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Experiments are performed to study the influence of an oblique jet on convective heat transfer to a vertical heated plate. The local heat transfer coefficient is investigated parametrically for Reynolds numbers ranging from 10000 to 75000 and jet to plate spacing of 3–20 d for different inclinations of a jet in the range of 15°–75°. The experiments are carried out on a flat rectangular surface of 600 × 300 × 0.1 mm3, hit by a jet of 11.5 mm diameter, with air as the working fluid. The infrared technique is used to examine the temperature contours over the entire section of the test plate. The present investigation deals with the effect of inclination on cooling at different locations over the plate locally in all directions from the point of impingement. The effect of Reynolds number and jet to plate spacing due to inclined jet at different orientations on the longitudinal, lateral, and intercardinal axial location is studied. The study also focuses on the heat transfer phenomenon happening on the far side of the test plate. The correlation analysis is performed on the current experimental data. The local Nusselt number correlations for the inclined jet are suggested that predict the present experimental results as a function of the geometrical and flow parameters. © 2022 Elsevier Masson SAS

World University root node

Department of Mechanical Engineering

International Journal of Thermal Sciences

Investigation of local heat transfer from a flat plate impinged by an inclined circular jet

Heat transfer over a heated plate positioned normal to the horizontal circular impinging jet is investigated experimentally. The area average and local heat transfer characteristics over a large plate impinged by jet of air are investigated for Reynolds numbers ranging from 10,000 to 45,000, with jet-to-plate distances of 3–20 d for a maximum range of 0 ≤ r/d ≤ 44. The experiment is carried out for two jet diameters of 5.88 mm and 11.5 mm on a plate of 600 × 300 mm2 impinged by a jet. The infrared (IR) thermal imaging method is used to find the temperature distribution over the heated plate. The combined heat transfer investigation of both local and area average gives a proper insight of the cooling phenomenon of the jet at any location over the plate. The investigation is effective in identifying areas with a greater rate of heat transfer. At various locations, the Nusselt number growth with increasing mass flow rate is studied. The area average heat transfer is discovered to be the highest at z/d = 3 for all Reynolds numbers in the A1 (0 &lt; r/d &lt; 2.17) region. However, for z/d = 6 and r/d = 0, the local Nusselt number is reported to be the highest. Reduction in area average heat transfer is reported in area A3, i.e., r/d = 6.5, and beyond this area, the profile of Nusselt number practically converges. © 2022 Taylor &amp; Francis.

Experimental Heat Transfer

Experimental analysis of local and average heat transfer between circular impinging jet and flat plate

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. © 2021 Elsevier Masson SAS

Aerospace Science and Technology

Starting transients in second throat vacuum ejectors for high altitude testing facilities

The present study focuses on the mitigation of shock wave using novel geometric passages in the flow field. The strategy is to produce multiple shock reflections and diffractions in the passage with minimum flow obstruction, which in turn is expected to reduce the shock wave strength at the target location. In the present study the interaction of a plane shock front (generated from a shock tube) with various geometric designs such as, 1) zig-zag geometric passage, 2) staggered cylindrical obstructions and 3) zig-zag passage with cylindrical obstructions have been investigated using computational technique. It is seen from the numerical simulation that, among the various designs, the maximum shock attenuation is produced by the zig-zag passage with cylindrical obstructions which is then followed by zig-zag passage and staggered cylindrical obstructions. A comprehensive investigation on the shock wave reflection and diffraction phenomena happening in the proposed complex passages have also been carried out. In the new zig-zag design, the initial shock wave undergoes shock wave reflection and diffraction process which swaps alternatively as the shock front moves from one turn to the other turn. This cyclic shock reflection and diffraction process helps in diffusing the shock wave energy with practically no obstruction to the flow field. It is found that by combining the shock attenuation ability of zig-zag passage (using shock reflection and diffraction) with the shock attenuation ability of cylindrical blocks (by flow obstruction), a drastic attenuation in shock strength can be achieved with moderate level of flow blocking. © 2020 The Authors

Fulltext

Defence Technology

Shock wave mitigation using zig-zag structures and cylindrical obstructions

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 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

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

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.

