Experiments on Flow Boiling Heat Transfer of Pure CO2 and CO2-Oil Mixtures in Horizontal Smooth and Micro-Fin Tubes

Experiments on the flow boiling heat transfer of almost pure CO2 and CO2-oil mixtures in horizontal smooth and micro-fin tubes have been carried out. The smooth tube is a stainless steel tube with an inside diameter of 3mm, and the micro-fin tube is a copper tube with a mean inside diameter of 3.04mm. Experiments were carried out at mass velocities from 190 to 1300kg/(ms), saturation temperature at 10 ̊C, heat fluxes from 5 to 30kW/m, the circulation ratio of the oil (PAG) from <0.01 to 0.72wt%. In the case of almost pure CO2, the flow boiling heat transfer is dominated by nucleate boiling for both tubes. In the case of CO2-oil mixtures, the heat transfer coefficient is much lower than that of almost pure CO2. The result is considered due to the nucleate boiling being suppressed by the oil film. The dryout quality decreases greatly with the increase in mass velocity for the smooth tube, and is almost not influenced by the mass velocity for the micro-fin tube.