狭缝射流撞击圆柱表面的湍流特性实验研究

实验研究了狭缝射流撞击圆柱表面后壁面射流区的平均流动和湍流特 性. 考察了雷诺数 Re (6000-20000), 喷口到受撞表面距 离 Y/W (5-13), 喷口宽度 W (6.25mm, 9.38mm), 受撞表 面曲率(半圆柱体直径 D = 150mm)对流动和湍流结构的影响. 通过分析 X 热线 在壁面射流区的测量结果发现，在近壁区域，表面曲率、 Re_{w} , Y/W 和 S/W 等 参数对 \sqrt {\overline{u^2}} / U_m 的影响比对 \sqrt {\overline{v^2}} / U_m 强，并且切 应力 \overline {uv} /U_m^2 对表面曲率变化最敏感. 当喷口与受撞击表面之间的距 离 Y/W 在一定范围内增加时， 沿圆柱表面流动的流向和横向的湍流强度增强. 用平板射流和圆柱体表面壁面射流的数据进行比较，从而得到表面曲率对壁面射流特 性的影响. 结果表明，曲率对壁面射流的影响较强， 并随着 S/W 的增大而增强. 随着雷诺数的增大，壁面曲率的影响也有强化的趋势.     

Experimental investigation of mean flow characteristics of slot jet impingement on a cylinder

An experimental investigation is made to study the flow characteristics of slot jet impingement on a cylinder. The velocity profiles and pressure distribution around the cylinder are reported for various parameters namely, the flow rate, width of the nozzle, distance of the cylinder from the jet exit and eccentricity of the cylinder to the jet axis.

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Experimentelle Untersuchung über die Strömungseigenschaften eines Düsenstrahls, der auf einen Zylinder aufprallt

Zusammenfassung Es wurde eine experimentelle Untersuchung gemacht, um die Strömungseigenschaften eines Düsenstrahls zu unterschen, der auf einen Zylinder prallt. Die Geschwindigkeitsprofile und die Druckverteilungen an dem Zylinder wurden für unterschiedliche Parameter dokumentiert. Die Parameter sind die Strömungsgeschwindigkeit, Düsengröße, Abstand zwischen Zylinder und Strahlaustritt und die Exzentrizität von Zylinder und Strahlachse.

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Nomenclature B breadth of the nozzle at the exit - D diameter of the cylinder - C _p pressure coefficient - g acceleration due to gravity - L distance of the cylinder from jet exit - P _a atmospheric pressure - P _c static pressure along the jet center-line - P ₀ stagnation pressure - P _W wall static pressure - Re _D Reynolds numberu _j D/ _a - Re _W Reynolds numberu _j W/ _a - r distance measured from cylinder surface in radial direction - r _m position of maximum velocity from cylinder surface - r _0.5 half width of the jet - u mean velocity - u _j mean velocity at the jet exit - u _m maximum velocity - W width of the nozzle - _a density of air - _m density of mercury - _w density of water - absolute viscosity - kinematic viscosity     

Experimental study of slot jet impingement heat transfer on a wedge-shaped surface

An experimental investigation was conducted to study the convective heat transfer rate from a wedge-shaped surface to a rectangular subsonic air jet impinging onto the apex of the wedge. The jet Reynolds number, nozzle-to-surface distance and the wedge angle were considered as the main parameters. Jet Reynolds number was ranged from 5,000 to 20,000 and two dimensionless nozzle-to-surface distances h/w?=?4 and 10 were examined. The apex angle of the wedge ranged from 30° to 180° where the latter case corresponds with that of a flat surface. Velocity profile and turbulence intensity were provided for free jet flow using hot wire anemometer. Local and average Nusselt numbers on the impinged surface are presented for all the configurations. Based on the results presented, the local Nusselt number at the stagnation region increases as the wedge angle is decreased but, it then decreases over the remaining area of the impinged surface. Average Nusselt number over the whole surface is maximum when the wedge angle is 180° (i.e. plane surface) for any jet and nozzle-to-surface configuration.     

