THE EFFECT OF CHANNEL DIAMETER ON HEAT TRANSFER CHARACTERISTICS OF TWO-PHASE FLOW IN MICROCHANNELS

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 mu m) 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 rn/s), and wall heat flux (0.34 - 0.95 MW/m(2)). The results showed that the Nusselt number increased with increasing gas flow rate for the 506 mu m and 334 mu m channels, while the Nusselt number decreased with increasing gas flow for the 222 mu m and 140 mu m channels. Based on these experimental results, a transition channel diameter of about 235 mu m to 260 mu m, 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.