Multispecies aerosol evolution and deposition in the human respiratory tract

Introduction: The transport and subsequent regional deposition of inhaled aerosol in the human lung depend on the characteristics of (1) the particles, such as size, shape, and density; (2) airway geometry and structure; and (3) air flow. It is widely accepted that the governing mechanisms of particle deposition in airways are inertial impaction, gravitational sedimentation, and Brownian diffusion. Impaction is a velocity-dependent mechanism, whereas sedimentation and diffusion depend on the residence time of the particles in the airways. In this work, we used detailed computational fluid dynamics analysis to differentiate the contribution of the mentioned mechanisms to regional aerosol deposition in the human respiratory tract. Methods: Computational simulations were performed using AeroSolved open-source software developed within the OpenFOAM framework. The poly-disperse aerosol transport and deposition were modeled using an Eulerian sectional approach. The details of our model equations were explained in (Frederix et al., 2017). Simulations of non-evolving spherical particles with a density of 1,000 kg/m3 were conducted in a human respiratory tract geometry up to six generations of the tracheobronchial tree, originally obtained and introduced in (Zhe et al., 2012). We considered an air flow rate of 2.25 L/min through an inlet connection of 6 mm at the mouth inlet. We simulated for a wide particle size range (0.05–40 μm), discretized into 24 size section bins. Results and Conclusions: Figure 1(a) shows regional and total deposition efficiency, defined as the number flux of deposited particles on the internal surfaces of the respiratory tract versus the number flux of particles at the inlet. Large particles (1–40 µm) mainly deposit in the upper part of the respiratory tract (throat and G1–3). Small particles (0.05–1 µm) are primarily delivered to the lower respiratory tract (G4–6). In order to differentiate the contributions of the deposition mechanisms, we have also shown the deposition efficiencies neglecting the influence of diffusion or sedimentation processes. The diffusion process governs deposition of small particles, whereas sedimentation due to gravity is responsible for deposition of large particles. Inertial impaction has a minor effect in this case due to the low flow rate. The contribution of gravitational sedimentation was also evaluated by rotation of the geometry axis (along trachea) with respect to the gravity direction (angle θ). Large particle filtration in the upper respiratory tract due to sedimentation is increased for the larger rotation angles. This rotation does not influence the small particle deposition pattern. The work is currently extended toward deposition results for higher flow rates representative of inhalation, including turbulence.