阵列射流冲击冷却流场与温度场的数值模拟

采用数值模拟方法对冲击冷却的流动和传热过程进行了三维数值研究。特别研究了在冲击孔叉排方式下,相邻孔间距、冲击距离以及射流入口雷诺数对冲击表面冷却流动传热特性的影响规律。     

阵列射流冲击冷却换热特性的数值研究

运用数值计算的方法对不同流动取向的多排孔冲击射流冷却特性进行了三维模拟,并对有初始横向流的多排孔冲击射流冷却特性进行了数值研究,揭示出射流雷诺数、流动方向、初始横向流对冲击冷却传热特性的影响规律。结果表明:研究范围内,射流雷诺数越大,冲击靶面换热效果越好;冲击腔室两端都设为出口时努赛尔数峰值所对应的射流驻点区向下游偏移最小且换热效果最好;当横流雷诺数与射流雷诺数之比大于0.5之后,有横流时的冲击射流冷却局部努赛尔数比无横流时有较为显著下降。     

Heat Transfer Control of an Impinging Inclined Slot Jets on a Moving Wall

This work is devoted to the numerical study of the interaction of an inclined plane turbulent jet with a moving horizontal isothermal hot wall. The inclination of the jet allows the control of the stagnation point location. The numerical predictions based on statistical modeling are achieved using second order Reynolds stress turbulence model coupled to the enhanced wall treatment. The jet Reynolds number (Re), surface‐to‐jet velocity ratio (R_sj); and optimal inclination angle of the jet (α) are varied. The calculations are in good agreement with the available data. The numerical results show that the heat transfer is greatly influenced by the jet Re and the velocity of the moving wall. The local Nusselt number (Nu) decreases with increasing R_sj (until R_sj = 1). However, the optimal inclination of the jet enhances heat transfer and modifies significantly the stagnation point location. Average Nu is correlated according with the problem parameters as .     

Heat Transfer Characteristics of Cooling High Temperature Steel Plate by Single Round Jet Impingement

The heat transfer characteristics of a single round air jet impingement on a high temperature steel plate were examined experimentally using a single‐point temperature measurement method, incorporated with solving the inverse heat conduction problem. During the experiments, the temperature of the steel plate varied from 1073 K to 373 K, the Reynolds number was set to 27,000, the nozzle to plate spacing was set to 4. The results indicated that the radial distribution of the local Nusselt number is bell‐shaped at the initial stage of the transient cooling process. As the cooling process continues, the local Nusselt numbers decrease and a second peak occurs at r/D = 2. The area averaged Nusselt number are in accordance with the correlation proposed by Hofmann and Martin at first and then decrease significantly, but this trend is not obvious at r/D > 10.     

高温差下卷吸作用对冲击射流换热影响数值研究

基于V2F湍流模型计算研究了卷吸作用对高温差下圆管冲击射流换热的影响,首先通过计算结果与实验值的对比验证模型方法的有效性,然后分析了基于绝热壁面温度计算的努赛尔数和射流有效度随射流和环境的温差以及雷诺数的变化,并研究了取不同定性温度对计算结果的影响。计算结果表明,高温差下定性温度取为射流温度时,基于绝热壁面温度计算的努赛尔数与射流和环境之间的温差近似无关,有效度也与雷诺数无关,但有效度随射流和环境的温差变化较大。因此,在温差较低时,依据射流和环境温度相同时的换热工况得到射流和环境温度不同时的换热工况是可行的,但温差越大,由该方法带来的误差也越大。     

倾斜射流对移动平板表面紊动和传热特性的影响

采用雷诺应力湍流模型和Simplic算法对半封闭槽道内倾斜射流冲击移动平板的流动和传热特性进行了数值模拟,研究了不同射流角度和不同平板移动速度下平板近壁湍动能和板面努塞尔数的变化.结果表明：射流角度和平板运动速度对平板近壁湍动能和表面努塞尔数值分布影响显著;当入射角与平板运动方向相同时,板速的升高提高了近壁面的湍动能,但是降低了冲击区域的局部努塞尔数值;平板表面的平均努塞尔数值随板速的提高先降低后大幅升高,高速下角度对平板表面的平均传热效果影响较小;当入射角为80°,平板运动方向与入射方向相反且板速和射流速度相同时,在移动平板表面能够获得较佳的紊动和传热效果.     

