Transient numerical simulation of airflow and fibrous particles in a human upper airway model

In this study, the unsteady airflow field in a realistic model of human upper airways during the breathing cycle and the fibrous particle transport and deposition were investigated using the CFD method. An anatomically realistic model of airway passage including vestibule to the end of the trachea was constructed from the CT images of a 24-year-old healthy woman. Several user-defined functions (UDFs) were developed and incorporated into the discrete phase model (DPM) of ANSYS Fluent code and were used for the evaluation of ellipsoidal particle motion in transient airflows. The developed UDF was used to solve the coupled translational and rotational equations of motion of ellipsoidal fibers and to analyze their dispersion and deposition in the upper airways. The total and regional depositions for a range of fiber sizes were evaluated. The transient particle deposition fraction was compared with those obtained for the equivalent steady flow condition. In addition, the fraction of ellipsoidal fibers that penetrate the lower respiratory tract under cyclic breathing was calculated and compared with the steady flow simulation. The presented results showed that the steady simulation with an appropriate equivalent airflow rate can predict the total fibrous particle deposition during cyclic breathing with reasonable accuracy. The steady simulations, however, cannot properly predict the regional particle deposition under transient flows. In particular, the comparison between steady and transient penetration fraction reveals that the steady breathing model fails to accurately predict the penetration fraction of ellipsoidal fibers into the lower respiratory tract.