Abstract
In order to obtain the data for designing and optimizing air conditioning systems using a CO2 refrigerant, the cooling heat transfer characteristics of CO2 were examined at a supercritical pressure condition 9.5 MPa and temperatures between 20 and 70°C. The obtained data revealed that heat transfer coefficient increased with mass flux and the maximum heat transfer coefficients were obtained at 45 °C. Two layer model for flow field near the tube wall were proposed the model was found to be valid according to the comparison of the predicted values with the measured data. A CO2-Oil mixture was also investigated, and was found to exhibit lower heat transfer coefficients than only CO2. A visualization experiment revealed that the oil near the tube wall became an obstacle for heat transfer coefficient and the distribution of oil changed according to the CO2 velocity and density variation due to its temperature variation.
