This study presents a combined numerical and experimental investigation of the starting process of a second throat diffuser during ground testing of a thrust-optimized parabolic (TOP). In this investigation, compressed air has been utilized as the operating fluid in a subscale experimental setup. The study examines three scenarios with varying nozzle pressure profile, including two cases of start and one case of un-start. Additionally, this study employs numerical simulations to identify and analyze the physical phenomena that occur at each stage of the start and un-start processes in these cases. The results for the case started at a relatively low nozzle pressure profile (24.5 bar Max) indicate that the vacuum generation process during high-altitude testing of TOP nozzles can be broken down into five stages. The first stage involves an increase in pressure within the vacuum chamber during the early moments of the starting process. In the second stage, vacuum generation occurs gradually as the nozzle operates under free shock separation (FSS). This is followed with the reappearance of small fluctuations in the vacuum chamber pressure due to transition from the FSS to restricted shock separation (RSS) flow pattern (third stage). The fourth stage begins with the predominance of the shock separation with recirculation (SSR) flow pattern inside the nozzle, resulting in gradual vacuum generation. This stage terminates upon transformation of the cap shock structure into a regular reflection (RR) structure. In the final stage of vacuum generation, the evacuation rate is almost half of the fourth stage. This is attributed to the establishment of expanded and under-expanded conditions, as well as the impingement of the nozzle outflow jet with the wall and the onset of start conditions. Next, the results of vacuum generation have been studied at higher nozzle pressures profile (34 bar Max). The results indicate that increasing the nozzle pressure rate not only reduced the starting time by 23 %, but also significantly reduced the pressure fluctuations in the evacuation process. In fact, at higher nozzle pressure, the third stage is almost eliminated. In the un-started case, where the nozzle pressure is lower than the minimum starting pressure, fluctuations occur in the vacuum chamber pressure due to the dynamics of the diffuser inlet recirculation bubble and the transition of the nozzle separation pattern from RSS to SSR and vice versa.

Keywords