ID 103

Effect of air swirl and nozzle geometry on flapping instability of liquid jet in an airblast atomizer.

 

Abstract:

The present work investigates the flapping instability of liquid jets in the presence of annular swirling air flow. The jet flapping is a large-scale instability, which may lead to vigorous lateral oscillation of the tail end of the jet core consequently influencing downstream spatio-temporal fluctuations of spray droplets. Though such instability has been reported in earlier works, the influence of the air swirl is yet unknown. The reported experiments consider a coaxial airblast atomizer that issues a centrally flowing water jet surrounded by a high-speed annular air jet. A swirler (geometric swirl number 0.84) is used in the air passage to impart angular momentum to the annular air flow. The primary breakup of the liquid jet is imaged using High Speed Shadowgraphic technique. Experiments are performed for a wide range of aerodynamic Weber number (83‒297) and air to liquid momentum flux ratio (1.1‒8.3). Measurements in the absence of air swirl are also reported as reference. The jet flapping, without or with air swirl, is characterized by temporal tracking of the liquid-air interface close to the breakup point. The amplitude and frequency of this instability was found to be different from the shear driven Kelvin-Helmholtz instability, which is characterized close to the injector exit. Since the nozzle geometry has a decisive influence on the jet breakup process, two different nozzles with different liquid exit diameter but the same outer diameter of the air flow are considered to identify the respective influence on the jet flapping behavior.