This study compares the heat transfer characteristics of gravity-assisted heat pipes constructed from micro-fin and smooth tubes. With refrigerant 11 (R-11) as the working fluid, heat transfer rates were measured for a variety of charge levels, evaporator temperatures of 98 deg F, 108 deg F, and 118 deg F (36.7 deg C, 42.2 deg C, and 47.8 deg C), and evaporator-to-condenser inlet water temperature differences ranging from 10.0 deg F to 40.0 deg F (5.5 deg C to 22.2 deg C). 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.

KEYWORDS: heat pipes, air air heat exchangers, comparing, refrigerants, R11, heat flow, evaporators, water temperature, condensers, temperature difference, smooth tube heat exchangers.