Effect of nozzle spacing in the formation of primary and secondary deposits in multi-nozzle inertial impactors part II: Numerical study

In this study CFD models are introduced and validated to describe the flow field on the experimental impactor (3 impinging jets) and to determine the particle deposition mechanisms (primary and secondary deposits) for each experimental condition carried out in Part I. For the determination of particle trajectories (d(p) between 1.5 and 6.0 mu m), a steady state numerical analysis was carried out for laminar and turbulent flow continuum regimes. In addition to that, the effect of gravity and the so-called stochastic turbulent models on particle trajectories was also studied. Regarding the effects of the jet interaction, the simulation results show that: - The half-moon shaped premature primary deposits are formed due to the deviation of the jet caused by the recirculation of the jet-to-jet interaction. - The straight lines deposits are formed by particles that are not able to follow the high velocities generated in the recirculation zone between pairs of jets and are separated inertially in the line of stagnation between them. - The nozzle plate backside deposits are formed with particles of the interaction region that ascend to the nozzle plate in the area in which the three jets converge. - With respect to the halo, from the combined analysis of the numerical and experimental results, it is possible to distinguish two types of halo occurring at low and at larger Reynolds numbers. Therefore, the analysis of the formation of the halo must take into account both inertial and gravitational effects and the balance between them. The impactor CFD models have been validated with the experimental data obtained for the same operating parameters, with good agreement for both the root Stk(50) and the steepness of the efficiency curves. These accurate simulations are obtained using a geometry of multiple nozzles, with an impaction plate sufficiently large that allowed the formation of the halo, and by using a two-equation turbulence model that included gravity and turbulent dispersion to calculate the particle trajectories.