < ηeg < = eη f 0. (13) A f 0 + 1 A f 0 + 1 Energies 2018, 11, x FOR PEER REVIEW 8 of 17 Energies 2019, 12, 1917 8 of 17

This air-driven fan system can change the BPR of the prototype turbofan. If the BPR of the

prototypeThis air-driventurbofan is fan B system, then its can mass change flow theat the BPR inner ofthe duct prototype is mBa / turbofan.. According If to the Equation BPR of the(1), prototypethe effective turbofan BPR of isADTFB, then is shown its mass as flow at the inner duct is ma/B. According to Equation (1), the effective BPR of ADTF is shown as +−+ηηη mmfa A f000 AA ffffa BBB==+. . (1 ) >. e + η (14) m f +mB afffm/1a A f A0 A f A Af000 + η f 0 η f ηa Be = . = (1 + − )B > B. (14) m /B A f A f + 1 η f The effective BPR of ADTFa is larger than the prototype0 BPR,0 and the air-driven fan system servesThe as eaff kindective of BPRBPR of“amplifier.” ADTF is largerWith both than para themeters prototype of efficiency BPR, and of theEARGB air-driven and effective fan system BPR, servesprimary as aperformance kind of BPR “amplifier.”parameters Withcan bothbe parameterseasily analyzed of effi ciencyvia the of EARGBexisting and mature effective overall BPR, primaryperformance performance analysis parametersmethod. can be easily analyzed via the existing mature overall performance analysis method. 3.3. Thermodynamic Analysis of High-BPR ADTF 3.3. Thermodynamic Analysis of High-BPR ADTF To thermodynamically analyze the main parameters of ADTF, a prototype turbofan model is usedTo (see thermodynamically Reference [18] and analyze Table 1 the fore main the parametersmain parameters). of ADTF, a prototype turbofan model is used (see Reference [18] and Table1 fore the main parameters). Table 1. Main parameters of prototype turbofan. Table 1. Main parameters of prototype turbofan. Parameters of Prototype Turbofan Values ParametersTotal of PrototypeMass Flow Turbofan (kg/s) 95.3 Values BPR 5.3 Total Mass Flow (kg/s) 95.3 Fan Pressure Ratio 1.7 BPR 5.3 Fan PressureFan Efficiency Ratio 0.90 1.7 FanHPC E ffiPressureciency Ratio 13.5 0.90 HPC PressureHPC Efficiency Ratio 0.85 13.5 HPCHPT Effi Efficiencyciency 0.88 0.85 HPTLPT Effi Efficiencyciency 0.88 0.88 BurnerLPT Exit ETotalfficiency Temperature (K) 1400 0.88 Burner Exit Total Temperature (K) 1400 The influence of efficiency of EARGB on fan pressure and SFC is shown in Figure 3. When otherThe main influence parameters of effi remainciency ofthe EARGB same, onas effici fan pressureency of andEARGB SFC drops, is shown the in energy Figure delivered3. When other from mainLPT to parameters fan goes remaindown, thedecreasing same, as fan effi ciencypressure of EARGBratio. The drops, decrease the energy of fan delivered pressure from ratio LPT also to fanincreases goes down, SFC. For decreasing example, fan when pressure efficiency ratio. Theof EARGB decrease is of approximately fan pressure ratio 0.9, alsoSFC increases increases SFC. by Forapproximately example, when 5%, which efficiency means of EARGBthat the iseconomy approximately of ADTF 0.9, is SFCacceptable. increases Equation by approximately (13) shows that 5%, η ==η η whichthe efficiency means that of theEARGB economy has ofa ADTFlow islimit. acceptable. When Equationfa0 (13)0.9 , showsthe minimum that the e ffiofciency eg ofis EARGBapproximately has a low 0.82, limit. and When SFC dropsη f 0 = ηbya =approximately0.9, the minimum 10%. of Thisηeg isfinding approximately means that 0.82, under and SFCthe dropscondition by approximatelyof high efficiency 10%. of Thisthe aerodynamic finding means pa thatrts, underthe economy the condition of ADTF of highis acceptable efficiency even of the in aerodynamicthe extreme case. parts, the economy of ADTF is acceptable even in the extreme case.

Figure 3. Fan pressure ratio and delta speciﬁc fuel consumption (SFC) versus eﬃciency of equivalent aerodynamicFigure 3. Fan reduction pressure gear ratio box and (EARGB). delta specific fuel consumption (SFC) versus efficiency of equivalent aerodynamic reduction gear box (EARGB).