Flow transients in un-started and started modes of vacuum ejector operation

In recent times, shock tube flows have been widely employed in many micro-scale devices in the fields of propulsion technology, micro-heat engines, particle delivery systems, and so on. The very small length scales in such micro-shock tubes make the flow physics more complicated compared to the ordinary macro-shock tubes. The major differences in the flow features are the profound influences of wall effects and rarefaction effects. The rarefaction effect alters the boundary layer structure by imparting additional velocity and thermal gradients to the wall-bounded fluid. These phenomena can strongly affect the micro-shock tube flow characteristics such as shock-contact wave speeds, wave propagations, hot and cold zone properties. The main objective of the present work is to produce a detailed understanding on the wave propagation characteristics in a micro-shock tube under rarefied conditions using computational fluid dynamics methods. The shock-contact interface movement under different operating conditions such as Knudsen number and pressure ratio are investigated in detail and compared with the macro-scale shock tube flows. The difference between the numerical and analytical works and their cause is identified and discussed. The results obtained show that the shock strength attenuates rapidly for micro-shock tubes compared to macro-shock tubes. The shock-contact propagation and the distance between them in a micro-shock tube have a strong dependence on rarefaction effects. The more the rarefaction effects are, lesser will be the shock-contact distance. The shock-contact distance decreases as the pressure ratio increases. A strong attenuation in shock strength can also be observed as the rarefaction increases. © IMechE 2013.

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

Computational analysis of the wave motions in micro-shock tube flow

Micro shock tubes are widely employed in many micro instruments which require high speed and high temperature flow field. The small flow dimension introduces additional flow physics such as rarefaction effects, viscous effects etc, which makes the micro shock tube different from conventional macro shock tubes. In the present study, a numerical investigation of the flow physics associated with shock propagation and reflection inside micro shock tubes was carried out using unsteady Navier Stokes equations. Maxwell's slip boundary conditions were incorporated to simulate the rarefaction effects produced due to low pressure and very small length scale. The effect of initial pressures on the shock propagation was investigated keeping the pressure ratio constant. The dependency of the shock tube diameter on shock propagation was also investigated. The results show that shock strength attenuates drastically in a micro shock tube compared to macro shock tubes. The viscous boundary layer becomes a governing parameter in controlling micro shock tube wave propagations. The implementation of slip velocity to model rarefaction effects increases the shock strength and aids in shock wave propagation. The simulation with slip wall exhibits a wider hot zone (shock-contact distance) compared to no-slip simulation. The contact surface propagation distance reduces under the slip effects. A drastic attenuation in shock propagation distance was observed with reduction in diameter. The shock wave when reflected from the end wall inhibits the rarefaction effects, generally happening at very low pressure micro shock tubes, and the associated slip effect vanishes for the post reflected shock flow field. © 2013 The Korean Society of Mechanical Engineers and Springer-Verlag Berlin Heidelberg.

Journal of Mechanical Science and Technology

Computational study of the unsteady flow characteristics of a micro shock tube

The study of flow physics in microshock tubes is of growing importance with the recent development of microscale technology. The flow characteristics in a microshock tube is considerably different from that of the conventional macroshock tube due to the boundary layer effects and high Knudsen number effects. In the present study an axisymmetric computational fluid dynamics (CFD) method was employed to simulate the microshock tube flow field with Maxwell's slip velocity and temperature jump boundary conditions, to accommodate the rarefaction effects. The effects of finite diaphragm rupture process and partial diaphragm rupture on the flow field and the wave propagations were investigated, in detail. The results show that the shock propagation distance attenuates rapidly for a microshock tube compared to a macroshock tube. For microshock tubes, the contact surface comes closer to the shock front compared to the analytical macroshock tube case. Due to the finite diaphragm rupture process the moving shock front will be generated after a certain distance ahead of the diaphragm and get attenuated rapidly as it propagates compared to the sudden rupture case. The shock-contact distance reduces considerably for the finite diaphragm rupture case compared to the sudden diaphragm rupture process. A partially burst diaphragm within a microshock tube initiates a supersonic flow in the vicinity of the diaphragm similar to that of a supersonic nozzle flow. The supersonic flow expansion leads to the formation of oblique shock cells ahead of the diaphragm and significantly attenuates the moving shock propagation speed. Copyright © 2013 by ASME.