涡轮叶片表面流动及换热特性实验研究

为深入了解叶片表面的流动和换热特性,在叶片表面布置静压测孔获得叶片中截面压力系数和速比系数分布规律,同时采用瞬态液晶技术获得叶片表面高分辨率的努塞尔数分布云图。结果表明,叶片前缘受来流直接冲击,努塞尔数中间高两侧低;在吸力面上努塞尔数在分离区域降低,在再附着区域升高;在压力面上努塞尔数先降低再逐渐升高。     

微尺度流动模型研究

对微尺度气体流动及其适用模型进行综述,并以微通道内气体流动为例,采用基于宏观连续介质假定的无滑移、一阶滑移和修正二阶滑移模型,以及基于分子运动的DSMC方法分别进行数值研究,分析不同努森数下各流动模型的适用性.     

烧结开孔金属泡沫壁的辐射和自然对流耦合换热试验研究

开展烧结开孔金属泡沫壁在大空间条件下的辐射和自然对流耦合换热试验研究,研究壁面倾角、瑞利数Ra*、泡沫厚度对换热性能的影响。试验所得竖直光铜板的结果与文献结果符合一致,验证了试验方案的有效性。试验结果表明:在一定的加热功率下,随着倾角的增加,纯自然对流努塞尔数和复合努塞尔数先增加后减小,在倾角为60o～80o达到极大值,且辐射换热量占总换热量的33%～44%;竖直放置时,烧结泡沫壁的辐射换热量占总换热量的比值小于光壁,且该值随泡沫厚度增加而增加。与光壁相比,烧结泡沫竖壁复合换热的平均努塞尔数和纯自然对流换热的平均努塞尔数分别提高了1.52～1.98倍和1.16～1.66倍。通过对红外相机拍摄的泡沫表面温度分布进行分析验证了基于泡沫表面平均温度的辐射换热量计算模型的有效性。     

非均匀加热条件下微矩形槽道内的滑移流动换热特性

利用正交函数法对定热流密度加热、壁面温度在周向可任意变化条件下,气体在微矩形槽道内的热充分发展滑移流动的换热特性进行理论分析,获得相应条件下的Nu数计算方法及换热特性,并与大尺度槽道的换热特性进行比较,探讨了Kn数、槽道高宽比及不同加热条件对微矩形槽道内滑移流动换热性能的影响。结果表明,在任何加热条件下,微矩形槽道内的平均Nu数均低于相同加热条件下大尺度矩形槽道中的Nu数,且随Kn数的增加而减小。高宽比越小,平均Nu数下降越大。在相同的高宽比和Kn数下,单边加热条件下的换热性能相比相同加热条件的常规大槽道内的换热性能下降最小。     

稀薄效应对空气静压导轨速度边界和温度特性的影响

当空气静压导轨气膜厚度为微米级别时,依据Kn数定义可知此时气膜内流动可能处于滑移区,气浮支撑的内部压力和承载特性会受到稀薄效应的影响,最典型的表现为边界速度滑移和温度跃迁现象。将基于格子Boltzmann方法研究稀薄效应对空气静压导轨速度边界和温度特性的影响,得到气膜径向速度和温度分布。通过机理分析和数值计算发现:随着Kn数的增大,速度滑移程度增大;而速度滑移间接引起黏温效应,导致边界温度跳跃现象。温度跳跃相当于在导轨与气膜之间的一个附加热阻,增加流动的摩擦损耗,影响散热。     

轻质点阵夹芯结构主动换热性能影响因素分析

针对点阵夹芯结构换热性能影响因素问题,采用翅片法推导四面体型点阵结构等热流密度和等壁面温度边界条件下的努塞尔数,以此表征结构的换热性能,并与实验结果进行对比。进而讨论结构支杆长度、支杆直径、导热系数和流动方向单胞个数对结构换热性能的影响,并比较相同孔隙率下四面体型、金字塔型和Kagome型点阵结构的换热性能。结果表明,点阵结构支杆长度和流动方向单胞个数对结构换热性能影响较大,Kagome型结构换热性能明显优于其他两类。     

