[Reza Zarghanishiraz]

Hello,
In the following problem, I have a geometry consisting of multiple layers of screens that are supposed to filter out the particles from the incoming contaminated flow.
A steady K-Omega SST turbulent model is used to solve the flow. The duct dimensions are 0.4*0.4*15 mm. There is a one-way interaction between the particles and the flow. Therefore, the flow is solved first. After convergence (errors down to 1e-10 or lower), the particles are injected at the inlet using the group feature (3 groups of 20 parcels) with the same velocity as the flow at the inlet. the particle size is 1 micron and their mass flow rate is 1e-20 (infinitesimal).
In the following, you can see a screenshot of the CFD mesh. 7 layers of inflation are added. The cell size close to the walls zone is 4e-6m and away from that (I call it downstream and upstream of the domain) is 8e-6 in one case and 16e-6m in the other. The problem is that the number of trapped particles on the walls is dramatically different in these two cases while the CFD mesh is different only away from the walls. I have checked the Z-velocity (the velocity along the duct) as a grid study parameter and the graphs align very well. So, the grid resolution seems to be sufficient. However, DPM results are different and we know that for Lagrangian solutions, as long as the flow is converged and consistent (for the drag forces), DPM results should not vary with the grid size since it just calculates the particles' trajectories by solving F=ma for each particle along its path.
additional information: BCs: velocity inlet (1.7 - 3 m/s) pressure outlet (0 gauge) Symmetry walls on 4 side walls of the duct, no-slip walls for the screen walls.
Please help me with what I should do.
Thanks in advance