Primary jet breakup dynamics in a coaxial air blast atomizer: Effect of nozzle geometry and air swirl on jet instabilities and atomization

Understanding the influence of nozzle geometry and swirling air on the liquid jet breakup length and various instabilities during the primary breakup process of the liquid jet in a coaxial airblast atomizer is the aim of this paper. Experiments were performed in three different nozzle geometries for a wide range of air and liquid flow rates both in the presence and absence of air swirl (swirl number, S = 0 and 0.8 respectively). High-speed shadowgraphic visualization of the liquid jet core is done near the nozzle exit. The mean jet breakup length showed a weak dependence on nozzle geometry. The amplitude and frequency corresponding to Kelvin-Helmholtz instability and flapping instability were measured by tracking the temporal variation of the liquid-air interface near the nozzle exit and near the breakup point respectively. The nozzle geometry was found to strongly influence the jet instabilities. The presence of swirling air enhances the atomization process such that the mean breakup length reduces and the amplitude and frequency corresponding to different jet instabilities are increased.