Mixed convection cooling of a heated circular cylinder by laminar upward-directed slot jet impingement

An experimental and numerical study has been carried out to investigate the heat transfer characteristics of a horizontal circular cylinder exposed to a slot jet impingement of air. A square-edged nozzle is mounted parallel with the cylinder axis and jet flow impinges on the bottom of the cylinder. The study is focused on low Reynolds numbers ranging from 120 to 1,210, Grashof numbers up to Gr = 10Re ² and slot-to-cylinder spacing from 2 to 8 of the slot width. The flow field is greatly influenced by the slot exit velocity and the buoyancy force due to density change. A Mach–Zehnder Interferometer is used for measurement of local Nusselt number around the cylinder at 10° interval. It is observed that the average Nusselt number decreases with increasing the jet spacing and increases with rising the Reynolds number. A finite volume method utilizing a curvilinear coordinate transformation is used for numerical modeling. The numerical results show good agreement with the experimental results. The flow and thermal field are seen to be stable and symmetric around the cylinder over the range of parameters studied.     

Experimental and numerical investigations on slot air jet impingement cooling of a heated cylindrical concave surface

Experimental and numerical studies have been carried out for slot air jet impingement on a heated concave surface of a partially opened-top horizontal cylinder of length L = 20 cm. The slot jet is situated at the symmetry line of the partially opened-top cylinder along the gravity vector and impinges to the bottom of the cylinder which is designated as θ = 0°. The width of the opening at the top of the horizontal cylinder is W = 3 cm which corresponds to a circumferential angle Δθ = 50.8°. The experiments are performed by a Mach–Zehnder interferometer which enables to measure the local convection heat transfer coefficient. Also, a finite volume method based on the SIMPLE algorithm and non-orthogonal grid discretization scheme is used to solve the continuity, momentum, and energy equations. The Poisson equations are solved for (x, y) to find the grid points which are distributed in a non-uniform manner with higher concentration close to the solid regions. The effects of jet Reynolds number (Re _j) in the range from 190 to 1,600 and the ratio of spacing between nozzle and cylinder surface to the jet width from H = 1.5 to H = 10.7 on the local and average Nusselt numbers are examined. It is observed that maximum Nusselt number occurs at the stagnation point at (θ = 0°) and the local heat transfer coefficient decreases on the circumferential surface of the cylinder with increase of θ as a result of thermal boundary layer thickness growth. Also results show that the local and average heat transfer coefficients are raised by increasing the jet Reynolds number and by decreasing the nozzle-to-surface spacing.     

Flow structure and heat transfer of impingement jet

This paper presents the characteristics of flow behavior and thermal fields of both free and impingement jets issued from circular orifice nozzle at Re = 9,700. The flow behavior of a single round jet and impingement jet were observed by smoke flow visualization recorded by a high speed video camera with 5,000 frames per second. Heat transfer coefficient on the impingement surface was calculated varying the Reynolds number and the separation distance between nozzle exit and plate. Time-series analysis was applied to the visualization image to get the information of time variation of flow behavior. Probability distribution of vortex scale induced by the jet at discrete positions was investigated. Experimental results show that the potential core is not a continuous phenomenon with time and the frequency of vortex ring formation have similar features regardless of whether the impingement plate was set on or not, furthermore the time-series analysis with flow visualization images makes clear the detailed flow behavior.     

Boiling heat transfer characteristics of nanofluids jet impingement on a plate surface

The jet boiling heat transfer of a bar water–CuO particle suspensions (nanofluids) jet impingement on a large flat surface was experimentally investigated. The experimental results were compared with those from water. The quantificational effects of the nanoparticles concentration and the flow conditions on the nucleate boiling heat transfer and the critical heat flux (CHF) were investigated. The experimental data showed that the jet boiling heat transfer for the water–CuO nanofluid is significantly different from those for water. The nanofluids have poor nucleate boiling heat transfer compared with the base fluid due to that a very thin nanoparticle sorption layer was formed on the heated surface. The CHF for the nanofluid increased compared with that of water. The reasons were that the solid–liquid contact angle decreased due to a very thin sorption layer on the heated surface and the jet and agitating effect of the nanoparticles on the subfilm layer enhance supply of liquid to the surface.     

