the differences between flow patterns of the two cases were addressed. It is noted from Triplett et al. [17,18] that based on different channel cross-sectional shapes, the adiabatic two-phase air– water flow characteristics in semi-triangular micro-channels were similar to those obtained from circular channels. The heat transfer of an air–water flow in parallel micro-channels of 0.1 mm in hydraulic diameter was experimentally investigated by Hetsroni et al. [19]. Their results showed a decrease in the Nusselt number with an increasing gas flow rate, which was opposite to the results