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22.1.3 Overview and Limitations of the Eulerian Model

Overview

The Eulerian multiphase model in 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 Section  22.7.3 for multiphase modeling strategies.

FLUENT's Eulerian multiphase model differs from the Eulerian model in FLUENT 4 in that there is no global distinction 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 FLUENT solution is based on the following:

Limitations

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

Stability and Convergence

The process of solving a multiphase system is inherently difficult, and you may encounter some stability or convergence problems, although the current algorithm is more stable than that used in FLUENT 4. If a time-dependent problem is being solved, and patched fields are used for the initial conditions, it is recommended that you perform a few iterations with a small time step, at least an order of magnitude smaller than the characteristic time of the flow. You can increase the size of the time step after performing a few time steps. For steady solutions it is recommended that you start with a small under-relaxation factor for the volume fraction. Another option is to start with a mixture multiphase calculation, and then switch to the Eulerian multiphase model.

Stratified flows of immiscible fluids should be solved with the VOF model (see Section  22.2). Some problems involving small volume fractions can be solved more efficiently with the Lagrangian discrete phase model (see Chapter  21).

Many stability and convergence problems can be minimized if care is taken during the setup and solution processes (see Section  22.7.3).


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