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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:


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