Large eddy simulation of metered dose inhaler sprays with low-GWP propellants

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-atomisation of a fluorocarbon propellant blended with drugs and other chemicals. MDIs produce complex multiphase flows in which turbulence, heat transfer and non-equilibrium flash evaporation play coequal roles in determining aerosol properties. Experimental studies of near-field spray structure are limited, as MDI sprays are not amenable to many conventional optical diagnostic techniques and the parameter space for the various propellants and nozzle geometries used in practice is large. This paper presents an alternative approach using multiphase large eddy simulation (LES). We employed a compressible 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 modelled using a Sigma-Y specific surface area transport model (Rachakonda et al., 2018). We show that the HRM model can approximately reproduce experimental droplet size distributions obtained using laser diffraction and experimental line-of-sight integrated density distributions obtained using X-ray scattering. Results were obtained for three commonly used propellant-ethanol mixtures at a fixed ethanol mass fraction of 15%. Our model provides new insight into the features of the non-equilibrium flash-evaporation of the jet, how this process varies depending on propellant thermophysical properties, and what non-dimensional parameters are best suited to capturing these variations. • Metered dose inhalers produce complex flash evaporating multiphase flows. • A compressible mixed-fluid approach for MDI sprays using OpenFOAM is developed. • Phase change is handled using a homogeneous relaxation model. • Atomisation is modelled with the Sigma-Y surface area transport approach. • Simulation results are validated against laser diffraction and X-ray scattering.