Paper presented at the 5th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 1-4 July, 2007.
Recent studies on evaporation of carbon dioxide in micro-channels reported that the heat transfer coefficient decreased drastically with increasing quality. To improve the evaporating heat transfer characteristics a grooved multi-channel micro-tube was suggested in this study, and the evaporating heat transfer characteristics have been experimentally investigated. The multi-channel aluminum tube, which is directly heated by the electricity, has 8 channels with a diameter of 0.8mm and a length of 1.1m. Each channel has eight micro-grooves with the width of 0.2mm and depth of 0.1mm. The heat transfer coefficients were measured in the range of heat fluxes from 12 to 18 kW/m2; mass fluxes 400 – 800 kg/m2s; evaporative temperature 5℃; and qualities from zero to superheated state. The measured values were compared with those in the plain multi-channel micro-tubes with the same diameter. The heat transfer coefficient was found to be increased at low qualities (x < 0.4). At high qualities, the sudden drop of the HTC due to the dry-out phenomena was not improved. In the evaporation process of CO2, the grooves applied to microchannels have a noticeable effect only in low quality regions.