A Monte Carlo simulation of convective dispersion in the large airways.

A numerical model describing the local interactions between convection and diffusion in the first 11 generations (0 to 10) of the human bronchial tree is presented. The model, based on a Monte Carlo procedure, is used to investigate the effects of four velocity profiles: (i) parabolic, (ii) asymmetrical, (iii) asymmetrical with swirling and (iv) flat. Behavior was investigated for three diffusivities: (i) 0.75 cm2/s (He/air), (ii) 0.25 cm2/s (N2/O2) and (iii) 0.1 cm2/s (SF6/air) on the convection-diffusion interaction. The results of these simulations showed that 'Taylor dispersion' is an important effect, with respect to tracer segregation, and that it is of major significance only in the largest airways. By generation 10, molecular diffusion begins to dominate over Taylor dispersion. It was also found that use of a parabolic velocity profile, or application of the Gill-Sburamanian dispersion theory seriously overestimates axial dispersion. On the other hand, the use of a flat velocity profile underestimates dispersion.