Effects of Vacuum Chamber and Reverse Flow on Supersonic Exhaust Diffuser Starting
26 Feb 2015-Journal of Propulsion and Power (American Institute of Aeronautics and Astronautics)-Vol. 31, Iss: 2, pp 750-754
About: This article is published in Journal of Propulsion and Power.The article was published on 2015-02-26. It has received 9 citations till now. The article focuses on the topics: Diffuser (thermodynamics) & Chamber pressure.
Citations
More filters
TL;DR: In this article, the authors investigate the secondary flow characteristics and the associated vacuum generation caused with an increase in the primary pressure ramping in zero-secondary flow ejectors, and they find that with the jet expansion reaching a critical level, the fluid supply from the reverse flow is suddenly entrained back into the main jet at the maximum jet expansion point.
Abstract: This paper aims to investigate the secondary flow characteristics and the associated vacuum generation caused with increase in the primary pressure ramping in zero-secondary flow ejectors. The sudden expansion of the primary jet into the diffuser during the ejector start-up results in flow separation from the shear layer formed between the primary and inducted flows and produces large recirculation bubbles in the top and bottom sides of the jet. These recirculation bubbles cause an induced flow from ambient air into the diffuser duct as well. The fluid supply from the reverse flow due to the shear layer separation and the induced flow from ambient air provide a counter momentum against fluid entrainment from a vacuum chamber. As a result of this, the initial vacuum generation process progresses in a slow rate. Thereafter, the primary jet expansion reaches a critical level and a rapid vacuum generation can be seen. It is found that with the jet expansion reaching a critical level, the fluid supply from the reverse flow is suddenly entrained back into the main jet at the maximum jet expansion point. This suddenly reduces the counter-momentum which has been prohibiting the entrainment of fluid from the vacuum chamber and results in rapid evacuation. This is followed by a stage in which the vacuum chamber pressure is increasing due to the attainment of a constant Mach number at the diffuser inlet and the jet pressure ramping. It is found that the secondary flow dynamics and the vacuum generation processes in rectangular and round ejectors show a close resemblance.
24 citations
TL;DR: In this paper, an experimental study has been carried out to investigate the nature of transients in vacuum ejector flows during start-up and the dynamics in flow characteristics, and the results show that the secondary stream induction progresses with non-uniform rates with the ramping primary jet pressure during startup.
Abstract: An experimental study has been carried out to investigate the nature of transients in vacuum ejector flows during start-up and the dynamics in flow characteristics. The results show that the secondary stream induction progresses with non-uniform rates with the ramping primary jet pressure during start-up. The initial evacuation period is subjected to gradual and highly perturbed secondary fluid entrainment. In this phase, the secondary stream induction by the shear layer is asymmetric leading to an un-even vacuum generation in the secondary chamber. In the second phase, the secondary pressure fluctuations are found to be ceased for a critical primary jet pressure followed by a rapid induction of the secondary fluid till the primary jet expands to the diffuser wall. The transition from the first phase to the second phase is caused by the secondary stream flow choking in the diffuser. Following the second phase, a stable stage exists in the third phase in which the vacuum pressure decreases only marginally. Any further attempt to increase the secondary chamber vacuum level beyond the third phase, by increasing the primary jet total pressure, results in flow reversal into the secondary chamber, spoiling the already achieved vacuum level. In the fourth phase of start-up, a complicated shock interaction transformation from a Mach reflection (MR) to regular reflection (RR) occurs within the diffuser. It is also observed that the primary jet pressures for the minimum secondary chamber pressure, the minimum secondary pressure, and the primary pressure for MR-RR transformation decrease initially with increase in diffuser length and then increase. It is found that the decreasing and increasing trends are caused by the pressure recovery and Fanno effects, respectively.
13 citations
TL;DR: In this article, an experimental analysis of a second-throat exhaust diffuser (STED) performance has been conducted for the high altitude test of a parabolic bell-type nozzle.
