Numerical investigation of slip flow across micro/nano pin fins

Gas flows in micro scale mostly lie in the slip flow regime, which correspond to a Knudsen number range from 0.001 to 0.1, where the velocity slip and temperature jump conditions at the walls play a major role in heat transfer and pressure drop. In this study, numerical simulations are conducted to investigate gas flow characteristics in microchannels with pin fins under velocity slip and temperature jump boundary conditions at uniform wall temperature and constant heat flux. Air is considered as the working fluid in the silicon based micro heat sink with channel cross sectional area ratio of 1:3 (minimum cross sectional area /maximum cross sectional area). The wall temperature is fixed at 303K, while the constant heat flux applied to the heat sink surface is kept as 20 kW/cm2. The computations were performed for five different Knudsen and Reynolds numbers. The flow pressure drop and heat transfer characteristics were investigated and compared to those of continuum flows with no-slip and no temperature jump conditions. Nusselt number trends in the absence and presence of temperature jump condition were revealed at isothermal and isoflux boundary conditions. The obtained results indicate that the velocity slip and temperature jump conditions have opposite effects on heat transfer. Although velocity slip condition tends increase Nusselt number, temperature jump condition tends to reduce it. The effect of surface enhancement with pin fins on heat transfer is also studied for the temperature jump boundary condition.