气枪喷嘴高速射流的除水效率研究

为揭示喷嘴除水的机理并进而对气枪喷嘴进行改进和优化设计,本文提出了利用图像分析处理对小尺度气枪喷嘴高速冲击乘风破浪的除水效率的研究方法。该方法将有效除水面积作为衡量喷嘴除水效率的标准,从面实现了对喷嘴整体除水效率的定量测量,并利用该方法对影响气枪喷嘴除水效率的各种因素（一次侧压力,喷嘴到平板的距离和射流攻角）进行了研究,并将实验结果与用热线风速仪及总压探头测量的结果进行了比较,得到冲击射流在平板水平速度分量是蚊蝇 嘴除尘除水效率的决定性因素等结论。     

Experimental investigation of an axisymmetric,impinging turbulent jet. 1. Velocity field

Mean and fluctuating velocities and shear stresses in an air jet impinging on a flat surface have been obtained by particle image velocimetry. A recirculation zone is revealed within the flow that carries material from the periphery of the wall jet back to its initial regions. Results within the wall jet agree with earlier data from laser Doppler anemometry, although significant differences occur with probe measurements. Data on the mixing characteristics of the flow are presented in a companion paper.     

Numerical analysis of flow pattern of impinging liquid sprays in a cold model for cooling a hot flat plate

This paper treats the numerical analysis of two-phase mist jet flow, which is commonly adopted to cool the solidified shell in the secondary cooling zone of the continuous casting process. Flow structures of the two-phase subsonic jet impinging on a flat plate normal to flow, corresponding to the present cooling situation, are solved on the assumption that particles are perfectly elastically reflected from a surface. Again, the numerical experiments concerning mist flows composed of air and water-droplets are made in a cold model. The flow fields for both gas and particle phases strongly depend upon the particle size. When waterdroplets mixing in the mist are very small, the impinging particles travel very closely to the surface. With increasing particle size, particles are reflected from the surface in a far distance. Therefore, also, the case is analysed where a low velocity annular gas-only flow surrounding a round nozzle co-axially is present so that such idle particles may be pushed back to the surface again. This is considered to result in an improvement of the mist cooling efficiency.     

A turbulent plane jet impinging nearby and far from a flat plate

Plane air jets presenting an impact find applications in many industrial devices. They can be found in installations of heating, cooling or drying, cleaning, pulverization, or containment of polluted environments. Other applications can be found in the ventilation of buildings. The correct design of these kinds of installations requires thorough knowledge of the structure of the jet from the cinematic point of view. With this intention a test bench with variable geometry was developed. Then, using laser Doppler velocimetry (LDV) and particle image velocimetry (PIV), it is possible to analyze the development of the jet for various geometrical and cinematic configurations. It appears that the development of the jet is independent of the Reynolds number, and the velocity decrease in the developed and impinging zones can be characterized by using very simple laws. Furthermore, by PIV visualization of the impinging zone, it has been possible to highlight the causes of mass transfer through the jet. Received: 19 January 2000 / Accepted: 14 May 2001     

Numerical simulation of an impinging jet on a flat plate

Two-dimensional normal impinging jet flowfields, with or without an upper plate, were analysed by employing an implicit bidiagonal numerical method developed by Lavante and Thompkins Jr. The Jones–Launder K–? two-equation turbulent model was employed to study the turbulent effects of the impinging jet flowfield. The upper plate surface pressure, the ground plane pressure and other physical parameters of the momentum flowfield were calculated at various jet exit height and jet inlet Reynolds numbers. These results were compared with those of Beam and Warming's numerical method, Hsiao and Chuang, and others, along with experimental data. The potential core length of the impinging jet without an upper plate is longer than that of the free jet because of the effects of the ground plane, while the potential core length of the impinging jet with an upper plate is shorter than that of the free jet because of the effects of the upper plate. This phenomenon in the present analysis provides a fundamental numerical study of an impinging jet and a basis for further analysis of impinging jet flowfields on a variable angle plate.     

Experimental investigation of an axisymmetric, impinging turbulent jet. 2. Scalar field

In this, the second part of a two-part study of an impinging air jet, measurements of mean and rms concentrations and concentration probability density functions obtained using a Mie scattering technique are reported. Results in the wall jet are in good agreement with earlier data obtained using laser Raman spectroscopy, although differences in the spreading rate of the wall jet do occur, most likely due to buoyancy. The data demonstrate the influence of the recirculation zone, identified in the first part of the study, on the mixing field in causing low levels of jet fluid to persist to large distances from the surface. This finding has important consequences for many mass transfer applications of impinging jets.     

