Experimental investigations of atomization characteristics of an airblast atomizer at high ambient pressure
Spray performance of swirling airblast atomizer is important to fuel droplet combustion, heat transfer and pollutant emission for aircraft engines. Optimizing atomization characteristics of fuel injectors is one of goal for next generation aircraft. A comprehensive experimental investigation motivated by improving the efficiency of aircraft engines and reducing pollutant emissions was performed in a high-pressure spray test facility which is built in Iowa State University to study spray characteristics of an airblast atomizer under different working conditions. The effects of pressure drop ratio, ambient pressure, and fuel flow rate on spray characteristics and fuel injector performance are studied using stereo particle image velocimetry (SPIV). It was found that while air velocity fields can be significantly affected by the pressure drop ratio, the influences of ambient pressure is very slightly. Furthermore, a theoretical analysis reveals that if the airflow is assumed to be isentropic, inviscid and steady, airflow velocity should be proportional to the square root of the pressure drop ratio. At atmospheric pressure, the flow field at various pressure drop ratios can satisfy it very well but under high ambient pressure (e.g., 105 psi), the velocity magnitude is not perfectly proportional to the pressure drop ratio. Next, it should be noted that both ambient pressure and pressure drop ratio can greatly affect the spray velocity field. Spray droplets can obtain a higher velocity with a larger ambient pressure or a higher pressure drop ratio. Furthermore, the velocity profile of spray droplets gradually approaches the airflow velocity profile as ambient pressure is increasing. This phenomenon indicates the tracing capacity of spray droplets is getting better at high ambient pressures. A theoretical model is built to show the effects of ambient pressure on the droplets tracing capacity by evaluating Stokes number of droplets under different ambient pressures. The analysis suggests that at high ambient pressure, Stokes number for droplets significantly gets smaller (i.e. better tracing capacity) due to the increase of air density. It reveals that droplets seem more possible to closely follow air stream motion under high ambient pressure. In addition, the shrinking size of droplets at high ambient pressure also provides better tracing capacity for droplets. These results and analysis suggest that future studies about fuel atomization should be conducted under high ambient pressure (e.g., >100 psi), since the airblast atomizer may have distinctive performance under different ambient pressures. Finally, the effects of the liquid flow rate on the spray velocity field are found to be very slight. When liquid flowrate increases from 24 ml/min to 90 ml/min, the velocity magnitude of spray droplets gets a little smaller due to the increase of the number of liquids but this change is within 5 m/s. This result shows the stable performance of the airblast atomizer under different operating conditions.