The Effect of Gap Distance on the Heat Transfer Between a Finned Surface and a Porous Plate

Abstract

<p> Experiments were performed to investigate the effect that a gap between a heated fin and a porous plate has on the heat transfer performance of a simulated capillary evaporator. The heat transfer performance was examined for two porous plates with average pore radii of 50 and 200 μm respectively. Tests were performed for gap distances between 0 and 900 μm and heat fluxes ranging from 17 to 260 kW/m^2. The heat transfer performance of the simulated capillary evaporator initially increased as the gap distance was increased. However, a further increase in the gap distance caused a decrease in performance. The maximum heat transfer performance occurred at a smaller gap distance for the plate with the smaller pore radius. For small gap distances, persistent high temperature regions were observed on the surface of the heated foil that grew and became more frequent at higher heat fluxes. For larger gap distances, saturated regions that appeared on the foil at moderate heat fluxes suggest that microlayer evaporation may be taking place within the gap. At high heat fluxes, these saturated regions are no longer present, but the temperature of the heated foil remained stable.</p> <p> The heat transfer process in the porous media was examined using thermocouples embedded within the porous plates. These temperature measurements indicate that a two phase region forms within the porous plate for a pore radius of 200 μm. Little evidence of vapour was observed within the plate with a pore radius of 50 μm. In that case, there was more evidence of vapour present at the surface of the porous plate. There was less evidence of vapour at the surface of the porous plate for the larger gap distances, suggesting that the vapour escapes more easily through the gap at larger gap distances.</p>