## 22.1.2 Limitations

Limitation on the Particle Volume Fraction

The discrete phase formulation used by FLUENT contains the assumption that the second phase is sufficiently dilute that particle-particle interactions and the effects of the particle volume fraction on the gas phase are negligible. In practice, these issues imply that the discrete phase must be present at a fairly low volume fraction, usually less than 10-12%. Note that the mass loading of the discrete phase may greatly exceed 10-12%: you may solve problems in which the mass flow of the discrete phase equals or exceeds that of the continuous phase. See Chapter  23 for information about when you might want to use one of the general multiphase models instead of the discrete phase model.

Limitation on Modeling Continuous Suspensions of Particles

The steady-particle Lagrangian discrete phase model described in this chapter is suited for flows in which particle streams are injected into a continuous phase flow with a well-defined entrance and exit condition. The Lagrangian model does not effectively model flows in which particles are suspended indefinitely in the continuum, as occurs in solid suspensions within closed systems such as stirred tanks, mixing vessels, or fluidized beds. The unsteady-particle discrete phase model, however, is capable of modeling continuous suspensions of particles. See Chapter  23 for information about when you might want to use one of the general multiphase models instead of the discrete phase models.

Limitations on Using the Discrete Phase Model with Other FLUENT Models

The following restrictions exist on the use of other models with the discrete phase model:

• When tracking particles in parallel, the DPM model cannot be used with any of the multiphase flow models (VOF, mixture, or Eulerian - see Chapter  23) if the shared memory option is enabled (Section  22.11.9). (Note that using the message passing option, when running in parallel, enables the compatibility of all multiphase flow models with the DPM model.)

• Streamwise periodic flow (either specified mass flow rate or specified pressure drop) cannot be modeled when the discrete phase model is used.

• Only nonreacting particles can be included when the premixed combustion model is used.

• Surface injections will be moved with the grid when a sliding mesh or a moving or deforming mesh is being used, however only those surfaces associated with a boundary will be recalculated. Injections from cut plane surfaces will not be moved with the mesh and will be deleted when remeshing occurs.

• The cloud model is not available for unsteady particle tracking, or in parallel, when using the message passing option for the particles.

• The wall-film model is only valid for liquid materials. If a nonliquid particle interacts with a wall-film boundary, the boundary condition will default to the reflect boundary condition.

• When multiple reference frames are used in conjunction with the discrete phase model, the display of particle tracks will not, by default, be meaningful. Similarly, coupled discrete-phase calculations are not meaningful.

An alternative approach for particle tracking and coupled discrete-phase calculations with multiple reference frames is to track particles based on absolute velocity instead of relative velocity. To make this change, use the define/models/dpm/options/track-in-absolute-frame text command. Note that the results may strongly depend on the location of walls inside the multiple reference frame.

The particle injection velocities (specified in the Set Injection Properties panel) are defined relative to the frame of reference in which the particles are tracked. By default, the injection velocities are specified relative to the local reference frame. If you enable the track-in-absolute-frame option, the injection velocities are specified relative to the absolute frame.

• Relative particle tracking cannot be used in combination with sliding and moving deforming meshes. If sliding and/or deforming meshes are used with the DPM model, the particles will always be tracked in the absolute frame. Switching to the relative frame is not permitted.

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