Condensation heat transfer of R32 in a horizontal circular microchannel with the inner diameter of 1 mm is numerically simulated with VOF model. The inlet saturation temperature of R32 is 40℃ and the wall temperature is 30℃. The vapor and liquid phases are assumed to be turbulent and laminar

respectively

and the effects of gravity and surface tension are considered. The simulation results show that the vapor quality decreases nearly linearly along the tube and reaches 0.62 at the outlet of the channel

and the liquid film thickness at the top of the tube increases first and then maintains almost constant while the liquid film thickness at the bottom of the channel increases consistently along the channel. As the thermal conductive resistance of the liquid film is dominant in condensation heat transfer

the heat transfer on the top surface tube is stronger than that on the bottom surface. The liquid film on the top surface flows to the bottom of the tube due to the effect of gravity. The results show that the gravity effect cannot be neglected when the hydraulic diameter of channel is 1 mm. Axial velocity distribution of the liquid film is linear with the vapor quality. Vapor-liquid interface velocity on the top surface in the tube increases first and then decreases a little along the channel while interfacial velocity on the bottom increases consistently. Simulated heat transfer coefficients increase with vapor quality

and the simulation results are lower than the experimental results in the overlapped vapor quality region while the deviations are still within the experimental error.