The effect of slot jet size on the confined transverse slot jet

The current study explored the flowfield characteristics of an induced recirculation zone resulting from a transverse slot jet issuing in a cross flow. The slot jet spanned 95% of the full channel spanwise dimension which resulted in a highly three-dimensional mean flowfield. Experimental investigations were carried out at various jet momentums for two different transverse slot jet thicknesses. Velocity field data were gathered using digital particle image velocimetry and hot-wire anemometry. The momentum ratio of the jet to the channel was found to be the leading parameter for scaling the dimensions of the mean recirculation zone. The jet thickness demonstrated an influence on the level of three-dimensionality, turbulence levels, and integral length scales.     

Unsteady flow with heat and mass transfer due to impingement of slot jet on an inclined plate

The unsteady laminar incompressible boundary layer flow due to a two-dimensional slot jet on a flat plate at an angle of attack has been studied. The unsteadiness in the flow field is due to the free stream velocity distribution or wall temperature (concentration) which varies with time. The governing partial differential equations in primitive variables have been solved numerically using an implicit finite-difference scheme in combination with the quasilinearization technique. The effect of the variation of the free stream velocity distribution with time is found to be more pronounced on the skin friction than on the heat or mass transfer. The Prandtl number and the variation of the wall temperature with time strongly affect the heat transfer. Similarly, the Schmidt number and the variation of the concentration at the wall with time strongly affect the mass transfer. Beyond a certain critical value of the viscous dissipation parameter, the plate gets heated instead of being cooled.     

Comparison of the flow-field characteristics of a slot synthetic jet with and without sidewalls

In this paper, experimental investigation of the flow-field of a slot synthetic jet (SJ) with and without sidewalls, issuing into a quiescent environment is systematically reported. Two parallel sidewalls were mounted along the shorter side of the slot extending in the streamwise direction to constrain the flow along the slot span. Hot-wire anemometry was used to explore the flow-field characteristics of both configurations at a Reynolds number of 4000 based on the slot-width and slot average exit velocity during the ejection phase. The present work is a first step towards the investigation of the SJ flow-field characteristics in a bounded region. In a number of generic situations, this work is of high importance as the SJ is deployed in constrained environments (e.g., in cooling applications) where sidewalls may be present. The relative difference in the magnitude of the distinct peaks in the near-field spanwise velocity profiles for both configurations reveals that the vortex does not get curled up towards the centerline in the case of the synthetic jet with sidewalls due to the presence of the no-slip walls. Spectral analysis in the near-wall region further confirms the absence of the phenomenon of axis-switching in the case of the synthetic jet with sidewalls. This behavior is also demonstrated with the help of the relative spreading rate in both configurations, where the unbounded synthetic jet spreads rapidly compared to the bounded one, due to greater entrainment of the surrounding fluid. The statistically two-dimensional region for a synthetic jet with sidewalls is found to extend over a longer axial distance in the downstream. The other jet properties such as turbulence intensity, skewness, and flatness factors further reveal the differences in the flow-field of the two configurations. The results show that the presence of the sidewalls strongly influences the SJ flow-field and hence, it would significantly impact the heat transfer capacity of the SJ.     

Planar collisionless jet impingement on a specular reflective plate

This paper presents a fundamental gas-kinetic study on a high speed planar rarefied jet impinging on a flat plate of specular reflections. Based on previous collisionless planar free jet results, it is straightforward to obtain jet impingement flowfield solutions, and jet impingement for specular reflective plate surface properties. Several direct simulation Monte Carlo simulation results are provided and they validate these analytical solutions of rarefied planar jet flows. The results can find applications in many disciplines, such as materials processing, molecular beams, and space engineering.     

Parametric analysis of a round jet impingement on a heated plate

In this study, the flow and heat transfer characteristics of a round air jet have been experimentally investigated in details using two techniques: Particle Image Velocimetry (PIV) and the Laser Doppler Velocimetry (LDV). The measurement of the mean velocity components are compared, and agree well with the experimental data obtained by Baydar (1999). The distributions of the velocity, turbulence quantities and temperature profiles are analyzed in the main characteristic regions of the jet where the heat transfer occurs. Parametric variations were conducted to produce information about the influences of the Reynolds number (Re = 1000, 2000, 3000), the distance between the pipe exit and the flat impingement plate (h/d = 1 and h/d = 2) and the temperature of the plane (T_p = 22 °C, 54 °C, 96 °C) on the impinging jet flow field.     

