微通道内给定壁面热流的气体换热研究

本文利用实施给定热流边界条件的DSMC方法,对短通道内给定壁面热流边界条件下的气体换热情况进行了模拟.结果表明,壁面热流密度增大导致通道内压力分布非线性程度增加.随着热流密度的增大,截面速度分布趋于平缓,滑移速度增大.给定热流密度的通道壁面温度与气流截面平均温度的差值沿程增大,温度梯度沿程下降,气体稀薄性增大时,通道换热减弱.     

Hydrodynamics and heat and mass transfer at chemical conversions in slot reactors

In this paper, experimental and numerical investigations of the hydrodynamics and heat transfer in a disk slot heat exchanger-reactor for a radial flow of a gas mixture reacting on the channel walls are described. Data for the coefficients of heat transfer from the wall being heated to the gas flowing inside the reactor are presented. The temperature field of a catalytically active reactor plate at heat release on it has been investigated experimentally. Calculations of the flow and heat transfer in a slot reactor element for a catalytic reaction with heat release have been performed. Partial oxidation of methane in an oxygen medium with the formation of a hydrogen-containing synthesis gas in a two-dimensional microchannel has been investigated numerically. Data for the extent of the chemical conversion of methane versus the initial mixture consumption and reaction temperature are presented.     

METHOD AND SOME RESULTS ON EXPERIMENTAL STUDY OF UNSTEADY TURBULENT HEAT TRANSFER

Abstract

An experimental setup is described for studying unsteady heat transfer where both the heat generation rate in the channel wall and the fluid flow rate vary in time. For measuring the surface temperature and the wall heat flux, the temperature field in the thermal sublayer is used. The results of the first experiments carried out for a sudden change in channel wall heat generation are discussed. Unsteadiness rates much greater than those in previous work known to the authors were achieved     

The effect of channel orientation on heat transfer and wall shear stress in the bubbly flow

Results of experimental investigation of heat transfer and wall friction in the upward bubble flow in a flat inclined channel are presented. Measurements were carried out in the range of superficial liquid velocities of 0.3–1.1 m/s and different values of volumetric void fraction. The hydrodynamic structure was measured by means of the electrochemical method with the use of miniature friction sensors. The values of average friction and heat transfer at different channel orientation were determined. It is shown that in the bubble gas-liquid flow we can observe a significant dependence of friction and heat transfer on the angle of channel inclination.     

Experimental study of temperature field evolution in the channel with corrugated wall at a jump-like change in the heat flux

Results of experimental investigation of nonstationary temperature field in the flow and on the surface of the channel with corrugated wall are presented at a jump-like change of heat release in time. The changes were performed at the air flow in a rectangular channel, whose one heated wall was made as a plate with triangular corrugation oriented at 45° relative to the main flow direction. The microthermocouples with the thickness of below 10 microns were used to perform these changes. Investigations revealed typical regularities of temperature evolution in the flow and along the perimeter of the heated rib. Experimental data on time-temperature dependence are approximated well by the exponential function.     

Large eddy simulation of compressible turbulent channel flow with spanwise wall oscillation

The influences of the modification of turbulent coherent structures on temperature field and heat transfer in turbulent channel flow are studied using large eddy simulation (LES) of compressible turbulent channel flows with spanwise wall oscillation (SWO). The reliability of the LES on such problems is proved by the comparisons of the drag reduction data with those of other researches. The high consistency of coherent velocity structures and temperature structures is found based on the analyses of the turbulent flow field. When the coherent velocity structures are suppressed, the transportations of momentum and heat are reduced simultaneously, demonstrating the same trend. This shows that the turbulent coherent structures have the same effects on the transportations of momentum and heat. The averaged wall heat flux can be reduced with appropriate oscillating parameters. Supported by the Key Subjects of National Natural Science Foundation of China (Grant No. 10732090), the National Natural Science Foundation of China (Grant No. 50476004), and the 111 Project (Grant No. B08009)     