Energies 2018, 11, x FOR PEER REVIEW 9 of 17

Energies 2019, 12, 1917 9 of 17

Given that ηa, η f 0, and η f remain constant and are equal to 0.9 (around the current design level in reality), the efficiency of EARGB and effective BPR only depend on the prototype fan pressure ratio and additional fan pressure ratio based on Equations (12) and (14). If ADTF is based on an existing, mature turbofan, as shown in Table1, then the prototype fan pressure ratio is fixed, and only the additional fan pressure ratio is a variable. As additional fan pressure decreases from 1.6 to 1.1, the efficiency of EARGB drops from 0.98 to 0.86 and effective BPR increases from 6 to 27 because the pressure-drop ratio of the air turbine and the mass flow rate of air the additional fan drives increase (See Figure4). In addition, Figure5 shows that as additional fan pressure decreases from 1.6 to 1.1, SFC drops from 3%Energies to 40%, 2018, whereas11, x FOR PEER netthrust REVIEW increases from 3% to 67%, mainly due to the influence of efficiency9 of of17 EARGB and effective BPR.

Figure 4. Efficiency of EARGB and effective bypass ratio (BPR) versus additional fan pressure ratio.

η η η Given that a , f 0 , and f remain constant and are equal to 0.9 (around the current design level in reality), the efficiency of EARGB and effective BPR only depend on the prototype fan pressure ratio and additional fan pressure ratio based on Equations (12) and (14). If ADTF is based on an existing, mature turbofan, as shown in Table 1, then the prototype fan pressure ratio is fixed, and only the additional fan pressure ratio is a variable. As additional fan pressure decreases from 1.6 to 1.1, the efficiency of EARGB drops from 0.98 to 0.86 and effective BPR increases from 6 to 27 because the pressure-drop ratio of the air turbine and the mass flow rate of air the additional fan drives increase (See Figure 4). In addition, Figure 5 shows that as additional fan pressure decreases from 1.6 to 1.1, SFC drops from 3% to 40%, whereas net thrust increases from 3% to 67%, mainly due to the influence of efficiency of EARGB and effective BPR. Figure 4. Eﬃciency of EARGB and eﬀective bypass ratio (BPR) versus additional fan pressure ratio. Figure 4. Efficiency of EARGB and effective bypass ratio (BPR) versus additional fan pressure ratio.

Figure 5. Delta SFC and delta net thrust versus additional fan pressure ratio. Figure 5. Delta SFC and delta net thrust versus additional fan pressure ratio. Figures3–5 show an apparent dilemma. When designing an ADTF, choosing low additional fan pressureFigures means 3–5 low show SFC an and apparent high net dilemma. thrust, and When it also designing means low an e ffiADTF,ciency choosing of EARGB, low which additional creates fanhigh pressure SFC increase. means However, low SFC and choosing high net high thrust, additional and it fan also pressure means meanslow efficiency that the eofff ectiveEARGB, BPR, which SFC, createsand net high thrust SFC cannot increase. be notably However, changed. choosing Therefore, high additional proper additional fan pressure fan pressuremeans that must the be effective chosen BPR,to remarkably SFC, and reducenet thrust SFC cannot and limit be thenotably counteraction changed. ofTherefore, SFC due toproper low eadditionalfficiency of fan EARGB. pressure An mustadditional be chosen fan pressure to remarkably of 1.2 is reduce proper SFC in this and case, limit whereas the counteraction the effective of BPR SFC is due approximately to low efficiency 14.

Figure 5. Delta SFC and delta net thrust versus additional fan pressure ratio.

Figures 3–5 show an apparent dilemma. When designing an ADTF, choosing low additional fan pressure means low SFC and high net thrust, and it also means low efficiency of EARGB, which creates high SFC increase. However, choosing high additional fan pressure means that the effective BPR, SFC, and net thrust cannot be notably changed. Therefore, proper additional fan pressure must be chosen to remarkably reduce SFC and limit the counteraction of SFC due to low efficiency

Energies 2019, 12, 1917 10 of 17

3.4. Brief Comparison between ADTF and GTF The concept of EARGB has already been discussed, and its efficiency can be calculated according to Equation (12). For example, based on the prototype turbofan described in Table1, a proper set of parameters, π = 1.2, π = 1.7, and ηa = η = η = 0.9, makes ηeg = 0.89. Compared with ηg 0.99 in f f 0 f 0 f ≈ GTF, the energy transfer efficiency of ADTF is slightly low because ηeg 0.9, and SFC increases by ≈ approximately 5%, as shown in Figure3. However, ADTF can greatly increase BPR, even making it larger than 12, and this is still difficult for GTF. To conclude, ADTF applies to very-high-BPR (more than 12) turbofan well, dodging the proper BPR range of GTF. Apart from the slightly low energy transfer efficiency, ADTF has more advantages in structure complexity, development difficulty, and manufacture difficulty, dealing better with the fan core speed incompatibility problem and low-cost contradiction. The SFC of ADTF is slightly higher than GTF at the same BPR. However, less development and manufacture difficulty makes ADTF more suitable for cost-sensitive and small-batch situations, such as regional jets and small UAVs. The comparison between ADTF and GTF is shown in Table2.

