气体在任意截面形状微尺度槽道中的滑移流动

利用正交函数法对气体在具有任意截面形状的微尺度槽道内的充分发展层流滑移流动特性进行了理论分析,获得了任意截面形状微槽道内的速度分布和流动阻力特性的分析解,并以矩形微槽为例分析了微槽截面上的速度分布和阻力特性.结果表明：随Kn数的增加,由于壁面处滑移流动的影响,气体流经微槽的流动阻力常数小于大尺度理论预测值;理论分析解的结果与实验结果吻合较好,表明在一定的Kn数范围内Navier-Stokes方程在考虑了速度滑移后可以描述微通道内的气体流动过程;正交函数法在微槽内滑移流动的分析中是可行的.     

微小三角形槽道内电渗流动特性分析

提出了微小三角形槽道内电渗流动理论计算方法，通过Galerkin法计算并分析了其内部的电势及速度分布，获得了温度、槽道尺寸、外加电势的电场强度、ζ电势以及电解质浓度对微小三角形槽道内电渗流动的影响规律。计算结果表明：微小三角形槽道内液体的质量流量随ζ电势、电场强度、流体温度及电解质浓度的增加而增加，随微小三角形槽道尺寸的增加先增加后减小。     

环形通道内层流入口段换热及流体变物性的影响

采用SIMPLER算法对环形通道内二维定常轴对称入口段流动与换热进行了数值计算, 研究了两种边界条件下的层流流动与换热规律, 给出了不同Prandtl数以及半径比率下沿程Nusselt数的变化曲线,同时还给出了流体物性随温度变化对流动与换热的影响, 最后还拟合出了不同半径比率下平均Nusselt数的关联式.计算结果还表明,环形通道能强化传热,强化程度随半径比率减小而增大,且入口段的换热强化与其较高的径向速度有关.     

轴向导热对微通道内传热特性的影响

以去离子水为工质,在当量直径为155.3 μm的三角形硅基微通道中进行了传热特性的实验研究;采用轴向导热与总加热量的比值和轴向导热准则数来分析轴向导热对微通道内传热特性的影响;基于实验结果,对入口段长度进行了计算。根据红外热像仪的测量结果,局部壁面温度沿流动方向上呈非线性规律分布,并计算得出局部Nusselt数沿流动方向上的分布规律。轴向导热与总加热量的比值的变化范围为0.106～0.275,表明轴向导热对微通道内传热特性的影响显著,特别是在Reynolds数较小的情况下。对轴向导热准则数进行了新的修正,在热流密度与入口温度相近的情况下,轴向导热准则数随Reynolds数的增加而减小。     

滑移壁面蛇形微通道两相流数值模拟

基于Fluent平台,采用CLSVOF方法对滑移壁面蛇形微通道气液两相流动进行了数值计算。计算选用的方法与理论结果具有较好的一致性,同时可以表明疏水壁面会产生滑移现象,且在高度较小的微通道内滑移效果更显著,从而减小通道内流体流动阻力,实现减阻;不同壁面性质通道内流体流动情况的计算结果表明,滑移壁面对截面速度分布趋势几乎没有影响,但上下壁面疏水性不同会影响通道截面最大速度分布。此外接触角及相对粗糙度对滑移特性影响较大,合理设计壁面润湿性及微粗糙元结构可以最大限度发挥滑移现象引起的减阻效果;与无滑移壁面相比,滑移壁面微通道内传热效果更好,且随滑移速度的增大,通道换热增强。     

滑移壁面蛇形微通道两相流数值模拟

基于Fluent平台,采用CLSVOF方法对滑移壁面蛇形微通道气液两相流动进行了数值计算。计算选用的方法与理论结果具有较好的一致性,同时可以表明疏水壁面会产生滑移现象,且在高度较小的微通道内滑移效果更显著,从而减小通道内流体流动阻力,实现减阻;不同壁面性质通道内流体流动情况的计算结果表明,滑移壁面对截面速度分布趋势几乎没有影响,但上下壁面疏水性不同会影响通道截面最大速度分布。此外接触角及相对粗糙度对滑移特性影响较大,合理设计壁面润湿性及微粗糙元结构可以最大限度发挥滑移现象引起的减阻效果;与无滑移壁面相比,滑移壁面微通道内传热效果更好,且随滑移速度的增大,通道换热增强。     

封闭腔内水自然对流换热数值模拟

为了揭示封闭腔内非Boussinesq流体在浮力驱动下所特有的流动换热现象和形成机理，采用CFD软件Fluent对封闭腔内水的自然对流进行数值模拟，得到矩形封闭腔高宽比、Rayleigh数、倾斜角度、壁面温度差对流动和传热的影响规律。研究结果表明：由于水的密度在3.98℃达到最大，两竖壁面温度跨越这一点时会引起流动图像反转；具有流动反转的双涡结构降低了对流换热平均Nusselt数；相同Rayleigh数下,高宽比为1对应对流换热平均Nusselt数最大值；倾斜角度对平均Nusselt数影响与Rayleigh数和温度边界条件有关。     

