| In this research the dynamics of a non-reactive and a reactive gas bubble under various pressure fields are studied. Comprehensive mathematical models have been developed to simulate the dynamic behavior of different gas bubbles. On the basis of these models several features of thermo-fluid mechanical behavior of gas bubbles are then discussed and their applications for the metals processing operations are described. The mathematical models form a set of coupled, highly nonlinear and stiff differential equations, which have been solved numerically by a modified Gear method.; The first part of this study is concerned with the transient dynamic behavior of a hydrogen gas bubble in a solidifying aluminium-3.4 wt pct copper alloy melt under various ultrasonic pressure fields. During the process of bubble collapse, the melt pressure surrounding the bubble is seen to increase very rapidly. The variations of pressure and supercooling in the melt surrounding the bubble can cause bulk crystallization. If the pressure in the vicinity of the dendrites exceeds a threshold value, dendrite fracturing takes place. Dendrite fragments become nuclei during metal crystallization in an ultrasonic field, which lead to the refined crystalline structure of the metal. The results show that adjacent to the bubble surface, the peak pressure generated in the melt is in the order of several hundreds to thousands atmospheres depending on the initial bubble size, pressure of undisturbed melt and the ultrasonic's specifications.; The second part of this study is related to the dynamics of a stable bubble under various ultrasonic pressure fields. When the imposed pressure field is beyond a threshold value, dissolved gas in the liquid flows into the gas bubbles by rectified diffusion. In this case, the bubbles grow sufficiently to float to the surface due to the hydrodynamic buoyancy force. The results show that the air bubble grows and the time-averaged bubble volume reaches several times of its initial value when the ultrasonic pressure amplitude is more than the threshold pressure.; The final part of this study consists of the complex dynamic behavior of a reactive gas bubble immersed in a nonreactive liquid under various liquid impulse pressures. Numerical investigations are conducted into the collapse and explosion of an isolated oxygen-hydrogen bubble immersed in water and in glycerin. The results show that, if the imposed pressure field is strong enough, the bubble is seen to explode. (Abstract shortened by UMI.)... |
