Surface Effect On Gas Flow Characteristics In Microchannels

With the rapid development of micoelectromechanical systems (MEMS), the study of the flow characteristics in microchannels has become a hot subject in the fluid mechanics. So it is very significant to investigate the flow characteristics in microchannels on both theoretical researches and applications. Based on large quantities of experimental data, theoretical analysis and numerical simulations,the gas flow characteristics were investigated. The main work is stated as follows.(1) An approximate theoretical model was proposed with partial consideration of compressibility. It is used to compute the mass flow and is suitable for the microchannels with a large ratio of depth to width. The applicability of the new model and the model usually used is provided.(2) The flaws on the one-dimension (1-D) compressible isothermal average-friction-factor were indicated. A friction factor computation method was proposed, which is not only suitable for compressible flow but also easy to achieve the average wall shear stress. Moreover, the applicability of the incompressible friction factor is discussed.(3) The effects of surface roughness in microchannels were taken into account in terms of a roughness-viscosity function. The compressible two-dimension (2-D) and three-dimension (3-D) models were proposed to interpret the experimental results. Better agreements between computation results and experimental data were found using the models.(4) Two phenomena in straight-convergent-divergent-straight microchannels were identified, which are different from the macroflow through experimental studies. One is that the first appearing sonic point position in microchannels is not near throat cross-section. The other is that the critical pressure ratio in microchannels is different from that in macroflow greatly. Numerical simulations were carried out to investigate the effects of the ratio of surface to volume on the two phenomena. (5) Experimental investigations for the long-constant-area microchannel indicate that the mass flow rate through the microchannel changes considerably small as the pressure ratio of inlet to outlet arrived at some critical pressure ratio. The phenomenon is defined as sub-choking and the corresponding pressure ratio is defined as the sub-choking-critical-pressure-ratio. Moreover, the effects of the ratio of surface to volume on the critical pressure ratio are studied.(6) Numerical simulations were conducted by using the Bhatnagar-Gross-Krook (BGK) method. Better agreements between computation results and experimental data were found about the pressure distribution and mass flow rate.