双电层效应对压力驱动微流体流动及传热的影响

在非对称壁面zeta电势及热通量边界条件下,研究双电层效应对平行微流道内压力驱动微流体流动及传热特性的影响.建立微流道内压力驱动流体的数学模型,双电层电势分布、流体流动及传热特性分别由Poisson-Boltzmann方程,修正的N-S方程,能量方程进行描述,对三个方程进行求解并得到微流道内电势,速度及温度分布的解析解.详细讨论动电参数、壁面zeta电势、上下壁面zeta电势比及热通量比等因素对电势场、流场、温度场及微流体传热性能的影响.结果表明,壁面zeta电势会影响微流道内电势分布,流动电势的改变会影响速度分布,进而影响微流道的温度分布与传热性能.在微尺度下,双电层效应对压力驱动流的影响很明显,与传统的无双电层效应的泊肃叶流相比,其流动及传热特性均有显著差异.     

平行板微通道内压力驱动流的流动机理

由于固-液界面双电层的作用,平行板微通道内的压力驱动流存在动电效应。平行板微通道可简化为二维截面,其截面上双电层电场和速度场的控制方程分别采用Poisson-Boltzmann方程和修正后的Navi-er-Stokes方程。应用有限元法对控制方程进行了数值求解,计算在微通道内流体的平均流速和动电效应形成的流动电势。研究表明,微通道高度和电解质溶液浓度是影响微流体流动的主要因素。动电参数越小,动电效应对微流体的影响越大,实际值偏离经典流体理论值越大;平均流速与通道两端的压力差线性相关。     

外加电场作用下双电层电粘度对水润滑特性的影响

根据双电层理论,建立了外电场作用下流体润滑中的双电层引起的电粘度效应的数学模型,并通过组合滑块水润滑试验考察了双电层的电粘度效应对流体润滑性能的影响。结果表明,外加电场作用下,双电层电粘度效应对摩擦因数具有明显影响;当速度较低时,随着外加电压的增加,摩擦因数明显增大;随着速度的增加,摩擦因数增大幅度减小;试验结果同所建立的数学模型相符。     

微通道内交变电场电渗流有限元分析

基于有限元方法,以粘性不可压缩流体N-S方程和双电层P-B方程为基础,建立交变电场驱动微通道电渗流模型,并进行数值模拟,分析溶液浓度、电场强度和微通道高度对电渗流的影响。结果表明,微通道内双电层滑移速度与电场强度成正比,受微通道高度影响较小,随溶液浓度的增大非线性减小。同时,微通道高度和溶液浓度的增加使双电层相对厚度减小,交变电场电渗流瞬时速度的波峰更加尖锐。该结论为微通道内交变电场电渗流精确控制提供了理论参考。     

微通道内电场频率对电渗流影响的数值模拟

为研究二维微通道内交变电场电渗流速度特性,以双电层P-B方程和粘性不可压缩流体N-S方程为基础,建立交变电场电渗流的数学模型。应用有限元方法,对不同频率的交变电场电渗流进行数值模拟。结果表明,交变电场电渗流速度分布呈"波浪状",微通道中心区域速度滞后于壁面附近,当流体离壁面距离增大时,流体流速的滞后量增大。电场频率小于1 000 Hz时,交变电场电渗流与稳定电场电渗流具有相似的速度分布。随着电场频率的增大,"波浪状"速度流型更为明显,并且双电层滑移速度减小,尤其当电场频率大于3 000 Hz时,双电层滑移速度迅速下降。     

双电层电粘度对薄膜润滑影响的试验研究与数值分析

由于固液界面处存在双电层,理论和试验研究都表明该效应对薄膜润滑有较为明显的影响。首先,利用自行设计的外加电场重构双电层装置进行了组合滑块水润滑试验,结果分析表明:双电层所引起的电粘度效应对流体润滑中摩擦因数有明显影响,另外当摩擦副的材料不同时,摩擦因数的变化规律不同。然后,又对KCl(氯化钾)溶液进行了组合滑块试验,进一步研究了离子浓度对电粘度效应的影响,在不同速度和溶液浓度的工况下,对摩擦因数进行了测量。结果表明:低浓度溶液使摩擦因数明显增加,浓度高时摩擦因数减小;随着速度的增加,双电层效应对摩擦因数的影响减小。最后,对试验工况进行了数值分析。     

疏水表面微通道交变电场电渗流数值模拟

基于有限元方法,针对具有特定滑移长度的疏水表面微通道,建立了控制方程,对交变电场电渗流进行了数值模拟,分析了电场强度、电场频率、微通道高度和溶液浓度等对电渗流的影响。结果显示,疏水表面和亲水表面微通道交变电场电渗流具有相似的速度分布,疏水表面微通道交变电场电渗流整体速度更大;双电层滑移速度与电场强度成正比,由于边界滑移,疏水表面微通道交变电场电渗流双电层滑移速度随溶液浓度增大先缓慢减小后迅速增大。     