Spatially Resolved Heat Transfer Rates in an Impinging Circular Microscale Jet

An experimental study of a submerged 125-μm circular microscale jet impingement is presented. These jets flows are associated with low exit Reynolds number and a correspondingly high, subsonic Mach number. Detailed distributions of heated and adiabatic wall temperature, and local and average Nusselt number variations are presented for five laminar exit Reynolds numbers in the range of 690 to 1770 at three nozzle-to-surface spacings of 2, 4, and 6 times the nozzle diameter. The corresponding jet exit Mach numbers range between 0.26 and 0.63. An infrared radiometer is used in conjunction with a heated-thin-foil technique to measure detailed surface temperatures. Results indicate that the adiabatic surface temperature distribution is relatively insensitive to nozzle-to-surface spacing within the parameter range studied. With an increase in Reynolds number, the adiabatic surface temperature decreases significantly near the stagnation point. The average Nusselt numbers are higher compared to Martin's correlation [1] Martin, H. 1977. Heat and Mass Transfer between Impinging Gas Jets and Solid Surfaces. Advances in Heat Transfer, vol. 13: pp. 1–60.  [Google Scholar] for large Reynolds numbers. A similar observation has been reported previously in a numerical study on microscale jet impingement [5] Pence, D. V., Boeschoten, P. A. and Liburdy, J. A. 2003. Simulation of Compressible Micro-Scale Jet Impingement Heat Transfer. Journal of Heat Transfer, vol. 125(no. 3): pp. 447–453. [CROSSREF] [Google Scholar] and attributed to compressibility of the flow and the possible existence of a slip flow.     

横流对振荡射流冲击冷却特性影响的数值研究

为了分析横流条件下振荡射流冲击靶面的换热特性,采用数值计算方法研究了横向排列和纵向排列方式下振荡射流冲击靶面的冷却性能,并对纵向排列下的射流振荡器进行了排布角度修正。结果表明:横流对冲击换热的作用主要受到两方面影响,即冲击点附近的马蹄涡和冲击点向后偏移程度;对于靶面上的冲击核心区,总体上纵向排列的高换热区域后移程度小于横向排列,而前者对流换热效果受马蹄涡的影响更大;在纵向排列、入口雷诺数为21 800、横流雷诺数为14 400的工况下,20°的排布角度下冲击核心区的平均努塞尔数增大21%。     

Numerical Investigation of Forced Convection Conjugate Heat Transfer from Offset Square Cylinders Placed in a Confined Channel Covered by Solid Wall

Flow over two isothermal offset square cylinders in a confined channel is simulated for different Reynolds numbers to disclose the forced convection heat transfer from the heated square cylinders to the ambient fluid. The spacing between the cylinder in the normal direction and the blockage ratio are fixed. The channel walls are covered by solid walls of thickness equal to the size of the cylinder and conjugate heat transfer is considered by including these walls. Heat transfer from the cylinders to the ambient fluid as well as that conducted within the solid wall through the conjugate interface boundary are investigated in connection with Reynolds number and are reported for both steady and periodic flows. Simulation is carried out for Reynolds number varying from 10 to 100 with air as the fluid. The onset of the vortex begins when the Reynolds number equals 48. The conjugate interface temperature declines when the Reynolds number grows. The isotherms in the solid wall show two dimensionality near the cylinder region.     

Optimal Homotopy Asymptotic Solutions for Nonlinear Ordinary Differential Equations Arising in Flow and Heat Transfer due to Nonlinear Stretching Sheet

Optimal homotopy asymptotic method (OHAM) is used to obtain solutions for nonlinear ordinary differential equations (ODEs) arising in fluid flow and heat transfer at a nonlinear stretching sheet. The solutions for skin friction and temperature gradient for some special cases are tabulated and compared with the available numerical results in the literature. Moreover, OHAM is found to be very easy to use and the technique could be used for solving coupled nonlinear systems of ordinary differential equations arising in science and engineering.     

