In automotive sector, in order to reduce the space occupied by engine radiators, the use of micro air channels with different cross-sections has been intensified. In these channels both; the hydrodynamic entry-length and the thermal entry-length are small enough compared to the total length of the channel. The flow in the channel occurs in laminar regime and it is plausible to assume it to be hydro-dynamically and thermally fully developed flow. In radiators, despite that several air channels with different cross-sections such as; tetragon, triangle, hexagon, sinusoid, parabola etc. are used, the channels with no sharp corners have advantages from manufacturing point of view and heat transfer point of view.