Flow transients in un-started and started modes of vacuum ejector operation
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.
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TL;DR: In this article, a low area ratio rectangular supersonic gaseous ejector is subjected to parametric evaluation to calculate the performance parameters like stagnation pressure ratio, compression ratio, entrainment ratio and the mixing parameter known as non-mixed length.
28 citations
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, the impinging shock of varying strengths on the free shear layer in a confined supersonic cavity flow is studied numerically using the detached-eddy simulation, and the resulting spatiotemporal variations are analyzed between the different cases using unsteady statistics, spectral analysis, and modal decomposition.
Abstract: The impinging shock of varying strengths on the free shear layer in a confined supersonic cavity flow is studied numerically using the detached-eddy simulation. The resulting spatiotemporal variations are analyzed between the different cases using unsteady statistics, $x-t$ diagrams, spectral analysis, and modal decomposition. A cavity of length to depth ratio $[L/D]=2$ at a freestream Mach number of $M_\infty = 1.71$ is considered to be in a confined passage. Impinging shock strength is controlled by changing the ramp angle ($\theta$) on the top-wall. The static pressure ratio across the impinging shock ($p_2/p_1$) is used to quantify the impinging shock strength. Five different impinging shock strengths are studied by changing the pressure ratio: $1.0,1.2,1.5,1.7$ and $2.0$. As the pressure ratio increases from 1.0 to 2.0, the cavity wall experiences a maximum pressure of 25% due to shock loading. At [$p_2/p_1]=1.5$, fundamental fluidic mode or Rossiter's frequency corresponding to $n=1$ mode vanishes whereas frequencies correspond to higher modes ($n=2$ and $4$) resonate. Wavefronts interaction from the longitudinal reflections inside the cavity with the transverse disturbances from the shock-shear layer interactions is identified to drive the strong resonant behavior. Due to Mach-reflections inside the confined passage at $[p_2/p_1]=2.0$, shock-cavity resonance is lost. Based on the present findings, an idea to use a shock-laden confined cavity flow in an enclosed supersonic wall-jet configuration as passive flow control or a fluidic device is also demonstrated.
21 citations
TL;DR: In this article, the impinging shock of varying strengths on the free shear layer in a confined supersonic cavity flow is studied numerically using the detached eddy simulation, and the resulting spatiotemporal variations are analyzed between the different cases using unsteady statistics, x-t diagrams, spectral analysis, and modal decomposition.
Abstract: The impinging shock of varying strengths on the free shear layer in a confined supersonic cavity flow is studied numerically using the detached eddy simulation. The resulting spatiotemporal variations are analyzed between the different cases using unsteady statistics, x–t diagrams, spectral analysis, and modal decomposition. A cavity of length to depth ratio [ L / D ] = 2 at a freestream Mach number of M ∞ = 1.71 is considered to be in a confined passage. Impinging shock strength is controlled by changing the ramp angle (θ) on the top wall. The static-pressure ratio across the impinging shock ( p 2 / p 1) is used to quantify the impinging shock strength. Five different impinging shock strengths are studied by changing the pressure ratio: 1.0 , 1.2 , 1.5 , 1.7, and 2.0. As the pressure ratio increases from 1.0 to 2.0, the cavity wall experiences a maximum pressure of 25% due to shock loading. At [ p 2 / p 1 ] = 1.5, fundamental fluidic mode or Rossiter's frequency corresponding to n = 1 mode vanishes whereas frequencies correspond to higher modes (n = 2 and 4) resonate. Wavefronts interaction from the longitudinal reflections inside the cavity with the transverse disturbances from the shock-shear layer interactions is identified to drive the strong resonant behavior. Due to Mach reflections inside the confined passage at [ p 2 / p 1 ] = 2.0, shock-cavity resonance is lost. Based on the present findings, an idea to use a shock-laden confined cavity flow in an enclosed supersonic wall-jet configuration as passive flow control or a fluidic device is also demonstrated.
21 citations
TL;DR: In this paper, the authors investigated the shock transformation in an underexpanded jet in a confined duct when the jet total pressure is increased, and they found that the Mach reflection in the fully undereexpanded jet transforms to a regular reflection (RR) at a certain pressure.
Abstract: This study investigates the shock transformation in an underexpanded jet in a confined duct when the jet total pressure is increased. Experimental study reveals that the Mach reflection (MR) in the fully underexpanded jet transforms to a regular reflection (RR) at a certain jet total pressure. It is observed that neither the incident shock angle nor the upstream Mach number varies during the MR–RR shock transformation. This is in contradiction to the classical MR–RR transformations in internal flow over wedges and in underexpanded open jets. This transformation is found to be a total pressure variation induced transformation, which is a new kind of shock transformation. The present study also reveals that the critical jet total pressures for MR–RR and RR–MR transformations are not the same when the primary pressure is increasing and decreasing, suggesting a hysteresis in the shock transformations.
15 citations
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TL;DR: In this article, a second throat ejector using nitrogen as the primary fluid is considered for the creation of a low vacuum in a high-altitude testing facility for large-area-ratio rocket motors.