Calculation of spatial flow past an inclined flat plate with a detached shock wave

The results of a numerical analysis of a supersonic underexpanded jet impinging on an inclined flat plate are presented. The effects of the angle between the plate and the jet symmetry axis, the distance from the nozzle exit section, the exit Mach number, and the off-design conditions on the distribution of the gasdynamic parameters in the jet flowfield and on the plate surface are demonstrated. Specific features of the compressed layer and obstacle surface flows are revealed. The three-dimensional flow is simulated using the large particle method on the basis of the nonstationary Euler equations written in the cylindrical coordinate system. The calculated results are compared with experimental data. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 31–35, January–February, 1997.     

Entrainment of fine particles from surfaces by gas jets impinging at oblique incidence

This paper describes an experimental study of the removal of fine (12 μm) polystyrene particles from a glass substrate, using a gas jet that impinges obliquely onto a particle-laden surface. In order to avoid transient affects associated with jet start-up, the sample was slowly translated under a steady jet. The translating gas jet produces a long, clean path that provides very good statistics for exploring the effect of jet parameters. This study focuses on the dependence of the spatial distribution of removal on the jet pressure ratio and impingement angle. The jet is translated over the sample both longitudinally and transversely to determine both the width and the length of the particle removal footprint. The width of the removal footprint increases and the length decreases as the impingement angle is increased. Previous researchers have reported seemingly contradictory results regarding the dependence of removal efficiency on impingement angle; this paper seeks to resolve these differences. For the steady jet, the threshold jet pressure ratio required for 50% particle removal increases with decreasing impingement angle. In addition, studies of the entrainment of well-characterized particles from well-characterized substrates provide insight into the surface shear stress imposed by the oblique jet. Received: 15 December 1998/Accepted: 15 March 2000     

Time resolved investigations on flow field and quasi wall shear stress of an impingement configuration with pulsating jets by means of high speed PIV and a surface hot wire array

The effects of jet pulsation on flow field and quasi wall shear stress of an impingement configuration were investigated experimentally. The excitation Strouhal number and amplitude were varied as the most influential parameters. A line-array with three submerged air jets, and a confining plate were used. The flow field analysis by means of time resolved particle image velocimetry shows that the controlled excitation can considerably affect the near-field flow of an impinging jet array. These effects are visualized as organization of the coherent flow structures. Augmentation of the Kelvin–Helmholtz vortices in the jet shear layer depends on the Strouhal number and pulsation magnitude and can be associated with pairing of small scale vortices in the jet. A total maximum of vortex strength was observed when exciting with Sr = 0.82 and coincident high amplitudes.Time resolved interaction between impinging vortices and impingement plate boundary layer due to jet excitation was verified by using an array of 5 μm surface hot wires. Corresponding to the global flow field modification due to periodic jet pulsation, the impact of the vortex rings on the wall boundary layer is highly influenced by the above mentioned excitation parameters and reaches a maximum at Sr = 0.82.     

Enhancement of turbulent heat transfer during interaction of an impinging axisymmetric mist jet with a target

The flow structure and heat transfer of a mist jet with a low mass concentration of droplets (within 1%) impinging onto a flat surface aligned normal to the jet are studied numerically. The mathematical model is based on solving a system of Reynolds-averaged Navier-Stokes equations for a two-phase flow with the kinetic equation of the probability density function for coordinates, velocity, and temperature of particles. Addition of droplets is demonstrated to enhance heat transfer substantially, as compared with an impinging single-phase air jet in the region directly adjacent to the stagnation point of the jet.     

