Effect of Geometric Configurations on the Starting Transients in Vacuum Ejector
27 Mar 2019-AIAA Journal (American Institute of Aeronautics and Astronautics)-Vol. 57, Iss: 7, pp 2905-2922
TL;DR: In this article, the starting transients in vacuum ejector and its dependence on various geometric configurations were investigated, and various geometric parameters were the ratio of diffuser to priors were investigated.
Abstract: This paper investigates the starting transients in vacuum ejector and its dependence on various geometric configurations. Various geometric parameters investigated were the ratio of diffuser to pri...
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TL;DR: In this article, the authors investigated the process of vacuum generation in a second throat vacuum ejector system employed for the high altitude testing of rocket motors and found that the various stages of evacuation are closely linked to the internal shock wave movement in the supersonic nozzle.
7 citations
TL;DR: In this article , the authors investigate the characteristics of a typical vacuum ejector's starting transient, steady-state, and shut-down transient, using time-resolved Schlieren images and oil flow visualization.
Abstract: Using simultaneous measurements of unsteady pressures in conjunction with time-resolved Schlieren images and oil flow visualization, we investigate the characteristics of a typical vacuum ejector's starting transient, steady-state, and shut-down transient. With varying primary jet chamber pressure, the pressure evolution in the secondary chamber shows smooth, perturbed, rapid, and steady evacuation stages, as well as hysteresis and rapid filling stages. It is noticed that the evacuation in the secondary chamber is improved during stopping transient just before the unstart event. By using oil flow images, we illustrate the separation bubble characteristics during each stage of the vacuum ejector operation and their influence on the pressure evolution. Through cross correlation, it has been determined that the primary jet flapping during the starting transient causes the jet to attach to one of the diffuser walls. We also demonstrate that the primary jet undergoes both longitudinal and lateral oscillations in the starting transient, the former having a major effect on unsteadiness in the secondary chamber using proper orthogonal decomposition and spectral proper orthogonal decomposition algorithms and power spectral density (PSD). Simultaneous acquisition of unsteady pressures and high-speed Schlieren images allow us to correlate the frequency peaks (PSD spectra) in the flow. Using magnitude-squared coherence and cross correlation analyses, we confirm communication of unsteadiness and its direction of propagation between the secondary chamber and the diffuser. In this study, we demonstrate that a high ramping rate of primary jet chamber pressure reduces the unsteadiness in the secondary chamber during the transient starting phase.
3 citations
TL;DR: In this paper , the authors present a combined numerical and experimental investigation of the starting process of a second throat diffuser during ground testing of a thrust-optimized parabolic (TOP).
Abstract: 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 and 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 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.
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TL;DR: In this article, the authors presented an analysis of the performance of ejector configurations under the first solution where the flow after mixing is subsonic and the second solution for mixing is supersonic, respectively, and provided a basis for selecting an optimal and a limiting ejector configuration for any given set of flight and injected gas characteristics.
Abstract: The solution of the equations representing the flow of a compressible fluid through an ejector has been shown to possess two distinctly different results after complete mixing. Part I of this study reported the analysis and described the performance of ejectors configured under the first solution where the flow after mixing is subsonic. The second solution to the ejector flow equations where the flow after mixing is supersonic provides a basis for selection of an optimal and a limiting ejector configuration for any given set of flight and injected gas characteristics. These special ejector configurations provide very high ideal thrust augmentations while translating from zero to supersonic velocities. Maps showing this ideal performance over large ranges of flight and injected gas characteristics are presented. Consideration is given to the influence of realistic, nonisentropic outlets as required for starting the supersonic flow (swallowing the starting shock wave) and for return of the supersonic mixed flow to ambient pressure. The data presented are limited to ejectors having a constant area mixing duct whose cross section is equal to 25 times that of the minimal area of the exhaust flow from the gas generator.
85 citations
TL;DR: In this article, a straight cylindrical supersonic exhaust diffusers (SED) using cold nitrogen and hot rocket exhaust gases as driving fluids were used to evaluate the effects of the ratios of the SED area to rocket nozzle throat area (Ad/At), SED areas to rocket exhaust manifold exit area, SED length to its diameter (L/D), and specific heat ratio of the driving gases (k) on the minimum starting pressure ratio, (Po/Pa)st, of SED.
64 citations
TL;DR: In this article, a discussion of the development of the compact jet-diffuser ejectors utilized for hovering and low speed flight propulsion has been presented, followed by a description of ideal ejector performance as derived from a compressible flow theory, over the range of flight speeds from zero to supersonic speed.
Abstract: : A discussion of the development of the compact jet-diffuser ejectors utilized for hovering and low speed flight propulsion has been presented This is followed by a description of ideal ejector performance as derived from a compressible flow theory, over the range of flight speeds from zero to supersonic speed These analyses introduced the concepts of ejector configuration optimization and the validity of the so-called 'second solution' to the mixing problem, wherein the flow after complete mixing is supersonic The ideal performance of thrust augmenting ejectors designed under this 'second solution' has been shown to be far superior to those designed by conventional methods The ability of properly designed ejectors to utilize the thermal energy of injected gas for the production of useful energy has also been described Finally, the influence of major losses has been discussed, including means for avoiding excessive performance degradation by proper optimization of the geometry of the ejector in view of these losses
52 citations
TL;DR: In this article, the starting transient and plume blowback at diffuser breakdown of a straight cylindrical supersonic exhaust diffuser with no externally supplied secondary flow are numerically investigated.
36 citations