Table 2. Comparison between ADTF and GTF.

Characteristics GTF ADTF Component Efficiency High High SFC (same BPR) Low A little higher than GTF Structure Complexity Very complex May be less complex Power Transfer Efficiency High ( 99%) Medium ( 90%) ≈ ≈ Development Difficulty High May be Medium Manufacture Difficulty High May be Medium Reliability and Durability Medium May be High Appropriate BPR High ( 10) High ( 12) ≈ ≥

4. Exploratory Design and Aerodynamic Analysis of an Air-Driven Fan System

4.1. Exploratory Design of An Air-Driven Fan System on the Basis of a Prototype Turbofan To demonstrate the feasibility of the ADTF concept, the case of air-driven fan system, which is the most innovative and doubtful section, is exploratorily designed on the basis of a prototype turbofan shown in Table1. Design targets are determined from the thermodynamic analysis. The appropriate total pressure ratio of the additional fan is selected as 1.2, and the mass ﬂow rate of air turbine is equal to that of the fan in the prototype turbofan (see Section 3.3), as shown in Table3.

Table 3. Design targets and simulation results of the air-driven fan system.

Components Parameters Design Targets Simulation Results Total Pressure Ratio 1.20 1.20 Additional Fan Mass Flow Rate (kg/s) 127.5 126.7 Isentropic Eﬃciency 90% 85.6% ≈ Total Pressure Drop Ratio 1.42 1.40 Air Turbine Mass Flow Rate (kg/s) 80.2 80.2 Isentropic Eﬃciency 90% 89.6% ≈

The design process of the air-driven fan system (shown in Figure6a) is the same as that of other turbomachineries. The process follows a preliminary design, throughflow design (design on S2 surface), 2D blading design (design on S1 surface), and 3D blading design [19,20]. During the throughflow design process, the rotational speed of the air-driven fan system is set as 5000 rpm at its design point. On the basis of the throughflow design, the 2D blade profiles of the fan and the air turbine at 0%, 50%, and 100% spans are designed. The blade profiles of the additional fan and the air turbine at the 50% span are shown in Figure7. Furthermore, the 3D blades are obtained by linear Energies 2018, 11, x FOR PEER REVIEW 11 of 17 Energies 2018, 11, x FOR PEER REVIEW 11 of 17 turbine at 0%, 50%, and 100% spans are designed. The blade profiles of the additional fan and the turbineair turbine at 0%, 50%,at the and 50% 100% span spans are shownare designed. in Figure Th e7. blade Furthermore, profiles of the the 3D additional blades are fan obtained and the by air turbinelinear interpolation at the 50% spanof these are threeshown sections. in Figure This 7. processFurthermore, is completed the 3D byblades the aerodynamicare obtained designby linearprogram interpolation of our group.of these The three camber sections. lines This of the process 2D blade is completed profiles are by madethe aerodynamic of multi-segment design arcs. programAlso, takingof our thegroup. angles The of camber attack andlines derivation of the 2D inbladeto consideration, profiles are madethe blade of multi-segment angles are determined arcs. Also,by taking iterations the anglesof numerical of attack simulation. and derivation Cubic in polynoto consideration,mial distribution the blade of anglesblade profileare determined thickness is by iterationsused, given of thenumerical radius ofsimulation. leading and Cubic trailing polyno edgemials, the distribution maximum of thickness, blade profile and itsthickness position. is For used,the given additional the radius fan, ofthe leading relative and Mach trailing number edge s,at theits maximumrotor tip is thickness, in a high and subsonic its position. state, Forso the the positionadditional of maximumfan, the relative curvature Mach and number thickness at itmusts rotor be shiftedtip is in backward a high subsonic appropriately state, toso avoidthe a positionstrong of shock maximum wave. curvature and thickness must be shifted backward appropriately to avoid a strong shockDue wave.to low pressure ratio in these parts, blade number and chord length greatly affect the performanceDue to low ofpressure these parts, ratio soin thethese parameters parts, blad aree optimizednumber and in thechord design length process. greatly The affect selection the of Energies 2019, 12, 1917 11 of 17 performancethe blade ofnumber these parts, should so ensurethe parameters that the aresolidity optimized of blades in the is designwithin process.a reasonable The selection range. In of this the paper,blade numberfor the additional should ensure fan, the that number the solidity of rotor of blbladesades is is 10 within and the a reasonablestator is 21; range. while Infor this the air paper,interpolationturbine, for the the of additional number these three of fan, turbine sections. the number blades This process isof 23 rotor and is bl completedtheades stator is 10 is by and 17. the Centroidthe aerodynamic stator stacking is 21; design while is used programfor mostlythe air of for turbine,ourthe group. blades. the Thenumber However, camber of turbine lines the fan of blades therotor 2D issweepsblade 23 and profilesforward the stator are in the madeis 17. middle Centroid of multi-segment and stacking sweeps backwardis arcs. used Also, mostly at taking the for tip to the weakenblades. angles ofHowever, the attack shock and the wave derivation fan inrotor the sweeps intopassage. consideration, forward In the end,in the the 3D middle blade geometry angles and sweepsof are the determined additional backwardby fanat iterationsthe and tip the to air weakenof numericalturbine the is shock simulation.obtained wave by in Cubiciteration the passage. polynomial design In based the distribution end, on numerical 3D geometry of blade simulation. profile of the thickness additional is used,fan and given the theair turbineradius ofisAfter leadingobtained the four and by iterationdesign trailing phases, edges,design an thebased entire maximum on air-driven numerical thickness, fan simulation. system, and its including position. the For additional the additional fan rotor, fan,additional theAfter relative the fourfan Mach stator,design number airphases, turbine at its an rotor entirerotor, tip air-drivenand is in air a highturbine fan subsonic system, stator, state,including is established so the the position additional and of3D maximum fanmodeled, rotor, as additionalcurvatureillustrated andfan in thicknessstator, Figure air 6b. must turbine be shifted rotor, backwardand air turbine appropriately stator, is to established avoid a strong and shock 3D modeled, wave. as illustrated in Figure 6b.

