In Japan, a carbon dioxide (CO_2) heat pump water heater for residential use was developed for sale in 2001 to conserve energy and help prevent global warming. In order to develop a more efficient and more compact CO_2 heat pump, it is necessary to develop a compact and highly efficient evaporator, and important to understand the CO_2 flow boiling heat transfer coefficient. An experimental study was conducted here to measure the pure CO_2 (99.999% purity, without oil) heat transfer coefficient in a horizontal copper tube (outer diameter : 6mm, thickness : 0.4mm, length : 4m). The test conditions were : CO_2 mass flux from about 200 to about 700kg/(m~2s) (heat flux from about 10 to about 40kW/m~2), quality at inlet of test section 0.17, CO_2 super heat at outlet of test section 5K, and saturation temperature from 0 to 10℃. The measured heat transfer coefficient was compared with calculated values by means of several correlations.
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