纳米流体在芯片微通道中的流动与换热特性

对去离子水及体积分数分别为0.15%和0.26%的水基γ-Al₂O₃纳米流体在当量直径为194.5 μm的硅基梯形芯片微通道内的层流流动和换热特性进行了实验研究。考察了Reynolds数、Prandtl数以及体积分数对流动换热的影响。结果发现,使用纳米流体后,压降无明显增加,纳米流体的流动阻力特性与去离子水基本相同;对流换热Nusselt数较去离子水有明显提高,且随着体积分数的增加而增加;相同泵功下换热热阻显著下降。实验还发现纳米流体的强化传热效果在较高温度时更加明显。根据实验数据得到了梯形硅微通道内低浓度纳米流体的层流对流换热关联式。研究结果对于集成高效芯片散热系统设计具有重要意义。     

黏度随温度变化对三角形螺旋夹套内湍流流体流动及换热的影响

考虑受热流体黏度随温度变化的影响,对三角形螺旋夹套内流体湍流换热进行了数值模拟分析。经与实验结果比对,证明了模拟方法可靠。基于模拟结果重点分析了受热流体黏度变化对流道内流体流动和换热特性的影响,剖析了受热流体的截面平均流动阻力(fRe_m)和平均Nusselt数(Nu_m)沿流动方向的发展特点,并比较了不同流动参数和不同结构参数下流道内受热流体定黏度和变黏度时流体总流动阻力和传热系数的差别。结果表明:考虑流体黏度随温度变化时会明显影响三角形螺旋夹套内流体的流动和换热特性。对比两种流体fRe_m和Nu_m沿流体流动方向的发展过程发现,在早期、中间和后期发展3个阶段中,后期发展阶段中两种流体的发展趋势明显不同;相同来流条件和热边界条件下,与定黏度流体相比,同一横截面上变黏度流体各点的局部流动阻力较小,而局部Nusselt数却较大,因此变黏度流体总的流动阻力较小,而总的传热系数较大;Reynolds数越低,流道量纲一曲率越小,变黏度受热流体的流动和换热性能与定黏度流体流动和换热性能相比差异愈明显,则考虑流体黏度随温度变化对流体流动和传热的影响更为重要。     

水/乙醇混合工质在硅基微通道中的流动与换热

以不同浓度的乙醇水溶液为工质,在5种不同几何尺寸的梯形硅基微通道中进行了层流流动与换热的实验研究。基于实验结果,分别得出了摩擦常数和平均Nusselt数的实验关联式。实验发现微通道的几何尺寸对微流体的流动与换热特性有着非常显著的影响,各通道中的换热随乙醇溶液浓度的变化而呈现不同的特性,而流动特性则不随溶液浓度的变化而变化。同时还发现在本实验参数范围内,入口段效应对流动与换热的影响都十分显著。     

Effects of roughness elements on laminar flow and heat transfer in microchannels

A model of laminar flow and heat transfer in rough microchannels is developed and analyzed numerically to compare the effect of roughness elements on the thermal and hydrodynamic characteristics. In this model, the rough surfaces are configured with triangular, rectangular and semicircular roughness elements, respectively. Here, the effects of the Reynolds number, roughness height, and roughness element spacing on pressure drop and heat transfer in rough microchannels are all investigated and discussed. The results indicate that the global heat transfer performance is improved by the roughness elements at the expense of pressure head when compared to the smooth channel. Differing from the smooth microchannels, both the Poiseuille number and average Nusselt number of rough microchannels are no longer constant with Reynolds numbers and are larger than the classical value. Especially, the difference from the effects of three types of roughness elements is identified. With the increasing roughness height, the flow over surfaces with semicircular and triangular roughness elements induces stronger recirculation and flow separation. This contributes to heat transfer enhancement but also increases the pressure drop. However, the influence of the rectangular roughness element height is weaker than that of semicircular and triangular elements. In addition, the effects of the spacing of roughness elements on the Poiseuille number and average Nusselt number are in decreasing order for semicircular, triangular and rectangular roughness elements, respectively.     

微小通道内低Reynolds数液-液两相流动与换热特性实验研究

实验研究了圆形微小通道内液-液两相流的流动和换热特性。选用去离子水为分散相,高黏度二甲基硅油为连续相。通过处理高速摄像所拍摄的可视化图像,总结了液-液两相流流型和液滴的长度/形状特征。并在此基础上考察了低Reynolds数下液-液弹状流对微小通道的换热作用。结果表明,平均Nusselt数随着Reynolds数的增加而增加,且油水比越大传热系数增加幅度越明显。Nu随着含水率的增加而降低。虽然含水率增加会使两相平均热容量提高,但在低Reynolds数下,这种提高被其长液滴内较弱的循环强度所抵消。选用三种不同形式接头在相同混合速度和含水率的情况下生成不同长度的液滴,发现短液滴更有利于换热。相同工况下,液滴长度的优化可以使整体传热系数提高近26%。     

Viscous dissipation effect on heat transfer characteristics of mixed electromagnetic/pressure driven liquid flows inside micropumps

