Modeling HFA-driven dense aerosol dynamics in pMDIs: Comparison of turbulence and interphase coupling models

In computational studies of pressurized metered-dose inhalers (pMDIs), aerosol transport is often modeled under a dilute-phase assumption that accounts only for the mass of the active pharmaceutical ingredient (API). This simplification overlooks the volumetric and dynamic contributions of the propellant, which play a critical role in spray development, droplet evaporation, and regional deposition. To capture these mechanisms, a transient, three-dimensional, two-phase flow model was developed using a Dense Discrete Phase Model (DDPM) with four-way coupling to represent both the API and propellant. Three modeling strategies were evaluated in terms of predictive capability and computational cost: LES–DPM (dilute phase), k–ω–DPM (dilute phase), and k–ω–DDPM (dense phase). Model predictions were validated against experimental deposition data obtained from a Next Generation Impactor (NGI) under identical operating conditions. For droplets depositing on the airway walls, with dominant deposition occurring in the 10–25 μm size range, the LES and k–ω models produced different airflow structures but resulted in similar deposition patterns. This indicates that, in this size regime, droplet transport is primarily governed by inertial impaction rather than by turbulence intensity. In contrast, the DDPM framework, which accounts for interphase momentum transfer and particle-particle collisions, reduced mouth-throat (MT) deposition by approximately 7%. This reduction arises from interphase and particle-particle interactions that weaken plume momentum and shift deposition downstream. Thermally, DDPM predicted airflow temperature 4–5 K lower and larger mean droplet diameters near the nozzle injection. Together, these coupled momentum–thermal effects reconcile differences between simplified dilute-phase assumptions and realistic dense-aerosol behavior, providing a more physically consistent and predictive framework for modeling pMDI spray dynamics.