Effect of air swirl and nozzle geometry on flapping instability of liquid jet in an air-blast atomizer

The present work investigates flapping instability in 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 region, and, as a consequece, may lead to spatio-temporal fluctuations spray droplet characteristics further downstream. Though such instability has been reported in earlier works, the influence of the air swirl and nozzle geoemtry are yet unknown. The reported experiments in the present paper consider two coaxial air-blast atomizers with the same annular gas jet diameter but different central liquid jet diameter. Experiments were performed over a wide range of aerodynamic Weber number, Weg (81-1200) and air to liquid momentum flux ratio, M (0.4-33). For each pair of air-liquid velocities, experiments were conducted in presence and absence of air swirl. The primary breakup of the liquid jet was imaged using High Speed Shadowgraphic technique. The liquid jet flapping was characterized by temporal tracking of the liquid-air interface close to the jet breakup point. The flapping frequency was found to be higher than the shear driven Kelvin-Helmholtz instability for lower range of M. It was also found that the air swirl does not influence the jet breakup mode, however, it enhances the amplitude and the frequency of the jet flapping. Also, the nozzle geometry has a strong influence on flapping characteristics.