An Experimental Investigation of Kerosene Droplet Breakup by Laser-Induced Blast Waves

The work presented in this paper intends to deepen our understanding of the mechanisms involved in the spark ignition of liquid fuel sprays. An experimental study is presented regarding the ignition of monodisperse droplet chains of Jet A-1 aviation kerosene in a generic model combustor under well-defined boundary conditions. Breakdowns created by focused laser radiation were used as ignition sparks. They featured rapid spatial expansion, resulting in the formation of spherical blast waves in the surrounding air. The focus of this study lay on the effect of the blast waves on the fuel droplets. Blast wave trajectories were investigated by Schlieren imaging. Their interaction with kerosene droplets was observed with a high speed camera via a long distance microscope; the droplets were visualized by laser-induced Mie scattering. Droplets within a distance of 10 mm from the breakdown position were deformed and disintegrated by the aerodynamic forces of the postshock flow field. Different breakup modes were observed, depending on the distance from the breakdown position: Catastrophic breakup was observed at a 5 mm distance, resonant breakup was observed at a 10 mm distance. Breakup by blast waves from ignition sparks is expected to be a crucial mechanism for spray ignition because it supports evaporation. Weber number calculations revealed that the breakup modes observed under lab conditions will also appear in aviation gas turbines at high altitude relight conditions.