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:
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.