Gravity-assisted heat pipes using micro-fin tubes for applications in air-to-air heat exchangers
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This study compares the heat transfer characteristics of gravity-assisted heat pipes constructed from micro-fin and smooth tubes. The vertical tubes had a 0.5 in. (12.7 mm) nominal outside diameter with a 35.5-in. (901.7-mm) overall length. Spiraling down the inside circumference of the micro-fin tube at an angle of 25 were 70 micro-fins, each with a depth of 0.0069 in. (0.175 mm). Energy was added at the bottom and extracted at the top of the gravity-assisted heat pipe by water flowing in surrounding annuli. The evaporator, adiabatic, and condenser sections of the heat pipe were 47.9%, 5.6%, and 46.5% of the total length, respectively. With refrigerant 11 (R-11) as the working fluid, heat transfer rates were measured for a variety of charge levels, evaporator temperatures of 98F, 108F, and 118F (36.7C, 42.2C, and 47.8C), and evaporator-to-condenser inlet water temperature differences ranging from 10.0F to 40.0F (5.5C to 22.2C). The heat transfer rate in the smooth tube gravity-assisted heat pipe was found to be independent of evaporator temperature and R-11 charge for the levels ranging from 44% to 102% of the evaporator section volume. The heat transfer rate in the micro-fin tube increased for larger evaporator temperatures and also varied with charge level. The heat transfer rate in the micro-fin tube was a maximum at a charge level of 29% of the evaporator volume. The inside heat transfer coefficient was found to be roughly five times greater in the evaporator and condenser of the micro-fin tube, depending on the charge level.
