X-ray Particle Tracking Velocimetry Research Articles

X-ray Particle Tracking Velocimetry in an Overflowing Foam

In mineral processing, froth flotation is based on recovering valuable mineral particles by means of the overflowing froth. Industrial-scale froth flotation cells are typically equipped with optical measurement systems, which monitor the bubble sizes and flow velocities at the froth surface. However, the velocity profile of the overflowing froth underneath its free surface is not accessible by optical observation. The present study combines X-ray radiography and particle tracking velocimetry in a laboratory-scale experiment aiming to measure local flow velocities within an optically opaque foam at a weir, which here describes a one-sided horizontal overflow. For this purpose, we prepared custom-tailored tracer particles: small 3D-printed polymer tetrahedra with tiny metal beads glued to the tetrahedral tips. In parallel to the velocity measurements by means of X-ray particle tracking, we determined the liquid fraction of the overflowing foam by electric conductivity measurements using electrode pairs. The experiment was performed with aqueous foams of two different surfactant concentrations but similar bubble size range and superficial gas velocity, yielding around 10% liquid fraction near the weir. Employing the particles as tools for flow tracing in X-ray image sequences, we measured the velocity profile in vertical direction above the weir crest and found that the maximum velocity is reached underneath the free surface of the overflowing foam.

Enhanced laboratory x-ray particle tracking velocimetry with newly developed tungsten-coated O(50 μm) tracers

Tracer particles designed specifically for X-ray particle tracking and imaging velocimetry (XPTV and XPIV) are necessary to widen the range of flows that can be studied with these techniques. In this study, we demonstrate in-lab XPTV using new, custom-designed O(50 μm) diameter tungsten-coated hollow carbon spheres and a single energy threshold photon counting detector. To the best knowledge of the authors, these are the first O(50 μm) tracer particles to be developed specifically for X-ray particle velocimetry. To explore the measurement quality enhancement enabled by the new tracer particles and photon counting detector, a well understood Poiseuille pipe flow is measured. The data show agreement with the analytical solution for the depth-averaged velocity profile. The experiment also shows that the tungsten-coated particles achieve higher contrast and are better localized than previously available silver-coated particles, making faster and more precise measurements attainable. The particles are manufactured with a readily scalable chemical vapor deposition process. We further show that laboratory XPTV is practical with currently available energy-resolving photon counting detectors (PCDs), despite their presently lower spatiotemporal resolution compared to scintillating detectors. This finding suggests that energy-thresholding identification of different classes of tracers is feasible, further motivating the exploration of the X-ray tracer particle design space. The latest generation of PCDs is incorporating multiple energy thresholds, and has higher count rate limits. In the near future one could potentially expand on the work presented and track multiple tracer species and scalar fields simultaneously.

Tomographic X-ray particle tracking velocimetry

We investigate the feasibility of in-laboratory tomographic X-ray particle tracking velocimetry (TXPTV) and consider creeping flows with nearly density matched flow tracers. Specifically, in these proof-of-concept experiments we examined a Poiseuille flow, flow through porous media and a multiphase flow with a Taylor bubble. For a full 360^circ computed tomography (CT) scan we show that the specially selected 60 micron tracer particles could be imaged in less than 3 seconds with a signal-to-noise ratio between the tracers and the fluid of 2.5, sufficient to achieve proper volumetric segmentation at each time step. In the pipe flow, continuous Lagrangian particle trajectories were obtained, after which all the standard techniques used for PTV or PIV (taken at visible wave lengths) could also be employed for TXPTV data. And, with TXPTV we can examine flows inaccessible with visible wave lengths due to opaque media or numerous refractive interfaces. In the case of opaque porous media we were able to observe material accumulation and pore clogging, and for flow with Taylor bubble we can trace the particles and hence obtain velocities in the liquid film between the wall and bubble, with thickness of liquid film itself also simultaneously obtained from the volumetric reconstruction after segmentation. While improvements in scan speed are anticipated due to continuing improvements in CT system components, we show that for the flows examined even the presently available CT systems could yield quantitative flow data with the primary limitation being the quality of available flow tracers.Graphic abstract

