Dr. Armine, thank you for your answer.

I was using the DPM option and refined the mesh especially close to the wall, but I still have the same question: why do I have this accumulation of particles in different areas?

I know that the DPM option doesn't consider the volume fraction; however, it is unclear why the concentration of particles in the cell is not homogeneous.

On the other hand, the slip condition is only fulfilled by the fluid flow. However, I noticed that in the velocity profile of the particle, the velocity at the wall is 0 m/s. Please could you tell me how to improve my simulation?

Many thanks for considering my request.