I think what you did for the material is right ... just edit one existing material if can't find yours. Also your setting is right, I think .. The issue is the physics ... high density particles will be at the outer edge of circulation, So they land on the wall and get trapped. You can try giving them a density that is close to the fluid and see where they land ... I think they will go much further ... Another trial could be injecting them with low velocity at the pipe centre line and see they just get thrown towards the wall and get trapped to the wall immediately. n