Effect of Azimuthal Velocity Fluctuation on Hollow Cone Spray

This paper reports an experimental investigation on the effect of imposed azimuthal velocity fluctuation on the spray characteristics of a hollow cone spray produced from a pressure-swirl nozzle. This effect is inferred by performing experiments separately on 0° and 60° axial straight vane swirlers oriented concentric to the spray nozzle for the air-flow subjected to upstream acoustic forcing. The hollow cone spray is subjected to external excitation using a loudspeaker at two different frequencies and two amplitudes. These frequencies correspond to the resonant frequencies of the plenum. The 60° vane swirler, when subjected to acoustic excitation, produces axial and azimuthal velocity fluctuation downstream of the swirler, whereas the 0° swirler produces only axial velocity fluctuation downstream of the swirler. In both 60° and 0° swirlers, the downstream velocity fluctuation due to acoustic disturbances propagate at the speed of sound. In addition, the velocity fluctuations produced due to the excitation are convected by the mean flow. This results in a combined effect of velocity fluctuations at the swirler exit. The hollow cone spray responds more readily to excitation at low frequencies than higher frequencies. This is observed from the high-speed shadowgraph images. The high-speed shadowgraph images acquired are processed to extract spray cone angle. The phase averaged spray cone angle fluctuates for different phase angle within an acoustic cycle. The analysis of the high-speed shadowgraph images shows that the cone angle fluctuation amplitude is higher for the 0° swirler as compared to the 60° swirler. The variation of the liquid sheet thickness and breakup length due to excitation is captured using high-speed planar laser induced fluorescence. The study clearly demonstrates that azimuthal velocity fluctuation affects the spray formation process.