Critical heat flux enhancement by a two-layer structured honeycomb porous plate in a saturated pool boiling of water

摘要

In the present paper, we propose a novel critical heat flux (CHF) enhancement technique using a two-layer structured honeycomb porous plate (HPP) that can be applied in principle regardless of heater orientation. In a previous study, the CHF during saturated pool boiling of water was investigated experimentally using an HPP attached to a heated surface and was shown to be enhanced to more than twice (2.0 MW/m~2) that for a plain surface. According to the proposed capillary limit model, the CHF can be increased by decreasing the thickness of the HPP because of the decrease in the frictional pressure drops caused by the liquid flow in the porous medium. However, the CHF could not be greatly enhanced when the thickness of the HPP was comparable to the thickness (approximately 100 μm) of the thin liquid film (the macro-layer thickness) formed beneath coalescent vapor bubbles. Based on the observation of the boiling configuration near the CHF, a large coalesced bubble forms on the heated surface and departs periodically. Therefore, the CHF may occur when water contained in a porous material disappears due to evaporation during the bubble hovering period. In order to prevent the dry-out phenomenon during the hovering period of a large coalesced bubble and enhance the CHF, we herein propose that the structure of HPPs should be improved by the superposition of two kinds of HPPs and that each of the HPPs must satisfy two conditions. First, an HPP simply attached to a heated surface should have very fine pores to supply water to the heated surface due to strong capillary action, and the HPPs should be as thin as possible in order to decrease the pressure drop caused by internal water flow. Second, the other HPP, which is stacked on top of the thin HPP, must be structured to hold a sufficient amount of water in order to prevent the inside of the HPP from drying out during the bubble hovering period over the plate. Moreover, for further CHF enhancement, we found that the gap between the HPP and the heated surface caused by the surface roughness is important because vapor escapes through this gap so that liquid is easily supplied to the heat transfer surface.