MASTER'S THESIS Atomisation of Liquid Jets in Gaseous Crossflow

A round liquid jet in a gaseous crossflow (water in air) was observed over a range of conditions in order to investigate the relationship between the final width of the droplet cloud and the momentum ratio between the jet and the crossflow. The motivation for this work is as it relates to aerial fire suppression – that is, the use of aircraft to drop water or other fire retardant products onto vegetation to control and contain bushfires (forest fires). The experiment was set up on a frame extending from the side of a car. The reservoir, pump and controls were placed inside the car and the jet was fixed on the end of the frame to the side of the car. The water was coloured with dye, and as it was sprayed onto the ground it was filmed from above with a video camera fixed to a stationary frame. The various breakup phenomena were observed in this manner, and the droplets fell onto a sheet of paper, weighed down on the ground. Still images were taken of the drop pattern made visible on the paper, which were later processed in MatLab and a mean profile and full width half maximum were extracted. Previous work, collating real scale drop tests, had shown a strong dependence of the width, normalised by a characteristic nozzle diameter, on the momentum ratio between the jet and the crossflow. A power law model was suggested, λ/ds = f2q 1/5 with different prefactors for free fall (or gravity fed) systems, f2 = 27, and pressurised jet systems, f2 = 58. Experiments were carried out over a range of momentum ratios with different nozzle diameters, flow rates and car velocities, and a reasonably good agreement was found between the data and the proposed model. Finally, a new component to the model is suggested. However, time constraints meant that insufficient data were collected to develop a complete model.