Effect of non-equilibrium homogeneous condensation on the self-induced flow oscillation of supersonic impinging jets

Supersonic impinging jets are characterized by a strong coupling between the flow and acoustic fields with a self-contained feedback mechanism. This self-induced oscillatory flow make thermal and mechanical loading more severe and produces severe noise at discrete frequencies, which may cause sonic fatigue of the structures and also may damage various instruments and equipments. These loads are also accompanied by dramatic lift loss, severe ground erosion and hot gas ingestion. To control the flow it is needed to clarify the characteristics and mechanism of oscillation. However, in actual jet flows, the working gas may contain condensable gas such as steam or moist air. In these cases, non-equilibrium condensation may occur at the region between nozzle exit and an object. The jet flow with non-equilibrium condensation may be quite different from that without condensation. Therefore, the numerical investigation of the self-induced flow oscillation of supersonic impinging moist air jet was carried out in this study. Moreover, this paper aims to clarify the effect of non-equilibrium condensation on the characteristic of pressure fluctuations during the impingement of under-expanded supersonic moist air jets on a perpendicular flat plate.     

Control of transonic flow fields using local occurrence of non-equilibrium condensation

Control of supersonic flow fields with shock wave is important for some industrial fields. There are many studies for control of the supersonic flow fields using active or passive control. When non-equilibrium condensation occurs in a supersonic flow field, the flow is affected by latent heat released. Many studies for the condensation have been conducted and the characteristics have been almost clarified. Further, it was found that non-equilibrium condensation can control the flow field. In these studies, the condensation occurs across the passage of the flow field and it causes the total pressure loss in the flow field. However, local occurrence of non-equilibrium condensation in the flow field may change the characteristics of total pressure loss compared with that by the condensation across the passage of the nozzle and there are few for researches of locally occurred non-equilibrium condensation in supersonic flow field. The purpose in the present study is to clarify the effect of local occurrence of non-equilibrium condensation on the transonic flow field in a nozzle with a circular bump. As a result, local occurrence of non-equilibrium condensation reduced the shock strength and total pressure loss in the transonic flow field by flowing the moist air from trailing edge of the circular bump to the mainstream.     

The Effect of Non-Equilibrium Condensation on Supersonic Air Flows over Rectangular Cavities

Numerical simulations have been carried out for a supersonic two-dimensional flow over open,rectangular cavi-ties(length-to-depth ratios are L/D=1.0 and 3.0)in order to investigate the effect of non-equilibrium condensa-tion of moist air on supersonic internal flows around the cavity for the flow Mach number 1.83 at the cavity en-trance.In the present computational investigation,a condensing flow was produced by an expansion of moist airin a Laval nozzle.The computational results showed that the upstream traveling compression waves becomeweaker than those without the condensation.Consequently,the weaker compression waves cannot excite theshear layer strongly and amplitudes of oscillation in the cavity became smaller than those without the condensa-tion.The occurrence of the non-equilibrium condensation in case of L/D=1.0 affected strongly the amplitude andfrequency of oscillation in the cavity compared with L/D=3.0.     

Experimental study and theoretical analysis of local heat transfer distribution between smooth flat surface and impinging air jet from a circular straight pipe nozzle

An experimental investigation is performed to study the effect of jet-to-plate spacing and Reynolds number on the local heat transfer distribution to normally impinging submerged circular air jet on a smooth and flat surface. A single jet from a straight circular nozzle of length-to-diameter ratio (l/d) of 83 is tested. Reynolds number based on nozzle exit condition is varied between 12,000 and 28,000 and jet-to-plate spacing between 0.5 and 8 nozzle diameters. The local heat transfer characteristics are estimated using thermal images obtained by infrared thermal imaging technique. Measurements for the static wall pressure distribution due to impinging jet at different jet-to-plate spacing are made. The local heat transfer distributions are analyzed based on theoretical predictions and experimental results of the fluid flow characteristics in the various regions of jet impingement. The heat transfer at the stagnation point is analyzed from the static wall pressure distribution. Semi-analytical solution for heat transfer in the stagnation region is obtained assuming an axisymmetric laminar boundary layer with favourable pressure gradient. The heat transfer in the wall jet region is studied considering fluid flow over a flat plate of constant heat flux. However, heat transfers in the transition region are explained from reported fluid dynamic behaviour in this region. Correlations for the local Nusselt numbers in different regions are obtained and compared with experimental results.     

