【作者】 谭德坤；

【导师】 刘莹；

【作者基本信息】 南昌大学 ， 机械设计及理论， 2014， 博士

【摘要】 随着微机电系统(Micro-electro-mechanical System，MEMS)技术的迅猛发展，诸如微流道散热器、微泵、微阀、微混合器、微喷嘴、微生物芯片等微流体器件的应用越来越广泛。在这些器件中，微流道是介质输运的基础，各种功能部件之间均由它连接。与宏观流动系统不同，随着特征尺度的减小，表面效应成为影响微流体系统性能的主要因素。深入了解微流道内流体流动和传热特性，对微流体器件的功能实现和优化设计具有重要作用。本文主要对微流道内动电效应、壁面滑移和壁面粗糙度三种表面效应的影响机理进行系统深入的研究，获得了表面效应影响下的流体流动及传热规律。论文的主要研究工作及获得的结论如下：(1)研究了壁面非对称边界条件下，动电效应对压力驱动微流体流动及热传递特性的影响。系统地分析了动电参数、壁面zeta电势、上下壁面zeta电势比及热通量比等参数对电势场、流场、温度场及微流体传热性能的影响。结果表明上下壁面zeta电势的大小相同、极性相反时，其引起的电场力相互抵消，动电效应消失；微流道内的温度场与双电层电势分布密切相关，上下壁面zeta电势取值不同时，造成双电层分布的不一致，从而影响壁面附近的温度场；对流传热性能与流体流速紧密相关，动电参数值小时，溶液浓度较低，此时双电层的厚度较大，努赛尔数随着zeta电势的增加而减小。而当动电参数值大时，双电层较薄，即使壁面zeta电势增加，它对努赛尔数的影响也很小。此结果表明可通过人工调控壁面zeta电势或改变溶液浓度来改善通道的流动和传热性能，为实现压力驱动下微流道内流体的精确操控、温度控制以及散热分析提供了依据。(2)研究了壁面滑移和动电效应两种因素耦合作用下微流体流动及传热特性，建立了相应的数学模型。运用电势分布的解析表达式，推导出流动电势及无量纲速度分布的解析表达式，将速度解代入能量方程，得到流道内的温度分布数值解。研究结果表明，流动电势阻滞流体流动，降低流速，而壁面滑移促进流动，使流速增加并放大电黏效应。在两种效应耦合作用下，定量分析了两者对流动及传热的影响大小，研究表明在流动中，动电效应占优，而在传热中，壁面滑移效应占优。在高壁面zeta电势下，壁面滑移和动电效应对滑移流速及努赛尔数的影响相互抵消。耦合分析和量化计算所得结果表明，为增强微流道的输运效率和散热性能，应采用疏水材料；而增加zeta电势，可大大改善疏水微流道内动电效应对流动和散热性能的不利影响。(3)采用几何形状描述法对微流道内的壁面粗糙度效应进行建模。构造了矩形、三角形、圆顶形和锯齿形等四种粗糙微流道模型，给出一种基于随机函数构造锯齿形随机粗糙元的方法。全面地分析了粗糙元形状、间距和高度对速度分布、压降、温度分布、摩擦因子及努赛尔数的影响规律。研究结果显示，壁面粗糙元的间隙区域有大量旋涡和回流，使壁面附近的流动发生明显改变，使主流区沿流动方向的压降增大，流阻增加。粗糙元的高度和密度会显著影响微流体流动及传热特性，粗糙元高度增加，对微流道传热及流动均不利，而粗糙元密度增加，增大了散热面积，微流道流阻增大但传热性能却增强。该结果合理解释了壁面粗糙度使努赛尔数增加的原因，对于人工粗糙元微流道散热器的优化设计具有指导意义。(4)最后还研究了壁面粗糙度及动电效应耦合作用下微流体的流动与传热特性。结果显示在粗糙微流道中，动电效应的存在使微流道流阻增大，而传热性能却增强，指出动电效应引起的逆向扰动是传热性能增强的原因。更多还原

