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.
Heat at high temperatures, in this work 400-650˝C, can be recovered by use of cooling panels/heat pipes in the walls of aluminum electrolysis cells. For this application a flat vertical heat pipe for heat transfer from a unilateral heat source was analyzed theoretically and in the laboratory, with special emphasis on the performance of the wick. In this heat pipe a wick of compressed nickel foam covered only the evaporator surface, and potassium was used as the working fluid. The magnitudes of key thermal resistances were estimated analytically and compared. Operating temperatures and wick performance limits obtained experimentally were compared to predictions. Thermal deformation due to unilateral heat flux was analyzed by the use of COMSOL Multiphysics ® . The consequences of hot spots at different locations on the wick were analyzed by use of a numerical 2D model. A vertical rectangular wick was shown to be most vulnerable to hot spots at the upper corners.
The use of wood stoves for space heating in energy effective residential buildings can be problematic due to the batch combustion giving a highly transient heat production and the limited regulation of the combustion process. Increasing the heat storage capacity and lowering the maximum heat release from the stove has been proposed to improve the utility of wood stoves. Latent Heat Storage (LHS) solutions will lower and even out the heat release from stoves. However, finding a suitable Phase Change Material (PCM) for a LHS solution can be problematic. In this work an analytical method for ranking PCM candidates for LHS solutions is proposed. The method takes into account PCM properties, in addition to LHS properties that have to be tailored to the selected PCM. The method is validated with numerical models using realistic heat production profiles from wood stoves. The numerical results show significant benefits of using PCMs in LHS solutions over traditional solutions. There exists significant work on PCMs and their properties, but little work on how to select a PCM for a given application. This work contributes to a more efficient selection process, decreasing the work required to select the optimum PCM for a LHS.
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