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Multiphase Flow Modeling in COMSOL
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Contents
Categorization
The multiphase flow interfaces
Dispersed multiphase flow models
Separated multiphase flow models
Multiphase flow and multiphysics
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Dispersed multiphase flow models
For bubbly flows with a large number of relatively small bubbles
For emulsions and aerosols (droplets in gas)
For large numbers of solid particles in fluids
For macroscopic multiphase flow (almost always required)
Separated multiphase flow models
For bubbles, droplets, phase boundaries, or particles that are relatively few and of the order of magnitude of the model domain
For multiphase flow in microfluidics
For free surfaces in otherwise single-phase fluids that are also in macroscopic systems
Categorization
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Dispersed multiphase flow models Separated multiphase flow models
Categorization
Volume fraction field, 0 < f < 1
Phase field, ff = 1 (red)f = -1 (blue)
Interfacedescribedin detail
Continuous phase
Dispersedphase
Isosurfaces
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Dispersed multiphase flow
Separated flow multiphase flow
Laminar and turbulent multiphase flows except for:
Two-phase flow with moving mesh
Three-phase flow
Only available as predefined for laminar flow but can be manually changed for turbulent flows
The Multiphase Flow Interfaces
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Dispersed Multiphase Flow
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Process industry
Chemicals, pharmaceuticals
Food and household supplies
Environmental sciences: Hydrogen technologies
Cooling systems and refrigeration
And more…
Defense and space
Gas-liquid systems for fuel, hydrogen-oxygen
Advanced cooling systems
And more…
Applications of Dispersed Multiphase Flow
Hydrogen/water separator modelVera et al, COMSOL Conference, Milano 2012
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Bubbly flow model:
Defines one dispersed phase, the gas phase, and one continuous phase, the liquid phase
The relative bubble velocity is described with an equation that balances the drag and pressure gradient
The momentum balance is defined for the continuous phase, while the pressure field is the same in the two phases
Relatively “inexpensive” two-phase flow model
Dispersed Multiphase Flow Models: Bubbly Flow
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Mixture multiphase flow model:
Defines one dispersed phase and one continuous phase
The relative velocity of the dispersed phase is described with an equation that balances the drag and pressure gradient
The momentum balance is defined for the mixture, while the pressure field is the same in the two phases
Relatively “inexpensive” two-phase flow model that can also handle phases with small differences in density
Dispersed Multiphase Flow Models: Mixture Model
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Euler-Euler multiphase flow model:
Defines dispersed and continuous phases
Separate momentum balances are defined for the different phases
Accounts for possible acceleration of the dispersed phase, while the mixture and bubbly flow models assume terminal velocity for bubbles, droplets, or particles
Accurate but relatively “expensive” multiphase flow model, which can be used for all cases of dispersed multiphase flow
Dispersed Multiphase Flow Models: Euler-Euler
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By nature, dispersed models are approximate in relation to the scale of the interface between phases
There is no additional loss of accuracy in combination with turbulence models
Turbulence models introduce a drift diffusion term in the dispersed phase, in addition to the eddy diffusivity contribution to viscosity
Dispersed Multiphase Flow Models and Turbulence
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Strengths
A large combination of possible descriptions is available for laminar and turbulent flow
Adaptable: Dispersed flow models may contain almost arbitrary interactions within and between phases, which are relatively easy to define in COMSOL Multiphysics
Relatively easy to tweak and define models
Can be combined with other physics phenomena
Weakness
The model equations are “nasty” and require good initial guesses and initial conditions to converge
Dispersed Multiphase Flow in COMSOL
Turbulent bubbly flow in a photo bioreactor with different separator plate configurations. Ramirez et al, COMSOL Conference, Curitiba 2014
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Separated Multiphase Flow
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Microfluidics in different fields and industries
Biotech and medical technology
Material science, such as electronics and semiconductors
Chemicals
Space and defense
And more
Larger scale flows to track interfaces between phases and free surfaces
Polymers: for extrusion and mold filling
Chemicals: mixers and reactors
Applications of Separated Multiphase Flow