Numerical Study of Heat Transfer and Separated Flow over Rectangular Obstacle in Jet

This paper is about a separated reattaching flow over a hot rectangular obstacle. Two types of incoming flow are examined in order to show the influence of the external zone of the flow on the reattachment process. It comes about due to a wall jet and a boundary layer. The inner region of these two flows is similar, but their external regions are extremely different. The separating and reattaching flow phenomena are of particular interest in engineering fields such as for an aeronautical application. Wall jet flow over an obstacle occurs in many engineering applications such as environmental discharges, heat exchangers, fluid injection systems, cooling of combustion chamber wall in a gas turbine, automobile design, and others. In electronics cooling, the prediction of the Nusselt number distribution along the obstacles is necessary before the design of the apparatus. For a heated obstacle at a constant temperature, T = 350 K and an aspect ratio of 10 (L = 10 H), the problem parameters are: (a) jet exit Reynolds number (Re) ranged from 1000 to 50000, (b) incoming flow configuration (boundary layer and wall jet). The ratio between the thickness of the nozzle (b) to the obstacle height (H) are examined simultaneously. The formulation is based on the SST k–ω turbulence model. The results show that the increasing of nozzle thickness; enhances the heat transfer and considerably modifies the stagnation point location. The highest incoming flow momentum provides the greatest values of average Nusselt number. Such as the boundary layer case in comparison with the wall jet cases. The average Nusselt number is correlated according to problem parameters .     

Effects of Variable Jet Nozzle Angles on Cross-Flow Suppression and Heat Transfer Enhancement of Swirl Chamber

This paper investigated the effects of variable jetting nozzle angles on the cross-flow suppression and heat transfer enhancement of swirl cooling in gas turbine leading edge. The swirl chamber with vertical jet nozzles was set as the baseline, and its flow fields and heat transfer characteristics were analyzed by 3D steady state Reynolds-averaged numerical methods to reveal the mechanism of cross-flow weakening the downstream jets and heat transfer. On this basis, the flow structure on different cross sections and heat transfer characteristics of swirl chamber with variable jetting nozzle angels were compared with the baseline swirl chamber. The results indicated that for the baseline swirl chamber the circumferential velocity gradually decreased and the axial velocity gradually increased, and the cross-flow gradually formed. The cross-flow deflected the downstream jets and drawn them to the center of the chamber, thus weakening the heat transfer. For swirl chamber with variable jetting nozzle angles, the air axial velocity is axial upstream, opposite to the mainstream, so that the impact effects of cross-flow on the jets were reduced, and the heat transfer was enhanced. Furthermore, with the increase of axial velocity along the swirl chamber, the jetting nozzle angle also gradually increased, as well as the effect of cross-flow suppression, which formed a relative balance. For all swirl chambers with variable jet nozzle angles, the thermal performance factors were all larger than 1, which indicated the heat transfer was enhanced with less friction increment.     

Heat Transfer and Fluid Flow over a Single Disk in a Fluid Rotating as a Rigid Body

Laminar heat transfer problem is analyzed for a disk rotating with the angular speed ωin a co-rotating fluid (with the angular speed Ω). The fluid is swirled in accordance with a forced-vortex law, it rotates as a solid body at β= Ω/ω= const. Radial variation of the disk's surface temperature follows a power law. An exact numerical solution of the problem is obtained basing on the self-similar profiles of the local temperature of fluid, its static pressure and velocity components. Numerical computations were done at the Prandtl numbers Pr = 1(?)0.71. It is shown that with increasing βboth radial and tangential components of shear stresses decrease, and to zero value at β= 1. Nusselt number is practically constant at β= 0(?) 0.3 (and even has a point of a maximum in this region); Nu decrease noticeably for larger βvalues.     

分离式热管倾斜蒸发段传热特性的试验研究

采用加热无缝钢管模拟倾斜布置的分离式热管蒸发段,分析了倾角、充液量、热流密度和工作压力对其传热特性的影响,并根据试验结果回归整理了相应的换热系数无量纲准则关系式,与试验数据吻合较好,计算误差小于15％。     

Heat Transfer Phenomena of Assemblages of Smooth Slip Spheres in Newtonian Fluids