Abstract: In the present study, the experimental analysis of a second-throat exhaust diffuser (STED) performance has been conducted for the high altitude test of a parabolic bell-type nozzle. The diffuser starting performance has been explored considering two different approaches, namely the gradual and instantaneous increase of the nozzle chamber pressure. Also, the influence of pre-evacuating the internal regions of the system on the diffuser starting performance has been studied. Numerical simulations have been carried out to have more physical insight into some test results. It is demonstrated that the non-dimensional hysteresis range in STED starting performance with a bell-type nozzle is up to 4 times larger than that with a conical nozzle. The pre-evacuation of the internal area does not affect the diffuser minimum starting pressure. However, starting the diffuser with pre-evacuation takes 50 to 70 percent less time than without pre-evacuation. Also, it is illustrated that in spite of large differences in minimum starting pressure of STED with conical and bell-type nozzles, the STED breakdown pressure (minimum operating pressure) is independent of nozzle profile. Moreover, it is shown with experimental evidences and numerical analyses that the creation of restricted shock separation (RSS) inside the bell nozzle leads to some unexpected behaviors in STED starting and breakdown behaviors. Finally, it is demonstrated that the instantaneous increase in the nozzle chamber pressure with steeper slope and pre-evacuation of the internal regions can eliminate separation pattern transition from common free shock separation to unwanted RSS.
8 citations
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.
Abstract: The present study aims to investigate the process of vacuum generation in a second throat vacuum ejector system employed for the high altitude testing of rocket motors. The study shows that the vacuum generation progresses with four distinct stages during the initial start-up process. The initial stage consists of a gradual evacuation process (first stage) which is followed by a transition region (second stage) in which the pressure starts to drop fastly and eventually leading to a rapid evacuation stage (third stage). This is followed with the reappearance of a gradual evacuation stage (fourth stage) which terminates with the onset of the started mode operation. It is found that the various stages of evacuation are closely linked to the internal shock wave movement in the supersonic nozzle. The gradual evacuation is found to be caused by the relatively large nozzle exit pressure caused by the multiple shock cells in the nozzle. The rapid evacuation occurs due to the sudden movement of the first shock cell to the nozzle exit and the reappearance of the gradual evacuation happens during the transformation of the nozzle jet from an overexpanded to underexpanded state. The parametric studies show that with increase in diffuser convergent cone angle, the contribution of vacuum generation from the fourth stage reduces due to a reduction in the maximum possible jet expansion. A same trend in vacuum generation is observed when nozzle distance from the second throat section of the ejector diffuser reduces.
7 citations
24 Aug 2020
4 citations
References
More filters
TL;DR: Theoretical methods for estimating the required starting pressure ratio and minimum base pressure ratio of axisymmetric ejector-diffuser systems used in rocket altitude simulation are presented in this paper.
Abstract: Theoretical methods are presented for estimating the required starting-pressure ratio and minimum base pressure ratio of axisymmetric ejector-diffuser systems used in rocket altitude simulation. The starting-pressure ratio is determined by applying over-all conservation relations; the theory also predicts the limiting second-throat contraction ratio, which can exceed the normal-shock-starting contraction limit for wind tunnels. Experimental starting data from 79 axisymmetric ejector systems agree with this theory within approximately 10%. The theory for the minimum base pressure ratio for zero secondary flow was developed by modifying Korst's two-dimensional theory to be applicable to axisymmetric systems. The modification consists of a new method for computing the peak recompression static pressure. Measured base pressures of 34 ejector configurations agreed with the latter within 6%.
46 citations
TL;DR: In this paper, a second-order high-resolution scheme for solving the new Lagrangian Euler equations is employed to accurately resolve the complicated shock patterns and associated slip lines and their interactions.
Abstract: A computational analysis of the two-dimensional supersonic inviscid flowfield in a second-throat ejector-diffuser (STED) system is presented. A second-order high-resolution scheme for solving the new Lagrangian Euler equations is employed to accurately resolve the complicated shock patterns and associated slip lines and their interactions. A parametric study covering a variety of Xst and Ost is implemented to investigate their effects on the flow structure in STED as well as its performance. Results suggest that the averaged Mach number along the entrance plane of the second throat is a suitable criterion for the justification of the performance of STED. With this criterion, an optimal design insuring the largest pressure recovery can be achieved.
