Research in this area has been in a predicament for a long time, and there is a great need for accurate experimental measurement to serve as the input for further theoretical study. Without information on the time-dependent morphology of the jet, it is not possible to either probe its speed or understand its dynamics and its subsequent breakup. However, owing to a dearth of experimental data to validate the initial and boundary conditions, and the complexity of these systems, current models are empirical, often oversimplified and mostly not validated by experiments 23.
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Computational fluid dynamics simulation is widely adopted in fluid-mechanics research. As far as we know, there has not been any experimental measurement on the time-dependent internal morphology or velocity fields in highly turbulent and optically dense jets. However, the use of tracer particles to map fluid velocities is an invasive approach because particles modify flow properties (especially in confined spaces), and their speeds often do not match the actual flow velocity owing to parameters such as buoyancy, particle interactions and shapes. We have also shown that X-ray imaging can be advantageous in tracking polydispersed particles in a creeping pipe flow 22. We used X-rays in our previous work on coaxial water jets to circumvent some of the light-scattering difficulties, owing to the weak interaction of X-rays with matter, but no dynamical study was then feasible and our results were limited to an averaged three-dimensional water-volume-fraction reconstruction 9. As a result, the dense sprays appear optically opaque and quantitative analysis of the images is difficult, if not impossible 1, 2, 15, 21. Existing measurement techniques are mainly based on light scattering and suffer from absorption, reflection and multiple scattering owing to the existence of a huge number of droplets and a complex air–liquid-interface morphology when the jet is breaking up 1, 2, 19, 20. Several additional breakup mechanisms have been proposed, but not clearly observed 16, 17, 18. It has long been known that the breakup of low-speed jets results from the unstable capillary-wave growth on the jet surface 15, but high-speed liquid-jet breakup seems to start earlier, at the nozzle exit 2, where the distance is too short for the unstable surface wave to develop, and is strongly dependent on the nozzle internal design, initial flow conditions and jet Weber number.
#Accurate 4 305 full
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. One of the most extensively studied multiphase flows is high-speed fuel injection into a combustion chamber. 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 13, 14.
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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 12. 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 11. The wide scientific and industrial interest in the study of multiphase flow ranges from blood flow in the human body 8 to industrial liquid sprays 9, and to the control of air pollution by dust particles 10. Daily-life examples include rain, volcanoes and aerosols.
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Multiphase flow is a very common phenomenon. This technique has tremendous potential for the study of transient phenomenon dynamics. 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. Here, we report a novel approach on the basis of ultrafast synchrotron-X-ray full-field phase-contrast imaging 7.
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Focused-X-ray-beam absorption measurements could provide only average quantitative density distributions using repeated imaging 6. 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 5. Great efforts have been devoted to understand their dynamics since the pioneering work of Rayleigh on low-speed jets 3, 4. High-speed liquid jets and sprays are complex multiphase flow phenomena with many important industrial applications 1, 2.