Abstract: DOI: 10.2514/1.39219 In the present work, a second throat ejector using nitrogen as the primary fluid is considered for the creation of a low vacuum in a high-altitude testing facility for large-area-ratio rocket motors. Detailed numerical investigations have been carried out to evaluate the performance of the ejector for various operational conditions and geometric parametersduring thenonpumpingandpumpingmodesof operation.Inthenonpumpingmode,the lowestvacuum chamberpressureisattainedwhentheprimaryjetjustexpandsuptothemixerthroatandtheresultingsingleshock cellsealsthethroatagainstanybackflow.Thestudyillustrateshoweachgeometricandoperationalparameterofthe ejector can be optimized to meet the test requirements in a high-altitude testing facility byensuring that the primary jet completely expands without a strong impact on the duct wall. When the rocket motor is fully started, due to the self-pumping action, the required vacuum is almost maintained by the exhaust flow itself and the external nitrogen ejector plays only a supplementary role. Numerical predictions for both nonpumping and pumping modes of operation have been validated with experimental data obtained from a scaled-down model of a high-altitude testing facility.
23 citations
01 Jul 2010
TL;DR: In this article, the authors analyzed the transient flow through the vacuum ejector system with the help of a computational fluid dynamics method and showed that the one and only condition in which a continuous mass entrainment can be possible in such types of ejectors is the generation of a recirculation zone near the primary nozzle exit.
Abstract: The objective of the present study is to analyse the transient flow through the vacuum ejector system with the help of a computational fluid dynamics method. An attempt is made to investigate the interesting and conflicting phenomenon of the continuous entrainment into the primary stream with limited mass supply from the secondary chamber. The results obtained show that the one and only condition in which a continuous mass entrainment can be possible in such types of ejectors is the generation of a recirculation zone near the primary nozzle exit. The flow in the secondary chamber attains a state of dynamic equilibrium of pressure at the onset of the recirculation zone. A steady flow assumption in such ejector systems is valid only after the dynamic equilibrium state.
22 citations
01 Aug 1961
TL;DR: In this paper, an investigation of ejectors without induced flow was made to determine the effects of varying diffuser lengths on ejector performance, and an empirical method was developed to estimate the starting and operating pressure ratios of such ejector configurations using simple-determined one-dimensional normal shock relationships.
Abstract: : An investigation of ejectors without induced flow was made to determine the effects of varying diffuser lengths on ejector performance. Four 18-deg half angle conical nozzles having constant exit diameters and different throat diameters and two contoured nozzles having zero-deg half angles at the exit were used as the ejector driving nozzles. Unheated air was used for all tests. The diffuser length-to-diameter ratios were varied between 0.7 and 21.5, and three cylindrical ducts of different diameters were used both with and without a subsonic diffuser. An empirical method was developed to estimate the starting and operating pressure ratios of such ejector configurations using simple-determined one-dimensional normal shock relationships.
22 citations
TL;DR: In this paper, an inertial effect is discovered due to the recirculation zone moving forward and backward during the transients, and subsequently, the pressure in the secondary chamber oscillates.
Abstract: In this endeavor, the transients persisting in a vacuum ejector system are studied by numerically simulating the flowfield and experimentally validating the steady-state results. An inertial effect is discovered in the study due to the recirculation zone moving forward and backward during the transients, and subsequently the pressure in the secondary chamber oscillates. The flow exhibits damped oscillations in which the direction of the mass flux through the secondary chamber keeps changing and finally settles down to a state in which there is no mass flux into or from the secondary chamber. It is seen that the characteristics of the short transients in pressure and mass flux in the secondary chamber depend highly on the thicknesses of the primary and secondary jets and the secondary chamber volume. The inertial effect reduces with the reduction in thicknesses of both primary and secondary jets. As the volume of the secondary chamber increases, the inertial effect decreases further. It is also seen that t...
13 citations
01 Aug 1961
TL;DR: In this paper, the variation of minimum cell pressure ratio with nozzle total pressure level for various ejector configurations having axially symmetric nozzles located on the centerline of cylindrical ducts was presented.
Abstract: : The variation of minimum cell pressure ratio with nozzle total pressure level is presented for various ejector configurations having axially symmetric nozzles located on the centerline of cylindrical ducts. Seven nozzles were tested, using unheated air, in combination with two cylindrical ducts of different diameter and of length to diameter ratio equal to or greater than five. The significant parameter involving nozzle total pressure level was found to be the unit Reynolds number at the nozzle exit times the nozzle throat diameter. An empirical method, which is usually accurate to within 20 percent of the experimental values, was developed for predicting the variation of minimum cell pressure ratio with nozzle total pressure level for ejectors using 18-deg conical nozzles. Curves are presented which permit the estimation of minimum cell pressure ratio for ejectors using isentropic or other contoured nozzles similar to those used in this investigation. A similarity parameter is presented which denotes the necessary condition for the equal performance of two geometrically similar ejector systems using different driving fluids.
12 citations