Numerical Approach for Transient and Steady-State Analysis of Second-Throat Ejector-Diffuser Systems

During ground testing campaigns of liquid rocket engines designed for altitude operations, high-altitude test facilities are employed to prevent overexpanded nozzle flows, which can lead to shock waves and flow separation. These phenomena can hinder thrust measurements and thermo-structural integrity assessments. High-altitude test facilities create vacuum-like conditions around the nozzle exit, often utilizing a second-throat ejector-diffuser to manage supersonic recompression and stabilize flow separation in a secondary throat. Auxiliary ejectors are commonly used to pre-evacuate the facility before engine starting, ensuring smooth and successful starting transients. When operated at steady-state, ejectors can also enhance overall performance by decreasing the system backpressure. The present study evaluates the effectiveness of a computational model based on Reynolds-Averaged Navier-Stokes (RANS) equations to simulate second-throat ejector-diffuser systems with auxiliary ejectors, focusing on both pre-evacuation and ejector operation during the engine nominal functioning. The results highlight the critical role of ejector mass flow rate in determining its suction effectiveness. Furthermore, unsteady simulations provide valuable insights into the shutdown transient, examining the dynamics of recirculation bubbles and oblique shock waves and assessing how ejector operation can address criticalities associated with the shutdown process.