X-Ray Cineradiography at MHz Frame Rates
Synchrotron Light Depicts Fast Processes in Opaque Objects
- Fig. 1: (a) Fuse schematic diagram. (b) Time-lapse X-ray radiographs of fuse element melting and vaporizing during electric arc ignition captured at 5 Mfps and 110 ns exposure (see video: http://bit.ly/video-Olbinado)
- Fig. 2: Visualization of fuel spray from an injection nozzle using (a) optical shadowgraphy at 1 Mfps and 0.25 µs exposure (adapted from Ref. 7 with permission from Springer), and (b) synchrotron X-ray radiography at 2 Mfps and 0.5 µs exposure.
Time-resolved imaging of transient dynamic processes inside opaque materials and optically dense sprays challenges the limits of ultra high-speed imaging using conventional light sources. Hard X-ray imaging using synchrotron light overcomes these limitations as it allows to probe inside dense or multi-scattering objects. Here, we exploit synchrotron-based X-ray imaging to visualize electric arc ignition during fuse operation, and spray structure of gasoline during injection with MHz frame rates (see video).
Real-time visualization of dynamic processes is an important tool for both basic research and applied science. The scope of high-speed photography and photonics covers applications such as atomic energy, fluid flow, combustion, advanced materials and manufacturing, machine vision, automotive, aeronautics, space, safety and defense technologies, medical and sports. Conventionally, real-time radiography using hard X-rays generated from single-flash X-ray systems, with pulse widths of 10-7 to 10-9 s, have been typically used since 1960s as a means for investigating transient processes in objects, that are opaque to other photon probes . Time-series of X-ray images are obtained by repeating experiments with varying time delays for each image acquisition. This stroboscopic approach works well, though only if the experiment can be precisely timed and where it is possible to reproduce the experiment over many repetitions: in many cases however, temporal and spatial evolution of dynamics has to be gathered in a single-shot experiment. Due to the limited brightness, high spatial resolutions are difficult to access. At third-generation synchrotron light sources, hard X-rays (10 -100 keV) with brilliance up to 1020 photons/s/0.1%bandwidth/mm2/mrad2 are available. At the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, time-resolved X-ray imaging with multiple-frame recordings (cineradiography) at mega-Hertz (MHz) frame rates can now be performed at beamline ID19 . This allows for the visualization of dynamics inside opaque materials such as electric arc ignition inside a fuse encased in a ceramic casing; and the visualization of fluid dynamics in gasoline spray, which is difficult to realize using optical shadowgraphy due to the absorption of light in the liquid and multiple scattering by interfaces.
Ultra high-speed X-ray imaging was performed using an indirect X-ray detector composed of a fast-decay scintillator (250 μm-thick, Ce-doped Lu3Al5O12 (LuAG:Ce) from Crytur, Czech Republic), which is lens-coupled to an ultra high-speed visible light camera (Hyper Vision HPV-X2, Shimadzu Corp., Japan), which achieves up to 5 million frames per second (Mfps).
The camera has a frame-transfer complementary metal-oxide-semiconductor (FT-CMOS) sensor with 400 × 250 pixels of 32 μm size. One recording sequence stores 128 images on-chip. A 4× magnification objective lens system (0.2 numerical aperture, Optique Peter, France) was used to achieve an effective field of view of 3.2 mm × 2.0 mm, and an effective pixel size of 8.0 μm. The beamline’s two U32 undulators operated at maximum power was used as a polychromatic X-ray source. The low-energy content of the beam was attenuated by 1.4 mm thick diamond and 0.7 mm thick aluminum filters. The experiments were performed during the uniform filling mode of the ESRF storage ring (maximum ring current 200 mA). These settings guaranteed a maximum photon flux (approximately 1013 ph/mm2/s) at the sample.
Electric Arc Ignition in Fuse
Fuses are found in most electrical circuits and functions to protect electrical components against current surges. Despite their widespread use, nobody has ever watched what happens inside a fuse during operation. Studies of fuses have often relied on theory and post-mortem analysis such as scanning electron microscopy and X-ray computed tomography. At industrial facilities, these methods of performance control need to be done on huge number of fuse prototypes, which is not practical. Time-resolved imaging during in-situ fuse operation is therefore a big leap towards understanding fuse operation for smarter fuse designs.