Figure 6. Air-driven fan: (a) 2D cut-open view, (b) 3D model, and (c) mesh for numerical simulation. Figure 6. Air-drivenAir-driven fan: fan: ( (aa)) 2D 2D cut-open cut-open view, view, ( (b)) 3D 3D model, model, and ( c) mesh for numerical simulation.

Figure 7. Blade proﬁles of the additional fan and the air turbine at the 50% span. Figure 7. Blade profiles of the additional fan and the air turbine at the 50% span. Figure 7. Blade profiles of the additional fan and the air turbine at the 50% span. Due to low pressure ratio in these parts, blade number and chord length greatly aﬀect the performance of these parts, so the parameters are optimized in the design process. The selection of the blade number should ensure that the solidity of blades is within a reasonable range. In this paper, for the additional fan, the number of rotor blades is 10 and the stator is 21; while for the air turbine, the number of turbine blades is 23 and the stator is 17. Centroid stacking is used mostly for the blades. However, the fan rotor sweeps forward in the middle and sweeps backward at the tip to weaken the shock wave in the passage. In the end, 3D geometry of the additional fan and the air turbine is obtained by iteration design based on numerical simulation. After the four design phases, an entire air-driven fan system, including the additional fan rotor, additional fan stator, air turbine rotor, and air turbine stator, is established and 3D modeled, as illustrated in Figure6b. EnergiesEnergies2019 2018, 12,, 11 1917, x FOR PEER REVIEW 1212 of of 17 17

