Experiments on flow boiling heat transfer of almost pure CO2 and CO2-oil mixtures in horizontal smooth and microfin tubes

Experiments on the flow boiling heat transfer of almost pure CO2 and CO2-oil mixtures in horizontal smooth and microfin tubes have been carried out. The smooth tube is a stainless steel tube with an inside diameter of 3 mm, and the microfin tube is a copper tube with a mean inside diameter of 3.04 mm. Experiments were carried out at mass velocities from 190 to 1300 kg/(m(2).s), a saturation temperature of 10 degrees C, heat fluxes from 5 to 30 kW/m(2), and an oil (PAG) circulation ratio from <0.01 to 0.72 wt. In the case of almost pure CO2, the flow boiling heat transfer is dominated by nucleate boiling for both smooth and microfin tubes. In the case of CO2-oil mixtures, the heat transfer coefficients are much lower than those of almost pure CO2. The strong dependence on heat flux disappeared due to the existence of oil. This is due to nucleate boiling being suppressed by the oil film: The dryout quality decreases greatly with an increase in mass velocity for the smooth tube and is little affected by mass velocity for the microfin tube. The effect of oil on dryout quality is very small in both tubes.