Numerical analysis of motion and deposition of particles in cascade impactors

The motion and deposition of small solid particles (1-10 μm) in an idealized six-stage cascade impactor is investigated numerically. The flow of air is simulated using commercial software, and the trajectories of solid particles are predicted by integrating equatious of motion that include gravitational forces, inertia and viscous friction. Particle-wall interactions are modeled in terms of a sticking probability and a restitution coefficient. The model correctly predicts the typical S-shaped trapping efficiency curves. As expected, the sticking probability has the greatest impact on trapping profiles. The restitution coefficient has only a very small effect, which becomes negligible for large sticking probability. However, a large unanticipated effect is uncovered by the model. The flow inside the impactor displays large recirculation regions that act as efficient traps of particles of specific sizes. Particles trapped in these regions do not deposit on collection plates, possibly biasing analysis results. This effect, which is magnified at low sticking probabilities, provides an alternative explanation for the 'wall losses' reported in several experimental studies that have examined impactors similar to the one studied here.