Studies of bubble growth and departure from artificial nucleation sites

grantor: University of TorontoA simulation of bubble growth and departure from an artificial nucleation site was developed using a numerical model. The generalized model was applied to both slow growing gas diffusion bubbles, and fast growing nucleate boiling bubbles. Dynamic effects due to the motion of the fluid surrounding the growing bubble as well as the pressure gradient due to gravity caused the bubbles to develop non-spherical shapes throughout the growth period. The profile of a gas bubble adjusts the interface curvature to balance the internal gas pressure with the surrounding fluid pressure. The changing bubble shapes lead to the onset of a necking formation at the bubble base causing detachment from the nucleation site. The numerical results were confirmed by experiments using carbon dioxide bubbles produced by gas diffusion at an artificial cavity on a horizontal plate. The simulated shapes of a growing bubble illustrate the importance of surface tension in the overall process as well as being the controlling factor in determining bubble departure diameters. The necking process during the bubble departure from the surface caused the remaining portion of the bubble to be pushed into the cavity. This caused a significant waiting time during which the bubble grew inside the cavity before reappearing on the surface. The restricted growth inside the cavity affected the boundary layer development surrounding the bubble and consequently the bubble growth on the surface as well. The role of buoyancy as defined by the common force balance type of analysis was examined and found to be inadequate in explaining trends for bubble departure diameters in micro-gravity. Buoyancy was often thought to be the sole driving force that pulls the bubble off the surface and surface tension as the major downward force that keeps the bubble attached to the surface. In micro-gravity, buoyancy is greatly reduced, yet bubbles can still depart from the surface and at a much smaller size than would be predicted using a force balance analysis. The numerical model showed that the surface tension aided departure by causing the neck formation which collapsed leading to the bubble departure from the surface.Ph.D