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 downtoastateinwhich thereisnomass flux intoorfrom the secondary chamber.It isseen that the characteristics ofthe short transientsin pressure and mass fluxinthe secondary chamber depend highlyonthe thicknessesof 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 the inertial effect decreases as the primary jet pressure increases, while other parameters remain constant. The movement of the recirculation zone during these initial transients is also studied in detail and found to be closely related to the mentioned ejector parameters. Copyright © 2014 by the American Institute of Aeronautics and Astronautics, Inc.

Rajesh G

Department of Aerospace Engineering

Mittal, A.

Lijo, V.

Kim, H.D.

EJECTORS (PUMPS)

Transients

DAMPED OSCILLATIONS

Diffuser systems

FORWARD-AND-BACKWARD

Inertial effect

Recirculation zones

SECONDARY CHAMBER

STARTING TRANSIENT

VACUUM EJECTORS

Pressure effects

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Starting Transients in Vacuum Ejector-Diffuser System

Journal of Propulsion and Power

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 primary duct height (D∕d), diffuser length–to–diffuser height ratio (L∕D), and secondary chamber height–to–secondary jet thickness ratio (hs∕ts). It is seen that the vacuum ejector start-up is associated with a gradual initial vacuum generation stage, which is followed by a rapid vacuum generation. The gradual vacuum generation is found to be caused by the mass supply by the recirculation bubble (existing in the diffuser) to the secondary chamber exit plane. The rapid evacuation happens when the initial recirculation bubble splits up, which results in a rapid reduction of the amount of reverse flow mass reaching the secondary chamber. It is found that with reduction in the D∕d ratio and increase in the L∕D ratio, the rapid evacuation stage reduces and eventually vanishes. The present investigation shows that both these configuration changes reduce the mass supply by reverse flow to the secondary chamber and this is attributed to the cause of the reduction in rapid evacuation stage. The present study also shows that the nature of vacuum generation does not vary with change in secondary chamber height. © 2019 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

AIAA Journal

Effect of geometric configurations on the starting transients in vacuum ejector

This paper investigates the mixing of two supersonic streams in a supersonic exhaust diffuser using theoretical, numerical, and experimental approaches. It has focused attention on the startup and stationary operation of the two-stream supersonic diffusers. The diffuser geometry consists of a constant area throat which connects two ducts of convergent and divergent cross-sections. The effects of diffuser contraction ratio (CR) and convergence angle (θ) on the starting process, pressure recovery, and flow features. Experiments have been performed at five different secondary Mach number (Ms) values in the range of 1.8 &lt; Ms &lt; 2.6, at a fixed primary Mach number (Mp) value of 2. Synchronized pressure measurements are used to study the diffuser performance characteristics. Numerical simulations were also carried out to obtain a better insight into the flow field and computed results have been compared with the experimental results. It was found that the starting behavior has a hysteresis. As a result, the tunnel can be operated at lower pressure ratio than the starting pressure ratio. It is observed that the settling chamber pressure required to start a Constant-Area Diffuser (CAD) is decreased with increase in the diffuser length up to an L/D ratio of 9; subsequently, starting pressure remains almost constant with the further increase in the length due to frictional and pressure recovery losses. The present study provides valuable insights into the multi-stream supersonic mixing and diffusion problem. © 2019 Elsevier Masson SAS

Aerospace Science and Technology

A study on the performance characteristics of two-stream supersonic diffusers

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. © 2018 Author(s).

Physics of Fluids

Physics of vacuum generation in zero-secondary flow ejectors

The performance and flow structure in a three-dimensional rectangular ejector have been numerically investigated by using air as the working fluid. Commercial software ANSYS Fluent 15.5 has been used for numerical simulation of supersonic, compressible, turbulent flow ejectors. In order to confirm the appropriate turbulence model, the computed results are validated against indigenous experimental data. The study aims 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 on the performance of the ejector. Also, the variation of entrainment ratio (ER) for a range of primary, secondary and exit pressure has been investigated. Both the on-design and the off-design characteristics and local flow structure of a rectangular ejector have been considered. © 2017

International Journal of Thermal Sciences

Three-dimensional numerical investigations on rectangular cross-section ejector

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. © 2017 Cambridge University Press.