NanoSat fuel delivery system. Influence of surface tensionMcDeWitt et al, COMSOL Conference, Boston 2010
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Two-phase flow with moving mesh:
A workhorse in microfluidics and free surface computation in rotating machinery
The interfaces between different phases are domain boundaries, where surface tension effects may be defined as boundary conditions
Very accurate for separated multiphase flows when topology changes do not occur
Separated Multiphase Flow Models: Moving Mesh
Topology changewhen this snaps
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Two-phase flow with level set:
The interface between different phases is represented by an isosurface of the level set function
Surface tension effects are added as sources and sinks over the thin volume around the isosurface that represents the interface between phases, i.e., as body forces
Allows for topology changes but requires a relatively dense mesh at the interface between phases
Separated Multiphase Flow Models: Level Set
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Two-phase flow with phase field:
The interface between phases is represented by an isosurface of the phase field function
The method contains a more accurate representation of the surface dynamics compared to the level set method
Surface tension effects are added as sources and sinks over the thin volume around the isosurface that represents the interface between phases, i.e., as body forces
Allows for topology changes but requires a relatively dense mesh at the interface between phases
Separated Multiphase Flow Models: Phase Field
Moving mesh
Phase field
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The main purpose is to describe the approximate position of the interface between phases on a relatively large length scale compared to that of surface tension effects
Accuracy in the description of the interface between phases is lost when turbulence models are combined with separated multiphase flow models: Surface tension effects cannot be properly described
Separated Multiphase Flow Models and Turbulence
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Strengths
Accurate: Effects of surface tension and contact angles are taken into account in a strict way
Can be combined with other physics phenomena; for example, Marangoni effects
Relatively easy-to-use
Weakness
Computationally, relatively expensive compared to the standard volume of fluids (VOF) method
Separated Multiphase Flow in COMSOL
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Separated multiphase flow:
Two-phase flow
Three-phase flow
Dispersed multiphase flow:
Ready-made two-phase flow interfaces
Relatively easy to extend to three-phase flow
Combination of dispersed and separated flows can also describe three-phase flow
Applicable for macroscopic models
Particle tracing can also be used for descriptions of solid particles and droplets in fluids and can be combined with multiphase flow models to describe three-phase flow
Applicable for both microfluidics and macroscopic flows
Sometimes referred to as Euler-Lagrange models
Possible Multiphase Flow Combinations
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Multiphase Flow and Multiphysics
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Taylor cone
Voltage applied between anode and cathode
Electrolyte with anions and cations
Thin layer of separated charges at the air-liquid interface subjected to forces from the electric field
The thin film with a net charge is displaced by the electric field
The thin film displaces the whole liquid with the help of surface tension and viscous forces
Multiphase Flow and Electrokinetic Flow
++
+
--
Chargeseparation
Cathode
AnodeLiquid
Air
Outletorifice
Inlet
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Laminar flow in both air and water
Phase field method for tracking the air-water interface
Structural mechanics for the deformation of the solid whisker
Moving mesh keeps track of the deformation of the fluid domain due to structural deformation of the whisker
Two-Phase Flow and Fluid-Structure Interaction (FSI)
Water
Air
Whisker
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Deforming domain only in the subdomain that is deformed by the whisker
Laminar flow in the two fluid subdomains
Solid mechanics in the solid subdomain
Phase field in the two fluid subdomains with initial conditions for air and water
Multiphysics:
Fluid-structure coupling on the solid’s outer boundary
Two-phase flow in the two fluid subdomains
Two-Phase Flow and Fluid-Structure Interaction (FSI)
Waterinitially (green)
Air, initially(blue)
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Fluid Flow and Structural Deformation
Flow field, phase boundary, and structural deformation
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Moving Mesh
Phase boundary, structural deformation and moving mesh
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Two-Phase Flow and Fluid-Structure Interaction (FSI)
Alternative witha thin shell instead of a whisker as obstacle
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