Developing natural convection gaseous flows in an open-ended parallel plate vertical microchannel with isothermal wall conditions are numerically investigated to analyze the rarefaction effects on heat transfer and flow characteristics in slip flow regime. The Navier-Stokes and energy equations are solve by a control volume technique subject to higher-order temperature jump and velocity slip conditions including thermal creep effects. The flow and thermal fields in the entrance and fully developed regions along with the axial variations of velocity slip, temperature jump, and heat transfer rates are examined in detail. It is found that rarefaction effects significantly influence the flow and thermal fields such that mass flow and heat transfer rates are increased considerably as compared to the continuum regime. Furthermore, thermal creep contribution to the velocity slip is found to be dominant close to the channel inlet and vanishes in the fully developed region, while velocity slip approaches a finite value there. Both Mass flow rate and thermal entrance length increase with increasing Knudsen number in slip flow regime.

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