This paper presents the effect of viscous dissipation on heat transfer characteristics of mixed electromagnetic/ pressure driven liquid slip flows inside parallel plate microchannels. Flow is governed by the Navier-Stokes equations subject to the imposition of electromagnetic field with the boundary condition appropriate to the slip flow regime. For isoflux walls, some closed form expressions for the local and bulk temperature profiles and the Nusselt number in terms of dimensionless slip length, Hartmann number and Brinkman number are given, while the viscous dissipation is also taken into account. Then the analytical solutions derived in this analysis are elaborated. It turns out that since the contribution of the viscous dissipation on the Nusselt number under the given circumstances, especially a stronger electromagnetic field, may reach to nearly 10%, therefore, the viscous heating should be taken into consideration. Otherwise, the heat transfer rate may be overestimated or underestimated depending on whether the fluid is being heated or cooled. Also, there are singularities in Nusselt number values, which move close together by including the viscous dissipation. Further, an increase in the Hartmann number increases the convection, which is especially reflected in smaller values of dimensionless slip length.     

Hydrothermal performance analysis of various surface roughness configurations in trapezoidal microchannels at slip flow regime

The effects of various surface roughness geometrical properties including roughness height (5%, 10%, 15%), number (3, 6), and shape (rectangular and triangular) on the flow and heat transfer of slip-flow in trapezoidal microchannels were investigated. The effects of mentioned parameters on the heat transfer coefficient through the microchannel, average Nusselt number and pressure drop for Reynolds number of 5, 10, 15 and 20 were examined. The obtained results showed that increasing the roughness height and number increases the pressure drop due to higher stagnation effects before and after roughness elements and decreases the Nusselt number due to higher recirculation zones effects than obstruction effects. The most reduction in Nusselt number and the most increment in pressure drop occur at the roughness height of 15%, roughness number of 6 and Reynolds number of 20 by about 10.6% and 52.8% than the smooth microchannel respectively.     

Laminar Flow and Heat Transfer Characteristics in Jackets of Triangular Flow Channels

Laminar flow and heat transfer characteristics of jacketed vessel with triangular flow channels were numerically studied under hydrodynamically and thermally fully developed conditions. Constant heat flux at theheated wall was assumed. The numerical program code interms of vorticity, stream function, axial velocity com ponent and energy equations was written based on a finite volume method. Based on the numerical results, the flow and temperature field were given, and the effects of Dean and Prandtl numbers on flow and heat transfer were ex amined, and the correlations of flow resistance and mean Nusselt number were developed for the jacket. The results show that the structure of secondary flow is steady two vortices in the investigated range of dimensionless curvatureratio and Reynolds number. Two peaks of local Nusselt number increase significantly with Prandtl and Dean num ber increasing, but the local Nusselt numbers near two ends and at the center of the heated wall increase only slightly. The center and two ends of heated wall are the poor positions for heat transfer in the jacket. Compared with the outer half coil jacket at the same area of heated wall, curvature radius, Reynolds number and Prandtl number, e jacket of triangular flow chmnel has lower flow resistance and less mean Nusselt number.     

错流降膜液体干燥剂除湿/再生传热传质数学模型及分析

分析了错流降膜液体干燥剂除湿及再生传热传质过程 ,建立了基于实际除湿系统的描述再生和除湿过程的数学模型 ,考虑到除湿过程中产生的热效应 ,以氯化钙溶液为除湿剂时 ,对气侧和液侧的传热传质系数进行了理论和数值求解 .计算结果表明 ,传热传质系数与气流流动状态、除湿剂的热物理性质等因素有关     

微型硅基内肋管内的流动与换热特性

目前内肋管强化换热研究大多基于铜、铝或者不锈钢等常规材质,其加工精度和加工尺寸均受到限制。本文采用MEMS工艺首次在硅片上制作成带有方形内肋的平行硅基微通道,并对其内部流动和换热特性进行了实验研究,结果表明：在通道深度和宽度相同的情况下,肋高和肋间距对工质在通道内的流动阻力和换热特性有显著的影响;微通道内加肋以后,通道内的流动阻力和换热性能有不同程度的增加,且提高程度与肋高和肋间距有关;通过合理的设计肋高和肋间距,可以起到明显的强化换热作用。结果还显示,进口段效应对硅基微通道内的流动和换热影响较为显著。     

Aerosol penetration in microchannels

New applications involving aerosol transport in microscale configurations requires the derivation of the penetration efficiency of aerosols in microchannels. Although many analytical solutions for the aerosol penetration in channels have been investigated, none of them are applicable for microchannels. Previously, the no-slip condition for the gas velocity and the zero particle concentration boundary condition have been applied to the convection diffusion equation. However, recent studies show these boundary conditions may not be appropriate for microscale geometries. The particle penetration through rectangular microchannels and cylindrical microtubes has been obtained using the numerical Crank Nicolson method with slip flow at the walls. Existing correlations for the aerosol penetration have been modified for the slip flow regime based on an optimization method. These correlations give the penetration as a function of the dimensionless deposition factor and Knudsen number of the gas. At large Knudsen numbers the penetration decreases relative to the case with continuum flow. Therefore, the aerosol penetration decreases in the slip flow regime. However, the non-zero boundary condition for the particle concentration at the walls does not have any significant effect on the model results of the particle penetration.