Film condensation heat transfer of low integral-fin tube.
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For condensation on horizontal low-finned tubes, the dependence of heat-transfer performance on fin spacing has been investigated experimentally for condensation of refrigerant 113 and ethylene glycol. Fourteen tubes have been used with inside diameter 9.78 mm and working length exposed to vapour 102 mm. The tube had rectangular section fins having the same width and height (0.5 mm and 1.59 mm) and with the spacing between fins varying from 0.25 mm to 20 mm. The diameter of the tube at the fin root was 12.7 mm. Tests were also made using a plain tube having the same inside diameter and an outside diameter equal to that at the root of the fins for the finned tubes. All tests were made at near atmospheric pressure with vapour flowing vertically downward with velocities of 0.24 m/s and 0.36 m/s for refrigerant 113 and ethylene glycol respectively. Optimum fin spacings were found at 0.5 mm and 1.0 mm for refrigerant 113 and ethylene glycol respectively. In earlier experiments for steam using the same tubes, the optimum fin spacing was found to be 1.5 mm. Maximum enhancement ratios of vapour-side heat-transfer coefficient (vapour-side coefficient for a finned tube / vapour-side coefficient for a plain tube. for the same vapour-side temperature difference) were 7.5, 5.2 and 3.0 for refrigerant 113, ethylene glycol and steam respectively. Enhancement phenomena have also been studied theoretically. Consideration has been given to a role of surface tension forces on the motion and configuration of condensate film. On the basis of this study, several semi-empirical equations, to predict heat-transfer performance, have been obtained. These are considered to represent recent reliable data (present and other recent works) satisfactorily.
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