1 Surface tension

Surface tension is the force that acts on the interface between a liquid and a gas, or between two immiscible liquids. It tends to minimize the surface area of the interface, which affects the shape and stability of bubbles. Surface tension also determines the balance between the pressure inside and outside the bubbles, which influences their growth and collapse. Moreover, surface tension affects the coalescence and breakup of bubbles, as it creates a resistance to the deformation and merging of the interfaces. CFD models need to specify the surface tension coefficient and the appropriate boundary conditions for the interface.

2 Fluid properties

Fluid properties, such as density, viscosity, and temperature, also affect the dynamics of bubbles in multiphase flows. Density and viscosity determine the buoyancy and drag forces that act on the bubbles, which influence their rise velocity and trajectory. Temperature affects the vapor pressure and solubility of the gas in the liquid, which influence the mass transfer and phase change processes that occur at the interface. Furthermore, fluid properties affect the turbulence and mixing of the fluid phases, which impact the distribution and size of the bubbles. CFD models need to account for the variation of fluid properties with temperature, pressure, and composition.

3 Flow regime

Flow regime is the pattern of flow that emerges from the interaction between the fluid phases and the geometry of the system. Flow regime affects the bubble dynamics in multiphase flows by determining the relative motion and contact between the bubbles and the fluid. For example, in bubbly flow, the bubbles are dispersed and move with the fluid; in slug flow, the bubbles form large elongated shapes that occupy most of the cross-section; and in annular flow, the gas forms a thin film along the wall and the liquid forms droplets or waves. Flow regime also affects the coalescence and breakup of bubbles, as different regimes have different levels of shear stress and turbulence. CFD models need to identify the flow regime and use appropriate closure models for the interfacial forces and mass transfer.

4 Geometry and boundary conditions

Geometry and boundary conditions are the external factors that define the shape and size of the system and the inlet and outlet conditions of the fluid phases. Geometry and boundary conditions affect the bubble dynamics in multiphase flows by influencing the flow field and the distribution of the bubbles. For example, the geometry of the system can create regions of high or low pressure, velocity, or curvature, which can affect the growth, collapse, or coalescence of bubbles. The boundary conditions of the system can specify the flow rate, pressure, temperature, or concentration of the fluid phases, which can affect the mass transfer, phase change, or turbulence of the flow. CFD models need to match the geometry and boundary conditions of the real system or the experimental setup.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?