The effect of the mucus layer and the inhaled air conditions on the droplets fate in the human nasal cavity: A numerical study

In this work, the behavior of inhaled water droplets with initial sizes of 1-30 mu m was studied under a sinusoidal cyclic breathing pattern. The maximum flow rate was 15 L/min, and the effect of inhaled air humidity and a humid mucus layer was considered. The geometry of the nasal airway model was created using magnetic resonance imaging (MRI) scans of a healthy male's nose. The airflow and droplet motion governing equations were solved using an Eulerian-Lagrangian computational fluid dynamics (CFD) approach. A convection-diffusion-controlled model was used to analyze the size change of droplets. The main focus of this study was on the behavior and fate of the droplets, including their changes in size, as well as their evaporation, deposition, and penetration patterns. However, other parameters, including airflow velocity, temperature, and vapor concentration, were also evaluated. Based on the results, the mucus layer can significantly reduce the droplets evaporation to the extent that evaporation of initially 12 mu m droplets reduces by about 90% due to the presence of the mucus layer. However, the inhaled air humidity and the mucus layer have little effect on the deposition fraction. The deposition rate mainly linearly depends on the initial droplet size. In the presence of the mucus layer, droplets with an initial size of 10 mu m are mostly susceptible to penetration; around 80% can exit the nasal cavity. The critical range of droplets initial size is 5-10 mu m. As the relative humidity of the inhaled air changes, the evaporation and penetration behavior of droplets with the critical range of size alters dramatically. Besides, in all cases, the deposition fraction of droplets smaller than 5 mu m is almost zero.