ENHANCEMENT OF HEAT TRANSFER BY HEAT PIPES WITH PIECEWISE UNIFORM LONGITUDINAL WICK PROFILES

Abstract

An equation (29) is derived to calculate heat transfer rate of a heat pipe evaporator in terms of liquid pressure loss along and temperature difference across the wick and thermo-fluid properties, which shows that various wick profiles transfer same amount of heat under the constraints of pressure loss and temperature difference. It is proved by calculus of variations that among these profiles, wick weight is minimized in case of uniform wick thickness. Case studies are applied for a copper-water heat pipe with a wick of 0.5 porosity, 1.5x10(-9) m(2) permeability, 8.65 mm outer radius and around 1.96 W/mK thermal conductivity. A case study shows that sum of the pressure losses of the liquid and vapor phases of the adiabatic region is minimized at a certain ratio of vapor core radius to wick outer radius. Finally, 1-D coupled flow and thermal analyses of the wick and vapor core of the heat pipe are performed for two types of designs with piecewise uniform wick thickness profiles which are proposed in this study. Under the constraint of constant total wick volume, heat transfer rate is plotted as function of wick thicknesses for each design. Without the wick volume constraint, increasing the adiabatic zone and condenser wick thicknesses while decreasing wick thickness of the evaporator enhances heat transfer rate up to 6.3%. On the other hand, increasing adiabatic zone wick thickness while decreasing that of the evaporator and condenser improves heat transfer rate up to 26.9% at capillary limit.