A Technology for Improving Regenerative Cooling in Advanced Cryogenic Rocket Engines for Space Transportation

Advances in Astronautics Science and Technology (2021) 4:11–18 https://doi.org/10.1007/s42423-020-00071-0

ORIGINAL PAPER

A. P. Baiju1 · N. Jayan1 · G. Nageswaran1 · M. S. Suresh1 · V. Narayanan1

Received: 28 August 2020 / Revised: 26 November 2020 / Accepted: 3 December 2020 / Published online: 19 January 2021 © Chinese Society of Astronautics 2021

Abstract Regenerative cooling of thrust chamber is the unique solution for the thermal management of high heat fux generated inside the combustion chamber of Cryogenic rocket engine. Heat is transferred from combustion hot gas to coolant through the channels provided on inner copper shell, thereby cools the inner wall of the nozzle. A novel technique of providing copper foam inside the channels will act as an infnite fn and also act as barrier for coolant stratifcation. This will improve the heat transfer to the coolant and reduce the nozzle wall temperature. Heat transfer improvement with copper foam inserts to the coolant channel is demonstrated through experiments with simulated fuids. Experiments are conducted with simulated hot gas chamber and coolant channels using water as the coolant. Copper foam with high porosity is selected to fll the channels. Hot tests are carried out with copper foam flled coolant channels and measured the coolant temperature rise and pressure drop across the channels. Tests are repeated with similar hot gas condition, but without inserting copper foam inside the channels. A substantial enhancement in heat transfer to the coolant is observed with copper foam inserts experiments, which will reduce the wall temperature. This gives a good handle on the life cycle improvement of multi-start cryogenic engines for future space transportation systems. This paper details the specifcation of copper foam, hardware design, experiments and measurements, and the application of the augmentation of heat transfer coefcient in operating cryogenic engines.

Keywords Regenerative cooling · Copper foam · Fin conduction · Life cycle · Cryogenic engine · Heat transfer

Abbreviations Twc Coolant side wall temperature αg Gas side heat transfer coefcient Twg Gas side wall temperature µ Viscosity Ma Mach number Pr Prandl number 1 Introduction Re Reynolds nummber At Area of throat Regenerative cooling [1] is typically used in high pressure C* Characteristic velocity thrust chambers of liquid rocket engines, in order to avoid Dt Throat diameter thermal failures due to high heat load. The role of regenera- k Ratio of specifc heats tive cooling in thrust chambers is to reduce the wall tempera- M Molecular weight ture with the available coolant and within the permissible Pc Chamber pressure temperature limits of the material selected. In a regenerative q Heat fux cooling system, the heat source is the combustion gas gen- qr Radiative heat fux erated inside the chamber. The hot gas is engulfed with the t Wall thickness copper inner chamber wall which is provided with coolant Taw Adiabatic wall temperature channels on its periphery. The coolant is passed through Tco Coolant temperature the channels to cool the wall by absorbing the heat from the chamber (Fig. 1). * A. P. Baiju In this process, heat is transferred from the hot gas to [email protected] the coolant through chamber wall provided in between them [2]. Convection from hot gas to the wall, conduction 1 Liquid Propulsion Systems Centre, Indian Space Research through the copper wall and again convection from wall to Organisation, Trivandrum 695547, India

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0.2 0.8 0.1 0.9 0.026 Cp (Pc)nsg Dt At g = × , D0.2 Pr0.6 C∗ R A t ns (1) with 1 1 = × , 0.68 0.12 1 TwG 1 + k−1 Ma2 + 1 1 + k−1 Ma2 2 (Tc)ns 2 2 2 (2) 4k Pr = , 9k − 5 (3)

= 46.6 × 10−10 M0.5T0.6. Fig. 1 Regenerative cooling of thrust chamber (4) The heat transfer process within the combustion chamber is complicated by the fact that the main stream of gas is a reacting mixture. Combustion of the fuel and the oxi- dizer will progressively release heat as the gases travel downstream and at the same time non uniform mixing in the gas mixture will cause circumferential variation in the heat transfer.

