FLOW BOILING HEAT TRANSFER OF CARBON DIOXIDE INSIDE ASMALL SIZE INTERNAL GROOVED TUBE

摘要

In this study, we investigate the flow boiling heat transfer coefficients of carbon dioxide inside a horizontal internally-grooved tube. The mean inner diameter of the grooved tube is 2.0 mm with a helix angle of 6.3o and an average area enlargement ratio of 2.0. The experiments were conducted at a saturation temperature of 15°C, the heat flux ranged from 4.5 to 18 kW/m~2, and the mass flux ranged from 360 to 720 kg/m~2s. Experimental results indicated that in the pre-dryout region, the effects of heat flux and mass flux on the heat transfer coefficient of the grooved tube are much more significant than those on the heat transfer coefficient of a smooth tube. Under particular conditions, the heat transfer coefficient at a higher mass flux was lower than that at a lower mass flux, indicating a suppression effect on nucleate boiling by convective heat transfer. The heat transfer coefficient of the grooved tube was 2.0 ～ 2.5 times higher than that of the smooth tube due to the enlarged heat transfer surface area of the grooved tube. In addition, the dryout occurred at a much higher quality ranging from 0.9 ～0.96, as compared to that of the smooth tube, the maximum dryout quality may increase by as much as 0.35. Pressure drops in the grooved tube were found to increase by 1.5 ～ 2.7 times under different conditions. The experimental results show that by using the grooved tube the overall heat transfer performance may increase significantly.