Ultrafast X-ray study of dense-liquid-jet flow dynamics using structure-tracking velocimetry

High-speed liquid jets and sprays are complex multiphase flow phenomena with many important industrial applications. Great efforts have been devoted to understand their dynamics since the pioneering work of Rayleigh on low-speed jets. Attempts to use conventional laser optical techniques to provide information about the internal structure of high-speed jets have been unsuccessful owing to the multiple scattering by droplets and interfaces, and the high density of the jet near the nozzle exit. Focused-X-ray-beam absorption measurements could provide only average quantitative density distributions using repeated imaging. Here, we report a novel approach on the basis of ultrafast synchrotron-X-ray full-field phase-contrast imaging. As illustrated in our case study, this technique reveals, for the first time, instantaneous velocity and internal structure of optically dense sprays with a combined unprecedented spatial and time resolution. This technique has tremendous potential for the study of transient phenomenon dynamics. Multiphase flow is a very common phenomenon. Daily-life examples include rain, volcanoes and aerosols. The wide scientific and industrial interest in the study of multiphase flow ranges from blood flow in the human body to industrial liquid sprays, and to the control of air pollution by dust particles. Despite its mundane existence, multiphase flow can have many non-intuitive behaviours owing to the complex interactions of its different phases, such as the appearance of a sand jet when a heavy sphere is dropped on a bed of sand. Sometimes, the importance of a phase in a multiphase phenomenon is recognized only when it is removed, as in the disappearance of splashing when a liquid drop impacts a solid surface in a vacuum environment. Also, in the past several years, the emerging fields of microfluidics and nanofluidics have stimulated great interest in understanding complex multiphase flows in small spatial dimensions. One of the most extensively studied multiphase flows is high-speed fuel injection into a combustion chamber. A key to successfully make the combustion cleaner and more efficient is a full understanding of the breakup and atomization mechanism of the fuel jet. It has long been known that the breakup of lowspeed jets results from the unstable capillary-wave growth on the jet surface, but high-speed liquid-jet breakup seems to start earlier, 472 ns 150 ps APS electron storage ring (hybrid-singlet mode)