扰流柱构型对层板冷却特性的研究

在扰流柱体积一定时,通过数值模拟研究了扰流柱构型对多孔层板冷却性能的影响.计算结果表明:扰流柱构型的改变对射流冲击和气膜冷却的影响较为微弱,对环腔内部的对流换热影响较大,故对层板结构的综合冷却效率有一定的影响.在文中研究的六种扰流柱构型的层板结构中,圆形扰流柱构型外表面相对较为顺滑,能够较好地匹配冲击射流形成的壁面射流...     

截面形状对旋转扰流柱通道流动换热特性的影响

为合理设计航空发动机涡轮动叶尾缘冷却结构,采用ANSYS FLUENT软件模拟了四种扰流柱截面形状在旋转状态下的流动换热特性。对比分析了不同旋转数(Ro)和不同扰流柱截面形状时通道内部的三维流场分布、湍流动能分布、努塞尔数(Nu)分布以及阻力系数。其中扰流柱的截面形状包含圆形、菱形、矩形和椭圆形,Ro数包含0、0.2、0.4和0.6四种数值。模拟过程中通道入口雷诺数为7 000,壁面恒定热流为1 000 W/m²。结果表明,截面形状和旋转数对流动和换热状态有着显著影响,矩形扰流柱通道的换热系数和阻力系数最高;随着旋转数的增加,迎风面和背风面的换热系数差异逐渐增大;在旋转作用下,扰流柱尾缘区域出现了纵向二次流,该二次流显著破坏了壁面附近的边界层,有利于背风面换热增强。     

分离结构扰流柱的叶片通道流动与换热数值研究

应用-两方程模型对分离结构扰流柱的叶片内冷通道的换热与流动进行了三维数值模拟研究,扰流柱的布置按简单叉排方式。计算结果表明:在本文的参数范围内,与完整扰流柱相比,分离结构扰流柱的换热效果略有增强,而阻力系数随之增大,分离位置居中的扰流柱通道的换热效果最好。随着开缝厚度的增加,通道表面换热效果和流动阻力系数均先略有增大而后逐渐减小。     

几种带冷却结构的双层管换热及流动对比

对相同质量流量下的光管、双层光管、带冷却结构（肋、扰流柱、凹坑、螺旋通道）的双层管等不同结构的管流动进行了流固耦合三维数值模拟,获取了固体壁温的分布特征;对各结构下,外层壁冷热侧温差、冷气温升、流动特性及综合换热效率进行了研究分析。研究结果表明：相同质量流量下,带螺旋通道双层管的外层壁冷热侧温差最小、综合换热效率最高;凹坑结构双层管与双层光管的流动及换热特性相似,流阻较小但换热效果也较差;扰流柱和肋结构双层管的流动换热特性相近,其温度分布均匀性、换热量介于双层光管和螺旋通道双层管之间,其流阻大且综合换热效率低。     

涡轮叶片尾缘端部冲击流动与换热特性研究

针对涡轮叶片尾缘吸力面热应力集中,容易造成叶片尾部烧毁的现象,提出端部冲击扰流柱结构,采用Realizable k-ε湍流模型和增强壁面函数分析涡轮叶片尾缘内部流场和吸力面换热特性,研究不同冲击孔与扰流柱排列方式的影响,揭示端部冲击扰流柱结构的流场与换热机理。研究表明,端部冲击扰流柱结构对于改善吸力面的换热效果要优于中间冲击扰流柱结构,对端壁的换热有显著提高;各表面平均换热系数均随着压比的增大而增大,顺排结构时,冲击孔换热最强,扰流柱换热次之;叉排结构时,冲击孔换热最强,隔板迎风换热次之;近距离冲击,顺排的综合效果优于其它几种结构,而远距离冲击,叉排的综合效果最好,其吸力面温度分布较均匀。     