Measurements of the Flow in the Vicinity of an Additively Manufactured Turbine Leading-Edge Using X-Ray Particle Tracking Velocimetry

AbstractX-ray particle tracking velocimetry (PTV) is performed, for the first time, to measure the velocity field inside a leading-edge of a turbine blade made by laser-additive-manufacturing (LAM) process. The traditional showerhead holes were replaced by a porous matrix in the leading-edge. The flow through such a leading-edge piece cannot be faithfully recreated by traditional prototype testing methods due to the surface roughness and imperfections caused by LAM process. Hence, direct measurement is the only option. However, it is difficult to measure flow inside such pieces with traditional velocimetry measurements due to the existence of metallic walls. Moreover, small internal size and high flow speeds call for a measurement technique with high spatial and temporal resolutions. To address these issues, we performed time-resolved X-ray PTV using the Advanced Photon Source (APS) synchrotron facility at the Argonne National Laboratory (ANL). A hydraulic system was constructed to run water, mixed with seeding particles, through the leading-edge piece. A high-speed camera captured the images of the seeding particles, which were later processed to create particle tracks. The time-averaged velocity field showed distinct pairs of vortices located in front of the porous outlet inside the leading-edge piece. The inlet channel showed reversed flow due to partial obstruction by the porous inlet of the test piece. Such knowledge of the flow field inside a leading-edge of a turbine blade will help us to design better cooling paths leading to higher cooling efficiency and increased life-span of a turbine blade.

X-ray particle tracking velocimetry in liquid foam flow.

In this work, we introduce a novel approach to measure the flow velocity of liquid foam by tracking custom-tailored 3D-printed tracers in X-ray radiography. In contrast to optical observations of foam flow in flat cells, the measurement depth equals 100 mm in the X-ray beam direction. Light-weight tracers of millimetric size and tetrapod-inspired shape are additively manufactured from stainless steel powder by selective laser melting. Matching with the foam structure and bubble size, these tracers follow the foam flow. An X-ray beam passes through the radiotransparent foam channel and is detected by an X-ray image intensifier. The X-ray transmission images show the two-dimensional projections of the radiopaque tracers. Utilizing particle tracking velocimetry algorithms, the tracer trajectories are measured with both high spatial (0.2 mm) and temporal (25 fps) resolution. Fine and coarse liquid foam flow of different velocities are studied in a partly curved channel with rectangular cross section. The simultaneous time-resolved measurements of the tracers' translational motion and their intrinsic rotation reveal both the local velocity and vorticity of the foam flow. In the semi-circular curved channel section, the rigid-body-like flow pattern is investigated. Moreover, a relaxation of the foam structure in the transition zone between straight and curved section is observed.

Orientation of cylindrical particles in a fluidized bed based on stereo X-ray particle tracking velocimetry (XPTV)

Fluidization of non-spherical particles commonly exists in the biomass utilization and municipal solid waste (MSW) processing industries. In this study, cylindrical particles are used as a typical type of non-spherical particle and its orientation distribution is investigated when being co-fluidized with small spherical bed material. X-ray particle tracking velocimetry (XPTV), based on an X-ray stereography imaging system, is used to measure the 3D orientation of a single tracer particle over a long time period in the fluidized bed. The effects of gas velocity (uf), cylindrical particle mass fraction (ω), particle sphericity (Φ), and bed material size on the orientation distribution of the cylindrical particle are investigated and discussed in detail. An orientation distribution probability density function (PDF) model is proposed based on all experimental results.The distribution probability P of the angle between the cylindrical particle central axis and vertical direction λ across the bed shows two minima in the ranges 0° ≤ λ < 10° and 40° ≤ λ < 50°, and two maxima in the ranges 20° ≤ λ < 30° and 70° ≤ λ < 80°. Increasing uf reduces λ, while the effect of particle sphericity Φ and cylindrical particle mass fraction ω increases with increasing uf.

