随机粗糙微通道内部流动与传质特性

为了研究微通道壁面随机粗糙度对流体流动和传质特性的影响,采用随机排布准则构建具有典型粗糙元类型的随机粗糙微通道壁面,利用有限元方法分析壁面随机粗糙度对流速、压降、流动阻力和传质性能的影响,并给出粗糙微通道内部Poiseuille数和分子传质扩散的近似变化规律。结果表明,流体在粗糙微通道近壁面区域和主流区的流速差异较大,近壁面区域流动分离现象明显;与光滑微通道相比,粗糙微通道内部各位置的压降和Poiseuille数沿着流动方向呈近似线性增大趋势;微通道壁面粗糙度的存在可以强化流体分子的传质扩散速率,但受粗糙度类型和相对粗糙度的影响较大。     

微流道壁面粗糙度影响的电渗流数值模拟

基于微流控动力学理论,应用有限元分析方法求解二维平板粗糙壁面微流道模型内矩形截面双电层场和速度场的耦合控制方程。从数值模拟角度研究不同矩形粗糙元对称分布微流道内的电渗流流动特性,分析了粗糙度对微流体流动的影响机理。结果表明:由于粗糙元的阻力作用,粗糙壁面流道内流体速度减小,引起的压力突变导致壁面附近速度出现波动。随着粗糙元高度、宽度的增加,电渗流流速相应地降低或升高。     

基于格子Boltzmann方法的微通道内气液两相流流型和压力降特性研究

微流体广泛应用于生物医学和化工等领域。采用格子Boltzmann方法对T型微通道内气液两相流的流动特性进行研究,分析壁面特性、气液流速和气液流速比等对两相流运动特性的影响。结果表明：壁面接触角越大越容易形成气泡,随着毛细数的增大,分散相脱离点逐渐远离两相入口,形成更长的分层流,不易形成气泡;当气相流速较大,生成气泡的位置远离T型微通道交叉处,分层流的长度增加;不同条件下沿微通道方向压力逐渐减小,在气液两相交汇区域压力存在波动;微通道轴线流速的峰值出现“滞后”现象,速度波动随气液流速比增大而增大;大密度比气液两相流模拟,可以对宏观实验现象的机制进行更深入的解释。     

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。     

微通道纳米结构的润湿接触状态对滑移减阻影响的分子动力学研究

针对流体在纳米通道的微尺度流效应,采用分子动力学方法以SPC/E水分子为纳米流动介质,分别计算模拟其在不同纳米结构的微通道内的润湿接触状态和Poiseuille流动行为,研究通过微通道壁面微纳结构改变而导致的不同润湿状态起到的滑移减阻效应.结果表明:纳米结构的周期性增加,会使得壁面的亲疏水性呈现马太效应,从而达到润湿性控制的目的.增加壁面亲水性,会使主流区密度、流体速度和滑移速度都减小;在增加壁面疏水性的过程中,主流区的密度增加;并且流体的状态由Wenzel向Cassie转变,流体速度和滑移长度先减小后增加;而亲疏水转变过程中,随着表征接触角的增加,当动静态流体与壁面的接触状态相同时,流体流动的壁面摩擦因数值呈现单调递减趋势;而当动静态流体与壁面的接触状态存在差异时,摩擦因数会出现轻度无规律波动.     

6kW静音型柴油发电机组排风通道风阻仿真研究

为了研究6 kW静音型柴油发电机组排风通道结构对空气流动阻力的影响,通过计算流体力学方法(CFD)分析排风通道内流体模型的压力场和流场分布,研究了排风通道内风阻的影响因素。通过改变阻风板、消声器相对位置和排风出口形状、尺寸,对比风阻变化及流体流速变化。结果表明移动阻风板,扩大排风通道体积及移动消声器相对阻风板的距离可以减小风阻,同时增大排风出口处U型挡板宽度可以减小风阻,当不考虑排风出口处吸音海绵表面粗糙度时其厚度对风阻影响不大。为排风通道结构设计和优化提供理论依据。     

用于高功率二极管激光器散热的变截面微通道数值模拟与性能分析

对一种屋脊式变截面微通道冷却器中的微通道进行了分析,采用Fluent 6.1软件对其中的流体流动和传热进行了数值模拟,并分析了流速、微通道入口大小对冷却性能的影响.     

