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.
Highlights
Heat pipes have found their applications in circumstances where dissipating large amounts of heat from relatively small areas is necessary
It may sometimes seem counterintuitive in the sense that if a heat pipe system is aided by a fan, the heat pipes break down and reduce the effectiveness of the thermal management system
To demonstrate and strengthen our proposition, consider for instance, if a certain design requires the dissipation for 50W of heat, the minimum temperature required would be about 70◦C. This is apparent through fig. 1, with 50W heat input, the maximum temperature achieved with a heat transfer coefficient of 15 Wm−2K−1 is 111◦C and with the same heat input and a heat transfer coefficient of 28 Wm−2K−1 the maximum operating temperature is 65◦C
Summary
Heat pipes have found their applications in circumstances where dissipating large amounts of heat from relatively small areas is necessary. They have excessively high effective thermal conductivity and exhibit very low temperature differences along its length (nearly isothermal profile) resulting in high thermal efficiencies. As the heat pipe must exhibit isothermal temperature profiles along its length, the theoretical operating temperature of the heat pipe can be determined by lumped parameter analysis, in addition to using the material properties (specific heat of heat pipe material, surface area exposed to heat transfer, specific heat of wick material and specific heat of the working fluid) and environmental properties (heat transfer coefficient and ambient temperature). This operating temperature is a parameter of design and environmental constraints
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