Structure of Airblast Sprays Under High Ambient Pressure Conditions

Abstract

A single-velocity-component Phase Doppler Particle Analyzer is used to survey and measure local variations in drop-size distributions and drop velocities in the near-nozzle region of a practical, contra-swirling, prefilming airblast atomizer. The technique of Laser Sheet Imaging is used to obtain global patterns of the spray. All measurements are taken with a constant pressure drop across the atomizer of 5 percent, at ambient air pressures of 1, 6 and 12 bar. The liquid employed is aviation kerosine at flow rates up to 75 g/s. The results show that increasing the air pressure from 1 to 12 bar at a constant air/fuel ratio causes the initial spray cone angle to widen from 85° to 105°. Further downstream the spray volume remains largely unaffected by variations in atomizer operating conditions. However, the radial distribution of fuel within the spray volume is such that increases in fuel flow rate cause a larger proportion of fuel to be contained in the outer regions of the spray. The effect of ambient pressure on the overall Sauter mean diameter is small. This is attributed to the fact that the rapid disintegration of the fuel sheet produced by the contra-swirling air streams ensures that the atomization process is dominated by the ‘prompt’ mechanism. For this mode of liquid breakup, theory predicts that mean drop sizes are independent of air pressure.