Effect of Nonequilibrium Homogenous Condensation on Flow Fields in a Supersonic Nozzle

INTRODUCTIONManystudies[1-131onthecondensationshockwaveoccurringinthecaseoftheraPidexPansionofmoistairorsteaminasupersonicnozzlehavebeenper-formed,andthecharacteristicsofcondensationshock'wavehavenearlybeenclarilied.Acondensationshockwavealsooccursinthebladepassageinasteamturbinel14,15]andsuchacondensationshockwavinteractswiththeboundarylayeronthesurfaceoftheblade.Thus,thefiowinthebladepassageofthesteamturbinewiththecondensationshockwavehasnotyetbeenclariliedl16'17].InthepreseDtstudythee…     

Generation of overpressure due to condensation in moist air jet

In the present study,a computational fluid dynamics method has been applied to investigate the effects of initialdegree of supersaturation at reservoir condition on under-expanded sonic jet structures,such as Mach disk loca-tion and diameter,barrel shock wave and jet boundary.The axisymmetfic nozzle geometry investigated was aconverging nozzle with straight part.As a result,it was found that the overpressures due to condensation generateand the characteristics of flow with generation of overpressure due to condensation in the jet were different fromthose without condensation.     

The Effect of Spherical Surface on Noise Suppression of a Supersonic Jet

Experiments were carried out to eliminate the screech tone generated from a supersonic jet. Compressed air was passed through a circular convergent nozzle preceded by a straight tube of same diameter. In order to reduce the jet screech a spherical reflector was used and placed at the nozzle exit. The placement of the spherical reflector at the nozzle exit controlled the location of the image source as well as minimized the sound pressure at the nozzle exit. The weak sound pressure did not excite the unstable disturbance at the exit. Thus the loop of the feedback mechanism could not be accomplished and the jet screech was eliminated. The technique of screech reduction with a flat plate was also examined and compared with the present method. A good and effective performance in canceling the screech component by the new method was found by the investigation. Experimental results indicate that the new system suppresses not only the screech tones but also the broadband noise components and reduces the overal     

Transonic moist air flow around a circular arc blade with bump

The unsteady phenomena in the transonic flow around airfoils are observed in the flow field of fan, compressor blades and butterfly valves, and this causes often serious problems such as aeroacoustic noise and the vibration. In recent years, the effect of bump wall on the flow field around an airfoil has been investigated experimentally and as a result, it was observed that the bump wall is effective for the control of shock wave on the airfoil. In the transonic or supersonic flow field, a rapid expansion of moist air or steam gives rise to non-equilibrium condensation. In the present study, the effect of non-equilibrium condensation of moist air on the self-excited shock wave oscillation around a circular arc blade with or without a bump on the blade was investigated numerically. The results showed that the non-equilibrium condensation significantly reduced the flow field unsteadiness such as root mean of pressure oscillation and frequency compared to the case without the non-equilibrium condensation.     

Experimental investigation of the flow and heat transfer of an impinging jet under acoustic excitation

Classic and high speed particle image velocimetry and infrared thermography are used to investigate the behavior of a round jet impinging on a flat plate for a Reynolds number 28,000, for orifice-to-plate distances of 3 or 5 nozzle diameters and for two different nozzles, a contraction and a long tube. The contraction nozzle reveals a different heat transfer distribution on the impinging plate compared to the long tube case. The jet can be excited by a loudspeaker at Strouhal numbers 0.26, 0.51 and 0.79. This acoustic forcing changes the jet structure, modifying annular vortex rings in the shear layer of the jet and increasing the turbulent values. The heat transfer is therefore modified, resulting in an increase of the Nusselt number near the jet axis and an alleviation or a shift of the secondary peak.     

