Abstract
The CO_2 heat pump water heater has a high potential for energy conservation and greenhouse abatement. To encourage the broader use of the heat pump water heater, it is necessary to develop space saving and high efficient equipment. The most important element is always the evaporator. Experimental studies have been conducted to measure the pure CO_2 (99.999% purity, without oil) flow boiling heat transfer coefficient in a horizontal smooth tube (outer diameter 6mm, thickness 0.4mm). In our previous report, the mean heat transfer coefficient had a peak value at a certain mass velocity or a heat flux, but which influence strongly affected the result was not understood. In this report, to distinguish the influence of mass velocity and that of heat flux to the heat transfer coefficient, the tube length is varied to 3.0m, 4.0m and 5.0m. The test conditions were: CO_2 mass velocity from about 150 to about 700kg/(m^2s), saturation temperature of CO_2 are about 0, 5 and 10℃, quality at inlet of test section is fixed to be 0.17 and CO_2 super heat at outlet of test section is fixed to be 5K. As a result, it has been understood that heat flux has a greater influence on the heat transfer coefficient.
