A study of hydrogen bubble growth during ultrasonic degassing of Al–Cu alloy melts

Abstract

A comprehensive mathematical model, combined with an aqueous physical model, has been developed to simulate the growth characteristics of a single hydrogen gas bubble in an aluminium–3.4 wt.% copper alloy melt by the process of rectified diffusion. The mathematical model is based on the coupled momentum, energy and mass transport equations. The model forms a set of coupled, highly non-linear and stiff differential equations and it is characterized by the fast moving boundary of the bubble. The model equations have been solved by the modified Gear method. The effects of initial bubble radius, initial concentration of dissolved gas in liquid, hydrostatic pressure and the amplitude and frequency of the imposed ultrasound field on the process of rectified diffusion are numerically studied. The results show that a hydrogen bubble in an aluminium–3.4 wt.% copper alloy melt grows when the ultrasonic pressure amplitude exceeds the threshold pressure. In this case, the bubble volume rapidly reaches several times its initial value and the gas bubble may float to the surface due to the buoyancy force. An aqueous experimental work has been carried out to simulate the growth of an air bubble in water under an ultrasonic pressure field. The experimental results for bubble growth are compared with the results of the mathematical model for air–water system. It showed a reasonable agreement between the experiments and the predictions.