微针肋槽道内去离子水换热特性

设计了槽肋比为1:2和2:1的矩形大长宽比微针肋散热器,并实验研究了去离子水在其内的流动换热性能。结果表明:当进口温度为40 ℃、微针肋槽道在雷诺数小于650、最高壁面温度低于77 ℃时,单位面积散热量可达21.32 W/cm2。当雷诺数一定时,同一个槽道壁面温度沿着流动方向不断增加、同一个位置壁面温度随着加热功率的增加而增大,局部努谢尔数沿着流动方向先减小后逐渐增加并趋于定值。当针肋流动换热长度较长时,其入口效应可以忽略,槽道平均努谢尔数随着雷诺数的增大而增大,与加热功率无关;为了更好地表达微针肋槽道内的换热特性,考虑了槽肋比、流动雷诺数等影响,拟合了去离子水在微针肋槽道内的对流换热关系式。     

Modeling of methane steam reforming in a microchannel with a heat flow distributed in length

Numerical modeling of a flow of a high-temperature mixture of methane with water vapors in a two-dimensional plane microchannel with activation of chemical conversions on the channel wall has been performed. The modeling was performed within the framework of Navier-Stokes equations for a laminar flow of a multicomponent compressible gas. The influence of the external heat flux supplied to the gas mixture and its distribution along the channel length on the properties of the methane steam reforming have been investigated. It has been shown that not only the amount of heat supplied to the reaction zone but also the method of heat supply along the channel length are important. All the reactions with the residence time of the mixture on the order of tens of milliseconds terminate several centimeters downstream from the channel inlet, which makes it possible to optimize a compact reactor for obtaining a synthesis gas.     

流道形状对铜基螺线管线圈冷却效果的影响

采用Fluent和ANSYS Workbench软件对水冷铜基螺线管线圈进行了流体分析及热流耦合分析。得到了水冷铜基螺线管线圈内壁面温度随曲率角和曲面角的变化关系。分析了3种不同流道截面(圆形、椭圆形、三叶形)的螺旋管线圈在同样工况下的出口及交界面的最大温度、平均温度和最小温度,发现流道形状会严重影响螺线管线圈的换热,椭圆截面和三叶形截面的螺旋管线圈相比圆形截面的螺旋管线圈具有更好的冷却效果,如平均壁表面温度均降低了近70℃,管壁上的最高温度也控制在了100℃以内,最大温差也控制在了60℃以内。     

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采用Fluent和ANSYS Workbench软件对水冷铜基螺线管线圈进行了流体分析及热流耦合分析.得到了水冷铜基螺线管线圈内壁面温度随曲率角和曲面角的变化关系.分析了3种不同流道截面(圆形、椭圆形、三叶形)的螺旋管线圈在同样工况下的出口及交界面的最大温度、平均温度和最小温度,发现流道形状会严重影响螺线管线圈的换热,椭圆截面和三叶形截面的螺旋管线圈相比圆形截面的螺旋管线圈具有更好的冷却效果,如平均壁表面温度均降低了近70℃,管壁上的最高温度也控制在了100℃以内,最大温差也控制在了60℃以内.     

Experimental Investigation of Mixed Convection in a Channel With an Open Cavity

Mixed convection in an open cavity with a heated wall bounded by a horizontally unheated plate is investigated experimentally. The cavity has the heated wall on the inflow side. Mixed convection fluid flow and heat transfer within the cavity is governed by the buoyancy parameter, Richardson number (Ri), and Reynolds number (Re). The results are reported in terms of wall temperature profiles of the heated wall and flow visualization for Re = 100 and 1000, Ri in the range 30–110 (for Re = 1000) and 2800–8700 (for Re = 100), the ratio of the length to the height of cavity (L/D) is in the range 0.5–1.5, and the ratio of the channel height to cavity height (H/D) is in the range of 0.5 and 1.0. The present results show that the maximum dimensional temperature rise values decrease as the Reynolds and the Richardson numbers decrease. The flow visualization points out that for Re = 1000 there are two nearly distinct fluid motions: a parallel forced flow in the channel and a recirculation flow inside the cavity. For Re = 100 the effect of a stronger buoyancy determines a penetration of thermal plume from the heated plate wall into the upper channel. Nusselt numbers increase when L/D increase in the considered range of Richardson numbers.     

