粗糙度与气体稀薄性对微尺度流动特性的影响

对氮气和氦气在粗糙微通道以及光滑微通道内流动进行了阻力特性实验研究。实验结果表明,即使在较小的相对粗糙度高度下,由于微通道中的粗糙度分布密集,会极大地增加流动阻力,这是导致文献中微通道流动阻力系数实验值相互偏差的主要原因之一;而对于滑移区的气体流动,气体稀薄性使流动阻力明显减小而导致流量增加。     

矩形微管内摩擦阻力特性的实验研究

以极性液体蒸馏水、无水乙醇及非极性液体R113作为实验工质,流过水力直径为231.91μm、250.88μm、297.14pm和210.89 pm,相应高宽比分别为0.90、0.70、0.90和0.42的由晶体生长方法得到的矩形紫铜微管,其相对粗糙度分别为1.47%、1.59%、1.48%和1.45%,测量其进出口压降与流量,从而获得摩擦阻力系数f与雷诺数Re的关系.实验结果表明:微管内流体的极性对微管流动阻力特性没有影响;同时实验结果显示对于粗糙度小于2%的矩形紫铜微管,其内壁面粗糙度对其流动阻力特性影响较小.当Re小于1600时,所有微管内的流动阻力特性与经典层流预测值几乎一致;而对于高宽比较小的矩形紫铜微管,当Re超过1600～1700时,微管的,值明显偏离经典层流预测值.     

微柱群阻力特性实验研究

以去离子水为工质,流经直径为0.5 aim,高度分别为1.0 mm、0.75 mm、0.5 mm和0.25 mm的圆柱组成的柱群板,其宽度与长宽分别为3.5 mm和40 mm,测量通道进出口压差及流量,研究微柱群内部分别在叉排和顺排时液体流动的阻力特性.研究表明,微柱群内流动阻力系数f,随Re数的增大而逐渐减小,当Re数大于500时,f基本不变;微柱高度和直径之间存在一个有利于流动的最佳比例,该值介于1到1.5之间;顺排微柱群的f明显小于叉排微柱群,其,值为叉排微柱群的0.5倍.     

硅基内肋阵列微通道内的流动和换热特性

制作了四种带有圆柱形内肋阵列的硅基微通道,以去离子水为工质对其内部流动和换热特性进行了实验研究,并与平直微通道进行了对比,分析了内肋阵列微通道中流动阻力提升和强化换热的机理。研究表明;内肋阵列带来较大阻力的同时也极大地改善了换热;流体流经内肋阵列微通道时,其阻力在低Re数下主要来自壁面效应产生的摩擦阻力,高Re数下则受绕肋产生尾涡的影响较大;不同内肋布置方式对流体流动和换热影响显著,叉排布置比顺排布置的内肋阵列微通道具有更大的摩擦因子和换热系数,且增大垂直于流动方向内肋密度更有利于增强换热;内肋排列最为紧密的微通道#2综合换热性能最好,相同泵功下,其换热热阻相对于平直微通道降低了53.4%。     

微通道内流体流动的阻力特性

本文重点研究了微通道内入口效应和粗糙度等因素对流动阻力特性的影响。实验结果表明;对于粗糙度3.5%,水力直径为345μm的微圆管,入口段长度比常规管道短,转捩雷诺数Re_c位于2000～2300,其内入口效应并不影响微管内流体从层流到湍流转捩的拐点位置;内径为112.6μm和169.8μm,高宽比为0.59和0.63以及粗糙分别为1.29%和2.03%的矩形微通道内流动由层流向湍流的转捩发生在Re≈1600～2800之间。实验结果表明,微尺度是微管内流体转捩提前的必要条件,但不是充分条件,拐点提前是由多因素综合作用所造成的结果,其机理有待于进一步研究。     

微通道冷板换热及流阻特性计算与结构优化

设计了一种适用于阵列多芯片冷却的微通道液冷冷板。对冷板进行数值分析,研究矩形截面的高宽比对冷板的换热和流阻特性的影响。选择最优矩形截面高宽比的冷板,并对其进行流道结构优化,改善其流阻性能。结果表明,矩形截面当量直径固定时,摩擦阻力系数f和努塞尔数Nu随着矩形截面高宽比变大而增加;当高宽比在2～4之间时,冷板有较好的换热及流动性能;优化流道截面尺寸、弯角半径和出口流道形式后,冷板表现出良好的低压降传热特性,实现有效散热及满足压降要求。     