考虑双电层力时微纳间隙动压润滑边界滑移研究

采用原子力显微镜对微纳米间隙下硅基片与纯角鲨烷液体固-液接触面的边界滑移进行试验研究,同时考虑固-液接触面处双电层力,拟合Si O2小球与固体试样的表面电荷密度以及角鲨烷流体动压力。结果表明,在不考虑小球重力、惯性力及分子间作用力的情况下,小球在垂直趋近固体试样的过程中主要受力为双电层力及流体动压力,在低速时,双电层力占主导地位;在角鲨烷液体环境中,探针趋近硅基片的过程中,双电层力表现为引力,且其大小与趋近速度无关;在试验的趋近速度范围内,Si(100)表面与角鲨烷的接触面均会产生边界滑移,且滑移长度随着速度的增大而升高。     

矩形微流道内流体电动效应研究

由于固-液界面双电层的作用,矩形微流道中的压力驱动流存在电动效应。矩形微流道截面上双电层场和速度场的控制方程分别是二维Poisson-Boltzmann方程和修正Navier-Stokes方程。应用有限控制容积法对控制方程进行了数值求解,并计算了压力梯度与雷诺数之间的关系,模型预测值与试验值之差在5%之内。相同尺寸的微流道中,考虑电动效应的模型预测液体摩擦系数的值大于宏观流体理论中液体摩擦系数的值,且电解质溶液浓度越低,摩擦系数偏离宏观流体理论值越大。     

电极材料对电加工8418钢表面质量的影响研究

采用紫铜电极、Cu W70在变化的峰值电流(I)、脉冲宽度(ton)电参数下加工8418钢,研究电极材料及电参数对电火花加工表面质量的影响。测得了工件表面粗糙度、微观硬度、白层厚度、微裂纹,结果表明:采用紫铜电极加工时工件表面粗糙度(Ra)及白层厚度(WT)均比采用Cu W70时略大,但在精加工放电参数下,电极材料对表面粗糙度的影响区别并不明显。工件表面粗糙度与白层厚度随着峰值电流和脉宽的增大而增大,且峰值电流对白层厚度的增加起主要作用。显微硬度随着与工件表面距离的增大而急剧减小。在低放电能量时,两种电极加工的8418钢表面基本上没有微裂纹,质量较好;在中高放电能量时,紫铜电极加工的工件表面微裂纹的数量比Cu W70电极的要多,但裂纹宽度差别不大。     

矩形微流道内电渗流影响因素的数值模拟

针对电渗泵的设计参数优化问题,建立电渗驱动流的数学模型,应用有限控制容积法对矩形流道断面上双电层场和速度场的耦合控制方程进行数值求解.详尽分析流道断面深宽比、流道当量直径、电解质溶液浓度和外加电场强度对电势、速度和流量的影响,并给出电渗流量和平均速度与影响因素之间的拟合公式.仿真结果表明:流量随流道深宽比的增大而减小,当流道深宽比为1时,流量最小;流量随流道当量直径的平方增加;平均速度与电解质溶液浓度和外加电场强度有关、与流道深宽比和当量直径的大小基本无关.     

The Effect of the Electric Double Layer on Very Thin Thermal Elastohydrodynamic Lubricating Film

The effect of the electric double layer (EDL) of friction surface on lubrication is significant under the condition of very thin lubricating film. This article presents a theoretical evaluation concerning the effect of the EDL on the film thickness and the pressure distribution of the elastohydrodynamic lubricating water film. These numerical analyses are based on the modified Reynolds equation that considers the EDL. Owing to the temperature risen readily in elastohydrodynamic lubrication (EHL) contact area, the influence of the temperature rise on the EDL effect was also investigated. The analysis results show that the EDL leads to a noticeable increase in the film thickness but has few influences on the pressure. Further, the analytical comparisons between isothermal and thermal conditions reveal that the temperature rise in the contact area weakens the effect of the EDL on the EHL film. Overall, consideration of the EDL effect gives a thicker EHL film, but once the temperature rise in the EHL regime is taken into account, the film thickness is correspondingly decreased.     

Experimental investigation of particle distribution in a flow through a stenosed artery

The particle distribution of a dilute solid-liquid suspension through a stenosed arterial geometry was investigated. Particle image velocimetry (PIV) was used to determine the velocity as well as to acquire the flow images. The light intensity scattered by particles was evaluated to determine the particle distribution. Flow separation exists where the flow emerges from the stenosis throat. From the PIV images, the particle density distribution exhibited differing non-uniform characteristics which vary with flow rate, particle size and particle concentration. At low flow rates, a particle-free layer is formed. As the flow rate is increased, particles accumulate in concentric recirculation orbits within the downstream vortex. Particles with larger size and higher concentration tend to accumulate more towards the center of the vortex.     