含润滑油R134a在微通道内沸腾-空化耦合实验研究

混入了矿物润滑油的制冷剂R134a,流经带有空化结构的微通道,发生了沸腾-空化耦合相变现象。无氧铜铜板上刻有长150 mm宽0.8 m深1 mm的微通道,微通道内置有长4 mm宽0.2mm的矩形空化结构。铜板上面覆盖有玻璃片。观察测量不同流量及不同加热量下通道内的流形及通道背面温度。实验结果表明:润滑油抑制了相变现象的发生,降低了换热效率;工作介质为混入润滑油的制冷剂时,通道背面温度远高于工作介质为纯净制冷剂时的通道背面温度。     

Heat transfer at a laminar free convection and separated flow past a rib in a vertical channel with isothermal walls

The results of numerical computations of a free laminar convection and heat transfer between two parallel isothermal plates in the presence of a single rib on the channel surface are presented. The investigations have been conducted for a channel with the aspect ratio AR = L/w = 10, where L is the channel height, and w is the distance between the plates. An infinitely thin adiabatic rib was located on one of the channel walls in the middle of its height. The relative rib height l/w was varied in the range 0÷0.8. The wall temperature was higher than the ambient temperature, and the Rayleigh number was varied in the range Ra = 10²÷10⁵. The main attention has been paid to the study of the influence of the rib height and the Rayleigh number on local and integral heat transfer and the Reynolds number in the channel (the convective thrust). A fundamental difference in the heat transfer over the channel height has been shown on the ribbed wall and on a smooth surface. The computational results have been compared with the case of a symmetric distribution of the ribs on the both walls with the integral height equal to a single rib.     

发汗冷却换热过程的实验研究与数值模拟

本文对水平矩形槽道内湍流对流换热与发汗冷却进行了实验研究和数值模拟。实验结果表明:随着冷却气体流量的增加,发汗冷却壁面温度、局部对流换热系数和Nu数都迅速下降;在注入率为1％时,壁温下降了约40％,对流换热系数降低至50％左右。随着注入率的增大,壁面热流先是增加,在F=0.7％-0.8％左右时达到一个最大值,随后下降。St/St0随着注入率的增大而降低; St／St0的实验值与由已有关联式以及数值计算得到的值基本吻合。     

圆管层流脉冲流动对流换热数值分析

对等热流和等壁温边界条件下圆管内层流脉冲流动对流换热问题进行了数值模拟。在等热流边界条件下的数值计算结果与理论解吻合很好。计算结果表明:在等热流和等壁温边界下脉冲流动可引起速度、温度以及努塞尔数随时间波动,振幅越大,脉冲频率越小,波动越大。但它们的时均值均等于在相同雷诺数下稳态流动的值,脉冲流动不能强化换热。     

滑移流区内微环缝槽道中的层流流动与换热

本文针对微环缝槽道采用速度滑移和温度跳跃边界条件求解了不可压缩气体的Ｎ－Ｓ方程和能量方程,理论分析了微环缝槽道在单侧或双侧不同热流密度加热条件下的流动与层流换热特性,讨论了Ｋｎ数、内外径比对流动阻力及换热特性的影响。结果表明：滑移流区微环继通道内的流阻和Ｎｕｓｓｅｌｔ数明显低于连续流区;且随着Ｋｎ数的增加,流阻和Ｎｕｓｓｅｌｔ数均减小;但其随内外径比ｒ＊的变化趋势与连续流区相似。     