三角形与梯形凹槽微通道流动换热特性试验研究

为研究壁面凹槽对微通道热沉流动和传热性能的影响，设计了两种侧壁具有不同凹槽结构的微通道热沉模型，以去离子水作为流体介质展开试验，并通过综合进出口压降、摩擦因子、加热面温度、努塞尔数和综合传热因子评价侧壁凹槽结构对流动换热特性的影响。结果表明，当凹槽开口长度同为1 mm、开口倾角同为25°时，三角形凹槽微通道的压降相对于梯形凹槽最高提高了9.36%；当加热功率同为240 W、入口温度同为20℃时，三角形凹槽微通道散热能力始终大于梯形凹槽微通道；当通道内雷诺数处于试验设定的500～3 500区间时，三角形凹槽微通道的流动与传热综合性能始终优于梯形凹槽微通道；设计微通道热沉侧壁凹槽结构应优先考虑三角形凹槽结构。     

双风冷外转子永磁同步电机不同动密封结构的传热特性

为加强基于定转子双风冷外转子永磁同步电机内部小间隙处密封性的同时保障间隙的散热性,在考虑旋转因素下,利用计算流体力学的方法对电机小间隙处不同迷宫密封结构在不同工况下的流场和传热特性进行数值模拟,通过对间隙处压力、速度的分析,探讨上下间隙处不同迷宫密封结构对电机整体密封性能以及定子侧壁面散热性的影响,并使用泄漏量和努塞尔数作为评价因子对其密封性能和散热性进行评估。结果表明：轴流风扇工况为吸风时更有利于提升电机密封性并同时保障间隙处散热性;在上下定转子间隙处添加迷宫密封的方式可以有效提高电机在双风冷系统下的密封性,且上下间隙处密封结构的压降总和越大,泄漏量越小;间隙处定子侧壁面的平均努塞尔数因迷宫密封的添加会有所下降,并且当来流温度低于壁温时其下降程度随密封结构阻流效果的增大而增大,高于壁温时随密封结构阻流效果、凹槽漩涡强度的减小而增大;相较于无密封结构,定子侧双凹槽结构在提高电机整体密封性的同时有效保障了间隙的散热性,研究结果为外转子永磁同步电机内部间隙处密封和散热结构的设计提供了参考。     

基于MATLAB的微轴承动力学特性的数值分析

微机电系统下,针对传统连续模型无法准确描述气体滑移对微轴承动力学特性影响规律的问题,引入Wu新滑移模型(任意克努森数下都与线性玻尔兹曼方程解有较高的吻合度)对传统连续模型和一阶滑移模型进行了修正。基于有限差分法,建立了用于数值分析微轴承动力学特性的动态数学模型,提出了Wu新滑移模型下动态数学模型的建立方法;在MATLAB软件平台上,对不同轴承参数下气体滑移与微小间隙下微轴承刚性转子系统动力学特性及其稳定性的变化关系进行了数值分析。研究结果表明:在微小气膜间隙下,微轴承在连续模型下预估的轴承动力学特性系数值最大,一阶滑移模型次之,Wu新滑移模型最小;即在设计MEMS下的微轴承时应采用Wu新滑移模型分析该类问题。     

基于仿生学的微通道耦合射流系统设计与研究

随着芯片热流密度的不断增长,散热问题日益严峻.文中以叶片脉络、斐波那契数列螺旋和雪花晶体结构为基础设计了3种由中心向四周拓扑的微通道耦合射流模型.通过仿真计算,分析了这3种仿生模型和典型肋柱模型的温度分布,并分析了这4种模型在不同雷诺数下的平均努塞尔数、在不同热流密度下的芯片温升和在不同泵功率下的综合性能.结果表明:3...     