41 citations
TL;DR: In this paper, the effects of essential performance parameters on the starting transient of a straight cylindrical supersonic exhaust diffuser (SED) are numerically investigated in terms of SED length and pre-evacuation configuration.
Abstract: Effects of essential performance parameters on the starting transient of a straight cylindrical supersonic exhaust diffuser (SED) are numerically investigated. Diffuser starting and evacuation transients are examined in terms of SED lengths and pre-evacuation configuration. Preconditioned Favre-averaged Navier–Stokes equations incorporated with a low Reynolds number turbulence model and Sakar’s method to treat turbulence compressibility is solved for strongly turbulent all-Mach diffuser flows. The numerical method is properly validated with the measurements with accuracy. Characteristic locus of diffuser-starting and diffuser-unstarting modes is constructed for the diffusers of three different lengths (L/D = 2, 5, and 20). Flow evolutions visualized in diffuser mode-transition regimes manifest a threshold L/D over which the SED starting transient is unique. An occurrence of plume blowback into the vacuum chamber due to lower initial pressure (PC,INIT/PA = 0.0027) expedites expansion of nozzle exhaust and diffuser choking, and causes faster chamber evacuation than the atmospheric starting.
24 citations
TL;DR: In this paper, the performance of a second-throat ejector diffuser system employed in high-altitude testing of large-area-ratio rocket motors is considered under various steady and transient operating conditions.
Abstract: The performance of a second-throat ejector―diffuser system employed in high-altitude testing of large-area-ratio rocket motors is considered under various steady and transient operating conditions. When the diffuser attains started condition, supersonic flow fills the entire inlet section and a series of oblique shock cells occurring in the diffuser duct seal the vacuum environment of the test chamber against backflow. The most sensitive parameter that influences the stagnation pressure needed for diffuser starting is the second-throat diameter. Between the throat and exit diameters of the nozzle, there exists a second-throat diameter value that corresponds to the lowest stagnation pressure for starting. When large radial/axial gaps exist between the nozzle exit and diffuser duct, significant reverse flow occurs for the unstarted cases, which spoils the vacuum in the test chamber. However, the starting stagnation- pressure value remains unaffected by the axial/radial gap. Numerical simulations establish that it is possible to arrive at an optimum diffuser geometry that facilitates early functioning of the high-altitude-test facility during motor ignition phase. The predicted axial variations of static pressure and temperature along the diffuser for the testing of a cryogenic upper-stage motor agree well with available experimental data.
24 citations
TL;DR: In this paper, the effects of four geometric parameters of an annular injection supersonic ejector, namely, the primary nozzle exit-to-throat area ratio, the contraction angle of the mixing chamber, the cross-sectional area and L/D ratio of the secondthroat on the performance parameters including the secondary flow pressure, the starting pressure and unstarting pressure were investigated experimentally.
Abstract: The effects of four geometric parameters of an annular injection supersonic ejector, namely, the primary nozzle exit-to-throat area ratio, the contraction angle of the mixing chamber, the cross-sectional area and L/D ratio of the second-throat on the performance parameters including the secondary flow pressure, the starting pressure and unstarting pressure were investigated experimentally. The starting pressure exhibits linearly proportional dependence on the throat area ratio when the mixing chamber length is less than a certain critical value. For a longer mixing chamber, the starting pressure is proportional to the mixing chamber length while the unstarting pressure depends on the throat area ratio only. The geometric parameters of the second-throat do not affect the static pressure of the secondary flow. This implies that the secondary flow is aerodynamically choked in the mixing chamber and the static pressure of the secondary flow is determined by the choking condition since the mixing chamber of the annular injection ejector is relatively long. Based on the findings by the experiment, a simplified analytical model was proposed to predict the secondary flow pressure. The predicted secondary flow pressure agrees reasonably well with the measurement for a small contraction angle of the mixing chamber.
20 citations