Dynamic Droplet Model
Many engineering flows involve a mixture of two fluids with different densities, such as air and water or oil and water, where one component is dispersed as droplets or bubbles within the other. In these flows, the size of the dispersed fluid elements is not fixed; droplets break up under shear forces and coalesce when they collide. The evolving size distribution affects how quickly the droplets rise or settle, how they interact with turbulence, and how the mixture as a whole behaves. FLOW-3D and FLOW-3D HYDRO include a Dynamic Droplet Model that tracks the evolution of dispersed droplet or bubble size throughout the simulation, providing a more physically realistic representation of two-fluid mixtures.
Oil droplets coalesce into larger droplets downstream of the energy dissipation plate, increasing buoyancy and allowing the oil to rise over the weir into the collection pipe.
How It Works
The Dynamic Droplet Model is an extension of the Drift Flux Model, which handles the relative motion between the dispersed and continuous phases due to buoyancy and viscous forces. This model accounts for the effect of breakup and coalesce on the evolution of the droplet or bubble size. The balance between these two competing mechanisms determines the local equilibrium droplet diameter at each point in the flow, which varies spatially and temporally as flow conditions change.
Because droplet size directly determines the drag force and therefore the drift velocity of the dispersed phase, a more accurate size prediction leads to more accurate predictions of phase separation, bubble rise rates, and the distribution of the dispersed phase throughout the fluid.
Applications
The Dynamic Droplet Model extends the two-component flow capabilities of FLOW-3D and FLOW-3D HYDRO to problems where droplet or bubble size evolution matters:
- Spillway and chute aeration for predicting the spatial distribution and size of entrained air bubbles in high-velocity flows over hydraulic structures
- Oil-water separation for modeling the breakup and coalescence of oil droplets in water as they move through separators, pipes, and process vessels
- Bubble column reactors for predicting bubble size distribution and its effect on mass transfer and mixing performance in chemical and biological reactors
- Liquid-liquid extraction for simulating the dispersal and coalescence of droplets in extraction columns and mixing-settling devices
- Emulsification and mixing for evaluating the effect of impeller geometry and operating speed on droplet size distribution in industrial mixing processes
- Aerated flows in hydraulic structures for assessing bubble size distributions in stilling basins, drop structures, and plunge pools where aeration affects energy dissipation and dissolved oxygen transfer
A bottle filling example uses the dynamic droplet model for highly viscous flows. These results compare three simulations that use different values of the critical capillary number, which defines the necessary shear stress for droplet breakup.