1.2 Hot Gas Radiation

Generally adopted method to predict radiative properties of Fig. 2 Thermal model gases and gas mixture is by direct measurement of the total energy emitted by an isothermal volume of gas; mainly on the two important combustion products viz. CO 2 and H 2O. The gas emissivity is defned as the ratio of the energy emit- coolant completes the heat transfer process. The heat fux ted by the gas to that of a black body. The radiative heat fux “q” will be the same passing through each layer of thermal can be evaluated from the Stefan–Boltzmann equation resistances (Fig. 2). q = E T4, As the fow takes place through the wall, a boundary r g g (5) layer is formed by the slow moving layers near the wall. The boundary layers act as a thermal resistance for the where, is the Stefan–Boltzmann constant, Eg is the appro- heat fow. Similarly the chamber wall also acts as a ther- priate gas emissivity, which is a function of the absolute mal resistance for the heat fow. In order to increase the temperature and the optical density of PgL, and Tg is the heat transfer, the favourable requirements are lower wall absolute temperature of gas. thickness, higher thermal conductivity of the material and higher velocity for the coolant. 1.3 Coolant Side Convective Heat Transfer

The coolant is passed through channels milled over the inner 1.1 Gas Side Convective Heat Transfer shell (copper alloy) for cooling the chamber (Fig. 3). The coolant channel dimensions are properly designed to attain The principal process by which heat is transferred to the the required velocity wherever it passes through. The maxi- walls of a thrust chamber from the hot combustion gas is mum velocity is at the throat region and is generally limited by forced convection. Heat is conducted out through the to 70 m/s to avoid excessive pressure drop. The heat transfer layers of hot gases immediately adjacent to the wall. Heat co-efcient is computed using Sedier Tate equation. transfer rates will be controlled by the fuid dynamic phe- 0.8 0.4 0.14 nomenon occurring within this boundary layer. The heat c = 0.023Re Pr Tco − Twc . (6) transfer coefcient on gas side is calculated using modifed Bartz [3] equation,

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Fig. 4 Channel flled with copper metal foam

the throat region, thermal stresses induced within the inner shell is also increases. Since the pressure inside coolant channel is more than the hot gas pressure inside the combustion chamber and the material strength properties reduce at elevated temperatures, Fig. 3 Coolant channel confguration the thermo mechanical stresses results in bulging of channel wall towards the hot gas side. The bulging of channel wall inside the chamber is generally termed as “dog house efect” 1.4 Heat Balance [5] which will increase with each hot test and fnally opens into the hot gas and failure occurs. The heat transfer from hot gas to coolant takes place with Multiple use engines require lower wall temperature, two convection and one conduction processes. From hot gas which will reduce the thermal stresses and the residual to copper inner shell is convection and gas radiation, con- strains in the channel wall and enhance the engine life duction through the copper inner shell and again convection cycles. from copper inner shell to coolant through convection. These three heat fux is iteratively made to balance to obtain the gas side and coolant side wall temperatures: 2.1 Regenerative Coolant Outlet Temperature Limitations q = + qr, g(Taw −Twg ) (7) In the present operating cryogenic engine cycles, expander q = t∕k Twg − Twc , (8) cycle engines and staged combustion cycle engines provide the maximum specifc impulse. Expander cycle engine is relatively more reliable and simple due to the absence of one q = T − T . c wc co (9) active ignition element, viz., gas generator. But expander cycle engine requires higher regenerative coolant outlet tem- The coolant heat absorption is computed from the bal- perature to drive the turbine. anced heat fux. A forward marching method is adopted Present expander cycle engines are provided, either with to obtain the entire heat fux profle along the combustion longer combustion chamber (VINCI) or Tubular chamber chamber and nozzle. walls (RL10) to achieve higher coolant (H 2) outlet temperature. However the longer chamber results in higher engine weight and tubular chamber is limited with low thrust 2 Limitations of Regenerative Cooling engines. in the Present Designs

In the present designs of regenerative cooling, the wall tem- 3 Introduction of ‘Copper Metal Foam’ peratures are relatively higher; approximately 800 K with which the number of engine operating cycles are limited. Copper metal foams are of high surface area, high porosity, At this operating temperatures, low cycle fatigue [4] (LCF) low density and sponge like structure (Fig. 4) which has got and creep efects are becoming predominant at the throat continuity of openings through which fow can take place location of the chamber. When the temperature increases at without much obstruction [6].

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3.1 Specifcation of “FOAM”

The specifcation of Copper foam is provided in the Table 1. The specifcation indicates that, porosity is 95% which means that majority of the foam volume is with voids or open. This feature will increase the surface area for convection with a lower fow resistance.

