dear developer,
The calculation I conducted was a continuous particle reactor, where fresh particulate material continuously entered the calculation domain from the mass flow boundary, and then particles of the same mass flowed out from the mass flow outlet at the bottom. However, I found that the number of particles in the calculation domain was constantly decreasing, and it seemed that the bottom outlet was still the free flow boundary or the pressure outlet boundary, without effectively controlling the mass flow rate.
I think I figured it out. Maybe there’s too little grid at the outlet. I couldn’t refine the mesh due to particle size limitations, so I increased the size of outlet to make the flow rate as same as what I set.