The present study focuses on measuring the flow boiling heat transfer coefficients of CO₂ in microchannels in the presence of a miscible oil at various oil concentrations and as a function of such operational conditions as refrigerant mass flux, saturation temperature, and vapor quality. Tests were conducted under subcritical conditions in a horizontal triangular microchannel with a hydraulic diameter of 0.86 mm. It was found that oil effect, among other parameters, is dependent upon oil concentration and vapor quality. Larger oil concentrations degrade the heat transfer coefficient significantly, while smaller oil concentrations (< 3% on mass basis) at low vapor qualities (x < 0.45) moderately (about 10% to 15%) enhance the heat transfer coefficient. The moderate augmentation of the heat transfer coefficient may be attributed to foaming action that improves wall-wetting. However, the heat transfer coefficient decreases significantly in the high vapor quality region. This reduction is caused by the formation of an oil-rich sublayer near the wall. The balance between the escape of refrigerant bubbles and the replenishment of fresh refrigerant at the sublayer plays an important role in the heat transfer process.

Units: SI