Experimental investigations of fluid flow and heat transfer characteristics of a slot jet impinging on a rectangular cylinder

Experimental investigations on flow characteristics and average heat transfer due to slot jet impinging on a rectangular cylinder have been carried out for different non-dimensional parameters. The minimum value of the pressure coefficient is found on the lower face of the rectangular cylinder at an angle of inclination of 15°. Drag coefficient calculated from the measured pressure distribution is found to be maximum within a range of breadth/width ratio of 0.67 to 1.5 of rectangular cylinders. The maximum value of heat transfer rate is obtained at the angle of inclination of 15° of the cylinder to the jet axis. An increasing trend of heat transfer rate is observed with higher Reynolds numbers. A correlation of average Nusselt number is presented for rectangular cylinders.

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Experimentelle Untersuchung der Strömungs- und Wärmeübergangs-charakteristik eines auf einen rechteckigen Zylinder auftreffenden Strahls aus einer Schlitzdüse

Zusammenfassung Es wurden experimentelle Untersuchungen des Strömungs- und Wärmeübergangsverhaltens an einem rechteckigen, durch einen Strahl aus einer Schlitzdüse beaufschlagten Zylinders für verschiedene dimensionslose Parameter durchgeführt. Der Kleinstwert des Druckbeiwertes tritt an der Unterfläche des rechteckigen Zylinders bei einem Neigungswinkel von 15° auf. Der ausder gemessenen Druckverteilung berechnete Widerstandsbeiwert erreicht bei einem Breiten-Dicken-Verhältnis des Zylinders zwischen 0,67 und 1,5 Maximalwerte. Den maximalen Wärmestrom erhält man bei einem Neigungswinkel zwischen Zylinder und Strahlachse von 15°. Mit steigenden Reynoldszahlen erhöht sich der abgeführte Wärmestrom. Eine Korrelation für die mittlere Nusseltzahl an rechteckigen Zylindern wird mitgeteilt.

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Nomenclature A surface area of the rectangular cylinder - a width of the rectangular cylinder - b breadth of the rectangular cylinder - C _D drag coefficient =D/ - C _p pressure coefficient = (p – p _a )/ - C _pb base pressure coefficient = (p _b –p _a )/ - D drag force - h _f free convection heat transfer coefficient - average heat transfer coefficient - k thermal conductivity of air - L distance of the axis of the square cylinder from the nozzle exit - l length of the rectangular cylinder - Pr Prandtl number - p static pressure - P _a atmospheric pressure - P _b base pressure on the rear face - Nu _f free convection Nusselt number - average Nusselt number - q heat loss - q _f heat loss due to free convection - Re Reynolds number =u _j W/ _a - T _a ambient air temperature - average surface temperature - u _j average jet velocity at the nozzle exit - W nozzle width - angle of inclination of the rectangular cylinder to the jet axis in degrees - _a kinematic viscosity of air - _a density of air     

Experimental investigations of fluid flow and heat transfer characteristics of a slot jet impinging on a square cylinder

Experimental investigations on pressure distributions and average heat transfer on square cylinders due to slot jet impingement have been carried out for different parameters such as, slot jet-width, distance of the square cylinder from the nozzle exit, angle of inclination of the cylinder to the jet axis and Reynolds numbers. The minimum value of the pressure coefficient is obtained on the lower face at an angle of inclination of 15° for all distances of the square cylinder from the nozzle exit. At the lowest Reynolds number the maximum average heat transfer rate is obtained at a distance of eight times the jet width from the nozzle exit. An increasing trend of the heat transfer rate is observed for higher Reynolds numbers. The maximum value of the heat transfer rate is obtained between the angles of inclination of 15° and 30° of the square cylinder to the jet axis. A correlation for the average Nusselt number is proposed in terms of the relevant non-dimensional parameters.

