Numerical Study On Rising Dynamics And Coalescence Of Bubbles

Dynamic characteristics of a bubble as it rises in a viscous liquid have been the focus of many industrial applications.In this dissertation,numerical studies were conducted for the rising of a single bubble and multiple bubbles.After being validated,the two-dimensional and three-dimensional numerical schemes incorporating the computational fluid dynamics(CFD)and the volume of fluid methods(VOF)were used,respectively.In the first study,the investigation of a methane(CH₄)bubble rising in deionized waterwith a pressure of 13 MPa and temperature of 5? was performed.Bubbles with diameters ranging from 3.0–6.0 mm were selected.The results show that the bubble trajectory,instantaneous liquid velocity distribution,and the wake structure manifest unique characteristics under high-pressure and low-temperature conditions.The bubbly wake involves complicated vortex structures.Moreover,the bubble velocity is relatively low relative to those obtained under ambient conditions.A non-dimensional analysis of bubble behaviors indicates that the bubble diameter significantly influences the drag coefficient of the bubble.A comparison of bubble behaviors associated with the bubbles rising in deionized water and seawater shows that the rising characteristics of the bubble in deionized water of 13 MPa and5? were complicated relative to those obtained under ambient conditions.Bubbles with diameters of 4.5 mm and 6.0 mm are accompanied by remarkable coexisting wake vortices.In the second study,three-dimensional unsteady numerical simulations were performed to reveal the rising behaviors of bubbles in a multi-bubble system.The effect of bubble size on rising behaviors and the interaction between bubbles were considered.Bubble sizes were ranged from 4.0 mm to 10.0 mm.Coaxial and triangle bubble configurations were investigated.The numerical scheme was validated through experimental results.Time-dependent bubble geometry,bubble velocity,and near-bubble flow patterns were obtained numerically.The results indicated that liquid flow structures depend on bubble shape and the rising process of the bubble.Participation of the lateral bubble causes a coalescence delay relative to the coaxial configuration.For small bubbles,the first coalescence arises between the trailing bubbles.With increasing bubble size,the first coalescence arises between the leading bubble and the left trailing bubble.The newly formed bubble undergoes an immediate decrease in the rising velocity,and the shape of the newly formed bubble depends significantly on the bubbles prior to coalescence.