锯齿型通道流动与换热的自维持振荡实验研究

本文对锯齿型通道内流动与换热的自维持振荡特性进行了实验研究。从可视化实验中观察到,Re300时,流场随时间几乎没有明显的变化;Re300,流动出现了明显的回流现象,且随着Re数增大,回流形成的涡增大,流动产生的涡的位置随时间呈有规律的周期性变化;在恒热流边界条件下,通道上、下壁面温度沿流动方向的变化与数值模拟所得到的Nu数、壁温的变化规律一致。     

直接模拟蒙特卡罗方法在微通道流动模拟中的应用

直接模拟蒙特卡罗方法是一种求解稀薄气体流动换热新的数值方法。本文采用该方法对Kn数跨越速度滑移区和过渡区的三个微通道内的流动进行了数值模拟,给出了通道内速度、压力及局部阻力系数的变化曲线.为了表明通道横纵比对流动的影响,还对每个算例在不同的横纵比下进行了比较。结果表明,微通道内的流动特性不仅与Kn数有关,而且与通道的横纵比也有很大的关系。     

DN板通道传热阻力特性的实验和数值研究

本文分别采用了实验和数值模拟两种方法对DN板通道的阻力特性和传热特性进行了研究。本文的实验采用了稳态实验法,数值模拟分别采用了三种典型的湍流模型(LBKE、SKE和SST)。结果显示,平均努塞尔数Nu随雷诺数Re增大而增大;表面换热因子j和阻力系数f随雷诺数Re的增大而增大。拟合得到了该型DN板的传热阻力特性关联式。同时,对数值模拟结果和实验结果进行了对比分析,对比发现,当Re2000时,三种模型对DN板换热能力和摩擦阻力的预测值均偏高;当Re2000时,LBKE模型和SST模型对DN板的换热预测值与实验值吻合较好,但三种模型对DN板通道摩擦阻力系数的预测值均偏低。     

光滑矩形微通道液体单相流动和传热的数值研究

采用CFD方法对水在矩形光滑微通道内的流动和传热特性进行了数值模拟.计算结果表明微通道的长径比、当量直径、高宽比和孔隙率都对其流动和传热有着不同程度的影响.在保持长径比大于70而使流动的入口效应可忽略的前提下,分别模拟了当量直径,高宽比和孔隙率对微通道流动和传热的影响,得到了各种工况下的流动和传热规律.     

FRICTION AND CONVECTION STUDIES OF R-134a IN MICROCHANNELS WITHIN THE TRANSITION AND TURBULENT FLOW REGIMES

Fluid flow and heat transfer characteristics of single-phase flows in microchannels for refrigerant R-134a were experimentally investigated. Experiments were conducted using rectangular channels micromilled in aluminum with hydraulic diameters ranging from approximately 112 to 210 w m and aspect ratios that varied from 1.0 to 1.5. Using overall temperature, flow rate, and pressure drop measurements, friction factors and convective heat transfer coefficients were experimentally determined for steady flow conditions. Effects of Reynolds number, relative roughness, and channel aspect ratio are examined in predicting friction factor and Nusselt number for the experiments. Experiment results indicated that transition from laminar to turbulent flow occurred between a Reynolds number of 2,000 and 4,000. Friction factor results were consistently lower than values predicted by macroscale correlations but exhibited the same trends with Reynolds numbers of macroscale correlations. Nusselt number results also exhibited a similar pattern of lower values obtained in the experiments than those predicted by commonly used macroscale correlations. Nusselt number results also indicated that channel size may suppress turbulent convective heat transfer and surface roughness may affect heat transfer characteristics in the turbulent regime.     