叶片直冷却通道宽高比对流动换热的数值研究

本文运用有限体积法,采用结构化六面体网格和k-ε湍流模型来求解三维N-S方程,对不同宽高比的内置V型扰流肋片的直冷却通道在入口雷诺数为20 000时的流动与换热特性进行数值模拟。分析了不同通道宽高比对肋间壁面的流动换热情况,并对W/H进行全局寻优。结果表明,带肋直通道的整体换热效果和综合冷却效率分别与W/H呈近似的函数关系,W/H为4.5时的通道整体换热效果最好,W/H为0.35时的通道综合冷却效率最佳。     

涡轮叶片尾缘典型结构的流动与换热特性研究

针对涡轮叶片尾缘"冲击+扰流柱"复合典型冷却结构,通过分析内部流场和换热特性,揭示"冲击+扰流柱"冷却结构中流动发展的过程以及冲击对压力分布和流场分布的影响,揭示涡轮叶片尾缘区内射流冲击扰流柱排通道内换热机理,详细分析了冲击下各个表面的换热情况。结果表明,压比的增大能够有效改善冷却通道端壁的换热性能,但同时增大了压力损失;在两种冲击距离下,n=3d换热效果优于n=6d,但是n=6d的下游换热覆盖效果优于n=3d。顺排时,冲击孔的平均换热系数大约是扰流柱的1.5倍;叉排时,冲击孔的平均换热系数大约是扰流柱的3倍,而其它部位的平均换热系数受排列方式的影响很小。因此,"冲击+扰流柱"冷却结构的匹配,对于优化涡轮叶片尾缘区域的换热及其重要。     

静叶前缘冲击-凸起扰流结构复合冷却的数值研究

为了改善燃气透平叶片冲击-凸起扰流结构流动与换热特性,构建了3种复合冷却叶片：无凸起结构V1,分别在前缘与冲击孔错列布置两种泪滴型扰流柱V2和V3,采用SST k-omega湍流模型,在8种不同质量流量比(MFR)工况下分析3种叶片的流动与换热特性。结果表明：针对C3X叶型布置的泪滴型扰流柱能有效改善叶片的换热特性;低MFR工况下,V3结构性能更优,冷却效率可提高9.29%;增加MFR可以同时改善流动与换热特性,当MFR从0.25%增加到1.00%时,3种叶片的压力损失系数先增大后减小;随着MFR从0.25%增加到2.00%,V2和V3结构冷却效率的提高率逐渐降低;在MFR≥1.00%时,V3结构的流动与换热特性均优于V2。     

内冷通道特性对外表面气膜冷却换热影响研究

采用混合网格和realizable к-ε紊流模型,求解三维N-S方程,对气膜孔入口无横流、仅有横流和横流通道中有扰流肋的情况下,外表面气膜冷却换热特性进行了数值研究.具体分析了横流及肋的存在对气膜冷却外表面换热系数的影响.结果表明,横流及肋的存在使通道流场变得非常复杂,横流提高了外表面的换热系数,且使流动及换热呈现不对称特性分布;而扰流肋的存在减弱了横流对外表面的影响,换热系数相对于仅有横流时降低,且使外表面流动及换热的对称性有所改善.     

具有浇注缺陷的扰流柱流动与换热特性的对比分析

采用数值模拟方法对两种具有浇注缺陷的扰流柱通道的换热和流动阻力特性进行了模拟,重点研究了断裂结构和束腰结构扰流柱的影响,并与完整扰流柱通道进行了比较。结果表明所研究的扰流柱通道与完整扰流柱通道相比,其换热效果和压力损失系数相差不大。具有断裂结构和束腰结构的扰流柱减轻了叶片的重量,降低加工精度,对通道的流动和换热特性的影响不大,甚至具有一定的强化传热作用。     

Non-Darcian effects on natural convection heat transfer in a wavy porous enclosure

