16 September 2011
The flow boiling heat transfer coefficients of pure CO2 and CO2-oil mixtures inside a micro-fin copper tube were measured at saturation temperatures of 0 and 10 °C using
electrical resistance method. The micro-fin tube is 5.43 mm in mean inner diameter and is heated using a film heater. In the case of pure CO2, the results showed that the flow boiling
heat transfer coefficients at the saturation temperature of 10 °C are slightly higher than those of 0 °C. The flow boiling heat transfer coefficients increase with the increase in heat flux, but are almost independent of mass velocity. These results imply that the flow boiling heat transfer in the case of pure CO2 is dominated by nucleate boiling. In the case of CO2-oil mixtures, the heat transfer coefficients depend on mass velocity, but are independent of heat flux. So the flow boiling heat transfer is considered to be dominated by forced convective
evaporation. From the flow visualization results, the foaming phenomenon was not observed, but it is found that an oil film was formed on the inner wall surface, and a lot of oil droplets are observed in the liquid phase of the refrigerant.