Prediction of the droplet size distribution in aerodynamic droplet breakup

The rim and bag dynamics in aerodynamic droplet breakup are investigated experimentally and theoretically. Three main modes contribute to the breakup sizes in aerodynamic droplet breakup: the rim node, the remaining rim and the bag breakup modes. However, existing models only consider one mode and are, therefore, unable to predict the size distribution. The present theoretical work seeks to model the dominant breakup mechanisms of each mode and to relate these mechanisms to the size distribution. It is shown that the nodes can be modelled using either the Rayleigh-Taylor or Rayleigh-Plateau instabilities with comparable results and that the variation in the node sizes results from the variation in the amount of mass in the rim that flows into the node prior to the rim breakup. The breakup of the rim is shown to be a result of a combination of the Rayleigh-Plateau instability and a newly proposed collision mechanism, wherein the impact of the corrugated receding rim of the bag with the main rim forces the main rim to break with the same wavelength as the receding rim. The resulting size distribution of the droplet breakup is estimated assuming that the relative weighting of the breakup mechanisms for each mode follows a two-parameter gamma distribution. The volume of each geometry is used to estimate the volume weighting of the modes, giving a reasonable prediction of the size distribution resulting from aerodynamic droplet breakup.