Experimental study in enhanced design of boiling surface used in jet impingement cooling and a complementary theoretical analysis for fully developed nucleate boiling.

摘要

An experimental investigation of nucleate boiling heat transfer from a surface cooled by a circular jet impinging normally to the surface is reported. To enhance the nucleate boiling and maximum heat fluxes, macro and micro modifications were made to the boiling surface. Water and Freon-113 were used as test fluids. With macro/micro-modification, the boiling curve is shifted to the left and upwards. At least two fold enhancement in nucleate boiling heat fluxes and CHF is observed over that for a plane surface. The data also delineate the effect of oxidation of the heated surface on nucleate boiling curve.;The static contact angle is a measure of surface wettability. The contact angle depends on the relative magnitude of solid-liquid adhesive forces and liquid-liquid cohesive forces, which can be represented by a well-known dispersion constant, A. The Girifalco-Good-Fowkes-Young equation is employed to quantify the relation between dispersion constant and contact angle. Water is used as the test fluid. The results show the variation of the dispersion constant with surface wettability and the variation of contact angle with system pressure.;The transport processes occurring in an evaporating two-dimensional vapor stem formed during saturated nucleate boiling on a heated surface are modeled and analyzed numerically. A balance between forces due to curvature of the liquid-vapor interface, disjoining pressure, hydrostatic head and liquid drag determines the shape of the interface. The vapor stem shape resembles a cup with a flat bottom. For a given wall superheat, several metastable states of the vapor stem between a minimum and maximum diameter are found to be possible. The effect of surface wettability and wall superheat on the size and shape of the vapor stem is parametrically analyzed and compared with limited data reported in the literature.;The model results are used to predict the heat flux in fully developed nucleate boiling. The boiling heat flux is obtained by multiplying the heat removal rate from one stem with the number density of stems (nucleation site density). The effect of system pressure on the prediction of fully developed nucleate boiling heat flux is also studied.