A numerical investigation is carried out to analyze natural convection heat transfer inside a cavity with a sinusoidal vertical wavy wall and filled with a porous medium. The vertical walls are isothermal while the top and bottom horizontal straight walls are kept adiabatic. The transport equations are solved using the finite element formulation based on the Galerkin method of weighted residuals. The validity of the numerical code used is ascertained by comparing our results with previously published results. The importance of non-Darcian effects on convection in a wavy porous cavity is analyzed in this work. Different flow models for porous media such, as Brinkman-extended Darcy, Forchheimer-extended Darcy, and the generalized flow models, are considered. Results are presented in terms of streamlines, isotherms, and local heat transfer. The implications of Rayleigh number, number of wavy surface undulation and amplitude of the wavy surface on the flow structure and heat transfer characteristics are investigated in detail while the Prandtl number is considered equal to unity.     

The thermal modeling of a matrix heat exchanger using a porous medium and the thermal non-equilibrium model

Heat transfer and fluid flow characteristics through a porous medium were investigated using numerical simulations and experiment. For the numerical simulations two models were created: a two-dimensional numerical model and a Fluent™ computational fluid dynamics (CFD) porous media model. The experimental investigation consisted of a flow channel with a porous medium section that was heated from below by a heat source. The results of the numerical models were compared to the experimental data in order to determine the accuracy of the models. The numerical model was then modified to better simulate a matrix heat exchanger. This numerical model then generated temperature profiles that were used to calculate the heat transfer coefficient of the matrix heat exchanger and develop a correlation between the Nusselt number and the Reynolds number.     

Evaporation heat transfer of falling water film on a horizontal tube bundle

A numerical simulation and experimental study were carried out for evaporation heat transfer of a falling water film on a smooth horizontal tube bundle evaporator. A laminar model and a turbulence model were respectively adopted to calculate the heat transfer coefficients of falling water film on horizontal heated tubes. The calculation zone on the heated tube was divided into the top stagnation zone and the lateral free film zone. The initial boundary conditions for the free film zone were determined from the calculated results of the stagnation zone. The modified wall function method was used for the turbulent flow. Comparisons between the experimental data and the numerical solutions by use of two flow models show that the experimental data lie between the laminar model solutions and the latter turbulence model solutions and that they are closer to the latter solutions. Finally, a simple dimensionless correction based on the numerical simulations is proposed for predicting the evaporation heat transfer of falling water film for actual engineering applications. © 2001 Scripta Technica, Heat Trans Asian Res, 31(1): 42–55, 2002     

基于遗传算法的燃气轮机叶片内部柱肋蒸汽冷却通道的优化分析

针对叶片尾缘内部柱肋冷却方式进行数值仿真和优化分析。采用CFX软件进行数值仿真计算,建立圆形柱肋、水滴形柱肋和正方形柱肋3种柱肋形状下,不同柱肋间距的矩形通道模型,验证数值模型的正确性以及网格无关性。分析了顺排和叉排的排列方式下,柱肋形状和柱肋间距对下底面努塞尔数以及整个通道内压力损失的影响,最后通过MATLAB的遗传算法对仿真结果进行优化。研究表明：柱肋模型中,横向和纵向柱肋间距最小时,换热效果最佳,压力损失最大;在顺排和叉排中,正方形柱肋对通道的换热强度的提升效果最明显,圆形柱肋提升效果最小。     

Turbulent natural convection in a vertical parallel-plate channel with asymmetric heating

Turbulent natural convection in a vertical parallel plate channel has been investigated both experimentally and numerically. The experimental channel is formed of a uniform temperature heater wall and an opposing glass wall. A fibre flow laser doppler anemometer (LDA) is used to measure velocity profiles along the channel. Simultaneous velocity and temperature profile measurements are made at the channel outlet. A commercial computational fluid dynamics (CFD) code is used to simulate heat transfer and fluid flow in the channel numerically. The code is customised building in some low Reynolds number (LRN) k–ε turbulence models. The numerical method used in this study is found to predict heat transfer and flow rate fairly accurately. It is also capable of capturing velocity and temperature profiles with some accuracy. Experimental and numerical data are presented comparatively in the form of velocity, temperature, and turbulent kinetic energy profiles along the channel for a case. Correlating equations are obtained from the numerical results for heat transfer and induced flow rate and, are presented graphically comparing with other studies available in the literature.     