非极性流体在微通道内的阻力特性分析

研究非极性流体在微通道内的流动特性。采用的微流动实验台通过测量不同极性液体流过矩形截面微通道中流量与压力,分析了不同极性流体的阻力特性。结果表明:对分子量较小的极性液体,微通道内流量-压差关系符合Hagen-Poiseuille定律,连续介质模型仍适用于微米尺度微通道;对分子量较大的非极性液体,常规理论在预测微通道内流量-压差、摩擦阻力系数-雷诺数关系时存在较大误差,须进行修正。     

平板微通道流动传热的粗糙元间距效应

微细尺度传热问题业已成为国际传热界的研究点,而壁面粗糙度对微细通道流动和传热特性有着重要影响。以粗糙平行平板微通道为研究对象,用三角形粗糙元模拟固体表面的粗糙度,通过采用CFD(computational fluid dynamics)流体固体共轭传热技术数值研究了粗糙元间距对平行平板微通道流动和传热特性的影响规律,同时研究了其间距对微通道流动的转捩雷诺数的影响规律;计算结果表明:随着三角形粗糙元间距的增大,粗糙平板微通道的阻力性能逐渐下降,层流向湍流转捩的雷诺数呈逐渐增大趋势,同时粗糙平板微通道的传热性能呈逐渐下降趋势。     

考虑粗糙度影响的微凹坑织构化机械密封润滑性能的数值模拟

采用正弦粗糙峰表征粗糙表面,建立了织构化粗糙机械密封端面流体动压润滑模型,研究了粗糙度、微凹坑织构几何参数对润滑性能的影响。结果表明:利用数值分析方法研究微凹坑织构对机械密封流体动压润滑性能影响时,不能忽略粗糙度、微凹坑之间的相互影响和边界压力的影响;存在最佳的微凹坑深度和面积密度使织构机械密封端面的流体动压润滑效应达到最佳。     

Non-lithographic fabrication of metallic micromold masters by laser machining and welding

A non-lithographic process of rapidly fabricating metallic micromold masters for the manufacture of disposable polymer microfluidic devices is presented in this paper. The developed technique exploits the precision material removal capabilities of industrial lasers to cut accurate profiles of microfeatures (e.g., liquid flow microchannels, reservoirs, passive micromixers) from thin metallic sheets. The machined micropatterns are then laser welded onto a metal substrate to form the final functional mold master. Multiple versions of the functional device are replicated from the assembled master by either soft molding polydimethylsiloxane or hot embossing polymethyl methacrylate. Several metallic micromold masters and polymer replicas are tested for dimensional accuracy and surface roughness to verify the developed microfabrication process.     

表面粗糙度对多孔端面机械密封膜压的影响

为了研究表面粗糙度对多孔端面机械密封膜压的影响,建立含粗糙元的矩形截面轮廓激光加工多孔端面机械密封微间隙液膜多尺度的三维几何模型和计算模型,采用有限体积法求解三维N-S方程,分析相对表面粗糙度、粗糙元密度、不同分布位置对膜压分布的影响规律。研究表明:表面粗糙元和微凹腔都能产生动压效应;分布在动环端面的粗糙元对静环膜压的影响较大,分布在静环端面的粗糙元对动压效应的影响相对较小;随着粗糙元密度的减小,动环上粗糙元的动压效应增强,影响区域将由动环表面沿膜厚方向扩大到静环表面;较小或较大的相对表面粗糙度对静环膜压的影响相对较小,而中等的相对表面粗糙度,兼具较强的动压效应效果和较大的影响范围,对静环膜压的影响相对较大,它使得微凹腔在静环上形成的低压区减弱甚至消失。     

Experimental investigations and analytical modeling of multi-pass CO2 laser processing on PMMA

In this research work, microchannels have been fabricated utilizing multi-pass CO₂ laser processing on Poly-methyl meth-acrylate (PMMA) substrates. CO₂ laser engraving machines are cost effective and less time consuming compared to other tools and methods of fabricating microchannels on PMMA. However, the basic problem of low surface finish of the microchannel walls still restricts thus fabricated product from many potential applications. In this work, experimental and theoretical investigations of multi-pass CO₂ laser processing on PMMA have been conducted. A number of experiments were performed to establish the relationship between laser power and scanning speed with microchannel parameters like width, depth, heat affected zone, surface roughness and surface profiles. Experiments were conducted at four different power settings with 50 mm/s of constant scanning speed and seven numbers of passes in each setting. Changes in thermo-physical properties of PMMA were observed for as-received PMMA sample and PMMA sample residing in heat affected zone (HAZ) for first pass and secondary passes respectively. Effect of different numbers of passes on microchannel width, depth, HAZ and surface roughness were explored for different power setting. Microchannel profiles resulting from different numbers of passes have been compared. Energy dispersive X-ray analysis was performed to determine elemental composition after each pass. Many advantages of multi-pass processing over single-pass processing were recorded including high aspect ratio, low heat affected zone, smoother microchannel walls and reduced tapering of microchannels. An energy balance based simple analytical model was developed and validated with experimental results for predicting microchannel profiles on PMMA substrate in multi-pass processing. Multi-pass processing was found to be time and cost effective method for producing smooth microchannels on PMMA.     

