dpm problem
 

dear friends,I am modeling a water flow which solid particles settle in channel. it is based on a paper, I did everthings but some thing is not clear to me.
"As a first step, the fluid mechanics problem was solved in the absence of particles to find the steady state flow field. The converged solution was defined as the solution for which the normalized residual for all variables was less than 10−6. In addition, the convergence was checked from the outflow rate calculated at each iteration of the run. The convergence was achieved when the flow rate calculated to exit the tank no longer changed. Then the particles, whose density and size could be assigned at will, are released from the inlet and are tracked along their trajectories.
The particles reaching the bottom were deemed trapped whereas the rest were considered escaped. Particle tracking is fast and 25,000 particles could be tracked in less than 10 min once the flow field had been computed.
The number of particles was selected after many trials in order to combine the solution accuracy with short computing time for convergence. Fig. 4 reveals that, as the number of particles increased from 2000 to 25,000, the number of iterations needed for the model to converge decreased, whereas an even higher number would not yield any significant improvement. Therefore, a number of 25,000 particles was selected as a suitable one. The converged solution was defined as the solution for which the normalized residual for all variables was less than 10−6. In addition, the convergence was checked from the particle number balance calculated at each iteration of the run. The convergence was achieved when the percentage of particles calculated to exit the tank no longer changed.
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this is what have been said in that paper and also it is one way coupling. I did fluid modeling but I dont know how to define particle number.

pls help :(((
best regards