A control-volume numerical approach has been used to study rarefaction effects in simultaneously hydrodynamically and thermally developing flow in rectangular microchannels with a prescribed uniform wall heat flux in the slip-flow regime (10-3 ≤ Kn ≤ 10-1). The effects of velocity slip and thermal creep on the key flow parameters are examined in detail. Low Reynolds number flows (Re ≤ 1) for different channel aspect ratios (0 ≤ α* ≤ 1) are considered. The effects of rarefaction on the global features of the flow and thermal development in the entrance region are examined. Dramatic reductions in the friction coefficient are observed in the entrance region due to rarefaction effects, which are enhanced by thermal creep. For the fluid heating cases considered here, thermal creep increases slip at the wall and thereby further reduces the friction coefficient and slightly enhances heat transfer at a given Reynolds number. For an identical heat flux applied to the microchannel walls, thermal creep effects become much more pronounced at lower Reynolds numbers since it results in higher axial temperature gradients.

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