Plane hydrogen jets

This study is focused on understanding the structure and behaviour of hydrogen under-expanded jets from plane nozzles and their differences with circular nozzle jets. Results of numerical simulations of hydrogen highly under-expanded jets from a storage vessel at pressure 40 MPa through a circular nozzle and two plane nozzles with aspect ratios 5.0 and 12.8 respectively, all of the same cross-section area, are presented. Two stages approach is applied to simulate under-expanded unignited jets and jet fires. At the first stage, the high Mach number flow in a near field to the nozzle is simulated by compressible flow solver. At the second stage, incompressible flow solver is applied to simulated either unignited or combusting jets in the far from the nozzle field with “inner” boundary conditions taken from the first stage. The structure and behaviour of hydrogen plane highly under-expanded jets is scrutinised, including the switch-of-axis phenomenon when the exiting jet expands in the vicinity of the nozzle only in the direction of the minor nozzle axis while it contracts in the major axis direction. Simulations demonstrated that plane jets may provide faster concentration decay compared to axisymmetric jets with the same mass flow rate due to the difference in air entrainment. The concentration decay rate is shown to be a function of the plane nozzle aspect ratio. The eddy break-up model is applied to simulate under-expanded hydrogen jet fires from the equipment at pressure of 40 MPa. The circular and plane nozzle jet fire simulations are validated against experiments by Mogi and Horiguchi (2009). The simulations are in a good agreement with the experiment.     

Boundary layer effects on the critical nozzle of hydrogen sonic jet

When hydrogen flows through a small finite length constant exit area nozzle the viscous effects create a fluid throat which acts as a converging-diverging nozzle and lead to Mach number greater than one at the exit if the jet is under-expanded. This phenomenon influences the mass flow rate and the dispersion cloud size. In this study, the boundary layer effect on the unsteady hydrogen sonic jet flow through a 1 mm diameter pipe from a high pressure reservoir (up to 70 MPa) is studied using computational fluid dynamics with a large eddy simulation turbulence model. This viscous flow simulation is compared with a non-viscous simulation to demonstrate that the velocity is supersonic at the exit of a small exit nozzle and that the mass flow is reduced.     

Experimental and Three—Dimensional Numerical Study on Under—Expanded Impinging Jets

IntroductionUnder-expanded impinging jets have attracted theinterest of many researchers not only because they havepotentially engineering applications such as surfacecooling devicesl'], and plasma spray coating['], alsobecause they are not fully understood yet[' 5]. Goldsteinet al. confirmed that the stagnation temperature near thestagnation region on the impinging plate is larger thanthat in the settling chamber despite no heat added to theflow during its process from the settling chamber to…     

Wall shear rates and mass transfer in impinging jets: Comparison of circular convergent and cross-shaped orifice nozzles

This article presents a study on the wall shear rate and mass transfer of impinging jets on a flat plate. The performance of a cross-shaped orifice nozzle was compared with a reference convergent circular nozzle having similar equivalent diameter. An array of electrodiffusion micro probes inserted into the plate was used for wall shear rates measurements. Mass transfer in the impinging region was calculated from the measured wall shear rates for a Reynolds number around 5500 and over a range of streamwise distances between the nozzle and the impinging plane within 1 to 5 nozzle equivalent diameters. The obtained Sherwood number of the reference convergent nozzle is close to the one given by Chin and Tsang (1978) [5]. The most important observation in the present investigation is that the wall shear rates and the mass transfer in the impingement region of the cross-shaped orifice nozzle are up to 175% and 40%, respectively, higher than that of the convergent nozzle. The performance of the cross-shaped orifice jet is probably related to its particular vortex dynamics characteristic of the near exit region. All the results confirm that the jet passive control enhance the mass transfer.     

Effect of non-equilibrium condensation of moist air on unsteady behaviour of shock waves around a circular arc blade

The unsteady phenomena in the transonic flow around airfoils are observed in the flow field of fan,compressorblades and butterfly valves,and this often causes serious problems such as the aeroacoustic noise,the vibration.In the transonic or supersonic flow where vapour is contained in the main flow,the rapid expansion of the flowmay give rise to a non-equilibrium condensation.However,the effect of non-equilibrium condensation on thetransonic internal flows around the airfoil has not yet been clarified satisfactorily.In the present study,the effectof non-equilibrium condensation of moist air on the self-excited shock wave oscillation on a circular arc bladewas investigated numerically.The results showed that in the case with non-equilibrium condensation,frequenciesof the flow oscillation became smaller than those without the non-equilibrium condensation.     

