Effect of Charging Amount on Evaporative Heat Transfer of CO₂ in a Multi-Channel Micro Tube

Carbon dioxide has gained a great interest as a promising alternative natural refrigerant due to good thermodynamic characteristics. One of the most important factors in applying micro tube to carbon dioxide refrigeration system is refrigerant distribution. In this paper, evaporating heat transfer characteristics were experimentally investigated relating the charge of carbon dioxide in a multi-channel micro tube. The heat transfer coefficient and the pressure drop of the 6-channel aluminum tube were measured at various charges. It was found that the dryout quality and heat transfer coefficient were heavily dependent on the maldistribution of carbon dioxide. If the charge of CO₂ was not enough, maldistribution in the multichannel micro tube became serious and a sudden drop of heat transfer coefficient was observed. These phenomena were conspicuous at high mass flux of CO₂. Also, the pressure drop was greater if the CO₂ was not properly distributed.