3.2 Copper Foam Act as an Extended ‘FIN’

The idea of inserting the copper foam into the regenerative coolant channels is to overcome the limitations of the present mode of cooling and to enhance the heat transfer and thereby to achieve a reduced chamber wall temperature. In the heat transfer process, the convection from hot gas and the conduction through chamber wall remains in the same Fig. 5 Heat transfer mechanism across hot gas and coolant channel manner. But convection from wall to coolant is substantially increases on two aspects. inserted, the heat is conducted through the fn and reaches 3.3 Copper Foam Acts as a ‘Super Fin’ to the cold areas of the fuid and makes the coolant with homogenous temperature (Fig. 6). The surface area of the metal foam is extremely high. When the coolant is passed through the pores of the metal foam, the conduction through the chamber wall and ribs are extended 4 Experiments with Copper Foam through the “copper foam fn”. Finally the heat is delivered to the coolant through the large exposed surface area of 4.1 Test Article Confguration metal foam. Since the foam is made of copper metal, the conduction through ‘foam fn’ is faster and the overall cool- A water cooled combustion chamber with nozzle is designed ant side heat transfer coefcient becomes higher (Fig. 5). for the demonstration of copper foam experiments. The coolant channels are milled over the inner shell and it is covered 3.4 Elimination of Coolant Stratifcation with an outer shell. At the aft end of the nozzle, provision for accommodating diferent throats are made, so that various In a normal coolant fow through a duct, a thermal bound- chamber pressure conditions can be experimented with the ary layer will be formed near the walls of the duct. It means same hot gas fow rate. The interfaces are made with fanged that, the core fuid temperature will not be much afected by confguration so that assembly and disassembly is become the heat transfer process from the duct wall. In other words, easy and fexible (Fig. 7). coolant stratifcation [7] will be occurring in the cross sec- Coolant is admitted in the aft end of the nozzle and moves tion of fow. It shows that the entire fuid will not take part in in the counter fow direction through 18 number of coolant the heat transfer process. A thermal gradient will be estab- lished from the walls to the axis of the fow. Fluid near the wall will be hotter (heat transfer from hot wall) and towards the axis of fow channel, fuid temperature will be reducing to the bulk fuid temperature [8]. But when the ‘foam’ is

Table 1 Foam specifcation Parameter Defnition

Material Copper Porosity 95% (total void volume/ (total solid volume + void volume) Pores per inch (PPI) 50 (indicates surface area/unit volume) Major feature Large surface area per unit volume Fig. 6 Formation of boundary layer with and without foam

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Fig. 7 Confguration of coolant milled chamber with outer shell

Fig. 10 Schematic diagram of advanced coolant channel experiment

Fig. 8 Fabricated hardware with copper foam inserted into the channels

Fig. 11 Experimental setup

convergent nozzle with throat segment is also integrated with the assembly. The test article is mounted on a test bed where it is connected to a kerosene/air combustor which generates the hot gas (Fig. 10). The water is passed through the coolant channels of the chamber prior to admission of hot gas into the chamber Fig. 9 Chamber with foam flled channels assembly (Fig. 11). Hot gas generated by the combustor is passed through the chamber which builds the hot gas pressure inside the chamber. channels. The test article is designed with SS 321 material (Fig. 8). The test article is fabricated and is proof tested to a pres- 4.3 Experiment Procedure sure of 20 bar to check the integrity of hardware. The temperature and pressure of hot gas and coolant water 4.2 Experimental Setup are measured after reaching thermal equilibrium. Mass fow rate of coolant water is also measured (Fig. 12). The Copper metal foams are inserted in each of the coolant experiments are repeated without insertion of copper foam channels (Fig. 9) and assembled with the outer shell. A to fnd out the heat fux increase due to copper foam.

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Fig. 12 Hot fre test with copper foam inserts inside the channels

Fig. 14 Comparison of coolant temperature rise with and without 5 Hot Test Results copper foam

