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. The region above the dried out portion of the wick continued to work with the large pores between the clusters being primarily occupied with vapor and the small pores between the particles being occupied with the liquid. In this work, we report our efforts to reduce the size of the wall-wick interface dry-out region by sintering a thin layer of uniform size particles on the wall as originally suggested in a thesis by Seminic (2007). 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. The presumed point of nucleation in both wicks is similar, with the heat flux increasing much more rapidly than the liquid superheat and it is clear that this slope is much steeper for the double layer wick. This finding has great potential to expand the performance capabilities of heat pipes and vapor chambers because the new double layered wick can transfer more heat with less superheat thereby increasing the effective thermal conductivity of the wick and decreasing the wall-wick interface temperature for a given heat flux.
A simple and effective model for prediction of effective thermal conductivity of vacuum insulation panels