Aerosol evolution and deposition in the human upper respiratory tract

Authored by  M Asgari, F Lucci, J Hoeng, A Kuczaj

Presented at RDD Europe 2019    
INTRODUCTION: When liquid aerosol is inhaled, the physical characteristics of particles, as well as the respiratory flow, determine the deposition rates in the human respiratory tract. The evolution of these liquid particles in the respiratory tract is temperature-driven and characterized by continuous condensation and evaporation processes that occur for inhaled aerosol. This impacts both the dynamics and also the deposited liquid mass. In this work, we have assessed the temperature-driven size dynamics of liquid particles and its influence on the subsequently deposited mass on the respiratory tract surface. Our results show that condensational growth of particles leads to a significant increase in the deposited liquid-mass. 

METHODS: The computational simulations were performed using the AeroSolved open-source software developed within the OpenFOAM framework. In a respiratory tract model, the poly-disperse aerosol transport, evolution, and deposition were modeled using an Eulerian sectional approach, which accounts for the mixture density variations due to the particles drift and evolution. The simulations of non-evolving glycerol aerosol were conducted in the human upper respiratory tract geometry up to nine generations of the tracheobronchial tree. We considered an air flow rate of 2.25 L/min through an inlet connection of 6 mm at the 2 mouth inlet [2]. In order to be able to compare our results with the available literature data, we considered a wide particle size range (0.1–40 μm), which was discretized into 24 size sections [3]. We considered a particle number density of 1.5–105 cm–3 including inertia and diffusion processes and neglecting coalescence. The simulation of evolving propylene glycol was conducted in the same geometry but up to the first generation, including the throat and trachea. Aerosol at the inlet had a temperature of 50°C and was modeled with a log-normal distribution with mass median diameter of 1.3 μm, geometric standard deviation of 1.33, and particle number density of 105 cm–3 . Size distribution was discretized in this case into 64 size sections to resolve the condensational growth/evaporation of the particles captured with a multispecies aerosol model. 

CONCLUSION AND OUTLOOK: We simulated transport, evolution, and deposition of liquid particles in a realistic anatomical model of the human upper respiratory tract. Simulation of non-evolving aerosol showed the size-dependent deposition of particles in the human upper respiratory tract. Our simulation of evolving aerosol demonstrated that condensational growth of the particles significantly increases the total deposited liquid mass. The computational results suggest that the thermo-control of the respiratory flow is crucial for aerosol deposition measurements of evolving particles. Therefore, we have developed an in vitro human respiratory tract cast model, shown in Figure 3, equipped with a flow temperature-control feature to represent the physiological conditions of the human lung. Future work will be dedicated to conducting aerosol deposition experiments using the cast.