Flow and pool boiling on porous coated surfaces

Abstract

Industrial demands for more efficient boilers and evaporators as well as economic incentives have encouraged the development of methods to increase boiling heat transfer coefficients, critical heat fluxes, and, where possible, to obtain the highest heat flux by applying the smallest wall superheat. The goal may be to reduce the heat exchanger size or pumping power required for a specified heat duty, and also to prevent excessive temperature, or even system destruction, in systems where heat generation rates are fixed – for instance, in nuclear fuel assemblies or in chemical reactors. Numerous enhanced surfaces have been developed to intensify flow as well as pool boiling heat transfer. Excellent reviews on that topic have been provided in textbooks. Porous coated surfaces, which this article deals with, belong to the efficient categories of enhanced boiling surfaces. This article reviews the selected results of a comprehensive study of flow and pool nucleate boiling on porous coated surfaces completed in our laboratory. Particularly, this article deals with the following: (1) nucleate pool boiling on flat circular plates and horizontal tubes covered with porous coatings. Particularly, the influence of coating thickness and porosity on heat transfer coefficient and burnout heat flux for distilled water at atmospheric pressure was studied; (2) nucleate pool boiling on horizontal tube partially coated with a porous metallic layer. The possibility of porous coating application to local heat transfer enhancement, for instance in order to smooth and alleviate circumferential temperature distribution during nucleate pool boiling on horizontal, electrically heated tube, was examined; (3) nucleate pool boiling from small horizontal tube bundles built of porous coated tubes. The effect of the tube pitch and operating pressure on local, i.e., for a single tube or selected row of tubes, and average, i.e., for the whole tube bundle, heat transfer coefficient for three working fluids (distilled water, methanol, and refrigerant R141b) was studied. The bundle factor and bundle effect were determined. Second, practical application of porous coated tube bundle as an evaporator in prototype two-phase thermosyphon heat exchanger was investigated; (4) flow boiling of pure refrigerants and refrigerant/oil mixture inside porous coated tubes. The average evaporation heat transfer coefficient and simultaneous pressure drop during evaporation of R22, R134a, and R407C and their oil mixture of different concentrations was studied. A correlation equation for heat transfer coefficient calculation during flow boiling of pure refrigerants inside a tube with porous coating is proposed.