Mass-Transfer Kinetics in the Presence of Cavitation

Abstract

The current hypotheses concerning the interrelationship of the degassing and cavitation processes are extremely conflicting. Some authors [70, 88] believe that the degassing of a liquid is possible only when cavitation is present, after the formation of vapor-gas cavities, which, increasing in size due to diffusion and coalescence, escape from the liquid. In the opinion of others [72] degassing has nothing to do with cavitation, but is determined by the diffusion of gas into pulsating bubbles already present in the liquid and their subsequent coalescence. If this problem is to be resolved unequivocally, we must compare the kinetics of the degassing process at low enough acoustic intensities that cavitation is clearly absent, as well as in the presence of cavitation. Of decisive interest in this connection is the report in [93] of the existence of an optimum range of intensities for degassing, within which the rate of change of the gas concentration is a maximum. This range is limited at the low-intensity end by the cavitation threshold. The authors ascribe the concurrent onset of cavitation and increase in the mass-transfer rate to the action of two factors: 1) an increase in the number of bubble nuclei due to the formation and detachment of microbubbles from the surface of the extant bubbles by the excitation of large-amplitude surface modes; 2) an increase in the diffusion flow of gas into the bubbles due to the increase in the interface area when the bubbles oscillate in higher modes. No explanation has been offered to account for the decrease in the mass-transfer rate as the acoustic intensity is further increased. It is also important to note that the entire paper [93] has more of a predictive than a descriptive mien and therefore does not supply a direct answer to the question of the role taken by cavitation in the degassing of liquids.