Surface Effects On Fluid Flow And Heat Transfer Characteristics In Microchannels

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.