Multiphase—Eulerian Approach

Multiphase flows, where several fluids flow in the domain of interest, play an important role in variety of industrial applications. In general, we associate phases with gases, liquids or solids and as such some simple examples of multiphase flows are: air bubbles rising in a glass of water, sand particles carried by wind, rain drops in air. The definition of phase can be generalized and applied to other fluid characteristics such as size, shape, density, and temperature.

Numerical simulations of such flows need to handle additional complexity in comparison to simulating single phase flow due to the presence of interface across which there is a jump in fluid-fluid properties and the exchange of mass, momentum and heat between the phases occurs as well. For modelling considerations, such flows can be classified based on increasing spatial scales of interface between the phases, into dispersed (bubbly flow, droplet flow), mixed / intermittent (slug flow, churn flow) and separated / stratified (film flow, annular flow, horizontal stratified flow).

Type	Flow Regime	Geometry	Characteristics	Examples
Stratified flows	Film Flow		Liquid film in gas Gas film in liquid	Film condensation Film boiling
	Annular Flow		Liquid core and gas film Gas core and liquid film	Film boiling Boilers
	Jet Flow		Gas pocket in liquid	Atomization Jet Condenser
	Free surface		Tank filled with liquid and gas above	Marine towing tank and waves Sloshing
Mixed or transitional flows	Slug flow		Gas pocket in liquid	Sodium boiling in forced convection
	Bubbly annular flow		Gas bubbles in liquid film with gas core	Evaporators with wall nucleation
	Droplet annular flow		Gas core with droplets and liquid film	Steam generator
	Bubbly droplet annular flow		Gas core with droplet and liquid film with gas bubbles	Boiling nuclear reactor channel
Dispersed flows	Bubbly flow		Gas bubbles in liqud	Chemical reactors
	Droplet flow		Liquid droplets in gas	Spray cooling
	Particulate flow		Solid particles in gas or liquid	Transportation of powder

In several practical applications, mostly more than one flow regimes exist within the domain. The following diagram shows an example of such flow:

The volume tracking / interface capturing methods such as VOF [523] or level-set methods [551] are well suited for simulation of stratified multiphase flows which are characterized by large scale interface. It must be noted that the size of computational cell is critical in classifying the interface as large scale or a small scale. Since the volume tracking / interface capturing methods rely on resolving the interface completely, they are prohibitively expensive for simulating multi-scale flows where modelling of dispersed regime accurately is equally important.

The two fluid model [453] [479], referred to as the Eulerian Multiphase (EMP) model in Simcenter STAR-CCM+, on the other hand, is successfully used for modelling dispersed flows as well as multi-scale flow situations where modeling of dispersed flow along with separated flow accurately is equally important.