Removal of high heat flux from solid surface by contact with water-containing porous plate

Abstract

When a plate made of a porous material containing liquid is brought into contact with a solid body subjected to a high heat load, the body surface will be cooled by evaporation. The resulting vapor escapes from the side of the solid body through the clearance between the contacting surfaces due to surface unevenness. If the opposite surface of the porous plate is immersed in bulk liquid during contact with the solid body, the wetting of and evaporation on the solid surface will be maintained by liquid flow toward the heated surface. This occurs via the drawing-in of liquid from the opposite surface induced by a capillary force, enhanced by the decrease in the liquid content of the porous plate. However, increasing the heat flux generates a dry region in the porous material close to the heated surface, which significantly deteriorates its performance as a heat sink due to the increase in thermal resistance. The present study assessed two-phase flow in a porous plate containing water in contact with a solid surface under a high heat load, based on a one-dimensional model, and derived an expression for the heat flux at the onset of drying. In addition, the drying-onset heat flux was determined experimentally and was found to be an order of magnitude higher than the critical heat flux for saturated pool boiling of water. The onset heat flux markedly increased with decreasing thickness of the porous plate and with increasing permeability of the plate, in keeping with predictions based on a theoretical model.