Abstract
New alternative refrigerants have been developed due to the ozone layer depletion and global warming. For this reason, carbon dioxide is believed to be a promising refrigerant for use in air conditioners and heat pumps. Evaporative heat transfer characteristics and pressure drop of CO₂ with outer diameter of 5 ㎜ in inclined (45°) smooth and micro-fin tubes have been investigated by the experiments with respect to several test conditions such as mass fluxes, heat fluxes, evaporation temperatures in this study. The inclined (45°) smooth and micro-fin tubes with length of 1.44 m were installed to measure the evaporative heat transfer coefficients of CO₂ and heat was supplied to the refrigerant by direct heating method where the test tube was uniformly heated by electricity. The tests were conducted at mass fluxes from 212 to 656 ㎏/㎡s, heat fluxes from 15 to 60 ㎾/㎡ and evaporation temperatures from -10 to 20℃. The heat transfer coefficients of CO₂ are slightly increased with increasing mass flux, and the heat transfer characteristics in the inclined (45°) tubes are enhanced about 5~10% compared with those in horizontal or vertical tubes.
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