For dispersed phases, you can specify a maximum packing parameter. This is the volume fraction of the phase at its state of maximum packing. It is most commonly used for compact solid dispersed phases, for example, as you would find in fluidized beds.

The maximum packing parameter is unity by default for a dispersed fluid. For a dispersed solid phase, it may range from 0.5 to 0.74, the latter being the maximum possible packing for solid spheres. For most applications, the default value 0.62 suffices.

The maximum packing parameter is used in correlations for certain drag laws, and models for particle collision forces. Unfortunately, it is not possible to numerically guarantee that volume fractions are bounded above by the maximum packing parameter. Consequently, you may observe volume fractions higher than the maximum packing.