The heat transfer and fluid flow characteristics of non-boiling two-phase flow in microchannels were experimentally investigated. The effects of channel diameter (140, 222, 334, and 506 μm) on the Nusselt number were considered. Air and water were used as the working fluids. Results were presented for the Nusselt number over a wide range of gas superficial velocity (1.24–40.1 m/s), liquid superficial velocity (0.57–2.13 m/s), and wall heat flux (0.34–0.95 MW/m2). The results showed that the Nusselt number increased with increasing gas flow rate for the 506 μm and 334 μm channels, while the Nusselt number decreased with increasing gas flow for the 222 μm and 140 μm channels. Based on these experimental results, a transition channel diameter of about 235 μm to 260 μm, which distinguishes microchannels from minichannels, was suggested. By observing two-phase flow patterns within the microchannels, viscosity and surface tension were identified as the key factors that caused the heat transfer characteristics to change. In addition, new correlations for the forced convection Nusselt number were developed.

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