16.5.1 Overview and Limitations of the Eulerian Model

Overview

The Eulerian multiphase model in ANSYS FLUENT allows for the modeling of multiple separate, yet interacting phases. The phases can be liquids, gases, or solids in nearly any combination. An Eulerian treatment is used for each phase, in contrast to the Eulerian-Lagrangian treatment that is used for the discrete phase model.

With the Eulerian multiphase model, the number of secondary phases is limited only by memory requirements and convergence behavior. Any number of secondary phases can be modeled, provided that sufficient memory is available. For complex multiphase flows, however, you may find that your solution is limited by convergence behavior. See this section in the separate User's Guide for multiphase modeling strategies.

ANSYS FLUENT's Eulerian multiphase model does not distinguish between fluid-fluid and fluid-solid (granular) multiphase flows. A granular flow is simply one that involves at least one phase that has been designated as a granular phase.

The ANSYS FLUENT solution is based on the following:

• A single pressure is shared by all phases.

• Momentum and continuity equations are solved for each phase.

• The following parameters are available for granular phases:
  • Granular temperature (solids fluctuating energy) can be calculated for each solid phase. You can select either an algebraic formulation, a constant, a user-defined function, or a partial differential equation.

  • Solid-phase shear and bulk viscosities are obtained by applying kinetic theory to granular flows. Frictional viscosity for modeling granular flow is also available. You can select appropriate models and user-defined functions for all properties.

• Several interphase drag coefficient functions are available, which are appropriate for various types of multiphase regimes. (You can also modify the interphase drag coefficient through user-defined functions, as described in the separate UDF Manual.)
• All of the $k$- $\epsilon$ turbulence models are available, and may apply to all phases or to the mixture.

Limitations

All other features available in ANSYS FLUENT can be used in conjunction with the Eulerian multiphase model, except for the following limitations:

• The Reynolds Stress turbulence model is not available on a per phase basis.

• Particle tracking (using the Lagrangian dispersed phase model) interacts only with the primary phase.

• Streamwise periodic flow with specified mass flow rate cannot be modeled when the Eulerian model is used (the user is allowed to specify a pressure drop).

• Inviscid flow is not allowed.

• Melting and solidification are not allowed.

• When tracking particles in parallel, the DPM model cannot be used with the Eulerian multiphase model if the shared memory option is enabled ( this section in the separate User's Guide). (Note that using the message passing option, when running in parallel, enables the compatibility of all multiphase flow models with the DPM model.)

To change from a single-phase model, where a single set of conservation equations for momentum, continuity and (optionally) energy is solved, to a multiphase model, additional sets of conservation equations must be introduced. In the process of introducing additional sets of conservation equations, the original set must also be modified. The modifications involve, among other things, the introduction of the volume fractions $\alpha_1, \alpha_2, \ldots\ \alpha_n$ for the multiple phases, as well as mechanisms for the exchange of momentum, heat, and mass between the phases.