Analytic heat and mass transfer of the mixed hydrodynamic/thermal slip MHD viscous flow over a stretching sheet

In this paper, the heat and mass transfer characteristics of the magnetohydrodynamic (MHD) viscous flow over a permeable stretching surface is solved analytically. The flow considered is under both the hydrodynamic and thermal slip conditions. The magnetohydrodynamic flow and heat transfer of an electrically conducting fluid, taking into account the effects of Joule and viscous dissipation, internal heat generation/absorption, work done due to deformation and thermal radiation is studied. The solution is expressed in a closed form equation and is an exact solution of the full governing Navier-Stokes and energy equations. Thermal transport is analyzed for two types of non-isothermal boundary conditions, i.e. prescribed surface temperature (PST) and prescribed surface heat flux (PHF) varying as a power of the distance from the origin. Results for some special cases of the present analysis are in excellent agreement with those existing in the literature. The effects of various physical parameters, such as magnetic parameter, thermal radiation parameter, heat source/sink parameter, Prandtl number, Eckert number and suction/injection parameter on the velocity and temperature profiles, skin friction coefficient and Nusselt number are examined and discussed in detail. Results show that there is only one physical solution for any combination of the slip together with all the parameters. The velocity/shear stress profiles and the temperature/heat transfer profiles are greatly influenced by these parameters.     

Hydrodynamic plane and axisymmetric slip stagnation-point flow with thermal radiation and temperature jump

This work focuses on the steady boundary layer flow and heat transfer near the forward stagnation point of plane and axisymmetric sheet towards a stretching sheet with velocity slip and temperature jump. The resulting nonlinear partial differential equations are reduced to the system of nonlinear ordinary differential equations by means of similarity transformations. The analytical solutions for the velocity and temperature distributions are obtained for the various values of the ratio of free stream velocity and stretching velocity, velocity slip parameter, magnetic parameter, the suction parameter, temperature jump parameter, Prandtl number, the radiation parameter and dimensionality index parameter in the series forms with the help of homotopy analysis method. Convergence of the series is explicitly discussed. The flow and shear stresses depend heavily on the velocity slip parameter. The temperature gradient at the wall increases with velocity slip parameter, temperature jump factor and decreased thermal radiation.     

An experimental study on heat transfer characteristics with turbulent swirling flow using uniform heat flux in a cylindrical annuli

An experimental study was performed to investigate heat transfer characteristics of turbulent swirling flow in an axisymmetric annuli. The static pressure, the local flow temperature, and the wall temperature with decaying swirl were measured by using tangential inlet conditions and the friction factor and the local Nusselt number were calculated for Re=30000-70000. The local Nusselt number was compared with that obtained from the Dittus-Boelter equation with swirl and without swirl. The results showed that the swirl enhances the heat transfer at the inlet and the outlet of the test tube.     

车用小通道平行流换热器传热流动试验与分析

小通道平行流换热器是燃料电池汽车的主要散热部件。吸收了电堆废热的冷却液(50%乙二醇溶液),流过小通道换热管,由换热器外侧空气冷却。在进液温度、进风温度、冷却液流量以及风速变化的试验工况下,测试了换热器的传热流动性能。引入量纲一参数k,评估了各工况参数对换热量、阻力影响的强弱。接着,分析液侧努谢尔数Nu和摩阻系数f随雷诺数Re的变化趋势,结果显示:在小通道内(当量直径D=2.685 mm),冷却液从层流到湍流的转浪点Re_c=1 750,介于微尺度与常规尺度的临界值之间。在此基础上,通过多元回归法,拟合得到层流和湍流的液侧换热系数,摩阻系数的关联式,以及空气侧阻力f_a公式。Nu和f的计算值与试验值误差分别在[-7.06%,5.93%]和[-3.95%,4.11%]内,f_a的误差在[-2.22%,3.62%]内。基于这些关联式,建立数学模型,可在广泛多变的运行条件下,对换热器的运行性能进行理论预测和评估。     

