Heat transfer characteristics of a non-boiling two-phase flow in micro-channels with different diameters were studied by Choo and Kim [21]. Air and water were used as working fluids to examine the dependence of Nusselt number on the channel diameter. They found that with channel diameters of 0.506 and 0.334 mm, the Nusselt number increased with the increment of gas flow rate, but decreased with increasing gas flow rate when the channel diameters of 0.222 and 0.140 were employed.