A typical high-breaking-capacity fuse has four main components: a metal strip (typically silver) with constricted regions, an arc-quenching material (usually silica sand), a ceramic cartridge, and two metal electrodes that connect to the circuit (fig. 1(a) - see video). During a fault current situation, the temperature inside the fuse element increases especially in the constrictions where ohmic resistance is highest (due to Joule effect). The metal at the constriction is melted and vaporized, leading to an electric arc ignition. This is called the pre-arcing period which ends in the sharp increase of the voltage across the fuse . A time series of X-ray radiographs obtained at 5 Mfps, depicting the pre-arcing period is shown in figure 1. The fuse was fabricated by Mersen, France. In order to visualize the fundamental phenomena during electric arc ignition, this fuse was especially made without its arc quenching material. The fuse operation was initiated by a high current delivered by a charged 8 mF capacitor and a gate turn-off thyristor switch. A pulse generator triggered both the fuse operation and the image acquisition, and an oscilloscope (Keysight Technologies, Canada) was used to monitor the current through and voltage across the fuse to confirm electric arc ignition. The melting of the silver metal strip started at the constriction at 20.0 µs after the fault current was generated to create Joule heating. The first frame in figure 1(b) shows the exact time of occurrence of electric arc ignition at 23.0 µs after the fault current, which was confirmed by a sharp increase of voltage across the fuse.
The appearance of the striation is not exactly understood, but could be explained by Laplace magnetic forces acting on the liquid metal column due to the high current densities . These results show that sub-microsecond image acquisition is crucial in visualizing the striation which appears during electric arc ignition and which must be understood to properly formulate physical models of the fuse operation.
Gasoline Spray During Injection
Innovations in designs of fuel injection and combustion engines are important to improve thermal efficiency of fossil-fueled engines, and to reduce exhaust emission, which is significant contributor to increased green house gases in the global environment. In gasoline engines, there has been a switch to high-pressure delivery of fuel via direct injection methodologies (GDI) in order to create better nebulization of the fuel, higher control of the combustion process, and reduce emissions and fuel consumption . Though beneficial from an efficiency and emissions standpoints, the stable performance of GDI engines, which rely heavily on carefully designed spray pattern and its interaction with the intake charge flow, are significantly affected by carbon deposits in and around the injector sprayholes (called injector coking effect) . Fuel injection is a fast phenomenon which occurs in the sub-milliseconds time regime, it is therefore necessary to investigate the effect of such deposits on GDI fuel injections using high-speed data acquisition. Ultra high-speed visualizations of fuel injection has been performed up to 1 Mfps camera frame rate and using an intense visible flash light called optical shadowgraphy. However, due to scattering of light by the transient droplet field, the spray body is rendered black and impenetrable (fig. 2 (a)) . This limitation is addressed by X-ray imaging. By using synchrotron X-rays at beamline ID19 of the ESRF, a visualization of GDI multi-hole fuel injection has been realized up to 2 Mfps. The X-ray radiograph is shown in figure 2 (b), side-by-side with an optical shadowgraph. The internal structures of the spray are clearly revealed. The X-ray image contrast comes from interfaces with different thicknesses and refractive indices, the very features that cause light-scattering in optical shadowgraphy. Injectors with various deposits have been investigated in this work. The results will be compared with predictions of computational fluid dynamics .
We have shown that X-ray cineradiography with MHz frame rates, made possible thanks to high brightness synchrotron X-rays and fast X-ray detection, opens new possibilities in visualizing transient dynamics inside dense, opaque, and multi-scattering objects.
Margie P. Olbinado1, Xavier Just2, Jean-Louis Gelet3, Peter Hutchins4, Hongming Xu5, Christopher Powell6, Alexander Rack1
1 ESRF – The European Synchrotron, Grenoble, France
2 Université Grenoble Alpes, Centre National de la Recherche Scientifique, Science Ingénierie des Matériaux et des Procédés, Grenoble, France
3 MERSEN France SB, Saint Bonnet de Mure, France
4 Prism Scientific Limited, Wallingford, UK
5 Mechanical Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
6 Energy Systems, Argonne National Laboratory, Argonne, IL, USA
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