Subcooled Pool Film Boiling Heat Transfer From Spheres With Superhydrophobic Surfaces: An Experimental Study

Pool film boiling was studied by visualized quenching experiments on stainless steel spheres in water at the atmospheric pressure. The surfaces of the spheres were coated to be superhydrophobic (SHB), having a static contact angle greater than 160 deg. Subcooled conditions were concerned parametrically with the subcooling degree being varied from 0 degrees C (saturated) to 70 degrees C. It was shown that film boiling is the overwhelming mode of heat transfer during the entire course of quenching as a result of the retention of stable vapor film surrounding the SHB spheres, even at very low wall superheat that normally corresponds to nucleate boiling. Pool boiling heat transfer is enhanced with increasing the subcooling degree, in agreement with the thinning trend of the vapor film thickness. The heat flux enhancement was found to be up to fivefold for the subcooling degree of 70 degrees C in comparison to the saturated case, at the wall superheat of 200 degrees C. A modified correlation in the ratio form was proposed to predict pool film boiling heat transfer from spheres as a function of the subcooling degree.