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Experimentelle Untersuchung der Strömungs- und Wärmeübergangscharakteristik bei Schlitzdüsenanblasung eines quadratischen Zylinders

Zusammenfassung Druckverteilung und gemittelter Wärmeübergang bei Schlitzanblasung eines quadratischen Zylinders wurden experimentell für folgende Parameter untersucht: Schlitzbreite; Abstand Düsenmündung vom Zylinder; Neigungswinkel des Zylinders zur Strahlachse; Reynoldszahl. Den Minimalwert des Druckkoeffizienten erhält man für alle Abstände an der Unterseite (bei einem Neigungswinkel von 15°). Bei der niedrigsten Reynoldszahl tritt der höchste Wert des gemittelten Wärmestroms in einem Abstand Düsenmündung/Zylinder von 8 Strahlbreiten auf. Mit steigender Reynoldszahl nimmt der Wärmestrom zu. Dessen höchster Wert tritt im Bereich 15 bis 30° des Neigungswinkels zwischen Zylinder und Strahlachse auf. Eine die Meßwerte korrelierende Nusscltbeziehung als Funktion dimensionsloser Parameter wird angegeben.

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Nomenclature A surface area of the square cylinder - a width of the square cylinder - C _p pressure coefficient=(p–p _a )/ - C _pb base pressure coefficient=(p _b –p _a )/ - h _f free convection heat transfer coefficient - average heat transfer coefficient - k thermal conductivity of air - L distance of the axis of the square cylinder from the nozzle exit - l length of the square cylinder - Pr Prandtl number - p static pressure - p _a atmospheric pressure - p _b base pressure on the rear face - Nu _f free convection Nusselt number - average Nusselt number - q heat loss - q _f heat loss due to free convection - Re Reynolds number=u _j W/v _a - T _a ambient air temperature - average surface temperature - u _j average jet velocity at the nozzle exit - W nozzle width - angle of inclination of the square cylinder to the jet axis in degrees - _a kinematic viscosity of air - _a density of air     

Flow characteristics of spray impingement in PFI injection systems

The present paper addresses an experimental study of the dynamic exchanges between the impact of an intermittent spray and the liquid film formed over the target, based on detailed phase Doppler anemometry (PDA) measurements of droplet size, velocity and volume flux in the vicinity of the impact. The flow configuration is that of a pulsed injector spraying gasoline onto a flat disc to simulate the port fuel injection (PFI) of an internal combustion engine operating at cold-start conditions. The measurements evidence that the outcome of impact cannot be accurately predicted based on the characteristics of the free spray, but requires precise knowledge of the flow structure, induced by the target. The implications for spray–wall interaction modelling are then discussed based on the application of conservation equations to the mass, momentum and energy exchanged between the impinging droplets and the liquid film. The results show that the liquid film starts to form in the vicinity of the stagnation region at early stages of injection and a non-negligible proportion of droplets impinging at outer regions splash after interaction with the film. Film disruption is mainly driven by the intermittent axial momentum of impinging droplets, which enhances the vertical oscillations. The radial momentum imparted to the liquid film at the stagnation region is fed back onto secondary droplets emerging later during the injection cycle at outwards locations, where momentum of impacting droplets is much smaller. As a consequence, although the number of splashed droplets is enhanced by normal momentum, their size and ejection velocity depends more on the radial spread induced onto the liquid film and, hence, on the radial momentum at impact. The analysis further shows that existing spray–wall interaction models can be improved if the dynamic exchanges between the impacting spray and the liquid film are accounted.     

Large eddy simulation of a slot jet impinging on a convex surface

A large eddy simulation is used to simulate flow and heat transfer in a turbulent plane jet with two distances from the jet-exit to impingement corresponding to twice and ten times the slot nozzle width. The resolved different unsteady vortex motions of the jet shear layers are studied and shown to have an important influence on heat transfer at the wall. They are used to explain existence of the second peak in Nusselt number for the case corresponding to twice the slot nozzle width. The predicted average surface Nusselt number profiles exhibit good agreement with experiments.     

Oblique impingement of a circular jet in a cross flow

The distributions of temperature and dye concentration produced by a cross flow jet impinging on a flat surface are examined experimentally. Measurements show that for sufficiently high jet injection speeds, a ground vortex region upstream of the jet develops. The dimensionless gross geometrical features of this recirculating region are related by a simple linear law and are independent of the jet — surface spacing or the injection speed ratio.