Starting Transients in Vacuum Ejector-Diffuser System
12 Sep 2014-Journal of Propulsion and Power (American Institute of Aeronautics and Astronautics)-Vol. 30, Iss: 5, pp 1213-1223
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...
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25 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 performance and flow structure in a three-dimensional rectangular ejector have been numerically investigated by using air as the working fluid, and the authors aim to optimize the convergent-divergent (C-D) nozzle position for each operating condition and to bring out the influence of reflected shock waves and boundary layer in the constant area mixing chamber.
16 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
Cites background from "Starting Transients in Vacuum Eject..."
...One major aspect of the confined jets is that the jet induces a secondary flow which changes the pressure levels in the outer duct, and the jet pressure ratio cannot be determined easily as the jet expansion is strongly coupled to the pressure levels and vice versa (Mittal et al. 2014)....
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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
References
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514 citations
"Starting Transients in Vacuum Eject..." refers methods in this paper
...T HE vacuum ejector-diffuser system consists of a primary chamber, nozzle, secondary chamber, mixing section, and the diffuser [1] as shown in Fig....
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TL;DR: In this paper, a review of the fundamental characteristics of the shock train and pseudo-shock is presented, and some simple predictions are made to simulate these very complicated phenomena, and control methods of the pseudo-shocks are also described.
478 citations
"Starting Transients in Vacuum Eject..." refers background in this paper
...The primary flow is underexpanded in the mixing chamber, and it further expands through a series of expansion waves, consequently leading to a shock train system formed by the consecutive reflections and interactions of expansion and shock waves [19]....
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TL;DR: In this paper, a small-capacity steam jet refrigerator has been tested with boiler temperatures in the range 120-140°C and the experimental data were found to be within 85% of the theoretical values.
Abstract: The paper provides the results of a theoretical and experimental study of a steam jet refrigerator. A small-capacity steam jet refrigerator has been tested with boiler temperatures in the range 120–140°C. The experimental data were found to be within 85% of the theoretical values. The experiments showed that choking of the secondary flow in the mixing chamber of the ejector plays an important role in the system performance. Maximum COP was obtained when the ejector was operated at its critical flow condition. Off-design performance characteristics of the system are provided.
317 citations
"Starting Transients in Vacuum Eject..." refers background in this paper
...Most of the works on the vacuum ejector systems with finite secondary chambers [3,8,10,11] focus mainly on the levels of the vacuumgenerated inside the secondary chamber after the steady state has been achieved, while other works [4–7,9] deal with secondary chambers that are infinite and are hence capable of supplying infinite mass during the entire ejector operation....
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302 citations
"Starting Transients in Vacuum Eject..." refers background in this paper
...Most of the works on the vacuum ejector systems with finite secondary chambers [3,8,10,11] focus mainly on the levels of the vacuumgenerated inside the secondary chamber after the steady state has been achieved, while other works [4–7,9] deal with secondary chambers that are infinite and are hence capable of supplying infinite mass during the entire ejector operation....
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168 citations
"Starting Transients in Vacuum Eject..." refers background in this paper
...Most of the works on the vacuum ejector systems with finite secondary chambers [3,8,10,11] focus mainly on the levels of the vacuumgenerated inside the secondary chamber after the steady state has been achieved, while other works [4–7,9] deal with secondary chambers that are infinite and are hence capable of supplying infinite mass during the entire ejector operation....
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...The ejector also proves its worth in a refrigeration facility [9], where it serves as a pump and also helps the refrigerant to expand....
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