Experimental study on the breakup of a ligament in a sheet cross flow

In a rotary bellcup atomizer, a liquid film is stretched into thin ligaments due to the centrifugal force of the high-speed bellcup rotations. The ligaments are then disintegrated into droplets due to a high-speed sheet cross flow, called a shaping air. To clarify the breakup mechanism, in the present experiment we simplified and highlighted the effect of the shaping air by considering the breakup of single ligaments only; with the aid of high-speed imaging and quantitative image analysis, we studied the breakup of liquid jets of about 0.1 mm in diameter exposed to high-speed sheet cross flows with velocities varying up to 120 m/s. We used water or glycerol ethanol solutions as liquid and air as gas. As a result, we obtained drop-size distributions characterized by remarkable double peaks both of which were found to be well described by gamma distributions. We clarified that the larger distribution is caused by the formation and aggregation of the blobs generated along the ligament of about the original diameter, while the smaller distribution originates from the breakup of thinner ligaments stretched by the cross flow between the blobs. Our time- and space-resolved image analysis clearly shows such time evolution of local thickening and thinning of the ligaments.