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.

Experimental investigations of atomization characteristics of an airblast atomizer at high ambient pressure

“…The effects of ambient pressure on the performance of atomizers have been investigated by some previous studies [14,19,20,36,42,43]. Jasuja [19] stated that mean velocity of spray droplets diminishes with increase in ambient pressure because this experiment keeps air-fuel mass ratio to be a constant so that at high ambient pressure, airflow velocity definitely will be smaller due to the increase of air density, and the airflow has less ability to scatter liquid sheets into droplets.…”

“…Leong et al [20] found that the increase in ambient pressure leads to the decrease of spray penetration depth when pressure drop and fuel flow rate are held constant. Zheng et al [42] used PDPA to measure droplet size distributions and droplet velocity of a prefilming airblast atomizer at different ambient pressures. The results showed that the effect of ambient pressure on the average size and velocity profiles of spray droplets is quite small.…”

“…The results showed that the effect of ambient pressure on the average size and velocity profiles of spray droplets is quite small. This is mainly attributed to the fact that for the specially-designed contraswirling airblast atomizers employed in their study, the atomization mechanism is independent of pressure [42]. Although some of recent studies have focused on the effects of ambient pressure on atomization process of airblast atomizers, the quantitative measurement of the velocity of spray droplets and airstreams in the flow fields with different ambient pressure has not been conducted yet, and influence of ambient pressure on the velocity of spray droplet is unknown.…”