Numerical analysis of motion and deposition of particles in cascade impactors

Prediction of the deposition of dry powder aerosols

The trajectory, mass fluxes, and deposition of aerosolized particles in a complex tubular system have been predicted. A procedure based on Lagrangian stochastic modeling is proposed to enable the anticipation of such phenomena, taking advantage of experimental results to characterize the air flow. The predictions have been obtained for pharmaceutical aerosols delivered by dry powder inhalers. A critical assessment of the dispersion model has been carried out using data available in the literature. The procedure assumes a low volume fraction of particles in the simulation of turbulent dispersion, but deposition is physically based on the interaction between the particles and both solid and liquid surfaces. The results were confirmed by experimental tests of powder deposition, run according to the European Pharmacopoeia. A parametric study was also carried out with the aim of providing a more complete evaluation of the model's performance. The comparison between predictions and experimental results has shown that the model properly describes the deposition of aerosolized particles.

Reducing bounce effects in the Andersen cascade impactor

The collection efficiency of the Andersen cascade impactor (ACI) can be affected by particle bounce, overload and re-entrainment (or blow-off), collectively referred to as bounce effects. Reduction of bounce effects in the ACI operated at 60 LPM was investigated for placebo large porous particles. Aerodynamic particle size distributions (aPSDs) obtained with the ACI and multi-stage liquid impinger (MSLI) were compared by observation of modes and statistical comparisons of the mass median aerodynamic diameter (MMAD) and geometric standard deviation (sigmag). Particle bounce effects were prevalent in the ACI with uncoated plates, i.e., bi-modal distribution with statistically significant differences in MMAD and sigmag (P<0.05). Coating the impaction plates with a thin layer of vacuum grease and decreasing the ACI stage jet velocities reduced, but did not minimize bounce effects. Bounce effects were minimized using 20-microm pore glass fiber filters saturated in water placed on inverted impaction plates, with good agreement obtained between the ACI and MSLI aPSDs, i.e., mono-modal with no statistically significant differences in MMAD and sigmag (P>0.05). Selection of the impaction substrate material and solvent must be evaluated with the drug product and analytical methods to minimize bounce effects and obtain an accurate measure of the aPSD.

Uniform resonant chaotic mixing in fluid flows

Laminar flows can produce particle trajectories that are chaotic, with nearby tracers separating exponentially in time. For time-periodic, two-dimensional flows and steady three-dimensional (3D) flows, enhancements in mixing due to chaotic advection are typically limited by impenetrable transport barriers that form at the boundaries between ordered and chaotic mixing regions. However, for time-dependent 3D flows, it has been proposed theoretically that completely uniform mixing is possible through a resonant mechanism called singularity-induced diffusion; this is thought to be the case even if the time-dependent and 3D perturbations are infinitesimally small. It is important to establish the conditions for which uniform mixing is possible and whether or not those conditions are met in flows that typically occur in nature. Here we report experimental and numerical studies of mixing in a laminar vortex flow that is weakly 3D and weakly time-periodic. The system is an oscillating horizontal vortex chain (produced by a magnetohydrodynamic technique) with a weak vertical secondary flow that is forced spontaneously by Ekman pumping--a mechanism common in vortical flows with rigid boundaries, occurring in many geophysical, industrial and biophysical flows. We observe completely uniform mixing, as predicted by singularity-induced diffusion, but only for oscillation periods close to typical circulation times.

Evaluation of preseparator performance for the 8-stage nonviable andersen impactor

The preseparator of an Andersen impactor with different coating treatments for a range of particle-size distributions was evaluated. Limited theoretical simulations constrained by simplifying assumptions of the airflow fields in the preseparator and upper stages of an 8-stage Andersen impactor were used to reveal low-velocity and high-pressure regions for potential deposition. These regions were then sampled in subsequent particle deposition experiments. Disodium fluorescein aerosols were sampled with different coating treatments of the preseparator floor. Particles collected at impactor stages determined particle size distributions. Stage deposition was compared between different preseparator treatments (buffer and silicon oil). Collection efficiency in the preseparator followed the pattern buffer > silicon oil > untreated. Statistical differences (P < 0.05) were noted in collection efficiency of large particles (45 micro m-75 microm) in the preseparator. The mass median aerodynamic diameters and geometric standard deviations showed some statistical differences when different preseparator treatments for large particles were used; therefore, preseparator coating was shown to influence performance and thereby estimates of particle size by inertial impaction.

Attraction of minute particles to invariant regions of volume preserving flows by transients

We find that tracer material can be concentrated into invariant regions of flows due exclusively to transient effects, as are produced when tracers temporarily become more buoyant than the surrounding fluid. This can occur either as a single event, e.g., if the tracer is initially weakly buoyant, or under periodic forcing, e.g., when external effects (such as solar heating) change the tracer density periodically. We study both cases in experiments, in a model, and in direct numerical simulations of laminar flow in a stirred tank. Focusing occurs for very small tracer size and inertia in flows that are instantaneously strictly volume conserving.