Characterization of Vapor Escape Restriction in Biporous Wicks With Monolayers for Thermal Ground Plane Optimization

Abstract

Developing better heat pipes requires advancement of technology in all aspects of construction. In this paper I am investigating the effect of vapor pathways on the performance of biporous wicks in heat pipes. Biporous evaporator wicks, generated by sintering copper particles into semi-uniform clusters, were demonstrated to achieve high flux, heat transfer performance for use in heat pipes by Semenic (2007). The effective thermal conductivity of thick biporous wicks at high heat fluxes was found to be reduced because the region next to the wall dried out prematurely, allowing the wall interface temperature to rise well above the saturation temperature. One possible way to reduce the size of the wall-wick interface dry-out region is to sinter a thin layer of uniform size particles on the wall as suggested by Seminic. The boiling curve for this “double layer” wick diverges from a standard “single layer” biporous wick at the point of nucleation by reducing the wall temperature, and concurrently the overall temperature drop across the wick needed to drive a given heat flux. The temperature drop across the wick is reduced because the thin layer of particles between the biporous wick and the wall reduces the wall-wick interface resistance and also provides additional capillary channels underneath the biporous wick. Experimental data supports this hypothesis by showing a clear divergence between measured wall temperatures for the double layer wick from its single layer counterpart with an indication that smaller cluster sizes in the biporous wicks perform better at lowering the superheat required to obtain high fluxes. In this work, we are looking to compare the performance of these wicks to similarly sized blocks of copper in order to investigate the performance increase offered by the wicks. In order to investigate this phenomenon we ran experiments in a similar manner to previous experiments done by Reilly (2009), but a plate was inserted into the chamber above the wick to restrict the vapor flow. To determine the behavior in the copper we ran several simulations in COMSOL (a finite element software used for doing conduction analysis) of copper disks at different representative thicknesses. We ran experiments with the plate at several heights above the wick, going so far as to place the plate flush with the upper surface of the wick to force vapor back through the wick laterally. By comparing the results between these two sets of experiments we were able to deduce that even in the case where there was no open space above the wick for vapor to escape, we were still able to double the performance with respect to a system of solid copper.