横流冲击射流尾迹涡结构的实验研究

本文采用LIF（激光诱导荧光）流动显示和PIV（粒子图像速度场仪）测量对横流冲击射流的尾迹涡结构进行了实验研究。水槽实验是在三种流速比和两种冲击高度实验工况下进行的。由实验结果可得到两种明显的尾迹涡结构、，即射流尾迹涡和横流尾迹涡。横流冲击射流中形成的主要尾迹涡结构主要依赖于流速比。本文还对横流冲击射流近区范围内射流尾迹涡和横流尾迹涡的形成机理和演化特征进行了分析。     

An experimental investigation of jet screech by air jet impingement on solid boundaries

An experimental investigation has been completed to study several methods of avoiding the jet screech phenomenon due to air jet impingement on solid boundaries. Measurements were completed in the Mach number region of M=0.5 using a 25 mm diameter nozzle with the air jet impinging on flat, concave and convex boundaries. Sound pressure levels were recorded in the plane of the nozzle outlet at a distance of 1.46 m from the jet axis. Hot wire studies and the stagnation pressure at the impingement zone of the jet were also recorded.With the air jet impinging on the flat board normal to its surface a maximum sound pressure occurred at a spacing of approximately two nozzle diameters producing a distinct screech at a sound level of 20 dB above that of the free jet. Three methods of preventing this screech were studied. First, by inserting disturbances into the shear layer at the nozzle exit; second, by changing the geometry of the boundary shape to improve the jet stability in the impingement region; and third, by introducing disturbances at the stagnation region which had the effect of displacing the distinct screech to another frequency range.     

Entrainment of fine particles from surfaces by gas jets impinging at normal incidence

 This paper describes an experimental study of the removal of fine (8.3 μm) polystyrene particles from a glass substrate using a gas jet at normal impingement. In order to avoid transient effects associated with jet startup, the sample was slowly translated under a steady jet. The translating gas jet produces a long clean path that provides very good statistics for exploring the effect of jet parameters. The dependence of the spatial distribution of removal efficiency on the jet pressure ratio, the jet height, and the translation speed is examined. Clean paths greater than 16 jet diameters wide are produced with a jet pressure ratio of 7 translating at 9.0 mm/s at a dimensionless height of 10. The path width is independent of the jet height at high pressure ratios and inversely dependent on the jet translation speed. A harmonic oscillator model for particle detachment accounts for the effect of translation speed. Results suggest that the particles act as nearly-quantized shear stress sensors that provide a direct, though as yet uncalibrated, measure of the surface shear stress. Further, knowledge of the pressure required to remove 50% of the particles from the central region of the path is sufficient to predict the extent of particle removal at higher pressures. Received: 30 June 1997/Accepted: 24 June 1998     

Turbulent flow structure and bubble distribution in an axisymmetric nonisothermal impinging gas-liquid jet

The flow structure of a bubbly impinging jet in the presence of heat transfer between the two-phase flow and the surface is numerically investigated on the basis of the Eulerian approach. The model uses the system of Reynolds-averaged Navier–Stokes equations in the axisymmetric approximation written with account for the inverse effect of the bubbles on the average and fluctuating flow parameters. The influence of the gas volumetric flow rate ratio and the dimensions of the bubbles on the flow structure in a gas-liquid impinging jet is studied, In the presence of gas bubbles the liquid velocity is higher than the corresponding value in the single-phase flow. A considerable, more than twofold, anisotropy between the axial and radial turbulent fluctuations in the gas-liquid impinging jet is shown to exist. An addition of air bubbles leads to a considerable growth in the liquid velocity fluctuations in the two-phase flow (up to 50% compared with the single-fluid liquid impinging jet). An increase in the disperse phase dimensions leads to intensification of turbulence of the liquid.     

Interpretation of PIV autocorrelation mesurements in complex particle-laden flows

Particle image velocimetry (PIV) was used to measure instantaneous and average particle velocity fields near the stagnation zone of a particle-laden impinging air jet. The results were compared with Lagrangian particle tracking measurements. Ensemble averages from the two methods agree well except in regions where particles have different histories, and a specific trajectory is dominant but not exclusive. The PIV autocorrelation method loses information regarding non-dominant particle trajectories. Thus, although instantaneous PIV measurements yield the dominant particle velocities correctly, the averaged measurements are biased in some regions.This work was supported by the Electric Power Research Institute under Contract RP 8034-01. We thank the 3M Corporation for their generous materials support.     

