定热通量加热下微三角形槽道中滑移流动的换热特性

微三角形截面通道是现代工程实际应用中常涉及到的流动通道.针对微三角形槽道利用正交函数法求解了滑移流区内带温度跳跃边界条件的能量方程,对不可压缩气体在微三角形槽道内充分发展层流滑移流动的换热特性进行了理论分析,获得了轴向定热通量加热、周向均匀壁面温度条件下微三角形槽道内的温度分布和换热特性的分析解.计算结果表明：正交函数法适用于微三角形槽道内滑移流动换热特性的分析计算;在滑移流区,微三角形槽道内的平均Nusselt数随Knudsen数的增加而减小,其随高宽比变化的分布曲线随Knudsen数的增加而平行下移,Nusselt数比随Knudsen数的变化关系基本不受高宽比的影响.     

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。     

采用DSMC方法对微槽流动统一滑移模型的讨论

基于扰动分析理论,在等温假设条件下,由二维Navier-Stokes方程和统一滑移模型推导得到压力分布、速度分布和质量流率理论表达式.以氩气为介质,采用直接模拟Monte Carlo(DSMC)方法计算了出口克努森数(Kno数)处于0.052～0.254的多个二维微槽流动,并将不同滑移模型的扰动分析解与DSMC结果进行了对比分析.结果表明,在模拟计算范围内,它们的压力分布基本一致;速度分布和质量流率均有一定差异,但在微槽道中心线附近流动高速部分仍十分接近.对于流动高速部分,当Kno数为0.052时,一阶滑移模型更接近于DSMC;当Kno数为0.073,0.101,0.15时,二阶模型更接近于DSMC;当Kno数为0.197,0.254时,所有滑移模型的速度和质量流率均小于DSMC.相比较,二阶滑移边界模型Cercignani与DSMC结果有更好的一致性.     

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。     

微槽道内单相流动阻力与传热特性

对去离子水在微槽道中的单相流动阻力和传热特性进行了实验研究.结果表明,微槽道中的流动阻力特性及传热特性与常规尺度通道相比存在明显的差异,实验的fRe值高于传统理论预测值,并且随Reynolds数的增加而增大,而传热系数则明显小于传统理论预测值,分析表明微槽道的表面粗糙度对流动和传热有重要的影响.在热阻相同的情况下,提高工质的入口温度或增加热流通量,可减小实验段的压降.同时给出了层流区的流动阻力计算关系式.     

三种板片结构热交换器的传热性能及流动阻力性能的数值模拟分析

基于Fluent软件,分析了相同雷诺数下,80o-圆弧、105°-圆弧和80o-平台三种波纹截面形状流道内速度场、压力场、温度场的分布规律,以分析波纹截面形状对热交换器传热及流动阻力性能的影响。分析结果表明:80o-平台波纹截面形状的热交换器传热系数最大,但同样流动阻力也最大;105°-圆弧波纹截面形状的热交换器传热系数最小,同样流动阻力最小;波纹截面形状对流动阻力性能的影响要大于对传热性能的影响。     

超疏水表面微通道内水的流动特性

在铝制微通道内壁上制造出超疏水表面,水滴在其表面上的接触角达到153°。对水在内径同为0.60 mm的超疏水微通道和超亲水微通道中流动的压降进行实验测定与对比,得出水在超疏水微通道内的流动阻力降有明显降低,降低的最大值可达25%。研究了水在超疏水微通道内的流动特性,发现水由层流向湍流转变发生在Reynolds数为2500左右,且在层流范围内fRe值基本保持不变。通过计算得出了不同流量下水在超疏水表面微通道壁面处的滑移速度和滑移长度,结果显示滑移速度和滑移长度均随流量的增大而增大。     

超疏水表面微通道内水的传热特性

微通道传热效率高但流动阻力大,超疏水表面因其与水具有滑移边界而表现出低流阻的特征,在微过程中具有应用前景。利用化学刻蚀法制备出具有微纳米阶层结构的铝基超疏水表面微通道(内径为0.68mm)。在超疏/亲水微通道内进行了水的流动传热实验研究,并将结果进行对比。研究发现存在于超疏水表面微纳米结构里的气泡层减小了水的流动阻力,也降低了表面传热系数,但降低程度明显小于流动阻力的降低,传热系数高于考虑纳米气泡层计算的传热系数。因此认为在水的滑移速度作用下,凹穴中微纳米级气泡内产生了气体的涡旋流动,一定程度上增强了传热效果。     

微注塑充模过程中壁面滑移对熔体流动影响的研究*

针对微型塑料件注塑充模过程中,壁面滑移对流动的影响不可忽略的情况,运用流体分析软件Fluent,以微阶梯圆形截面通道为模型,在考虑和不考虑壁面滑移的情况下,对微注塑充模流动过程中壁面滑移的影响进行了数值模拟。分析了细通道近壁面处熔体的剪切速率和黏度,发现考虑壁面滑移时近壁面处的剪切速率略大,黏度略低。研究了熔体在粗通道和细通道中沿径向的流动速度和温度分布,以及沿微通道流动方向上的压力分布。结果表明,考虑壁面滑移时熔体流动速度较大,与壁面接触的熔体流动速度不再为零,且微通道截面尺寸越小,这种现象越明显;考虑壁面滑移时近壁面处熔体温度略高,并且粗通道中的这种现象更明显一些;壁面滑移对微通道中的压力分布几乎没有影响。总体而言,壁面滑移有利于微注塑充模。     