Combined effects of slip velocity and volume fraction of slip spheres on the heat transfer characteristics of multiple slip spheres are numerically investigated within the framework of a free surface cell model combined with a linear slip velocity along the surface of the slip spheres. The governing conservation equations of the mass, momentum, and energy are solved by a segregated approach using a simplified marker and cell algorithm implemented on a staggered grid arrangement in spherical coordinates. The convection and diffusion terms of conservation equations are discretized using quadratic upstream interpolation for convective kinematics and second‐order central differencing schemes, respectively. Prior to obtaining new results, this numerical solver is validated by comparison of present results with the existing literature values. Further new results are obtained for a range of conditions as; Reynolds number, Re: 0.1–200; Prandtl number, Pr: 1–100; volume fraction of slip spheres, Φ: 0.1–0.5 and slip parameter, λ: 0.01–100. The effects of these dimensionless parameters on isotherm contours and local and average Nusselt numbers are thoroughly delineated. Finally, a new empirical correlation for the average Nusselt number of multiple smooth slip spheres is proposed on the basis of present numerical results.     

圆形液体射流冲击单相局部对流传热的实验研究

以强润湿性液体为工质，就圆形自由和浸没射流冲击r／d=0.2，3．5和5处的换热系数及局部换热系数的径向分布进行了系统测定并予以关联。详细研究了射流出口Re数和液体温度等对局部换热系数的影响，最后就Nu0～Re关系式中Re数指数的物理含义作了说明。图4表3参14     

On the Performance of Ag/Oil Nanofluids in Heat Transfer Enhancement in a Sinusoidal Tube: Constant Heat Flux Boundary Condition

The present work provides an empirical investigation on the thermal characteristics of Ag/oil nanofluids flow inside a sinusoidal tube under a constant heat flux boundary condition. Ag/oil nanofluids have been prepared in low‐volume concentrations of 0.011%, 0.044%, and 0.171%. The average size of the nanoparticles was 20 nm. A heated coil was attached to the upper and lower surface of the tube that satisfied the constant thermal boundary condition of 204 W. The experiment has been pursued at low Reynolds numbers less than 160. A loop was designed to keep the flow hydrodynamically fully developed during the experiment. The test case was a sinusoidal tube. Upper and lower surfaces of the tube have been designed sinusoidally. Moreover, the width of the plates was long enough, so the problem was not considerably affected by the three‐dimensional releasing effect. Convective heat transfer coefficient and Nusselt number were calculated. It has been observed that based on the acquired data of the present work, convective heat transfer coefficient increased up to 23% for the best case (nanofluid with a volume concentration of 0.171%) compared to the base fluid. This happened while the rise of the friction factor was very low. In addition, a comparison between the new results and the previous work by authors showed the positive performance of sinusoidal tubes in increasing the convective heat transfer coefficient (the average increase was calculated to be about 82%) compared to the annular tube.     

Towards the Investigation of Heat Transfer Characteristics in a Viscous Dissipative Shear Driven Flow Through an Annulus: An Analytical Study

In the present study, the laminar forced convective heat transfer of a shear‐driven flow through an annulus in a viscous dissipative environment is investigated. The temperature distribution for different constant heat flux boundary conditions is studied, and so are the heat transfer characteristics of a Newtonian fluid under consideration. Following an analytical methodology, closed‐form expressions of the Nusselt number on both walls of the annulus have been devised. It is shown that the effect of viscous dissipation significantly alters the temperature profile, and the consequential effect is reflected in the variation of the Nusselt number. In most of the cases, variation of the Nusselt number exhibits a discontinuity in behavior at different locations within the annulus where an equilibrium state between the energy is attained due to shear heating and the heat supplied by the wall. However, it has been observed that the greater viscous dissipative heat causes a rise in the temperature and consequently a fall in the rate of heat transfer. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 42(7): 569–588, 2013; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21057     

受限空气射流冲击矩形柱鳍热沉换热实验研究

采用实验方法研究了受限空气冲击射流与矩形柱鳍热沉相结合的散热方式应用于芯片冷却的换热规律,采用最小二乘法对实验数据进行了拟合,并最终获得平均努塞尔数关于雷诺数、喷口高度-孔径比及普朗特数的实验准则方程。在此基础上将这种散热方式与其他空冷方式进行了换热能力的比较,结果表明此种散热方式的换热能力大大超过其他空冷方式。最后,对实验系统误差进行了分析,根据误差传递理论求得的平均努塞尔数的实验相对误差不超过6%。