Heat transfer analysis of stratified flow in rotating heat pipes with cylindrical and stepped walls

This paper deals with the flow behavior and the related heat transfer characteristics of stratified flow in axially rotating heat pipes with cylindrical and stepped wall configurations. Flow patterns are presented with existing experimental data of heat transfer in cylindrical and stepped wall rotating heat pipes. Theoretical and semi-empirical models for calculation of the condensation and evaporation heat transfer coefficients are developed. Key dimensionless numbers such as Froude, Galileo, G and ξ-number are identified. Existing experimental data from a rotating cylindrical heat pipe are analyzed and used for regression based on semi-empirical models. Good agreement between the predicted results and experimental data was obtained. Comparison between the present heat transfer models rotating cylindrical wall heat pipes and experimental data from a stepped wall heat pipe shows that the present models can be used to predict the condensation and evaporation heat transfer coefficients in a rotating stepped wall heat pipe.     

Heat transfer from multiple row arrays of low aspect ratio pin fins

Pin fin arrays are used in many applications to enhance heat transfer. In modern gas turbines, for example, airfoils are designed with sophisticated internal and external cooling techniques. One method for cooling is routing air from the compressor through intricate cooling channels embedded in turbine airfoils. Heat transfer from the blade to the coolant air can be increased by installing arrays of cylindrical pedestals often referred to as pin fins. Pin fin arrays increase heat transfer by increasing the flow turbulence and surface area of the airfoil exposed to the coolant.For the current study, experiments were conducted to determine the effects of pin spacing on heat transfer and pressure loss through pin fin arrays for a range of Reynolds numbers between 5000 and 30,000. Results showed that spanwise pin spacing had a larger effect than streamwise spacing on array pressure loss while streamwise spacing had a larger effect than spanwise spacing on array heat transfer.     

Numerical study of the hydrodynamics and heat transfer characteristics of liquid–liquid Taylor flow in microchannel

A numerical study of an oil–water Taylor flow is presented in this paper to explore its flow and heat transfer characteristics. Due to the large surface area to volume ratio in narrow channels, using slug flows, high heat and mass transfer rates could be achieved. Sound knowledge of the underlying physics of slug flow is required for the practical design of microfluidic devices. In this study, hydrodynamics and heat transfer characteristics of dispersed oil droplets flowing inside a vertically upward circular microchannel (D = 0.1 mm) with water being the carrier phase have been explored numerically. ANSYS Fluent was employed to capture the liquid–liquid interface using volume of fluid method. Two different boundary conditions were considered in the present study. First, an isothermal wall of 373 K and later a constant wall heat flux (420 kW/m²) were, respectively, prescribed over the wall of the microchannel. The numerical code was validated against the results available in the literature, and the significant results in the form of pressure drop and heat transfer rates have been discussed. A considerable increase in Nusselt number, up to 180% and 210%, was observed with the oil–water slug flow in contrast to the liquid‐only single‐phase flow inside the microchannel for isothermal and constant wall heat flux conditions, respectively.     

不同搅混翼定位格架5x5棒束通道内流动与传热特性数值研究

建立了一组不同搅混格架的5×5棒束通道数值计算模型,以压降值、轴向及周向Nu分布为参考,将数值计算结果与文献中实验结果进行了对比验证,均取得了良好的吻合度。对两种具有不同形状搅混翼的棒束通道进行了数值模拟,比较分析了其流动及传热特性;通过引入三个无量纲因子：涡流搅混因子、交叉流搅混因子以及湍流强度因子,对其搅混作用进行了进一步的评价和比较。