Phase change and deposition of inhaled droplets in the human nasal cavity under cyclic inspiratory airflow

In many situations, inhaled droplets experience phase change in the human nasal passages during the breathing cycle. The associated size change of droplets can significantly influence the transport and deposition patterns in the nasal cavity. In this study, the transport, evaporation, and deposition of inhaled droplets under unsteady cyclic breathing conditions were simulated. A computational model of the nasal cavity was developed from magnetic resonance imaging (MRI) scans of the nose of a healthy male and a computational fluid dynamics (CFD) method was used to solve the governing equations of airflow and droplet motion. The Lagrangian trajectory approach was then used to investigate the transport and deposition of droplets. The size change of droplets was simulated using a convection-diffusion controlled model. In order to evaluate the effects of evaporation on transport and deposition of droplets, two inhaled compounds (water and linalool) with different saturation vapor pressures were simulated. The results showed substantial differences between the evaporation behavior of water and linalool droplets. Water droplets, which is more volatile than linalool drops, with the initial diameter ≤ 8 µm completely evaporated while complete evaporation of linalool droplets was observed for the initial diameter < 2 µm. During the time-varying inspiratory airflow, the minimum evaporation rates were observed when the airflow rate was near its maximum value. Comparing the deposition fractions of water droplets in the presence and absence of evaporation showed that droplet evaporation decreases the deposition of droplets by about 10-20% mainly due to the reduced inertial impaction effects. Furthermore, it was shown that a steady airflow simulation does not accurately estimate the deposition and evaporation of droplets in the human nasal cavity.