The capability of ECDM in creating effective microchannel on the PDMS

Today, polydimethylsiloxane (PDMS) is widely used in medical and industrial applications. Lithographic processes are commonly used to create microchannels in PDMS, which have encountered several limitations such as high fabrication time and cost. In this study, the effect of tool characteristics (distance, geometry and diameter) and electrolyte properties (temperature and type) on the surface quality, surface roughness and dimensional accuracy of the created microchannel on PDMS through electrochemical discharge machining (ECDM) are investigated for the first time. The results stated the capability of ECDM in the formation of channels with the similar quality of created channels by lithography. In addition, it has been declared that increasing the tool/workpiece distance would lead to an increase in the surface roughness as well as deteriorated surface quality of the grooves due to the longer traveled distance by the particles and the subsequent energy increase by colliding the workpiece surface. According to the obtained results, increasing the mentioned distance from 20 μm to 150 μm was followed by a 356% increase in the surface roughness. Moreover, by the utilization of grooved tools instead of simple ones, thicker gas film would be formed at the vicinity of the tool, and consequently, intensified stray sparks and the resultant increased material removal area would be achieved. Furthermore, rising the electrolyte temperature from 25 °C to 65 °C led to an increase in surface roughness from 0.109 to 0.140 μm. Additionally, respective reductions of 33% and 3% in surface roughness and microchannel width were attained by 20% reduction in the tool diameter.     

Mold surface roughness effects on cavity filling of polymer melt in micro injection molding

Micro injection molding presents many challenges in the injection-molding community. When the dimensions of the part (and thus the cavity of the mold) are small, micro-scale factors such as mold surface roughness may play an important role in the filling of polymer melt. This paper investigates the effects of mold surface roughness on cavity filling of polymer melt in micro injection molding. A disk insert, which has two halves with different surface roughness but with the same roughness mean lines, was used in the investigations. The ratio of flow area of the rougher half with the total flow area of the molded part is used to evaluate the significance of surface roughness effect. The experimental results revealed that mold surface roughness does resist the cavity filling of polymer melt in micro injection molding. For the limited range of injection rate investigated, it is not significant on the surface roughness effects. The increase of mold temperature will decrease surface roughness effects. The change of melt temperature within the range allowed by the process is insignificant for surface roughness effects.     

壁面粗糙度效应对微流体流动特性的影响

壁面粗糙度对微流道流动特性有重要影响。分别用矩形、三角形和圆顶形粗糙元对壁面粗糙度进行模拟,详细讨论了雷诺数、粗糙元高度、粗糙元间距等因素对流速、压降及流动阻力的影响。结果表明:与光滑流道相比,粗糙度使壁面附近的流动发生明显改变,从而导致微流道内流速、压降及流阻高于经典理论预测值;微流道内流动阻力随着雷诺数及粗糙元高度的增大而增大,而随着粗糙元间距的增大,流动阻力逐渐减小。三种粗糙元相比,矩形粗糙元的影响最大,圆顶形次之,而三角形粗糙元的影响最小,可见在实际应用场合,确立合适的粗糙元形状对分析结果非常重要。     

钛合金薄壁曲面液态金属-磨粒流加工仿真与试验研究

针对钛合金薄壁曲面工件磨粒流抛光后表面粗糙度分布不均匀的问题,提出一种基于液态金属的磨粒流加工方法。基于SST k-ω模型、OKA冲蚀模型,流体流动颗粒追踪模型,采用COMSOL有限元软件对不同电场布置下的液态金属-磨粒流动力学特性开展深入研究。仿真结果表明,通过电场的合理布置可以控制液态金属颗粒在流场中运动;合理的电场布置可以有效提高工件表面加工均匀性,并通过仿真得出了一组冲蚀较好的试验参数。基于仿真结果开展了液态金属-磨粒流加工试验,试验结果表明：液态金属-磨粒流加工方法可有效提高工件表面加工的均匀性。在加工14 h后,不加电场的磨粒流加工表面不同区域的粗糙度分布不均,工件凹陷处粗糙度明显大于凸起处,各区域表面粗糙度极差达到66.1 nm。使用液态金属-磨粒流加工后的工件表面各区域粗糙度的均匀性明显提高,各区域表面粗糙度极差减小为20.3 nm,为液态金属-磨粒流加工的开展及其调控提供了理论和试验依据。     

Finite Volume Modeling of Gas Flow in Microbearings with Rough Surface Topography

The random surface roughness effects on the performances of gas-lubricated slider microbearings are investigated using finite volume analysis. The rough surface morphologies with the atomic force microscope (AFM) data and the numerical procedures used to generate self-affine surfaces by the midpoint displacement method are proposed and compared. Three-dimensional finite volume modeling of the gas microbearing and its flow field mesh are described considering the velocity-slip boundary condition. The results indicate that surface roughness causes a deviation in the pressure distribution, load-carrying capacity, velocity profile, and local Reynolds number from conventional theory with various values depending on the degree of surface roughness used. The pressure decreases irregularly due to the fractal surface and produces a larger change. The larger the roughness exponent is, the larger the gas slip velocity at the bottom of the wall. In addition, the velocity-slip boundary condition can cause a decrease in the gas flow velocity. The surface roughness effect also leads to random variations in the local Reynolds number. This demonstrates that the random surface roughness potentially causes very complicated flow behavior in the ultra-thin-film bearing lubrication in microelectromechanical systems (MEMS) and should be adequately characterized in terms of the fractal nature of the bearing surfaces.