Abstract
The fabrication and performance of wicks for flat heat pipe applications produced by sintering a filamentary nickel powder has been investigated. Tape casting was used as an intermediate step in the wick production process. Thermogravimetric analysis was used to study the burn-off of the organic binder used and to study the oxidation and reduction processes of the nickel. The wicks produced were flat, rectangular and intended for liquid transport in the upwards vertical direction. Rate-of-rise experiments using heptane were used to test the flow characteristics of the wicks. The wick porosities were measured using isopropanol. The heat transfer limitation constituted by the vapour static pressure and the capillary pressure was discussed. The influence on wick performance by using pore former in the manufacturing was studied. When Pcap/Psat > 1, the use of a pore former to increase the wick permeability will always improve the wick performance. When Pcap/Psat < 1, it was shown that if the effective pore radius and the permeability increase with an equal percentage the overall influence on the wick capacity is negative. A criterion for a successful pore former introduction is proposed and the concept of a pore former evaluation plot is presented.
Highlights
The concept of heat pipes was first described in 1942, but the real development of heat pipes started in the 1960s [1]
The driving force in the condensate transport process is the capillary pressure created by the wick and working fluid interaction, in some cases the capillary pressure is supported by the gravity force
The capillary pressure inside a heat pipe can never exceed the vapour static pressure of the working fluid. This will limit the heat transfer capacity and the useful operating temperature range for heat pipes with wicks with small effective pore radii operated with working fluids with low vapour static pressures, for instance alkali metals
Summary
The concept of heat pipes was first described in 1942, but the real development of heat pipes started in the 1960s [1]. Designed heat pipes can transport heat at very high rates with a small temperature difference between the cold and the warm end. This is possible because the heat transport inside the heat pipe is based on evaporation, transport and condensation of a suitable working fluid, and thereby obtaining an apparent thermal conductivity several hundred times that of solid materials such as copper [2]. The heat pipe interior contains a wick, in which the condensate returns from the condenser section to the evaporator section. The gravity force contributes to the transport process when the condenser section is located above the evaporator section, i.e., the heat pipe is gravity-assisted [3]
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