Thank you for your answers.
I have read the link you provided me, and I wanted to tell you that I have been reviewing the mesh; my pipe is 30 mm in diameter, and the particle diameter is 0.2mm.
The structured mesh I have made has a square in the center (10mm x 10mm), and each cell is approximately 0.7mm (just over 3 times the diameter of the particle).
I am noticing that just in the central area (where the square is), the volume fraction distribution is more homogeneous, but after that, its values fluctuate, concentrating closer to the walls of the square. I understand that it is due to the geometry of my pipe (I have drawn some red lines so that the dimensions of the square can be better reflected)
1) Please, could you give me some recommendations to improve my results?
2) I understand that my cell must be larger than the diameter of my particle; however, my question is: How can I get better results close to the wall if I have to keep that cell size condition?
I appreciate any help you can provide.