Characterizing 3D granular flow structures in a double screw mixer using X-ray particle tracking velocimetry

Granular flows are commonly encountered in many industrial processes, but are difficult to characterize due to the opaque nature of the flow. For instance, screw pyrolyzers are being developed for the thermochemical conversion of biomass into bio-oil, but the granular flow and mixing process inside the reactor lacks fundamental understanding. In this study, X-ray particle tracking velocimetry (XPTV) is used to qualitatively and quantitatively characterize the three-dimensional (3D) granular flow structures in a double screw mixer, which geometrically replicates double screw pyrolyzers, by visualizing the position and speed profiles and quantifying the dimensionless pathlength and dimensionless residence time of individual tracer particles. The influence of screw rotation speed, dimensionless screw pitch, screw rotation orientation, and material injection configuration are investigated. Certain operating conditions are shown to significantly influence the granular flow structures and, in some instances, cause the double screw mixer to behave similar to two single screw conveyors. Comparisons with previous granular mixing studies are made to provide a link between granular flow behavior and mixing effectiveness.

A cone-beam compensated back-projection algorithm for X-ray particle tracking velocimetry

Characterizing multiphase or granular flows is difficult due to the opaque nature of the system. While invasive measurement techniques provide detailed information about a single point, assessing the entire system is a laborious task due to the large number of samples required. Therefore, significant work has gone into developing noninvasive methods of measuring these flow systems. In this study, identical pairs of X-ray source/detector systems are used to provide two simultaneous but independent X-ray radiographic projections, which are then coupled together to perform X-ray stereographic imaging of a granular flow. A cone-beam compensated back-projection algorithm is developed for X-ray particle tracking velocimetry (XPTV). This method accurately corrects for the X-ray׳s cone-beam geometry, which is ignored in parallel-beam back-projection methods. To demonstrate the need for the cone-beam compensation, a direct comparison between the cone-beam and parallel-beam back-projection algorithms is used, and significant differences are presented. These methods are then used to perform XPTV in a double screw mixer, allowing the position and velocity of individual tracer particles to be characterized.

Simultaneous measurement of size and velocity of microbubbles moving in an opaque tube using an X-ray particle tracking velocimetry technique

An X-ray particle tracking velocimetry (PTV) technique was developed to simultaneously measure the sizes and velocities of microbubbles in a fluid without optical aberration. This technique is based on a combination of in-line X-ray holography and PTV. The X-ray PTV technique uses a configuration similar to that of conventional optical imaging techniques, and is easy to implement. In the present work, microbubbles generated from a fine wire by electrical heating were used as tracer particles. The X-ray PTV technique simultaneously recorded size and velocity data for microbubbles (φb=10–60 μm) moving upward in an opaque tube (inner diameter φ=2.7 mm). Due to the different refractive indices of water and air, phase contrast X-ray images clearly show the exact size and shape of overlapped microbubbles. In all of the working fluids tested (deionised water and 0.01 M and 0.10 M NaCl solutions), the measured terminal velocity of the microbubbles rising through the solution was proportional to the square of the bubble diameter. The proposed technique can be used to extract useful information on the behaviour of various bio/microscale fluid flows that are not amenable to analysis using conventional methods.

X-ray-based assessment of the three-dimensional velocity of the liquid phase in a bubble column

Information regarding the local liquid velocity in bubble columns is of great interest for research into its performance. Common optical methods fail in bubble flows that have large void fractions because of the different refraction indices of the liquid and gaseous phases. The new X-ray particle tracking velocimetry (XPTV) described here solves the problem by the use of X-rays instead of light. X-rays penetrate a gas/liquid flow in straight lines. XPTV enables us to measure the three-dimensional velocity of the liquid phase. This method was applied and validated in two bubble columns. The same method is also applicable to opaque liquids.