An experimental study was conducted to characterize the performance of a CO² evaporator over a range of operating conditions encountered in typical residential heat pumps. The heat exchanger evaporator involved internal flow boiling of CO² refrigerant in a microchannel heat exchanger, which was heated by an external airflow. The experiments were conducted to evaluate the evaporator capacity (heat transfer rate) for various airflow rates, refrigerant inlet qualities, saturation temperatures, and mass flow rates. The microchannels used in the present study had a hydraulic diameter of approximately 1 mm. For the range of parameters reported here, CO² operated under subcritical flow boiling conditions. Utilizing the experimental results, a semi-empirical model was also developed to predict the capacities of CO² microchannel evaporators. The model takes into account the effects of air-side moisture condensation and refrigerant outlet superheat. All experiments were successfully conducted with an energy balance of ±3%.

Units: SI