Rarefaction and thermal creep effects in square cross-section microchannels

Abstract

Fluid flow and heat transfer in MEMS systems are numerically investigated in this study for potential rarefaction and thermal creep effects using our recently developed implicit, incompressible, hybrid (finite volume/finite element) flow solver. Rarefied flows characterized by the Knudsen number in the range of 0 ≤ Kn ≤ 0.1 are analyzed in detail for thermal creep effects in square cross-section microchannels with constant wall temperature boundary condition. Axial conduction becomes important for very low Reynolds number flows thereby enhancing the thermal creep effect. Three dimensional numerical simulations are conducted for simultaneously developing flows at very low Reynolds numbers in the range of 0.2 ≤ Re ≤ 5. Extended inlet boundary conditions are used to avoid entrance region singularity and also to account for axial heat conduction near the entrance. Friction coefficients are reduced with rarefaction in the slip flow regime. The reduction in friction coefficients was more pronounced due to thermal creep in the entrance region. Effects of rarefaction and thermal creep on heat transfer are studied for different gas-wall surface combinations as defined by the choice of momentum and thermal accommodation coefficients. It was found that heat transfer can increase or decrease with rarefaction. For very small values of gas-wall surface interaction parameter (β), it was observed that velocity slip dominates the temperature jump and heat transfer is enhanced with rarefaction. The opposite effect is observed for higher values of β. Nusselt number values increased slightly with thermal creep as Reynolds number was decreased.