Experimental Study of the Influence of the Inside Surface Characteristics on the Carbon Dioxide Flow Boiling Heat Transfer Cofficient in a Horizontal Tube

摘要

The CO2 heat pump water heater "ECO CUTE", which was commercialized in 2001, has a significant potential for energy conservation and greenhouse abatement. Shipments in fiscal 2007 expanded to 413,000 units. In order to encourage the broader use of "ECO CUTE", there is a need to develop a more compact and higher efficiency heat pump unit. The key element of the apparatus is always the evaporator. CRIEPI and Sumitomo Light Metal, Ltd. have studied CO_2 flow boiling heat transfer coefficient experimentally. In the previous report, mean heat flux was understood as being equivalent to mass velocity in significantly affecting the mean evaporating heat transfer coefficient (99.999% purity, without oil) in a horizontal smooth tube (outer diameter 6 mm, thickness 0.4 mm). In this report, the heat transfer coefficient and pressure drop of a smooth tube and 3 kinds of grooved tubes are measured. The test conditions are : CO_2 mass velocity from about 160 to about 660 kg/(m~2s)(heat flux from about 10 to about 40 kW/m2), pressures of CO_2 3.5MPa, 4.0MPa and 4.5MPa (saturation temperatures of 0, 5 and 10℃, respectively), tube lengths of 3.0m and 4.0m, quality at the inlet of the test section of 0.17 and CO_2 super heat at the outlet of test section of 5 K. Consequently, it is understood that the heat transfer coefficient of grooved tubes is higher than that of the smooth tube and in the higher mass velocity region, the heat transfer coefficient does not go below 400kg/m~2s. The difference in the heat transfer coefficient caused by the groove features is not clear.