弓形截面螺旋半管夹套内流体流动与换热

采用数值方法对弓形横截面螺旋半管夹套内流体的湍流流动与换热特性进行了研究。基于正交螺旋坐标系给出了夹套内换热流体充分发展的湍流流场与温度场分布。研究了雷诺数Re和无量纲曲率δ对流动阻力fRe及壁面平均换热努塞尔数Num的影响。结果表明:充分发展湍流条件下,弓形截面螺旋半管夹套的横截面上,平均轴向速度的最大值位置靠近弯曲的外壁侧;二次流为旋转方向相反的两涡结构。相同换热面积时,在0.058≤δ≤0.12,6000≤Re≤12 000的范围内,弓形截面夹套的流动阻力fRe是半圆形截面夹套的0.794～0.947倍,平均努塞尔数Num为0.86～0.98倍。     

Laminar Drag Reduction in Hydrophobic Microchannels

The apparent slip effects of laminar water flow in smooth hydrophobic microchannels and patterned hydrophobic microchannels were investigated. A series of experiments were performed to demonstrate the drag reductions for laminar water flow in hydrophobic microchannels. These microchannels were fabricated from silicon wafers using photolithography and were coated with hydrophobic octadecyltrichlorosilane (OTS). To generate a larger drag reduction, the patterned hydrophobic microchannels were fabricated to allow the liquid to flow over a region of trapped air in the cavity between the microridges. With the geometrical dimensions used, pressure drop reductions ranging from 10 to 30 % were found in the smooth microchannels and patterned microchannels. The pressure drop reduction was shown to increase with increasing microridge spacing and decreasing microchannel width. Using micro‐particle image velocimetry (PIV), we measured an apparent slip velocity at the wall of approximately 8 % of the centerline velocity, yielding a slip length of approximately 2 μm in the smooth hydrophobic microchannel. Theoretically, the analytical solution derived for three‐dimensional flow in a rectangular duct is presented to predict the slip velocity and slip length at the wall based on the pressure drop measurement. These results are in agreement with the experimental data obtained using micro‐PIV.     

Numerical Analysis of Isothermal Gaseous Flows in Microchannel

Two‐dimensional compressible momentum equations were solved by a perturbation analysis and the PISO algorithm to investigate the effects of compressibility and rarefaction on the local flow resistance of isothermal gas flow in circular microchannels. The computations were performed for a wide range of Reynolds numbers and inlet Mach numbers. The explicit expression of the normalized local Fanning friction factor along the microchannel was derived in the present paper. The results reveal that the local Fanning friction factor is a function of the inlet Mach number, the Reynolds number and the length‐diameter ratio of the channel. For larger Reynolds and inlet Mach numbers, the friction coefficient in the microchannel is higher than the value in a macrotube, and the gas flow in the microchannel is dominated only by compressibility. For smaller Reynolds and inlet Mach numbers, the Fanning friction factor of gas flow in the microchannel is lower than that in a circular tube of conventional size due to slip flow at the wall and thus, rarefaction has a significant effect on the fluid flow characteristics in a microchannel.     

TWO DIMENSIONAL MODEL ANALYSIS OF TURBULENT FLOW IN AN AGITATED VESSEL WITH PADDLE IMPELLER*

A two-dimensional model for the turbulent flow in an agitated vessel is developed by modifying a two-dimensional laminar flow model. The proposed model is based on the assumption that the tangential velocity adjacent to the vessel wall satisfies the slip condition. Specifically, it is assumed that the tangential velocity is inversely proportional to the radius. The eddy diffusivity of momentum is assumed to be constant. The eddy diffusivity calculated from the model is in good agreement with the available experimental data, while the calculated tangential velocity depends on the Reynolds number in a manner similar to that observed experimentally.     

管内稠密气固两相流数值模拟计算:颗粒动力学方法

基于稠密气固两相流流体动力学模型、颗粒动力学的颗粒湍动能模型 (KTGF)和气体湍动能模型(SGS) ,模拟计算结果得到了实验研究所揭示的环 -核流动结构 .模拟计算与Miller和Gidaspow的实验结果进行了比较 ,颗粒相浓度、速度和颗粒相动力黏性系数分布与实验值基本吻合 .     

COMBINED FORCED AND FREE CONVECTION IN BOUNDARY LAYER FLOW OF NON-NEWTONIAN FLUID ON A HORIZONTAL PLATE

An analysis is presented to study the effects of buoyancy-induced streamwise pressure gradients on laminar forced convection heat transfer to non-Newtonian fluids from a horizontal plate. Numerical solution of the transformed boundary layer equations has been carried out for different values of the flow behavior index and the buoyancy parameter by means of the local nonsimilarity method. A discussion is provided for the effect of the buoyancy force on the friction factor, the dimensionless heat transfer rate and the details of the velocity as well as temperature fields.     

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.     

基于抛物线形气-液界面的超疏水微通道减阻特性

针对超疏水表面微通道中的流动减阻特性,基于抛物线形气-液界面假设,采用VOF模型模拟了微通道中的二维层流流动,分析了流动和结构参数对减阻效果的影响。结果表明,含矩形微坑的超疏水表面微通道具有显著减阻作用,fRe随Reynolds数增大而略有提高,量纲1压降比随入口速度增大而略有下降。当增大微坑面积比或减小微通道高度时,fRe减小,量纲1压降比增大;且微通道高度越小,微坑面积比对fRe的影响越显著。随抛物线形高度增加,压降比和滑移长度均线性减小,而fRe则线性增加。当微坑深度大于其宽度的40%时,压降比和滑移长度趋于定值。微坑形状对减阻效果的影响依次是燕尾形、矩形、梯形和三角形。