Multiphase Large Eddy Simulation of Metered Dose Inhaler Sprays

Abstract: Metered dose inhalers (MDIs) have been used for 60 years to treat a range of respiratory diseases, including asthma. These portable, low-cost medical devices produce a respirable aerosol of micron-size droplets via flash-atomization of a fluorocarbon propellant blended with drugs and other chemicals. MDIs produce complex multiphase flows in which turbulence, heat transfer, evaporation, and non-equilibrium flash evaporation play coequal roles in determining aerosol properties. Experimental measurements are primarily limited to post-hoc aerosol sampling, as MDI sprays are not amenable to many conventional optical diagnostic techniques. This paper presents an alternative approach using multiphase large eddy simulation (LES). We employed an Eulerian mixed-fluid approach using a custom solver written in OpenFOAM. Phase change was handled using a homogeneous relaxation model (HRM) previously developed for flash-evaporating fuel sprays [Neroorkar et al., 2012]. Droplet formation, collisions, and transport were modeled using a Sigma-Y specific surface area transport model [Rachakonda et al., 2018]. We show that the HRM model can approximately reproduce experimental MDI plume morphology and droplet size distributions. Our model provides new insight into non-equilibrium flash-evaporation of the jet and how this varies depending on propellant properties. Keywords: Medical, aerosol, inhaler, computational fluid dynamics, large eddy simulation