Two-fluid non-Newtonian models for blood flow in catheterized arteries — A comparative study

Steady flow of blood through catheterized arteries is studied by assuming the blood as a two-fluid model with the suspension of all the erythrocytes in the core region as a non-Newtonian fluid and the plasma in the peripheral layer as a Newtonian fluid. The non-Newtonian fluid in the core region of the artery is modeled as (i) Casson fluid and (ii) Herschel-Bulkley fluid. The expressions for the shear stress, velocity, flow rate, wall shear stress and flow resistance, obtained by Sankar and Lee (2008a, 2008b) for the two-fluid Casson model and two-fluid Herschel-Bulkley model are used to get the data for comparison. It is noticed that the plug flow velocity, velocity distribution and flow rate for the two-fluid H-B model are considerably higher than that of the two-fluid Casson model for a given set of values of the parameters. Further, it is found that the resistance to flow is significantly lower for the two-fluid H-B model than that of the two-fluid Casson model. Thus, the two-fluid H-B model is more useful than the two-fluid Casson model to analyze the blood flow through catheterized arteries.     

微小尺度下平板间气体流动机理及压力特性分析

基于平板间气膜内气体分子运动和碰撞的规律,提出气膜分层理论,将板间气膜内的气体划分为近壁层、稀薄层、连续流层。给出了划分稀薄层和连续流层的依据,建立分层物理模型并提出每层的控制方程,验证了分层理论的合理性。通过大规模原子/分子大型并行模拟器仿真板间气膜内气体流态并计算沿高度方向的压力,得出了如下结论：随着板间气体流速的增大,板间气膜有效压力减小,连续流层的厚度增大,稀薄层的厚度减小;当气体流速到达一定值时,气膜内压力不再分层,速度滑移现象可以忽略。     

毛细管电渗流微泵的流型及驱动

采用二维纳维一斯托克方程、拉普拉斯方程、泊松-玻尔兹曼方程表述电渗流微泵的计算模型,讨论了电渗流在各种工况下的流型,分析了双电层(EDL)厚度、外加垂直电势、Zeta电势、背压以及管径对电渗流(EOF)驱动时间的影响。仿真结果表明:影响驱动时间的主要是管径,其他因素可以忽略;无载驱动与反向驱动时间是毫秒级,有载驱动时间是在微秒级。通过改变外加垂直电势或Zeta电势的极性,可以实现双向驱动。     

Experimental Research on Salt-out Particle Motion and Concentration Distribution in a Vortex Pump Volute

The vortex pump is suitable for salt solution transportation. But the salt-out flow mechanism in the pump has not been understood fully. Salt-out layer formation and growth rate are closely related to crystal particle motion and concentration distribution. Study on the particle hydrodynamic characteristics in the pump volute becomes a key problem, because the crystal particles are mainly distributing in this zone after they enter the pump. Phase Doppler particle analyzer(PDPA) is used to measure the two-phase flow field in a model pump volute to get more understanding about the salt-out phenomenon. The crystal particle velocities are obtained in all three peripheral, radial and axial directions. Particle size and particle number density(PND) measurements are also performed in the experiment. Results are presented and discussed along the radial direction under different pump operating conditions, as well as various axial measurement positions. It is found that particle velocity gradient of peripheral component varies with the pump discharge. There is a turning point of relation between peripheral velocity component and discharge. Radial flow velocity curves look like a saddle shape and velocity magnitudes are changing greatly with the discharge. The non-equilibrium velocity feature between liquid and solid phase on this direction is also remarkable. Particles flow into the impeller at radial position R1, and the axial velocity component increases in this region. The particle size curve shows an open-up parabola distribution. The largest particles are distributing near the casing peripheral wall. As flow rate increases, accordingly PND increases. It also grows up in the axial-outward direction towards the suction cover. Crystal particle aggregation phenomenon can be revealed from the analysis of particle size and PND distribution, and the aggregation region is determined as well. Research results are helpful for optimal design of this kind of pump preventing salt-out.     

口模压缩段对塑料挤出流动影响的有限元分析

在塑料挤出成形过程中，压缩段是调节模头流道各部分流量(流速)的主要区段，对熔体的流动具有重要影响。本文采用有限元数值分析方法，计算了熔体在口模内流动的速度场和压力场，定量分析了压缩比和压缩角等压缩段模具结构参数对挤出速度分布、挤出流量和挤出流动均匀性等的影响规律，为优化流道结构参数，提高挤出流动均匀性的研究提供了基础。     

Electroosmotic flows in channel with two symmetric periodic arrays of square-sectioned ribs

The present study has numerically investigated two-dimensional electroosmotic flows in channel equipped with two symmetric periodic arrays of square-sectioned ribs with one-fifth of the channel half-width in size. For the simulation, the ionic-species and electric-potential equations as well as the continuity and momentum ones are solved using the finite volume method. Instead of assuming the Boltzmann distribution, the Nernst-Plank equation is applied for the ionic species. Results show that the steady electroosmotic flow and ionic distributions depend strongly on the EDL length and streamwise periodic length. For a sufficiently large periodic length, the fluid flows along the wall as in the inviscid flow at a small EDL length compared with the rib size, whereas it flows with involving two recirculation bubbles around the rib as in the pressure-driven flow at a large EDL length. At an intermediate EDL length comparable to the rib size, a very intricate flow pattern is observed around the rib. With decreasing periodic length, on the other hand, the interaction between two adjacent ribs gets stronger and thus the flow pattern significantly changes. This study would contribute to further understanding electroosmotic flows in micro- and nanofluidic devices of complicated geometries.