4.2. CFD-Based Aerodynamic Analysis of the Air-Driven Fan System 4.2. CFD-Based Aerodynamic Analysis of the Air-Driven Fan System To evaluate the aerodynamic design of the air-driven fan system, a CFD simulation is employedTo evaluate here. the NUMECA aerodynamic packages design are of the used air-driven for the fansimulation. system, a For CFD both simulation rotors isand employed stators, a here.single NUMECA blade passage packages is taken are used as forthe thecalculation simulation. domain For bothand rotorsAutogrid and is stators, used ato single generate blade the passagestructured is taken grid. as An the O-type calculation mesh domain is used and around Autogrid the blade is used while to generate a butterfly-type the structured mesh (an grid. H-type An O-typemesh meshsurrounded is used by around an O-type the blade mesh) while in the a butterfly-typerotor clearance mesh region. (an The H-type total mesh number surrounded of grid nodes by 6 an O-type mesh)6 in the rotor clearance6 region. The total number6 of grid nodes is 2.2 10 , in which is 2.2 × 10 , in which 1.0 × 10 for the fan domain and 1.2 × 10 for the air turbine. The× entire mesh of 1.0 106 for the fan domain and 1.2 106 for the air turbine. The entire mesh of the air-driven fan the× air-driven fan system is illustrated× in Figure 6c. systemMoreover, is illustrated Fine/Turbo in Figure 6isc. used to simulate the flow field of the whole air-driven fan system. The boundaryMoreover, conditions Fine/Turbo are is set used as tofollows: simulate Total the flowpressure field of of the wholefan inlet air-driven is 101,325 fan system.Pa, total Thetemperature boundary of conditions the fan inlet are is set 288 as K, follows: and static Total pres pressuresure of the of outlet the fan is inletgiven; is total 101,325 pressure Pa,total of the temperatureair turbine ofinlet the is fan 17,250 inlet Pa, is 288 total K, temperature and static pressure of the air of theturbine outlet inlet is given; is 341 total K, and pressure static pressure of the air of turbineoutlet inlet is given; is 17,250 adiabatic Pa, total and temperature non-slip boundaries of the air turbine are set inlet for is solid 341 K,walls; and staticrotating pressure boundaries of outlet are isadopted given; adiabatic for rotors and and non-slip their hubs boundaries while stationary are set for boundaries solid walls; for rotating stators, boundaries their hubs, are and adopted casings; forperiodical rotors and boundaries their hubs whileare used stationary on both boundaries circumferential for stators, sides their of hubs,the blade and casings; passages; periodical and the boundariesconservative are usedcoupling on both by circumferentialpitchwise row is sides used of for the the blade rotor–stator passages; andinterfaces. the conservative Also, the couplingturbulence bymodel pitchwise used row in this is used paper for is the Spalart-Allmaras rotor–stator interfaces. (S-A) model. Also, the The turbulence solver Euranus model usedis used in thisto solve paper the is Spalart-Allmarassteady N-S equation, (S-A) and model. the convergence The solver Euranus is accelerated is used by to the solve multigrid the steady technique. N-S equation, and the convergenceThe characteristics is accelerated of by the the air-driven multigrid technique.fan system at the design speed are studied by CFD. The commonThe characteristics working point of the is obtained air-driven according fan system to at the the power design balance speed are relation. studied As by CFD.illustrated The common in Figure working8, as the point outlet is obtained static pressure according (back to thepressure) power balanceincreases, relation. the power As illustrated produced in by Figure the air8, as turbine the outletdrops, static while pressure that needed (back pressure)by the additional increases, fan the incr powereases. produced When the by outlet the air static turbine pressure drops, is while about that101 needed kPa (around by the additionalthe standard fan increases.atmospheric When pressure), the outlet power static balance pressure is is achieved about 101 and kPa the (around design thepoint standard at the atmospheric design speed pressure), is obtained. power The balanceCFD resu islts achieved at the design and the point design are pointshown at in the Table design 3. At speedthe design is obtained. point, The the CFDtotal resultspressure at theratio, design mass point flow arerate, shown and isentropic in Table3 .efficiency At the design of the point, additional the totalfan pressure are 1.2, 126.7 ratio, kg/s, mass and flow 85.6%, rate, respectively, and isentropic and effi theciency total of pressure the additional drop ratio, fan aremass 1.2, flow 126.7 rate, kg /ands, andisentropic 85.6%, respectively, efficiency of and the theair totalturbine pressure are 1.40, drop 80.2 ratio, kg/s, mass and flow89.6%, rate, respectively. and isentropic Compared efficiency with ofthe the design air turbine targets, are 1.40,the differences 80.2 kg/s, andbetween 89.6%, the respectively. design targets Compared and the withsimulation the design results targets, are within the diffpermissionerences between because the it design is an exploratory targets and design the simulation at this stage. results are within permission because it is an exploratory design at this stage.

Figure 8. Power balance between the additional fan and the air turbine at their design speed. Figure 8. Power balance between the additional fan and the air turbine at their design speed. The characteristic lines of the additional fan and the air turbine at their design speed are shown in FigureThe9. characteristic According to lines Figure of9 thea, the additional additional fan fan and at the the air design turbine point at their works design near speed its maximum are shown effiinciency Figure point. 9. According Due to the to factFigure that 9a, the the mass additional flow rate fan of at the the air design turbine point under works the conditions near its maximum we are concernedefficiency with point. remains Due to at 80.2the fact kg/ s,that the the power mass instead flow rate of mass of the flow air rate turbine is used under as the the abscissa conditions in the we characteristicare concerned map with of the remains air turbine, at 80.2 as kg/s, illustrated the power in Figure instead9b. Fromof mass this flow figure, rate it is can used be seenas the that abscissa the in the characteristic map of the air turbine, as illustrated in Figure 9b. From this figure, it can be

EnergiesEnergies 2018 2018, 11, 11, x, FORx FOR PEER PEER REVIEW REVIEW 1313 of of17 17 Energies 2019, 12, 1917 13 of 17 seenseen that that the the efficiency efficiency of of the the air air turbine turbine has has littl little changee change (keeps (keeps around around 0.9) 0.9) within within the the scope scope of of our our concern;concern; this this highly highly efficient efficient air air turbine turbine guarantees guarantees the the energy energy transfer transfer with with less less losses losses according according to to eﬃciency of the air turbine has little change (keeps around 0.9) within the scope of our concern; this EquationEquation (12). (12). highly eﬃcient air turbine guarantees the energy transfer with less losses according to Equation (12).

