Evaluation of a Drift Flux Model for Simulating Submicrometer Aerosol Dynamics in Human Upper Tracheobronchial Airways

Abstract

In this study, a hybrid drift flux velocity correction (DF-VC) model that accounts for both submicrometer particle diffusion and inertia was extended to transient conditions and was tested against existing experimental deposition data measured in a replica cast of the human tracheobronchial (TB) region for laminar and turbulent flow. To evaluate the effectiveness of the DF-VC model, deposition results were compared with a standard chemical species (CS) approach that neglects particle inertia. A numerical model of the TB cast was constructed from CT images and extended from the larynx to approximately the sixth respiratory generation. Experimentally determined inlet and outlet flow conditions were implemented in the computational model to ensure direct comparisons between simulations and measurements for the deposition of 40 and 200 nm particles. A low Reynolds number k-omega turbulence model was employed to resolve the laminar and turbulent flow regimes that coexist in the TB geometry. Interesting flow characteristics were observed due to the presence of the larynx, asymmetrical ventilation, and left-right asymmetry, which created a right-skewed laryngeal jet and flow reversal in the trachea that persist over a majority of the transient flow cycle. In comparison with the CS model, deposition results of the DF-VC approach persistently agreed better with experimental findings on a total and sub-branch basis, which indicated that the DF-VC model effectively captured the influence of finite particle inertia. For the submicrometer aerosols considered, transient flows were observed to increase deposition arising from impaction and decrease deposition arising from diffusion on a total and segmental basis compared with steady state conditions. However, the maximum deposition enhancement factor was significantly increased under transient conditions for both 40 nm (factor of 2) and 200 nm (factor of 7) aerosols. Results of this study indicate that a drift flux particle transport model with near-wall velocity corrections can provide an effective continuous-field approach for simulating the transport and deposition of submicrometer respiratory aerosols in human upper TB airways.