BOILING INCIPIENCE AND HEAT TRANSFER ON SMOOTH AND ENHANCED SURFACES

A comprehensive experimental study in nucleate pool boiling of binary mixtures was carried out to investigate the effects of mixture composition on boiling incipient and deactivation superheats and heat transfer coefficients. All experiments were performed at a pressure of 1.01 bar on conventional smooth surfaces and an enhanced surface (High Flux of Union Carbide Corp.). Contact angles were also measured for the same mixtures on the smooth surfaces of brass and copper. The incipience and deactivation of boiling sites on the enhanced surface occurred at much lower wall superheats than on the smooth ones. For the mixture systems investigated, the incipient superheats were observed to be higher than the corresponding deactivation superheats. The classical boiling nucleation criterion was found to be inadequate in predicting the measured incipient superheats. The variation in the measured contact angles was not sufficient to explain the maximum in the incipient superheats observed at intermediate mixture compositions. Several new factors were identified for boiling nucleation in mixtures which could qualitatively explain these maxima in incipient superheats observed for the mixtures.;The boiling heat transfer coefficients obtained on the smooth surfaces showed a deterioration when compared with the values obtained from a simple linear mixing law between the single component values. The enhanced surface heat transfer coefficients for boiling of the same mixtures showed both positive and negative deviations from the linear mixing law between the pure component values. The heat transfer coefficients on the enhanced surface showed appreciable augmentation when compared to those obtained on conventional smooth surfaces under similar conditions. The Schluender correlation was tested against the observed values of heat transfer coefficients and reasonable agreement was observed for only two of the four mixture systems tested on smooth surfaces. Enhanced surface heat transfer coefficients could not be satisfactorily correlated by the Schluender correlation.