Heat exchange at laminar flow in rectangular channels

Numerical modeling of heat exchange at a laminar stationary and pulsatile flow in rectangular channels with different aspect ratios of side lengths γ has been carried out by a finite difference method for two boundary conditions: a constant wall temperature and a constant heat flux density on the wall. For the boundary condition of the first kind, the similarity of distributions of the heat flux density and shear stress on the walls over the channel perimeter has been established. The reasons for a nonmonotonous dependence of the initial thermal interval length on γ are discussed. For the boundary condition of the second kind, the difference of the Nusselt number averaged over the perimeter at γ → 0 from its value for a flow in a flat channel has been explained. An increase in the Nusselt number averaged over the perimeter and the period of oscillations has been revealed for a pulsatile flow in the quasi-stationary regime at large amplitudes of the oscillations of the velocity averaged over the cross section.     

DNS of turbulent flow with hemispherical wall roughness

The present study of the effect of roughness density on the mean flow turbulence parameters is motivated by the need for new generation of boundary conditions for multiphase computational multiphase fluid dynamics (CMFD) models applied to boiling flows. Effect of roughness element density on the turbulent flow in a channel is quantified through direct numerical simulations (DNSs). The Navier--Stokes equations are solved using finite element method and bubbles are approximated as rigid near-hemispherical obstacles at the wall. Six different cases were analysed including channel flow with smooth wall and channel flow with rough wall for five different bubble nucleation site densities. Friction factor and the law of the wall was calculated and compared with the previously published results. Existing correlations for nucleating bubble site density dependency on a wall heat flux were used to obtain a relation between the heat flux and the friction factor, leading to the law of the wall dependency on the heat flux. This separate effect study provides new guidelines on how the heat flux in subcooled boiling regime affects the turbulence behaviour near the wall and guides the computational fluid dynamics model development for boiling two-phase flows.     

流体直接冷却薄板条介质温度及应力的解析表达

基于对流传热和热传导原理, 建立了流体直接冷却均匀抽运薄板条激光工作介质的热效应分析模型, 采用平面应力近似和最小功原理, 得到了板条工作介质内部温度分布和应力分布的解析表达式. 研究了不同流道厚度时对流热交换系数和冷却液温升与流体流速的关系, 分析了流道厚度对工作介质的温度分布和应力分布的影响规律, 讨论了之字形和直通光路时, 热致波前畸变随产热功率的变化趋势. 结果表明: 层流和湍流时, 较厚的流道可以实现更好的热管理效率; 增益介质中的热分布关于中心平面对称, 纵向最大温升出现在出水口端, 最大应力畸变集中在板条两端及其侧边; 流道厚度较大时, 工作介质更易形成一维的温度梯度, 产生的应力更小; 之字形光路可以明显缓解热光效应导致的波前畸变.     

Numerical simulation of heat transfer and fluid flow of Water-CuO Nanofluid in a sinusoidal channel with a porous medium

This study has compared the convection heat transfer of Water-based fluid flow with that of Water-Copper oxide (CuO) nanofluid in a sinusoidal channel with a porous medium. The heat flux in the lower and upper walls has been assumed constant, and the flow has been assumed to be two-dimensional, steady, laminar, and incompressible. The governing equations include equations of continuity, momentum, and energy. The assumption of thermal equilibrium has been considered between the porous medium and the fluid. The effects of the parameters, Reynolds number and Darcy number on the thermal performance of the channel, have been investigated. The results of this study show that the presence of a porous medium in a channel, as well as adding nanoparticles to the base fluid, increases the Nusselt number and the convection heat transfer coefficient. Also the results show that As the Reynolds number increases, the temperature gradient increases. In addition, changes in this parameter are greater in the throat of the flow than in convex regions due to changes in the channel geometry. In addition, porous regions reduce the temperature difference, which in turn increases the convective heat transfer coefficient.