Flow and Heat Transfer in Rough Micro Steel Tubes

Abstract

This article investigates the flow and heat transfer characteristics in micro steel tubes with inner diameters of 168 μm, 399 μm and relative roughness of 3.5% and 2.7%, respectively, by measuring the friction factors and the Nusselt number from laminar state to transitional state. Experiments show that the experimental Nusselt numbers are less than those predicted by the classical laminar correlation due to the effect of the variation of the thermophysical properties with temperature when Reynolds number is low. As the Reynolds number is higher than 800, the experimental Nusselt number are 25–50% higher than the predictions of the classical laminar and transitional correlations due to the effects of the roughness and the entrance length. The transition from laminar to turbulent flow occurs at the Reynolds number of 1,100–1,500.     

疏水表面减阻环带实验研究

针对疏水功能材料在流动减阻方面的应用, 选取典型不同粗糙度、不同疏水性的功能涂层表面, 通过新型环带实验研究了其阻力特性, 并获得了相应的扭矩和减阻率曲线. 实验采用测量圆盘带动环带旋转时的扭矩的方法间接计算环带表面所受的摩阻, 突破了传统微管道实验在尺度上的限制, 避免了水洞实验中影响因素过多的弊端, 对疏水材料的宏观应用有着重要意义. 实验证实了在宏观尺度下疏水涂层在低雷诺数时的减阻作用; 但在高雷诺数时, 减阻作用减弱, 甚至部分涂层有增阻作用, 而压差阻力的迅速增大是造成增阻的主要原因. 通过对比分析认为: 低雷诺数时, 疏水特性对于减阻效果影响更大; 而高雷诺数时, 粗糙度起更大作用, 甚至可能起到增阻的反效果. 关键词： 疏水表面 环带实验 粗糙度 减阻     

Experimental Study on Flow and Heat Transfer in a 19.6-μm Microtube

The flow and the heat transfer characteristics in a quartz microtube with an inner diameter of 0.0196 mm are investigated experimentally. Measuring the pressure drop between the inlet and outlet of the microtube and the average temperature of the microtube wall heated by steam when the working fluid is distilled water, the corresponding friction factors and Nusselt number are obtained. The experimental results show the friction factors in the microtube exceed those of the Hagen–Poiseuille prediction due to the predominance of the effects of the electrical double layer and the entrance. Also, the experimental Nusselt number is less than the classical laminar at Reynolds number < 500 due to the effect of the variation of the thermophysical properties with the temperature.     

Numerical Simulation of Heat Transfer in Rectangular Microchannel

Numerical simulation of heat transfer in a high aspect ratio rectangular microchannel with heat sinks has been conducted, similar to an experimental study. Three channel heights measuring 0.3 mm, 0.6 mm and 1 mm are considered and the Reynolds number varies from 300 to 2360, based on the hydraulic diameter. Simulation starts with the validation study on the Nusselt number and the Poiseuille number variations along the channel streamwise direction. It is found that the predicted Nusselt number has shown very good agreement with the theoretical estimation, but some discrepancies are noted in the Poiseuille number comparison. This observation however is in consistent with conclusions made by other researchers for the same flow problem. Simulation continues on the evaluation of heat transfer characteristics, namely the friction factor and the thermal resistance. It is found that noticeable scaling effect happens at small channel height of 0.3 mm and the predicted friction factor agrees fairly well with an experimental based correlation. Present simulation further reveals that the thermal resistance is low at small channel height, indicating that the heat transfer performance can be enhanced with the decrease of the channel height.     

The variations of heat transfer and slip velocity of FMWNT-water nano-fluid along the micro-channel in the lack and presence of a magnetic field

Simulation of forced convection of FMWNT-water (functionalized multi-walled carbon nano-tubes) nano-fluid in a micro-channel under a magnetic field in slip flow regime is performed. The micro-channel wall is divided into two portions. The micro-channel entrance is insulated while the rest of length of the micro-channel has constant temperature (T_C). Moreover, the micro-channel domain is exposed to a magnetic field with constant strength of B₀. High temperature nano-fluid (T_H) enters the micro-channel and exposed to its cold walls. Slip velocity boundary condition along the walls of the micro-channel is considered. Governing equations are numerically solved using FORTRAN computer code based on the SIMPLE algorithm. Results are presented as the velocity, temperature, and Nusselt number profiles. Greater Reynolds number, Hartmann number, and volume fraction related to more heat transfer rate; however, the effects of Ha and ϕ are more noteworthy at higher Re.     