Journal of Fluid Mechanics

Shock transformation and hysteresis in underexpanded confined jets

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 parameters during the nonpumping and pumping modesof operation.In the nonpumping mode, the lowest vacuum chamber pressure is attained when the primary jet just expands up to the mixer throat and the resulting single shock cell seals the throat against any backflow. The study illustrates how each geometric and operational parameter of the ejector can be optimized to meet the test requirements in a high-altitude testing facility by ensuring 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.

Optimization of second throat ejectors for high-altitude test facility

Experiments were carried out on straight cylindrical supersonic exhaust diffusers (SED) using cold nitrogen and hot rocket exhaust gases as driving fluids, in order to evaluate the effects of the ratios of the SED area to rocket nozzle throat area (Ad/At), SED area to rocket nozzle exit area (Ad/Ac), 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. The rocket nozzle and SED starting transients were also simulated in the models. The study reveals that (Po/Pa)st increases monotonically with increase in (Ad/At) and k. One-dimensional normal shock relations were used in predicting the (Po/Pa)st since the compression in long ducts is basically a normal shock process. Predicted values of (Po/Pa)st were validated with experimental data. SED efficiency factors(ηns) were arrived at based on one-dimensional normal shock relations. ηns goes down at higher values of (Ad/Ac). (Po/Pa)st is lower for lower k values for the same (Ad/At). Cylindrical SEDs exhibit no hysteresis. The results of this investigation were utilised in validating the design of high altitude test (HAT) facility for testing the third stage motor (PS-3) of Polar Satellite Launch Vehicle (PSLV). The simulation of starting transients in the model revealed that the HAT facility shall not be operated in the unstarted phase, because the rocket nozzle may fail due to violent oscillations of the vacuum chamber pressure. These experimental data were also utilised for designing a SED for PS-3 sub-scale motor, the results of which are covered in this paper. The accuracy of measurements are within a range of ± 0.4%. Error analysis of the data were carried out and are presented in Appendix A.Experiments were carried out on straight cylindrical supersonic exhaust diffusers (SED) using cold nitrogen and hot rocket exhaust gases as driving fluids, in order to evaluate the effects of the ratios of the SED area to rocket nozzle throat area (Ad/At), SED area to rocket nozzle exit area (Ad/Ae), 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. The rocket nozzle and SED starting transients were also simulated in the models. The study reveals that (Po/Pa)st increases monotonically with increase in (Ad/At) and k. One-dimensional normal shock relations were used in predicting the (Po/Pa)st since the compression in long ducts is basically a normal shock process. Predicted values of (Po/Pa)st were validated with experimental data. SED efficiency factors (ηns) were arrived at based on one-dimensional normal shock relations. ηns goes down at higher values of (Ad/Ae). (Po/Pa)st is lower for lower k values for the same (Ad/At). Cylindrical SEDs exhibit no hysteresis. The results of this investigation were utilized in validating the design of high altitude test (HAT) facility for testing the third stage motor (PS-3) of Polar Satellite Launch Vehicle (PSLV). The simulation of starting transients in the model revealed that the HAT facility shall not be operated in the unstarted phase, because the rocket nozzle may fail due to violent oscillations of the vacuum chamber pressure. These experimental data were also utilized for designing a SED for PS-3 sub-scale motor, the results of which are covered in this paper. The accuracy of measurements are within a range of ±0.4%. Error analysis of the data were carried out and are presented in Appendix A.

Experimental Thermal and Fluid Science

Evaluation of the performance of supersonic exhaust diffuser using scaled down models

Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering

Numerical simulation of transient flows in a vacuum ejector-diffuser system

International Journal of Heat and Fluid Flow

Studies on the starting transient of a straight cylindrical supersonic exhaust diffuser: Effects of diffuser length and pre-evacuation state

Numerical Investigations on Dynamics and Heat Transfer in a Turbulent Underexpanded Jet

Starting transients in vacuum ejector-diffuser system

Journal	Data powered by TypesetJournal of Propulsion and Power
Publisher	Data powered by TypesetAmerican Institute of Aeronautics and Astronautics Inc.
ISSN	07484658
Impact Factor	1.868
Open Access	No
Citation Style	unsrt
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