Experimental characterization of secondary atomization at high Ohnesorge numbers

Abstract

A droplet subjected to external aerodynamic perturbations and fragmenting into smaller droplets is defined as secondary atomization. In the past decades, droplet breakup at low Ohnesorge numbers (Oh < 0.1) had been characterized with unanimity in the community regarding its physics. However, droplet breakup at high Oh number, i.e. atomization where the influence of viscosity is significant, has not been studied in detail, and there is a poor agreement in conclusions by various researchers. This work presents a thorough analysis of the modes of deformation and breakup exhibited by a high viscous (Oh = 0.5, Oh =1) droplet subjected to continuous airflow (We = 30-120). Several modes of droplet breakup are presented and transition case with no droplet fragmentation has been discovered. Size-velocity pdfs of droplets at different Ohnesorge numbers for varying Weber numbers were obtained by processing the images acquired using Digital in-line holography (DIH). Particle image velocimetry (PIV) was employed to characterize the airflow in cases where the droplets exhibited no breakup and cases with multiple bag formation. The percentage of minimum backflow velocity required for droplet breakup is also presented.