This work concerns the instability behavior of a circular water sheet that breaks up into drops by orthogonal impingement of a vertical water jet against a conical boss sprinkler,a canonical geometry adopted in fire suppression applications.A cone angle of 90° with a nozzle K-factor of 58 lpm bar –0.5 is experimentally investigated.The goal is to correlate the instability characteristics of the water sheet with the ligaments that the sheet breaks up into and the drop size distribution,in turn,with the ligament statistics.Time-resolved imaging at KHz framing rate is adopted in terms of shadowgraphy,Mie scattering of high-repetition rate laser light sheet,and planar laser induced fluorescence(PLIF)of the liquid.A magnified view is used in tandem with the high-speed imaging to obtain very high spatio-temporal resolution in the region where the sheet breaks up in order to quantify the ligament characteristics statistically.Further,the liquid sheet wave propagation obtained by image processing is correlated with the subsequent ligament and drop movement radially outward by means of PLIF.The high-speed shadowgraphy images clearly show the propagation of both radial and azimuthal waves on the liquid sheet superposed on each other.The high repetition rate laser sheet imaging shows that the radial wave amplitude spatially grows until it shears abruptly over a narrow radial zone into ligaments.The wavelength of radial waves and their spatial growth rate along with the amplitude coefficient are deduced from these images,and are directly correlated with the ligament size.The propagation of the azimuthal waves superposed over the radial waves on the liquid sheet causes the later to flap axially in the laser sheet images,and the flapping amplitude is reported with injection pressure drop as indicative of the azimuthal wave amplitude.