Flow in a terminal alveolar sac model with expanding walls using computational fluid dynamics

Area of Science:

• Pulmonary physiology
• Aerosol science
• Computational fluid dynamics

Background:

• Particle deposition on alveolar walls raises concerns about translocation across the blood-gas barrier.
• Current whole-lung dosimetry models lack detailed alveolar flow fields, potentially leading to inaccurate deposition predictions.
• Limited and inconsistent studies exist on intra-alveolar flow patterns in realistic geometries.

Purpose of the Study:

• To investigate fluid dynamics within terminal air sacs.
• To quantify alveolar mouth to depth flow rate ratio, residual air penetration, and particle motion (diffusive vs. convective).
• To improve understanding of particle behavior in alveoli for accurate deposition modeling.

Main Methods:

• A computational model of a terminal alveolar sac with 13 alveoli was created using published morphometry.
• Numerical analysis of the flow field was performed using an in vivo breathing curve (2 and 4 seconds).
• Particle motion was assessed using Péclet numbers to differentiate between diffusion and convection.

Main Results:

• No recirculation was observed within the alveolar sac, aligning with previous findings.
• Flow rate ratios ranged from 0.18-0.36, consistent with reported literature values.
• Particle penetration depth was <33% during inhalation, with diffusion dominating for submicron particles; convection became significant for particles >0.5 micron at the entrance and >1 micron within the sac.

Conclusions:

• Diffusion is the primary driver for submicron particle motion in alveoli.
• Convective forces become increasingly important for larger particles (>0.5-1 micron) and at specific locations.
• Neglecting wall motion-induced convection may compromise deposition prediction accuracy for certain particle sizes and breathing conditions.