Liquid sprays play a key role in many engineering processes (e.g., food processing, coating and painting, 3D printing, fire suppression, combustion systems, etc.). The dynamics at the nozzle exit have a large impact on the downstream spray characteristics. However, visualizing the spray in this region is extremely challenging because, under most operating conditions, the spray is optically dense. Specialized X-ray diagnostics, like those found at the Advanced Photon Source (APS) at Argonne National Laboratory, can be used to produce time-resolved planar visualizations of the liquid-gas structures. These high-speed, high spatially resolved, images are analyzed to yield a qualitative understanding of the phenomena at play and quantitative information about the spray formation process in the near-field region.
A canonical airblast atomizer consisting of coaxial water and air jets is investigated with this novel imaging technique. The inner water flow is kept laminar at Rel = 1000, while the outer air jet is turbulent with Reg = 16,700. An air swirl ratio (SR), defined as tangential air flow rate divided by parallel air flow rate, was fixed at SR = 1. High-speed X-ray imaging of the flow obtained at O(104 Hz) shows the dynamics of the near-field region. This unique imaging allows for visualizations inside the aluminum nozzle and shows the liquid wetting the outside of the injection needle, where the gas flow is separated, particularly in the presence of gas swirl. The morphology of liquid break-up transitions from bag breakup to ligament formation are clearly observed. High-speed X-ray imaging captures air bubbles entrained inside the water jet prior to breakup. Also, air bubbles are found inside water droplets as these are formed and convected downstream, entrained by high amplitude distortions of the liquid-gas interface folding over themselves. The unique flow features of the atomization process presented in this video are captured thanks to the novel use of high-speed X-ray imaging.
This work was sponsored by the Office of Naval Research (ONR) as part of the Multidisciplinary University Research Initiatives (MURI) Program, under grant number N00014-16-1-2617. The views and conclusions contained herein are those of the authors only and should not be interpreted as representing those of ONR, the U.S. Navy or the U.S. Government.
This work was performed at the 7-BM beamline of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.