Multiphase Flows
Multiphase flows are present in virtually every industrial process involving fluids. From air bubbles entrained by a hydraulic structure to gas venting from a casting die as it fills with liquid metal to a coating process that deposits two fluid layers onto a moving surface, each of these scenarios requires a different approach to represent multiphase flows to provide efficient engineering solutions.
The FLOW-3D product family contains a comprehensive set of tools for simulating the interaction between liquids, gases, and solids, from large-scale free surface dynamics to dispersed bubbles, droplets, and phase change. The core capability of this toolset is TruVOF, the volume-of-fluid method that accurately resolves the movement of sharp liquid-gas free surfaces.
Free Surface Representation
TruVOF measures the local fraction of fluid versus gas to reconstruct a sharp surface that does not numerically diffuse. This allows the algorithm to capture the sharp three-dimensional dynamics of breaking waves, droplet formation, and processes such as filling and draining with high accuracy.
Free surface flow over a spillway. The surface is resolved as a sharp interface rather than a diffuse band.
Two Fluid Modeling
When the densities of the two fluid phases are similar, like an oil and water mixture, or there is significant momentum transfer between the phases, like in a vortex drop shaft; it becomes important to account for the physical interaction between the fluids. The Two Fluid Model does this by resolving the motion of both fluid phases in the domain through solving the full governing equations of each phase and their interaction at the interface.
One example is an oscillating water column (OWC) wave energy device. Waves entering the structure push air through a turbine. Because the motion of the air is directly influenced by the local motion of the water, resolving the velocity field of the gas phase, as well as its interaction with the free surface, is necessary to predict turbine performance and energy capture.
Oscillating water column wave energy converter simulated with the Two Fluid Model. Waves drive air through a simple turbine; air velocity, turbine kinetic energy, and air flow rate are plotted alongside the free surface.
Dispersed Phases
When two phases are mixed well enough, fine bubbles, droplets, or grains are introduced into the main liquid phase. Those dispersed phases move relative to the fluid that carries them; for example, suspended sediment settling out of wastewater in a clarifier or entrained air drifting up through molten metal in a gravity sandcasting. The Drift Flux Model accounts for the independent motion of the dispersed phase without having to resolve each individual piece of that phase.
When filling bottles, drug vials, or casting dies, the goal is often to fill as fast as possible without mixing air into the product. The Air Entrainment Model, which is an extension of the Drift Flux Model, tracks entrained air as a scalar field, capturing the effect of the small-scale bubbles on the overall filling process without needing to model each individual bubble.
Bottle filling on a production line. Entrained air bulks up the liquid volume during the fill; as the Drift Flux Model lets the air rise and escape, the reduction in liquid volume is visible.
The Oxygen Dissolution Model is another example of a dispersed phase model, used in wastewater treatment processes and in applications where dissolved oxygen levels affect aquatic life.
Plunging jet with the Air Entrainment Model and its Oxygen Dissolution extension active. Entrained air volume fraction is shown in the lower half and dissolved oxygen concentration in the upper half.
The dispersed phase can also be solid. In the transport of mining tailings, fine silt or mud particles change the local viscosity of the flow. The Tailings Model addresses this using a non-Newtonian rheology framework suitable for consequence analysis and dam safety review.
The Lagrangian Particle Model offers another approach for modeling small scale bubbles, droplets, or solids if their individual motion needs to be tracked.
One Fluid Modeling
Not all analyses require a detailed solution of the gas phase. When the liquid has enough inertia to be unaffected by the local motion of the surrounding gas, the One Fluid Model defines a gas region using a uniform pressure. The effect of the gas on the liquid is accounted for at the interface between the two. This is one of the most common and computationally efficient modeling choices.
When compression dynamics are needed to represent bubbles accurately, the Ideal Gas Bubble Model allows the pressure of a gas region to vary with changes in volume and temperature, while retaining the One Fluid framework. This captures processes such as bubbles forming due to liquid evaporating at a hot wall, rising through a column of water, and cooling as they move upward.
The Void Particle Tracking option extends the sharp bubble representation to the smallest bubble sizes by converting them to gas particles when they fall below a threshold, allowing fast computation without losing track of bubble location.
Evaporation and Condensation
FLOW-3D products account for the mass and heat transfer that occurs during vaporization and condensation, within either the One Fluid or Two Fluid framework. This applies to cavitation in closed conduits such as pump intakes and turbine exhausts, LNG and propellant transfer, laser metal vaporization, thermal reactors, distillation systems, and heat exchangers.
Two-fluid air and water simulation with heat transfer from a solid boundary. Gas bubbles nucleate at the heated wall and carry thermal energy to the surface, where part is released to the air above and part warms the water near the surface.
Consider water flowing through a valve constriction. In a venturi section, if the flow is fast enough, pressure can drop below the saturation pressure and vapor bubbles form. The subsequent collapse of these bubbles creates pressure spikes that cause pitting and wear.
In propellant management, a pump at the bottom of a tank lowers the internal pressure as fuel drains. Without replacing the lost liquid volume, pressure can drop to an unsafe level. A common solution is to release pressurized helium into the tank. The Active Simulation Control Model can simulate the operation of a check valve between the helium and propellant tanks, restoring pressure in the main tank.
Non-Condensable Gas Model
Within the Two Fluid framework, the Non-Condensable Gas Model tracks an additional gas phase that does not condense. This is used to represent inert gases such as helium in propellant pressurization systems, and also allows the quantity of evaporated fuel vapor in the gas phase to be tracked separately.
Vaporization Residue
The Vaporization Residue Model tracks solid material left behind after a carrying fluid evaporates. Applications include dry ink residue in inkjet printer heads, drying of coating liquids for electronics including spent soldering flux, and solute deposition on hot surfaces during spray coating, drug manufacturing, or desalination.
Applications
The multiphase flow capabilities of FLOW-3D products support a wide range of engineering and scientific applications:
- Wave energy converter design and performance assessment
- Hydraulic structure design including spillways, drop shafts, gates, and energy dissipators
- Consumer product filling and packaging, including bottles, vials, and containers
- Wastewater treatment and dissolved oxygen modeling
- Mining tailings transport and dam safety analysis
- Cavitation prediction in pumps, valves, and turbines
- Cryogenic and propellant tank management in aerospace applications
- Inkjet printing, spray coating, and evaporative deposition processes
- LNG, fuel, and propellant transfer systems
- Heat exchanger design including evaporators and condensers
- Metal casting
- Welding
- Additive manufacturing