Other affiliations: Andong National University, Indian Institute of Space Science and Technology, Keimyung University
Bio: G. Rajesh is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Projectile & Aerodynamics. The author has an hindex of 8, co-authored 32 publications receiving 141 citations. Previous affiliations of G. Rajesh include Andong National University & Indian Institute of Space Science and Technology.
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.
••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.
TL;DR: In this article, the authors investigated the influence of various geometric parameters and pressure ratios on the Coanda ejector performance, such as, the pressure ratio, primary nozzle and ejector configurations on the system performance.
Abstract: The Coanda effect has long been employed in the aerospace applications to improve the performances of various devices. This effect is the ability of a flow to follow a curved contour without separation and has well been utilized in ejectors where a high speed jet of fluid emerges from a nozzle in the ejector body, follows a curved surface and drags the secondary flow into the ejector. In Coanda ejectors, the secondary flow is dragged in the ejector due to the primary flow momentum. The transfer of momentum from the primary flow to the secondary flow takes place through turbulent mixing and viscous effects. The secondary flow is then dragged by turbulent shear force of the ejector while being mixed with the primary flow by the persistence of a large turbulent intensity throughout the ejector. The performance of a Coanda ejector is studied mainly based on how well it drags the secondary flow and the amount of mixing between the two flows at the ejector exit. The aim of the present study is to investigate the influence of various geometric parameters and pressure ratios on the Coanda ejector performance. The effect of various factors, such as, the pressure ratio, primary nozzle and ejector configurations on the system performance has been evaluated based on a performance parameter defined elsewhere. The performance of the Coanda ejector strongly depends on the primary nozzle configuration and the pressure ratio. The mixing layer growth plays a major role in optimizing the performance of the Coanda ejector as it decides the ratio of secondary mass flow rate to primary mass flow rate and the mixing length.
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.
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.
Lawrence Livermore National Laboratory1, Atomic Weapons Establishment2, University of North Carolina at Charlotte3, University of Sydney4, University of Exeter5, Missouri University of Science and Technology6, Princeton University7, Embry-Riddle Aeronautical University, Daytona Beach8, University of Florida9, Arizona State University10
TL;DR: In this article, the authors provide an extensive survey of the applications and examples where hydrodynamic instabilities play a central role, including solar prominences, ionospheric flows in space, supernovae, inertial fusion and pulsed-power experiments, pulsed detonation engines and Scramjets.
TL;DR: In this article, a variable geometry ejector (VGE) using isobutane (R600a) is presented, where the area ratio and the nozzle exit position can be actively controlled by a movable spindle installed in the primary nozzle.
Abstract: Experimental results with the first laboratory scale variable geometry ejector (VGE) using isobutane (R600a) are presented. Two geometrical factors, the area ratio and the nozzle exit position, can be actively controlled. The control of the area ratio is achieved by a movable spindle installed in the primary nozzle. The influence of the spindle position (SP) and condenser pressure on ejector performance are studied. The results indicate very good ejector performance for a generator and evaporator temperature of 83 °C and 9 °C, respectively. COP varied between 0.4 and 0.8, depending on operating conditions. The existence of an optimal SP, depending on the back pressure, is identified. A comparison of the benefit of applying the variable geometry design over a fixed geometry configuration is assessed. For example, for a condenser pressure of 3 bar, an 80% increase in the COP was obtained when compared to the performance of a fixed geometry ejector.
TL;DR: In this article, the wave structure is mainly composed of expansion waves, reflected shock waves and normal shock waves as ETMT moves for enough time at the speed of 1250 km/h.
TL;DR: In this paper, a Coanda effect nozzle supported in two fluid streams is proposed to orient the jet that is free of moving elements, also incorporating boundary layer plasma actuators to achieve larger deflection angles.
Abstract: The advantages associated to Vertical Short-Take-Off and Landing (V/STOL) have been demonstrated since the early days of aviation, with the initial technolology being based on airships and later on helicopters and planes. Its operational advantages are enormous, being it in the field of military, humanitarian and rescue operations, or even in general aviation. Helicopters have limits in their maximum horizontal speed and classic V/STOL airplanes have problems associated with their large weight, due to the implementation of moving elements, when based on tilting rotors or turbojet vector mechanical oriented nozzles. A new alternative is proposed within the European Union Project ACHEON (Aerial Coanda High Efficiency Orienting-jet Nozzle). The project introduces a novel scheme to orient the jet that is free of moving elements. This is based on a Coanda effect nozzle supported in two fluid streams, also incorporating boundary layer plasma actuators to achieve larger deflection angles. Herein we introduce a state-of-the-art review of the concepts that have been proposed in the framework of jet orienting propulsion systems. This review allows to demonstrate the advantages of the new concept in comparison to competing technologies in use at present day, or of competing technologies under development worldwide.
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.