A Theoretical and Experimental Investigation of the Effect of Surface and Flow Conditions on Nucleate and Transition Boiling

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An Analytical and experimental study of fully developed nucleate and transition boiling under pool and forced flow conditions on a vertical surface is proposed. A three region model of boiling forms the basis of the theoretical effort. An analysis will be carried out for each of the three regions, namely, the wall region, the intermediate region, and the vapor flow dynamics dominated region. Experiments will be conducted on surfaces having different degrees of wettability and roughness. The size distribution of cavities on the surface will be determined with an electron microscope and generalized Weierstrass type of functions will be developed for the surface. The experiments will be conducted with saturated water. In the flow boiling experiments, the void fraction will be varied from 0 to 20 percent. The experimental work will be complementary to the analytical effort. When completed, the study will allow a nearly theoretical prediction of fully developed nucleate and transition boiling heat fluxes as a function of wall superheat. The input to the model will be the surface wettability, the cavity size distribution function, the fluid thermophysical properties, and geometrical and flow conditions. The study will pave the way for carrying out a standard problem. It will also guide the development of two-tiered surfaces for passive enhancement of heat fluxes over the complete range of nucleate boiling.