Numerical and experimental study of choking phenomenon on RBCC engines in ejector mode

Abstract

This study investigates the impact of the choking phenomenon on the mixing, combustion, and thrust performance of rocket-based combined-cycle (RBCC) engines operating in ejector mode. Free-suction experiments were conducted on an axisymmetric kerosene-fueled RBCC engine equipped with two different mixer types (an expansion mixer and a constant-section mixer) under low backpressure conditions, and data on wall pressure distribution and net thrust were obtained. To supplement the experimental findings, numerical methods that incorporate compressibility effects and chemical non-equilibrium processes in the rocket nozzle were also utilized. The simulated results showed a high level of agreement with the experimental data, with relative errors of 3.6 % and 1.3 % for pressure and 3.42 % and 0.05 % for thrust, respectively. The simulations revealed two distinct combustion zones in the mixer, namely the chemical non-equilibrium zone within the rocket exhaust and the mixing zone between primary and secondary streams. The former zone was found to be highly susceptible to static pressure mismatch, which could result in significant oscillations. In contrast, the latter zone was almost uniformly distributed in the axial direction. Moreover, the study highlights the negative impact of inlet choking, which can result in low mixing intensity and significant total pressure loss due to the shock structure. Therefore, avoiding inlet choking is recommended for RBCC engines. Finally, the study also shows that exit choking in the constant section mixer, although reducing the entrained secondary air, could achieve higher mixing and combustion intensity than the expansion mixer, resulting in 18.87 % more specific impulse than a conventional rocket at the simulated condition of H5.1 km / Mach 1.0. In conclusion, this research provides critical insights into the choking phenomena on RBCC engines through rigorous numerical and experimental methods.