Heat transfer studies of the oblique impingement of round jets upon a curved surface

 The effect of jet inclination of the local heat transfer under an obliquely impinging round air jet striking on isothermal circular cylinder is experimentally investigated. The circumferential heat transfer distribution as well as axial Nusselt number is measured. The considered parameters are jet Reynolds number in range of 3800–40,000, and jet inclination angle, ranging from 90 to 20. The experiments are carried out for nozzle sizes, d=3, 5 and 7 mm, and separation distance from 7 to 30 of the nozzle diameter. The output results indicated that the point of maximum heat transfer along the x-axis is shifted upstream and the local heat transfer distribution changed as a function of jet inclination. The magnitude of the shift was found to be significantly higher than that observe for a flat plate. The increasing inclination caused increasing asymmetry around the point of maximum heat transfer, with the upstream side of heat transfer profile dropping off more rapidly than the downstream side. Correlations of both the magnitude and shift of maximum heat transfer point are presented. The surface average heat transfer rate is calculated and compared with the normal impingement. Received on 5 June 2000 / Published online: 29 November 2001     

Heat transfer from an obliquely impinging circular, air jet to a flat plate

A series of experiments was conducted for the measurement of local convective heat transfer coefficients for an obliquely impinging circular air jet to a flat plate. In the experiments, the oblique angles selected were 90°, 75°, 60° and 45°, with 90° being a vertical jet. Two different Reynolds numbers of 10,000 and 23,000 were considered for the purpose of comparison with previous data available in the literature. Another parameter varied in the measurements was the dimensionless jet-to-plate distance, L/D. Four values of L/D(2, 4, 7, and 10) were considered in the experiments. The experiments were conducted using the preheated wall transient liquid-crystal technique. Liquid-crystal color changes were recorded with a video system. Local convective heat transfer coefficients were obtained through the surface transient temperatures that were related to the recorded color information. Detailed local heat transfer coefficients were presented and discussed in relation to the asymmetric wall jet upon impingement of the jet flow. Results of experiments show that, for a given flow situation, the point of maximum heat transfer shifts away from the geometrical impingement point toward the compression side of the wall jet on the axis of symmetry. The shift is more pronounced with a smaller oblique angle (larger jet inclination) and a smaller jet-to-plate distance. Comparisons of experimental results with existing heat transfer data for both obliquely impinging jets and vertical impinging jets are made. The effect of oblique angles on heat transfer was assessed.     

Impingement of a low Reynolds number turbulent circular jet onto a flat plate at normal incidence

The velocity field of a circular water jet impinging onto a flat plate has been measured using particle image velocimetry, or PIV. The velocity field has been recorded at several instants in time, producing thousands of simultaneous two-dimensional velocity measurements for each realization. The instantaneous velocity, vorticity and rate-of-strain fields reveal the interaction of vortices near the impinging wall within the radial wall jet downstream from the stagnation point. An ensemble average of the instantaneous fields produces a mean velocity field of the jet flow, which reveals many of the processes leading to boundary layer separation and vortex breakaway within the wall jet. The PIV system extracts the velocity measurements using a two-dimensional autocorrelation method, and can obtain thousands of highly accurate velocity measurements within a few minutes. The structure found in these experiments may be similar to the ground level structure of atmospheric microburst phenomena.A version of this paper was presented at the 11th Symposium on Turbulence, University of Missouri-Rolla, 17–19 October 1988     

Numerical computation of an impinging jet with uniform wall suction

The paper presents numerical predictions of a turbulent axisymmetric jet impinging onto a porous plate, based on a finite volume method of solving the Navier-Stokes equations for an incompressible air jet with the K–ε turbulence model. The velocity and pressure terms of the momentum equations are solved by the SIMPLE (semi-implicit method for pressure-linked equation) method. In this study, non-uniform staggered grids are used. The parameters of interest include the nozzle-to-wall distance and the suction velocity. The results of the present calculations are compared with available data reported in the literature. It is found that suction effects reduce the boundary layer thickness and increase the velocity gradient near the wall.     

超音速冲击射流离散频率噪声的屏蔽抑制方法

根据导致超音速冲击射流离散频率噪声的反馈机理，提出了一种能够有效地破坏反馈环的形成，从而抑制超音速冲击射流离散频率噪声的喷嘴屏蔽方法。这种方法是通过阻隔反馈波使其不能到达喷嘴唇口从而破坏反馈环、同时屏蔽罩不与射流接触来实现降噪的目的的。本文介绍了这种方法的基本思想并提出了屏蔽罩的设计要点。实验结果表明，对于合适的屏蔽罩的参数，降噪效果达5分贝以上。应用LDV方法对超声速射流轴线速度进行了测量和比较，发现应用屏蔽降噪方法以后射流轴线速度显著增加，核心区长度增加50%左右。分析表明这种降噪方法对射流冲击障碍物的推力和除尘除水效率的提高有帮助。