Investigation On Characteristics Of Flow In Different Microfluidic Devices With Random Surface Roughness

As the Micro-Electro-Mechanic System (MEMS) develops,microfluidic devices have a great development. Some of microfluidicdevices realize industrialization, such as microchannels and micronozzles.However, the characteristics of flow in these devices have not beenunderstood well.Microfluidic devices have a characteristic length greater than1μmand less than1mm. Fluid in these devices is the so-called microfluid. Dueto the scale effect, the characteristics of microfluid are different from thoseof marcofluid. For example, microfluid has a slip velocity at the gas-solidsurface. The reduction of scale enhances the effect of surface roughness,which can usually been neglected in the macrofluid.In this paper, the characteristics of gas flow in different microfluidicdevices have been investigated under the coupled effects of random surfaceroughness and velocity slip by the method of computational fluid dynamics(CFD). The random surface roughness is characterized by fractal geometry,and the slip velocity on boundary is described by the second-order slip law.The main work and results are as below:1) Combined with the commercial CFD code, develop a numericalmethod to calculate the slip velocity at the gas-solid surface with a randomnormal vector. With this method, the coupled effects of velocity slip andrandom surface roughness can be investigated.2) In the rectangular microchannels, the simulated curves obtained byconsidering both second-order slip law and random surface roughness are in good agreement with experimental data in a range of averaged Knudsennumber from0.025to0.59. Random surface roughness has an obviouseffect on velocity slip at the wall: the velocity slip coefficients decrease asthe surface roughness increases.3) Investigate the random roughness on the subsonic and supersonicflow in micronozzles. On one hand, surface roughness has no obvious effecton the subsonic flow. On the other hand, it significantly disturbs thesupersonic flow. The thrust loss and thrust deviation caused by randomsurface roughness are also analyzed: as the surface roughness increases,both thrust loss and thrust deviation increase, which reduces theperformance of micronozzles.4) Propose a method to model a three-dimensional random roughmicrotube based on fractal geometry. After modelling, the effect ofroughness on the slip velocities, pressure and velocity profiles areinvestigated.