Effects of Operating Conditions and Geometry in Augmentation of Initial Transients and Hysteresis in Supersonic Vacuum Ejector

The vacuum ejector-diffuser system is widely used in high-altitude test (HAT) facilities to generate desired vacuum conditions. The persistence of initial transients has always been a challenge while operating HAT facilities. Hence, a numerical study is conducted to investigate the nature of short-duration initial transients that persist during vacuum ejector start-up. The time scale of these transients is in order of milliseconds. The dynamics of important features of this transient flow like unsteady oscillations, recirculation bubbles, shock structure transitions, and flow hysteresis have been explored. A detailed parametric study is conducted to analyze the effect of chamber conditions and geometric configurations on the evolution of flow features. The unsteady oscillations are observed in flow quantities like mass flow rate and pressure. These unsteady oscillations are found to be closely related to the dynamics of recirculation bubble and primary jet characteristics. It was observed that oscillations die down quickly with an increase in primary chamber stagnation pressure and a decrease in stagnation temperature. It is also seen that decreasing lateral chamber exit height results in rapid ceasing of oscillations. The behavior of static pressure during pressure ramping cycles confirms the presence of flow hysteresis. The study of hysteresis in shock transitions revealed that the geometric configuration has a profound effect on the evolution of shock transitions. The transition of Mach reflection to regular reflection and its reverse transition was observed for one geometric configuration, but the latter transition was not observed for another geometric configuration.