Transient flashing propellant flow models to predict internal flow characteristics, spray velocity and aerosol droplet size of a pMDI

Despite of the popularity of the pMDI as an asthma remedy, the mechanism leading to spray generation is elusive, mainly due to small length scales and short time scale, causing experimental difficulties to obtain flow information. This mechanism involves transient development of two-phase flashing propellant flow inside pMDI actuator as well as transfer of heat, mass and momentum between the liquid and vapour phase. Variations in the rate of such interphase phenomena dictate the two-phase mass flow rate emission, which itself determines spray velocity and droplet size. In this work we compare the performance of existing two-phase flow models to predict the flow conditions and the rate of propellant flow through a pMDI actuator: the homogeneous equilibrium model (HEM), the slip equilibrium model (SEM) and the homogeneous frozen model (HFM). The velocity prediction of the HFM was found to be in good agreement with Phase Doppler Anemometry (PDA) data indicating the metastable nature of the emitted propellant spray. This work also considers Clark’s correlation for the aerosol droplet size based on the results of the flow model. The results of the correlation were compared with phase Doppler anemometry (PDA) droplet size measurements. Clark’s correlation was found to be effective in predictions of the temporal droplet size variations. However, the value of an empirical constant had to be tuned to fix the droplet size for a given combination of formulation, device and to a lesser extent also the distance from the spray orifice