Heat Transfer from a High-Temperature Solid to a Water-Containing Porous Plate Separated by a Narrow Gap

摘要

Heat transfer from a high-temperature solid surface to a porous plate,which is separated by a narrow gap and contains water,and the resulting evaporation rate of water from the porous plate are investigated experimentally.The vapor flow induced in the clearance establishes a pressure field,which makes a high-temperature solid hover without contacting the plate,as previously reported by the authors(Kojima and Okuyama,2004).This hovering phenomenon is expected to be applied to high-temperature material processes such as the fabrication of flat glass.In the present report,the experiment is conducted under fixed clearance conditions.The heat transfer and evaporation rates increase with the increase in the solid surface temperature up to 1273 K and with the decrease in the clearance from 1.0 to 0.25 mm,while the temperature near the upper surface in the plate remains close to the saturation temperature throughout the parameter ranges.These findings imply that the water is sufficiently distributed over the entire thickness of the plate.The resulting heat flux to the porous plate is shown to be much smaller than the maximum heat flux,which is estimated as the limit of the evaporation at the capillary meniscus in pores adjacent to the heated surface.The transferred heat is mainly consumed as the latent and sensible heats of the vapor and as the sensible heat of the liquid taken up for evaporation from the lower surface of the porous plate.Even after the cessation of water supply from the lower surface,the upper surface temperature of the porous plate and the evaporation rate remain close to the values before cessation,until the critical moisture content of the material is reached.