基于格子Boltzmann方法的动压气体轴承黏性热耗散研究

箔片气体轴承微间隙内的流场常处于滑移区,甚至过渡区,会出现一些微观效应,其热特性的研究采用宏观方法已不再合适。为研究不同工况下动压气体轴承间隙热特性变化规律,基于格子Boltzmann方法建立包含黏性热耗散项的径向轴承间隙传热数值模型;采用总能形式的双分布函数热模型,通过有限差分离散将其应用到贴体网格中,同时引入速度滑移和温度阶跃边界条件,通过数值计算得到不同参数下的轴承间隙气膜温度分布,并分析了不同埃克特数（Ec）、偏心率和转速条件以及温度阶跃对黏性热耗散的影响。结果表明,当Ec数、偏心率和转速增大时,气膜最高温度增加,两侧的温度阶跃增加;温度阶跃效应的忽略均会导致黏性热耗散量不同程度的低估。     

Numerical investigation on developing laminar forced convective heat transfer and entropy generation in an annular helicoidal tube

In this study, numerical investigations are conducted for forced convective heat transfer in an annular helicoidal tube under uniform wall temperature condition for laminar flow including developing region. The numerical computations reveal the developments and distributions of heat transfer and flow fields in the annular helicoidal tube when the outer tube wall is heated and the inner tube wall is insulated. The effects of Reynolds number, curvature ratio, and coil pitch on the circumferential average friction factor and Nusselt number at different axial locations, and the non-dimensional entropy generation number of laminar convection in an annular helicoidal tube are investigated. In addition, the differences of flow and heat transfer characteristics between the annular helicoidal tube and circular helicoidal tube are also described.     

Extended Graetz Problem Including Axial Conduction and Viscous Dissipation in Microtube

Extended Graetz problem in microtube is analyzed by using eigenfunction expansion to solve the energy equation. For the eigenvalue problem we applied the shooting method and Galerkin method. The hydrodynamically isothermal developed flow is assumed to enter the microtube with uniform temperature or uniform heat flux boundary condition. The effects of velocity and temperature jump boundary condition on the microtube wall, axial conduction and viscous dissipation are included. From the temperature field obtained, the local Nusselt number distributions on the tube wall are obtained as the dimensionless parameters (Peclet number, Knudsen number, Brinkman number) vary. The fully developed Nusselt number for each boundary condition is obtained also in terms of these parameters.     

Effect of thermal radiation on convection heat transfer in cooling channel of photovoltaic thermal system

The effect of thermal radiation on convection heat transfer in flat-box type cooling channel of photovoltaic thermal system with tilt angle of 30 degree was studied by 3D numerical simulation under constant heat flux boundary condition. The temperature contours and velocity fields of fluid near the outlet were obtained. The variations of wall temperature and convection Nusselt number along flow direction for all the separate walls composing the cooling channel were compared and analyzed. The results show that due to thermal radiation, the deflection of the maximum velocity region to heated top wall, together with the asymmetry of temperature field is weakened. For natural convection, radiation promotes the formation of multi-vortices. For mixed convection, heat transfer on all the cooling channel walls is enhanced under the condition of lower heat flux while heat transfer on heated top wall is deteriorated when the heat flux is relative high. Also, pressure re-rising is promoted by thermal radiation.     

Effect of guide wall on jet impingement cooling in blade leading edge channel

The characteristics of fluid flow and heat transfer, which are affected by the guide wall in a jet impinged leading edge channel, have been investigated numerically using three-dimensional Reynolds-averaged Navier–Stokes analysis via the shear stress transport turbulence model and gamma theta transitional turbulence model. A constant wall heat flux condition has been applied to the leading edge surface. The jet-to-surface distance is constant, which is three times that of the jet diameter. The arrangement of the guide wall near the jet hole is set as a variable. Results presented in this study include the Nusselt number contour, velocity vector, streamline with velocity, and local Nusselt number distribution along the central line on the leading edge surface. The average Nusselt number and average pressure loss between jet nozzle and channel exit are calculated to assess the thermal performance. The application of the guide wall is aimed at improving heat transfer uniformity on the leading edge surface. Results indicated that the streamwise guide wall ensures the vertical jet impingement flow intensity and prevents the flow after impingement to reflux into jet flow. Thus, a combined rectangular guide wall benefits the average heat transfer, thermal performance and heat transfer distribution uniformity.