Numerical investigation of transient transport and deposition of microparticles under unsteady inspiratory flow in human upper airways.

In the present study, unsteady airflow patterns and particle deposition in healthy human upper airways were simulated. A realistic 3-D computational model of the upper airways including the vestibule to the end of the trachea was developed using a series of CT scan images of a healthy human. Unsteady simulations of the inhaled and exhaled airflow fields in the upper airway passages were performed by solving the Navier-Stokes and continuity equations for low breathing rates corresponding to low and moderate activities. The Lagrangian trajectory analysis approach was utilized to investigate the transient particle transport and deposition under cyclic breathing condition. Particles were released uniformly at the nostrils' entrance during the inhalation phase, and the total and regional depositions for various micro-particle sizes were evaluated. The transient particle deposition fractions for various regions of the human upper airways were compared with those obtained from the equivalent steady flow condition. The presented results revealed that the equivalent constant airflow simulation can approximately predict the total particle deposition during cyclic breathing in human upper airways. While the trends of steady and unsteady model predictions for local deposition were similar, there were noticeable differences in the predicted amount of deposition. In addition, it was shown that a steady simulation cannot properly predict some critical parameters, such as the penetration fraction. Finally, the presented results showed that using a detached nasal cavity (commonly used in earlier studies) for evaluation of total deposition fraction of particles in the nasal cavity was reasonably accurate for the steady flow simulations. However, in transient simulation for predicting the deposition fraction in a specific region, such as the nasal cavity, using the full airway system geometry becomes necessary.