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 arnhorizontal internally-grooved tube. The mean inner diameter of the grooved tube is 2.0 mm with a helixrnangle of 6.3o and an average area enlargement ratio of 2.0. The experiments were conducted at arnsaturation temperature of 15℃, the heat flux ranged from 4.5 to 18 kW/m~2, and the mass flux rangedrnfrom 360 to 720 kg/m~2s. Experimental results indicated that in the pre-dryout region, the effects of heatrnflux and mass flux on the heat transfer coefficient of the grooved tube are much more significant thanrnthose on the heat transfer coefficient of a smooth tube. Under particular conditions, the heat transferrncoefficient at a higher mass flux was lower than that at a lower mass flux, indicating a suppression effectrnon nucleate boiling by convective heat transfer. The heat transfer coefficient of the grooved tube was 2.0rn～ 2.5 times higher than that of the smooth tube due to the enlarged heat transfer surface area of therngrooved tube. In addition, the dryout occurred at a much higher quality ranging from 0.9 ～0.96, asrncompared to that of the smooth tube, the maximum dryout quality may increase by as much as 0.35.rnPressure drops in the grooved tube were found to increase by 1.5 ～ 2.7 times under different conditions.rnThe experimental results show that by using the grooved tube the overall heat transfer performance mayrnincrease significantly.