Advances in Astronautics Science and Technology (2021) 4:11–18 https://doi.org/10.1007/s42423-020-00071-0 ORIGINAL PAPER A Technology for Improving Regenerative Cooling in Advanced Cryogenic Rocket Engines for Space Transportation 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 Vol.:(0123456789)1 3 12 Advances in Astronautics Science and Technology (2021) 4:11–18 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 cham- ber 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, 1 3 Advances in Astronautics Science and Technology (2021) 4:11–18 13 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 tempera- ture. 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]. 1 3 14 Advances in Astronautics Science and Technology (2021) 4:11–18 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.