Abstract

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.

The present work studies low viscosity twin-fluid atomization experimentally and analytically to characterize and predict the droplet size distribution of the spray. The study is based on experiments conducted using commercially available twin-fluid nozzles with water as the liquid. Shadowgraph images were used to visualize the near-nozzle flow while the droplet size distribution was measured in the far field using a Malvern Spraytec. To analytically model the atomization of the spray, the authors’ recent works on aerodynamic droplet breakup, which describe the formation and breakup of ligament and bag structures by multiple mechanisms, are extended to provide an analytical prediction of the droplet size distribution of the spray that is validated against the present experiments. The present model is developed to be a good physical representation of the spray behaviour at practical operating conditions. A Python implementation of the model has been deposited in a GitHub repository to accompany this work.

No abstract

The present work studies low-viscosity two-fluid atomization experimentally and analytically in order to characterize and predict the formation and breakup of ligaments in the spray. The study is based on experiments conducted using commercially available two-fluid nozzles with water as the liquid. Shadowgraph images were used to visualize and characterize the nearnozzle flow and ligaments while the droplet size distribution was measured in the far-field using a Malvern Spraytec. Our images reveal that two different wave structures contribute to the breakup: surface waves, and bulk waves. These waves eventually result in the formation of structures that breakup with a mechanism similar to droplet breakup, where small droplets are formed from the surface waves and large droplets form from the bulk waves. A recent deformation-rate based droplet breakup model is applied to the surface and bulk wave geometries to predict the formation and breakup of ligaments. These predictions are compared to measurements of the ligament sizes from the shadowgraph images as well as the droplet size distributions with good agreement in the order of magnitude and trends of each mode.