Hot test results are analysed (Fig. 13). The source hot gas temperature is 1000 °C which is passed through the cham- Three experiments are conducted with and without cop- ber. Coolant temperature rise without copper foam insert per foam inserts to fnd out the consistency of the results is 58 °C from ambient temperature of 31 °C. Coolant tem- (Fig. 16). perature with copper foam insert is 74 °C. The above graphs indicate that, with the copper foam, Coolant pressure drop increase with copper foam insert, the heat transfer rate is higher by 32–40% compared to the is provided (Fig. 14). Coolant pressure drop without cop- tests without copper foam. Similarly the pressure drop with per foam is 4 mbar whereas with foam insert it increases ‘foam’ is higher by 14 mbar compared to that without foam. to 18 mbar. Diferent experiments are conducted and the consistency of Results indicate that the coolant temperature rise with results is obtained. copper foam insert is 43 °C compared to that of without foam is 27 °C only. One dimensional thermal analysis has been carried out for the experimented condition without foam and attained a close match with the measured values 6 Discussion in the test (Fig. 15). The experiments show a clear trend and magnitude of aug- mented heat transfer [9] rate during the hot test with ‘Copper foam’ compared to other without ‘foam’. The salient infer- ences obtained by the experiments are summarised.

• Experiments conducted are providing excellent results and are in good agreement with the theoretical studies [6]. • Results of experiments are consistent. • Simulation runs are carried out for the experimental condition without foam and obtained a close match with the measured data. • It is observed that an increase in coolant side heat transfer [10] coefcient by 34% is prevailing with copper foam insert. • Increase of coolant side heat transfer [11] will reduce the gas side wall temperature. • The pressure drop increase with foam is marginal, which Fig. 13 Comparison of coolant temperature rise with and without can be compensated by increasing the channel height foam accordingly.

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Fig. 15 Comparison of esti- mated coolant temperature rise and measured values

Fig. 16 Summary of perfor- mance comparison of heat fux with & without copper metal foam (normalised)

• Advanced regenerative coolant channels [12] with foam 7 Conclusion inserts is providing a defnite merit over the existing designs in terms of engine life cycle (lower wall tem- The copper foam insert technology can be adopted for the perature) and higher coolant outlet temperature (for future designs of regenerative cooled thrust chambers. Life expander cycle engine).

1 3 18 Advances in Astronautics Science and Technology (2021) 4:11–18 cycle of cryogenic engine is limited by the throat wall 3. Bartz DR (1957) A simple equation for rapid estimation of rocket temperature only, which means that foam insert is also nozzle convective heat transfer coefcients. Jet Propuls 27:49–51 4. Quentmeyer RJ (1977) Experimental fatigue life investigation of required inside the coolant channels at the throat region cylinder thrust chamber. AIAA, pp 77–893 only. Viscosity of Hydrogen (cryo rocket engine cool- 5. Cook RT, Fryk EE, Newell JF (1983) SSME main combustion ant) is two order less compared to water (experiment), chamber life prediction. NASA Technical Report hence pressure drop will be less in actual rocket engines 6. Bai MO (2007) Numerical evaluation of heat transfer and pressure drop in open cell foams. Thesis material for Master of Science with Hydrogen as coolant. No design change is required University of Florida for the presently working cryo engines for implementing 7. Kecynski KK (1992) Thermal stratifcation potential in rocket this copper foam technology and achieve the life cycle cooling channels. NASA TM-4378 enhancement. 8. Carlile JA, Quentmeyer RJ (1992) An experimental investigation of high aspect ratio cooling passages. In: AIAA-92–3154, AIAA/ / Acknowledgements SAE ASME/ASEE 28th Joint Propulsion Conference The experiments are conducted at PRS facility 9. Koh JCY, Colony R (1974) Analysis of cooling efectiveness by of LPSC and acknowledging the contributions by Dr. Sunil kumar S porous materials in coolant passage. J Heat Transfer 96:324–330 and JC Pisharady. 10. Koh JCY, Stevens RL (1975) Enhancement of cooling efectiveness by porous materials in coolant passage. J Heat Transfer 97:309–311 References 11. Calmidi VV, Mahajan RL (2000) Forced convection in high porousity metal foams. J Heat Transfer 122:557–565 12. Chung JN, Tully L, Kim JH (2006) Evaluation of open cell foam 1. Niino AM, Kumakawa A, Yatsuyamagi N, Susuki A (1982) heat transfer enhancement for liquid rocket engines. In: 42nd Heat transfer characteristic of liquid hydrogen as coolant for the AIAA/ASME/SAE/ASEE Joint Propulsion Conference and LO 2/LH2 rocket thrusts chamber with channel wall construction. Exhibit, Sacramento, California AIAA, pp 82–1107 2. Riccius JR, Zametaev EB (2002) Stationary and dynamic thermal analysis of cryogenic liquid rocket combustion chamber wall. AIAA

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