A MONTE-CARLO CALCULATION OF THE DEPOSITION EFFICIENCY OF INHALED PARTICLES IN LOWER AIRWAYS

Steady airflow and particle transport in a straight cylindrical tube and in a three-dimensional model of a bifurcating airway were calculated. The model domains were based on human lung data. For simplicity, airways in the bifurcating model were assumed to have a rectangular cross-section with hydraulic diameters similar to those of generations 16 and 17 of the Weibel model. Steady flow with a Reynolds number of unity within this domain was solved numerically by a finite element method using a commercial software package. Parabolic inlet conditions were assumed. Using the calculated flow profile, individual particle trajectories within the air stream were simulated by numerically solving the fundamental differential equations of particle motion. The simulation included the effects of diffusion and sedimentation. Deposition efficiencies by individual mechanisms and the total deposition efficiency were obtained using a Monte Carlo method. For parabolic flow in a straight tube, the results compared very well with existing formulas in the literature. For both geometries, the sum of the efficiencies due to individual mechanisms was greater than the numerical prediction of total efficiency. The total deposition efficiency, eta(total), was accurately expressed in terms of the efficiencies of sedimentation, eta(s), and diffusion, eta(d), using the empirical formula eta(total) = (eta(d)p + eta(s)p)1/p. The best value of p depended on the particular geometry.