STUDY OF INCIPIENCE IN NUCLEATE POOL BOILING

Abstract

An investigation has been carried out both experiment-ally and analytically to determine point of incipience in nu-cleate pool boiling of refrigerant Freon-12 and Freon-22 at different pressures, on electrically heated (uniform heat flux) brass tube, The heating surface was suitable for visual observation. The vapour formed due to heating was condensed in a refrigerant cooled condenser which permits boiling at desired operating con-ditions. The condensed liquid returns back to the cylindrical vessel under subcooled conditions forming a closed circuit. The degree of subcooling of liquid refrigerant in the main cir-cuit was controlled by throttle valve setting of amxilary re-frigerator circuit ( secondary circuit ) connected to the con-defter. The heat flux has been varied from 100 to 7000 Kcal/m2-hr for Freon-12 and 200 to 1020 Kcal/m2-hr for Freon-22 at different pressures ranging from 4.005 to 7.68 kg/cm2 abs. for Freon-12 and 4.53 to 5.93 kg/cm2abs. for Freon-22. The experiment was conducted by gradually increasing the heat flux during heating and decreasing the heat flux during cooling process while keeping the pressure constant. It has been found that the heat flux and wall superheat at the point of incipience in nucleate pool boiling during heating process were different from the cooling process. In other words the point of ebullition and the point of subsidence are different. It was also observed that the predicted point of incipience in nucleate pool boiling (theoretiCal ebullition point) obtained from the intersection of boil-ing curve and natural convection curve, is close to the experimental value of the point at which boiling subsides, rather than the experimental ebullition point. The wall superheat and the heat flux at ebullition are functions of pressure; both decreasing with increase in pressure.