(a)( a) (b()b )

FigureFigureFigure 9. 9. 9.The TheThe characteristic characteristic characteristic lines lines lines of of the of the theadditional additional additional fans fans fans and and andthe the air the air turb air turb turbinesinesines at attheir attheir theirdesign design design speed: speed: speed: (a )( a) Additional(Additionala) Additional fan; fan; ( fan;b ()b air) ( bair )turbine airturbine turbine (the (the mass (the mass mass flow flow flowrate rate is rate ismaintained maintained is maintained as as 80.2 as80.2 80.2kg/s). kg/s). kg / s). The overall flow feature of the air-driven fan system at the design point is shown in Figure 10a. TheThe overall overall flow flow feature feature of of the the air-driven air-driven fan fan syst systemem at at the the design design point point is isshown shown in in Figure Figure 10a. 10a. Also, Figure 10b,c show the flow field in the additional fan and air turbine. The additional fan stator Also,Also, Figure Figure 10b,c 10b,c show show the the flow flow field field in in the the additional additional fan fan and and air air turbine. turbine. The The additional additional fan fan stator stator is employed to make the flow direction axial, whereas the air turbine stator is more like a support is isemployed employed to to make make the the flow flow direction direction axial, axial, wher whereaseas the the air air turbine turbine stator stator is ismore more like like a asupport support plate because small flow turning is required in this design. Under the circumstances with higher plateplate because because small small flow flow turning turning is isrequired required in in this this design. design. Under Under the the circumstances circumstances with with higher higher effective BPR, additional fan rotor will function like a ducted propeller, so circumferential velocity effectiveeffective BPR, BPR, additional additional fan fan rotor rotor will will function function like like a aducted ducted propeller, propeller, so so circumferential circumferential velocity velocity at the outlet can be ignored, which implies the absence of the need to introduce a stator (a tradeoff atat the the outlet outlet can can be be ignored, ignored, which which implies implies the the abse absencence of of the the need need to to introduce introduce a astator stator (a (a tradeoff tradeoff between structural complexity and propulsion efficiency). In these situations, additional fan stator and betweenbetween structural structural complexity complexity and and propulsion propulsion effici efficiency).ency). In In these these situations situations, additional, additional fan fan stator stator air turbine stator can be replaced by a small number of support plates to reduce weight. andand air air turbine turbine stator stator can can be be replaced replaced by by a smalla small number number of of support support plates plates to to reduce reduce weight. weight.

Figure 10. Static pressure contour and stream lines of the air-driven fan system: (a) Whole system, Figure 10. Static pressure contour and stream lines of the air-driven fan system: (a) Whole system, (Figureb) additional 10. Static fan, pressure (c) air turbine. contour and stream lines of the air-driven fan system: (a) Whole system, (b()b additional) additional fan, fan, (c )( cair) air turbine. turbine. Figure 11a shows a relative Mach number contour of the additional fan at 50% span. It is indicated by thisFigureFigure ﬁgure 11a 11a thatshows shows the a ﬂow arelative relative is supersonic Mach Mach number number at the inlet.contour contour There of of the existsthe additional additional a shock fan wave fan at at that50% 50% increases span. span. It It theis is indicatedpressureindicated overby by this pressurethis figure figure side, that that along the the flow with flow is the issuperson expansionsupersonic ic wavesat at the the whichinlet. inlet. There decrease There exists exists that a over ashock shock the suctionwave wave that side.that increasesTheseincreases two the the kinds pressure pressure of waves over over pressure both pressure make side, side, a contribution along along wi withth the to the theexpansion expansion aerodynamic waves waves loadwhich which of decrease the decrease fan. Inthat that order over over to thereducethe suction suction the side. loss side. ofThese These shock two two wave, kinds kinds pre-shock of of waves waves wave both both Mach make make number a contributiona contribution is restricted to to the the below aerodynamic aerodynamic 1.2 during load theload of design. of the the fan.fan. In In order order to to reduce reduce the the loss loss of of shock shock wave, wave, pr pre-shocke-shock wave wave Mach Mach number number is isrestricted restricted below below 1.2 1.2

