### 6.6.4. Limitations of Particle-Particle Collision Model in ANSYS CFX

General model limitations and underlying assumptions are mentioned in Range of Applicability of Particle-Particle Collision Model and in [149], [150] and [152]. When using the particle-particle collision model in ANSYS CFX, the following limitations apply:

- Particle collisions only take place between particles of the same type.

- Particle collisions are currently only supported for fully coupled particles.

- Collisions are taken into account in transient simulations only after the second time step, because the PPCM relies on volume-averaged particle properties, which are available only after the completed first time step. Thus, averaged particle properties used in the PPCM for the generation of the virtual collision partner in binary particle collisions lag behind by one time step. With , the model converges to the correct particle property representation for the virtual collision partner in the PPCM, but for large integration time steps the particle phase information seen by the tracked particle in the PPCM might be inaccurate.

Also see Requirements for the Applicability of Particle-Particle Collision Model in the CFX-Solver Modeling Guide.

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