X-RAY VISUALIZATION OF AN AIRBLAST ATOMIZER UNDER VARIOUS AMBIENT PRESSURES

Atomization has been studied extensively over the past century, but obtaining quality data in the near-field region is a challenge because the region is optically dense. This region, which typically extends just a few nozzle diameters below the exit, is where the liquid core first begins to break apart and is critical in the initiation of the atomization process. Studying the effects of pressure in the near-field region is even more challenging. In this study, a chamber for assorted pressurized spray surveillance (CAPSULE) is used with phase contrast x-ray imaging to characterize the near-field region of an airblast atomizer. The ambient pressure varies from P = 1 to P = 5 bar for three different momentum flux ratios of MFR = 5, 25, and 56. The gas flow through the outer gas chamber of the airblast atomizer also varies to impart swirl in the exiting gas. The swirl ratio, quantifying the ratio of swirl flow to coflow while maintaining a constant total gas flow rate, varies from swirl ratio SR = 0 to SR = 1.5. High-speed x-ray white beam imaging shows that the momentum flux ratio has a significant effect on spray formation, while the effects of ambient pressure are less observable. The swirl ratio enhances the radial spread of the liquid region. The liquid jet flapping frequency increases with increasing momentum flux ratio. Increasing the ambient pressure produces a small increase in flapping frequency. As swirl ratio increases, the flapping frequency increases until a critical swirl ratio is reached and then the flapping frequency drops significantly. The critical swirl ratio also decreases with an increasing momentum flux ratio.