Numerical Study of Ejector–Diffuser Coupling and Hysteresis in an Active-Ejection High-Altitude Simulation Test Stand

Engine high-altitude simulation tests require establishing a stable low test-chamber pressure across a wide range of operating conditions. Existing studies focus predominantly on the local starting mechanisms—namely, shock-wave swallowing, aerodynamic choking, and boundary-layer separation—of individual ejector or diffuser components, with limited investigation of the coupling among shock evolution, chamber pressure response, starting hysteresis, and diffuser starting thresholds during continuous ejection pressure regulation. To address this gap, a two-dimensional axisymmetric Reynolds-averaged Navier–Stokes model with the standard k-ε turbulence model is employed, encompassing the test chamber, engine nozzle, supersonic diffuser, and annular ejector of a large-scale active-ejection test stand (where an annular ejector actively evacuates the test chamber to simulate high-altitude backpressure). The formation, migration, and stabilization of the shock system are revealed under increasing ejection pressure. Hysteresis characteristics are elucidated by comparing pressure-increasing and pressure-decreasing paths. The non-monotonic relationship between chamber pressure and ejector total pressure ratio under zero-secondary-flow conditions is established. The mechanism governing diffuser starting threshold variation under coupled engine gas and ejection flows is identified. Quantitatively, the ejector starting pressure ratio is found to be approximately 16.78 with a hysteresis width of ~13.6% between the starting and unstarting thresholds, and the diffuser starting threshold scales linearly with the ejector total pressure ratio (R2 = 0.976). The results reveal the coupling among shock evolution, hysteresis, chamber pressure response, and diffuser starting thresholds and clarify the dual influence of ejection pressure on both chamber pressure regulation and diffuser starting thresholds. These findings provide a theoretical basis for parameter matching and operational control of high-altitude simulation facilities.