| Two separate parts form this dissertation. In the first part, the flow around two in-line spherical bubbles is studied numerically and the free rise of bubbles under gravity field is simulated under different terminal velocities. It is found that the trailing bubble always experiences a smaller viscous drag than the leading one. This difference in viscous drag on the two bubbles balances the repelling forces due to the potential part of the flow filed. Therefore, with the approximations used, the bubbles are predicted to reach a constant distance from each other. This conclusion is at variance with experiment, presumably due to the neglect of the bubble deformation.; Axisymmetric inhomogeneous flow past a single bubble is also studied in the large Reynolds number assumption. Unlike the case of uniform flow, the result shows that the viscous force on a bubble is not proportional to the relative velocity in general.; In the second part, theoretical studies of accidental ignitions of liquid gun propellants are carried out with emphasis on compression heating of an entrained gas bubble in the liquid and viscous heating due to extremely large rate of shearing. The compression of an entrained gas bubble heats up the liquid around the bubble. Since most of the stagnant bubbles in a liquid are found near a wall, the problem is studied by looking at the axisymmetric collapsing of a gas bubble near a solid wall under a sudden pressurization of the liquid. The temperature and velocity field in the bubble are calculated to give an estimate of the instantaneous heat transfer rate across the bubble wall, and of the corresponding temperature rise in the liquid. High temperature rises can be observed for high ratio of pressure change, but the high temperatures only last for a very short time, which is considered not enough for the ignition induction process to be developed.; For the viscous heating aspect of the problem, weight impact experiments on a drop of propellant are simulated numerically based on a parabolic Navier Stokes equation with conjugate heat transfer. The momentum and energy equations are coupled through the temperature-dependent viscosity and are calculated simultaneously. Although it is found that, in the framework of the model, high temperatures can be generated when the liquid film is very thin (a few {dollar}mu{dollar}m), this conclusion is unlikely to be significant in practice due to slight misalignment of the plates, surface imperfections and, most of all, local deformation due to the extremely high normal stresses that arise. It is thus concluded that ignition is unlikely to be caused by the viscous heating mechanism studied in this work. |