【Abstract】 With the rapid development of micro-electro-mechanical systems(MEMS), themicrofluidic devices are widely used in many engineering fields, such asmicrochannel heat sinks, micropumps, microvalves, micromixers and microfluidicchips. Microchannels, however, are the basic structures in these devices. Transportphenomena at the microscale reveal many features that are not observed in themacroscale devices, the surface effects will become a dominant factor to control thebehavior of microfluidic system. It is very important to deeply understand the liquidflow and heat transfer characteristics in microchannels in order to guide the designand application of the microfluidic devices. This thesis mainly studies the mechanismof electrokinetic effect, wall slip and surface roughness, and reveals the nature ofsurface effects on liquid flow and heat transfer in microchannels. The main topics inthis thesis are stated as follows:(i) This work investigates the electrokinetic effect on pressure-driven liquid flowand heat transfer in microchannels under asymmetric wall boundary conditions. Theinfluences of electrokinetic parameter, zeta potential, ratio of two wall zeta potentialsand heat flux ratio on electrical potential field, flow field, temperature field and heattransfer performance are analyzed in detail. The results show that the influence of theelectrokinetic effect in the microchannel disapperas due to the cancellation of reverseeffects stemming from two walls at zeta potentials of the same magnitude butopposite polarity. Temperature field is closely related to the electrical potentialdistribution of electrical double layer(EDL) in the microchannels, the potentialdistribution is inconsistent in two parts of microchannels because the value of upperand lower wall zeta potentials is different, thus affecting the temperature field nearthe channel wall. The convective heat transfer performance is also colsely related tothe flow velocity, if the electrokinetic parameter is small, which means a lowersolution concentrations and a larger thickness of the EDL, then the Nusselt numberdecreases with increasing zeta potential, this is due to the EDL is thicker, then theelectro-viscous become stronger, thereby causes flow velocity to decrease dramatically, and consequently lowers the heat transfer performance. But if theelectrokinetic parameter is large, which means the EDL is thin, the counter-ions aretightly bound to the channel wall, charge density in the bulk fluid area tends to zero,the EDL field only affects a small area near the wall, even if increasing zeta potential,its impact on nusselt number is still very small. The results indicates that the heattransfer performance in microchannels can be enhanced by changing the solutionconcentration or regulating the wall zeta potential manually. It provides the referencefor precise fluid and temperature control in pressure-driven microchannel flow, whichcan also be used to analyze the impact factors of heat transfer through microchannelstheoretically.(ii) This work carries out the investigation on the fluid flow and convective heattransfer with consideration of wall slip and streaming potential effects simultaneously,the corresponding mathematical model is established. The dimensionless analyticalexpressions of streaming potential and velocity distribution are derived byintroducing the analytical solution of Poisson-Boltzmann equation withoutconsideration of Debye-Huckel approximation, after that the velocity solution issubstituted into energy equation, then the numerical solution of temperaturedistribution in microchannels can be obtained. The numerical results show that theflow-induced streaming potential retards the pressure-driven flow, however, wall sliptends to increase the flow velocity and amplifies electroviscous effect. When theabove two effects are both considered, the impacts of them on liquid flow and heattransfer behavior are analyzed quantitatively, the electrokinetic effect is dominant inmicrochannel flow, while wall slip effect is the main influence factor on heat transfer.In case of high wall zeta potential, the impacts of wall slip and electrokinetic effect onthe Nusselt number and slip velocity will counteract each other. It can be seen fromthe above conclusions, in order to enhance transport efficiency and heat transferperformance in microchannels, the hydrophobic materials should be given priority forthe design of microfluidic devices, the adverse influence of electrokinetic effect onflow and heat transfer performance can be greatly improved by increasing wall zetapotential in hydrophobic microchannels.(iii) The geometry method is used to model the surface roughness in microchannels. Four types of rough surface are modeled by rectangular, dome-shaped,triangular and random sawtooth roughness elements, a method of constructingrandom roughness elements with random function is presented. The influences ofroughness element shape, roughness height and roughness element spacing onvelocity distribution, pressure drop, temperature distribution, friction factor andNusselt number are discussed in detail. The study indicates that there is a largeamount of vortex and reflux in the gap of wall roughness elements, so that the liquidflow near the channel wall is changed significantly, thereby the pressure drop in bulkfluid area increases along the flow direction, that is, the flow resistance becomes large.The height and density of roughness elements can also significantly affect fluid flowand heat transfer characteristics in microchannels, it is negative for flow and heattransfer when the height of roughness elements increase, but when the density ofroughness elements increases, then increasing the cooling area, it is positive forenhancing heat transfer performance and increasing the flow resistance. The resultsgive a reasonable explanation that Nusselt number increases with increasing wallroughness, it is helpful for the design and optimization of microchannel heat sinkwith artificial roughness elements.(iv) Finally, the thesis studies the wall roughness and electrokinetic effects onfluid flow and heat transfer characteristics. The results indicate that the electrokineticeffect increases the flow resistance in rough microchannels, and enhances the heattransfer performance. The inverse disturbance caused by the electrokinetic effect isthe reason for improved heat transfer performance.更多还原