Experimental investigation of impingement heat transfer on a flat and dimpled plate with different crossflow schemes

A nine-by-nine jet array impinging on a flat and dimpled plate at Reynolds numbers from 15,000 to 35,000 has been studied by the transient liquid crystal method. The distance between the impingement plate and target plate is adjusted to be 3, 4 and 5 jet diameters. Three jet-induced crossflow schemes, referred as minimum, medium and maximum crossflow correspondingly, have been measured. The local air jet temperature is measured at several positions on the impingement plate to account for an appropriate reference temperature of the heat transfer coefficient. The heat transfer results of the dimpled plate are compared with those of the flat plate. The best heat transfer performance is obtained with the minimum crossflow and narrow jet-to-plate spacing no matter on a flat or dimpled plate. The presence of dimples on the target plate produce higher heat transfer coefficients than the flat plate for maximum and minimum crossflow.     

Shock wave in supersonic moist air jet for a low pressure ratio

In the present study,a computational fluid dynamics work was performed to investigate the occurrence of the shock wave by condensation in supersonic moist air jet.The unsteady,compressible axisymmetric Navier-Stokes equation is solved by TVD(Total Variation Diminishing) scheme in this study.The numerical simulations have been performed for low pressure ratio and various humidities.The results show the occurrence of the shock wave in supersonic moist air jet for a low pressure ratio when Mach disk does not occur,depending on humidity of the air.     

Passive Control of Transonic Flow Fields with Shock Wave Using Non—equilibrium Condensation and Porous wall

When non-equilibrium condensation occurs in a supersonic flow field, the flow is affected by the latent heat released. In the present study, in order to control the transonic flow field with shock wave, a condensing flow was produced by an expansion of moist air on a circular bump model and shock waves were occurred in the supersonic parts of the fields. Furthermore, the additional passive technique of shock / boundary layer interaction using the porous wall with a cavity underneath was adopted in this flow field. The effects of these methods on the shock wave characteristics were investigated numerically and experimentally. The result obtained showed that the total pressure loss in the flow fields might be effectively reduced by the suitable combination between non-equilibrium condensation and the position of porous wall.     

Streamwise distribution of the recovery factor and the local heat transfer coefficient to an impinging circular air jet

An investigation of the radial distribution of the recovery factor and the local heat transfer for an axisymmetric impinging air jet formed by a smooth nozzle is described. The recovery factor is dependent on the jet nozzle to impingement plate spacing, but is independent of jet Reynolds number. The maximum stagnation region heat transfer occurs at a nozzle to impingement plate spacing of about eight jet diameters. A correlation is obtained for the average heat transfer from the surface.     

Effect of nonequilibrium condensation of moist air on transonic flow fields

luttoductionMany studies on condensation occwhng in the caseof the rapid expansion of moist air mr steam in asupersonic nozzle have been performed experimentallyand numerically, and the characteristics of condensationhave been nearly clchfiedll4]. Schnerr et al.[5] and lriya atal.le] investigated the effect of condensation on thestrength of shock wave on suiface of wing, drag and liftnumerically. However, the. effect of condensation on theshock wave on s~e of wing and talulences behindshock wa…     

Blow-off process of highly under-expanded hydrogen non-premixed jet flame

Relationships between flame lift-off heights and reservoir pressure were experimentally investigated in order to clarify blow-off process of hydrogen non-premixed jet flames with a highly under-expanded jet structure. In this study, straight nozzles with diameters of 0.34, 0.53, 0.75 and 1.12 mm were used with maximum reservoir pressure for spouting hydrogen of 13.2 MPa. Experimental results are shown that lift-off heights in stable under-expanded jet flames do not vary significantly and are independent of the reservoir pressure in the range of studied pressure. However, the lifted heights are affected by the nozzle diameters and become smaller as the nozzle diameters increase. From experimental results, the condition for the blow-off process of under-expanded subsonic jet flames was proposed. It was concluded that the under-expanded jet flame could be blown off when the maximum waistline position, where radial distance from the jet axis to an elliptic stoichiometric contour reaches its maximum comes closer to the nozzle exit than the edge of the jet flame base.