Experiments with Gravity-Assisted Heat Pipes with and without Circumferential Grooves

Abstract

The results of a series of experiments on the performance of gravity-assis ted heat pipes with and without circumferential grooves are reported. Heat-transfer rates and temperature gradients were measured, so that heat pipe conductance and evaporator and condenser heat-transfer coefficients were determined. Such actual performance data is useful to determine the improved performance of grooved heat pipes compared to wickless heat pipes or thermosyphons. Such performance data is important since the added cost of manufacturing the grooves is a significant part of the cost of the heat pipe, especially for long pipes. For a 91.4 cm long by 1.27 cm o.d. heat pfpe with 40 grooves per cm, the maximum thermal conductance measured was 22.7 W/°C, and the heat transfer coefficients in the evaporator and condenser were 4700 and 11,992 W/m2°C. The same size and type of heat pipe without grooves had a thermal conductance of 4.2 W/°C and evaporator and condenser heat-transfer coefficients of 1000 and 1500 W/m2 °C. Experiments were also conducted on a 6.4 m long by 3.18 cm o.d. ironwater heat pipe without grooves and on a U-shaped copper water heat pipe with grooves. H Nomenclature = evaporator heat-transfer coefficient, W/m2°C = condenser heat-transfer coefficient, W/m2°C = height of condenser elevation above evaporator, cm K=Q/AT ^thermal conductance, W/°C Q = heat-transfer rate, W R = AT/Q = thermal resistance, °C/W T = temperature, °C AT = temperature difference, °C x — length, cm a = angle of incline from horizontal, deg