Energies 2018, 11, x FOR PEER REVIEW 14 of 17 Energies 2019, 12, 1917 14 of 17 duringEnergies the 2018 design., 11, x FOR Moreover, PEER REVIEW the relative Mach number contour of the air turbine at 50%14 of span17 is illustratedMoreover, in Figure the relative 11b. Mach According number to contour this figure of the, air the turbine relative at 50% Mach span number is illustrated rises in over Figure 1.1 11 atb. the during the design. Moreover, the relative Mach number contour of the air turbine at 50% span is outletAccording of the stator, to this and figure, a theshock relative wave Mach exists number near rises the overtrailing 1.1 at edge. the outlet Although of the stator, the existence and a shock of the illustrated in Figure 11b. According to this figure, the relative Mach number rises over 1.1 at the shockwave wave exists brings near themore trailing flow edge. losses, Although the efficien the existencecy of ofturbine the shock remains wave bringshigh because more flow of losses, the low outletthe effi ofciency the stator, of turbine and remainsa shock highwave because exists near of the the low trailing pre-shock edge. wave Although Mach number.the existence of the pre-shockshock wavewave bringsMach number.more flow losses, the efficiency of turbine remains high because of the low pre-shock wave Mach number.

FigureFigure 11. 11. MachMach number number contour contour of of the the air-driven air-driven fan fan system: system: (a (a) )Additional Additional fan fan at at 50% 50% span, span, (b (b) )air air Figure 11. Mach number contour of the air-driven fan system: (a) Additional fan at 50% span, (b) air turbineturbine at at 50% 50% span. span. turbine at 50% span. AnotherAnother important important issue issue is is the the flow flow mixing mixing at at the the outlet outlet (after (after the the stators) stators) of of the the additional additional fan fan andandAnother the the air important turbine.turbine. FigureFigure issue 12 is12 shows theshows flow the the spanwisemixing spanwise at distribution the distribution outlet of(after circumferential-averaged of circumferential-averaged the stators) of the additional flow angleflow fan and angleandthe velocityair and turbine. velocity magnitude magnitudeFigure at the12 atshows outlet the outlet of the the ofspanwise air-driven the air-driven fandistribution system. fan system. In thisof Incircumferential-averaged figure, this figure, the air the turbine air turbine locates flow anglelocatesbetween and velocitybetween 0% to 50%magnitude0% to span 50% while span at the thewhile outlet additional the additionof the fan ai betweenalr-driven fan between 50% fan to system. 50% 100% to span.100% In this Accordingspan. figure, According the this air figure, this turbine locatesfigure,the between flow the angle flow 0% is angle to approximately 50% is approximately span while zero atthe mostzero addition at of most theal span, offan the whichbetween span, means which 50% the meansto flow 100% isthe roughlyspan. flow Accordingis axial roughly with this figure,axialthe the help with flow of the the angle help stators ofis (kineticapproximatelythe stators energy (kinetic loss zero is energy reduced). at most loss Also, ofis reduced).the the span, velocity Also,which magnitude the means velocity is the between magnitude flow 150is roughly m is/s axialbetweento with 200 mthe /150s athelp m/s most toof of 200 the m/s span.stators at most So, (kinetic the of flow the span. mixingenergy So, mainly lossthe flowis happens reduced). mixing after mainly Also, the casing happens the velocity and after the mainstream themagnitude casing is andradial the flow mainstream mixing is radial not the flow leading mixing cause is not of the loss. leading cause of loss. between 150 m/s to 200 m/s at most of the span. So, the flow mixing mainly happens after the casing and the mainstream radial flow mixing is not the leading cause of loss.

Figure 12. Outlet ﬂow angle and velocity magnitude of the additional fan and the air turbine. Figure 12. Outlet flow angle and velocity magnitude of the additional fan and the air turbine.

Figure 12. Outlet flow angle and velocity magnitude of the additional fan and the air turbine.

Energies 2019, 12, 1917 15 of 17

In conclusion, the numerical results show that the design of the air-driven fan system satisﬁes the design targets. Therefore, the feasibility of the core part of ADTF, that is, the air-driven fan system, is demonstrated by the numerical simulation to a certain degree.

4.3. Comparison of Performance between the Prototype Turbofan and Demo ADTF Given the CFD results of the air-driven fan system, the comparison of performance between the prototype turbofan and demo ADTF can then be made to exhibit the performance advantages of ADTF. According to Equation (12), the isentropic efficiency of the prototype fan rotor is 0.9, and the efficiency of EARGB (ηeg) is 0.90. This result means that the increase of SFC due to the air-driven system, approximately 5%, is acceptable. Table4 shows that when the air-driven fan system is added to the prototype turbofan, BPR can be improved by 156.6%, SFC can be reduced by 27.6%, and thrust can be improved by 38.4%. Therefore, the air-driven fan system, which serves as the so-called BPR “amplifier,” is able to greatly improve the high-BPR turbofan performance. Also, as shown in Table4, the demo GTF (the same BPR with the demo ADTF) with the gearbox efficiency of 99% will reduce the SFC by 5.7% compared with the demo ADTF, verifying the analysis in Section 3.3 that the air-driven system will increase the SFC by 5% approximately.

