On the operating temperature of heat pipes

Abstract The relationship between the operating temperature of heat pipes and the maximum heat transport capacity posed by the heat pipe capillary limit is often overlooked. It is demonstrated through heat pipe experiments that for a given heat input, there exists a minimum temperature for the heat pipe system to operate. This phenomenon occurs due to the temperature dependence of the thermo-physical properties of the working fluid in the heat pipes and the working temperature range of the heat pipe system can thus be discerned by the capillary limit equation in conjunction with the heat pipe transient equation obtained by energy conservation. It may sometimes seem counterintuitive in the sense that if a heat pipe system is aided by a fan (and therefore increase the heat transfer coefficient), then the heat pipes break down and reduce the effectiveness of the thermal management system. This is due to the fact that heat pipes have excessively high effective thermal conductivity and their breakdown leads to heat transfer only through their constituent materials, whose thermal conductivities are lesser by at least an order of magnitude. Heat pipes in a thermal management system must therefore be meticulously designed for precise temperature ranges.