| This research investigated particle inhalability in low velocities typical of occupational settings. Because of difficulties suspending uniform particle concentrations in low velocity wind tunnels, the goal was to develop a computational fluid dynamic (CFD) model suitable to study inhalability. The first step in this study was to determine the appropriate simplifications to the human form. An experimental study compared the velocity field and particle aspiration for a 2/3-scale human mannequin and a stacked elliptical cylinder: velocity field differences were limited to a region within 20 mm upstream of the mouth orifice and, more importantly, the fraction of aspirated particles was significantly larger for the simpler geometric surrogate than for the fully-featured mannequin. Knowing that detailed facial features were critical to large particle aspiration, a small-scale humanoid CFD model was developed and its predictions were compared to the experimental results. The standard k-epsilon model provided reasonable estimates of the velocity field in the forward-facing orientation, even though rigorous studies have reported poor performance of this model in the wake of a surrogate human. Particle aspiration simulations using this model confirmed that the moderate under-estimations of vertical velocity field affected particle transport from a source, but that the model is still useful for the exploration of particle inhalability using the assumption of a uniform particle concentration and laminar particle transport. The final step in this study examined a human-scale version of this CFD model, where three velocity conditions were simulated to investigate particle inhalability. One condition was matched to data in the literature, and agreement was found for particles < 68 mum. Simulations of aspiration efficiency for larger particles under-estimated those reported by others. However, the other authors' experimental biases may have over-estimated aspiration efficiency by under-sampling the reference concentration. Although this research does not consider other orientations needed to fully define aspiration efficiency, the facing-the-wind orientation studied here is associated with the maximum aspiration efficiency. These results provide evidence that the omni-directional aspiration efficiency curve for low velocity environments decreases below the 50% asymptote given by the American Conference of Governmental Industrial Hygienists' inhalable particulate mass collection efficiency curve guideline. |