Numerical study of flow and heat transfer from a torus placed in a uniform flow

Forced convection heat transfer characteristics of a torus (maintained at a constant temperature) immersed in a streaming fluid normal to the plane of the torus are studied numerically. The governing equations, namely, continuity, momentum and thermal energy in toroidal coordinate system, are solved using a finite difference method over ranges of parameters (aspect ratio of torus, 1.4 ≤ Ar ≤ 20; Reynolds number, 20 ≤ Re ≤ 40; Prandtl number, 0.7 ≤ Pr ≤ 10). Over the ranges of parameters considered herein, the nature of flow is assumed to be steady. In particular, numerical results elucidating the influence of Reynolds number, Prandtl number and aspect ratio on the isotherm patterns, local and average Nusselt numbers for the constant temperature (on the surface of the torus) boundary condition. As expected, at large aspect ratio the flow pattern and heat transfer are similar to the case of flow and heat transfer over a single circular cylinder.     

Effects of Heating Load on Flow Resistance and Convective Heat Transfer in Micro-pin-fin Heat Sinks with Different Cross-section Shapes

The flow and convective heat transfer characteristics under different heating loads in micro-pin-fins of circle, diamond and triangle are experimentally investigated with Reynolds number ranging from 0–1,000. The pressure drops, friction factors, thermal resistance and Nusselt number in micro-pin-fins with different cross-section shapes are obtained when the heating load changes from 50 to 150 W. Basing on the experimental results, the mechanisms of the influence of heating load on the resistance and heat transfer characteristics in micro-pin-fins with different cross-section shapes are detailed analysed. It is found that pressure drops in three types of micro-pin-fins all become large with the increase of the heating load, and the change of pressure drop in triangular micro-pin-fins is larger than those in the other two micro-pin-fins. At low Re, the friction factors in the three types of micro-pin-fins become large with the increase of the heating load, but this phenomenon disappears when Re>400 for the circle and diamond micro-pin-fins, and Re>250 for the triangular micro-pin-fins. The convective heat transfer in micro-pin-fins with cross-section shapes of circle, diamond and is enhanced by increasing the heating load, but the convective heat transfer coefficients and Nu in the triangular micro-pin-fins becomes slightly smaller when Re>250.     

细小圆管内液体流动特性实验研究

本文实验研究了去离子水在直径为 1.0～2.0 mm 的细小圆管中的流动阻力特性.通过比较相同直径、不同管长的压差,获得了摩擦因子及局部损失系数随 Re 数变化关系.本文实验条件下,对于不同直径的细小圆管,沿程压降及局部压降随流速变化趋势一致;随着管径减小,摩擦因子也减小,且在层流区和湍流区内均要比常规尺度条件下的小;对于不同直径的细小圆管,局部损失系数随 Re 数变化趋势基本一致,当 Re 数相同时,管径越小,对应的局部损失系数越大;对于直径为 1.0～2.0 mm 的细小圆管,临界雷诺数 Rec=2050～2300.     

基于拉格朗日跟踪法的波纹流道流体特性研究

采用正交实验方法考察了具有不同结构参数的三维周期波纹流道中的流体性能,并采用Webb评价方法对其进行性能评价。比较了不同波纹宽度的波纹流道的阻力因子e_f、传热因子e_Nu和能效因子η的值,结果表明三者都随Re的增大而增大,波纹宽度最小时能效因子η最大。流体在波纹流道中垂直于主流方向的横截面上产生二次流,随着Re增大,二次流增强,阻力增大,温度边界层减薄,温度等值线分布变得不均匀,传热增强。采用拉格朗日粒子跟踪技术分析了不同Re下,流体粒子在波纹流道内的运动轨迹,绘制了不同周期出口流体粒子的庞加莱截面图,结果表明流体粒子在波纹流道中被反复拉伸和折叠,增加了流体粒子的接触面积,提高混合效率,强化了传热。