Table 4. Comparison of performance among the prototype turbofan, demo GTF, and demo ADTF.

Parameters Prototype Turbofan Demo GTF Alteration (%) Demo ADTF Alteration (%) BPR 5.3 13.6 156.6 13.6 156.6 SFC (kg/dN/h) 1.05 0.70 33.3 0.76 27.6 − − Net Thrust (kN) 25.5 38.5 51.0% 35.3 38.4

Therefore, after the overall thermodynamic analysis and detailed aerodynamic design, ADTF is proven as a feasible method to improve the performance of a high-BPR turbofan, better dealing with the fan core speed incompatibility problem and low-cost contradiction, especially in cost-sensitive and small batch situations, such as regional jets and small UAVs. Further work remains to be done to improve the theory and practice of ADTF.

5. Conclusions This research presents the thermodynamic and aerodynamic analysis of an air-driven fan for high-BPR turbofan engines. According to the above work, we can make the following conclusions:

(1) A novel concept of ADTF is proposed in this research. ADTF can solve fan core speed incompatibility problem and relieve low-cost contradiction, serving as a promising candidate for high-BPR turbofan, especially for cost-sensitive and small batch applications, such as regional jets and small UAVs. (2) The design criteria of ADTF, namely, power balance relation, thrust condition, and total pressure relation, are proposed. To easily thermodynamically analyze ADTF and compare ADTF and GTF, the theoretical expression of the efficiency of EARGB and effective BPR are proposed using the equivalent method. (3) The parameter analysis indicates that ADTF is more suitable for higher BPR compared with GTF. The efficiency of EARGB of more than 0.9 with fine part design brings approximately 5% SFC increase over the GTF, which is acceptable given great performance improvement due to BPR increase. Moreover, ADTF may have the potential to outstand GTF in structural complexity, reliability, durability, and cost of development, manufacture, and maintenance. (4) After a thermodynamic analysis and primary parameter optimization, an air-driven fan system, the most important part of ADTF, is exploratorily designed and aerodynamically analyzed by numerical simulation. The calculated results and flow fields indicate the feasibility of ADTF. Energies 2019, 12, 1917 16 of 17

The addition of an air-driven fan system improves the thrust of a prototype turbofan by 38.4% and reduces its SFC by 27.6%.

Author Contributions: W.L. performed the theoretical analyses and wrote the manuscript; G.H. proposed the research route and method; X.X. and J.W. contributed to simulation and data analyses; Y.Y contributed to manuscript preparation. Funding: This research was funded by the National Basic Research Program of China (No. 2014CB239602) and the National Natural Science Foundation of China (No. 51176072). Acknowledgments: A preliminary idea of the air-driven turbofan concept is presented in 51st AIAA/SAE/ASEE Joint Propulsion Conference, AIAA Propulsion and Energy Forum. The authors wish to express their gratitude to Jiangsu Province Key Laboratory of Aerospace Power System (affiliated with the College of Energy and Power Engineering, Nanjing University of Aeronautics and Astronautics) for technical support. The team members of College of Energy and Power Engineering of Nanjing University of Aeronautics and Astronautics are also gratefully acknowledged for their cooperation. Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature

ADTF Air-Driven TurboFan BPR ByPass Ratio CFD Computational Fluid Dynamics EARGB Equivalent Aerodynamic Reduction Gear Box HP/ HPC/ HPT High Pressure/ High Pressure Compressor/ High Pressure Turbine GTF Geared TurboFan LP/ LPC/ LPT Low Pressure/ Low Pressure Compressor/ Low Pressure Turbine SFC Specific Fuel Consumption UAV Unmanned Air Vehicle A Function of Pressure Ratio B ByPass Ratio C Absolute Velocity Cp Isobaric Specific Heat Capacity of Air F Thrust k Ratio of Specific Heats of Air m Mass Flow Rate T* Total Temperature W Relative Velocity Efficiency (for compressors, η = Wis/Ws; for turbines, η = Ws/Wis. Where, Wis is η stagnant isentropic work and Ws is shaft work. For others, η = Wo/Wi, which is the output work over input work.) ηeg Efficiency of Equivalent Gearbox eη f 0 Power Transfer Efficiency of Prototype Fan ηg Gear Box Efficiency ηm Mechanical Efficiency ηo Power Transfer Efficiency of Bypass Duct ηt Power Transfer Efficiency π Total Pressure Ratio a Air Turbine f Additional Fan f 0 Prototype Fan m Axial Component of Velocity u Circumferential Component of Velocity Energies 2019, 12, 1917 17 of 17

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