Positron Emission Particle Tracking Research Articles

On the autonomous validation and comparison of particle models for a Newtonian laminar flow mixing model using PEPT

A methodology for the validation of a Computational Fluid Dynamics (CFD) mixing model using Positron Emission Particle Tracking (PEPT) was applied to a horizontal laminar stirred tank containing Newtonian glycerol. Particle trajectories recorded using PEPT were compared to three one-way particle-coupled CFD models (CFD-DEM, and CFD-Lagrangian, and CFD-Massless) through time-averaging and binning into a grid of 3-D voxels. A cell-by-cell comparison of particle velocities between the computational models and PEPT highlighted the accuracy of the CFD-DEM and CFD-Massless models over the Lagrangian model. Furthermore, using PEPT as a validation dataset the Pearson r-squared was used as a cost function to guide an autonomous calibration of a 5-dimensional DEM parameter space. A novel statistic, the simulation quality coefficient Qsim, was applied to assess accuracy between trajectory datasets and revealed the extent to which a model can be considered calibrated. Considering the balance between computational cost and precision, the CFD-massless model was the most effective particle model for this validation workflow. Further investigations will apply this methodology to calibrate non-Newtonian laminar CFD models.

Comparative evaluation of electrical resistance tomography, positron emission particle tracking and high-speed imaging for analysing horizontal particle-liquid flow in a pipe

We evaluate three experimental techniques - electrical resistance tomography (ERT), positron emission particle tracking (PEPT) and high-speed imaging (HSI) – for analysing the local particle velocity field and spatial distribution in a horizontal particle-liquid pipe flow under varying conditions of solid concentration. A new ERT methodology is devised for estimating particle velocity, circumventing the limitations of the conventional cross-correlation technique. Furthermore, an enhanced HSI approach is introduced and systematically compared with PEPT and ERT. Results show that, under all conditions, PEPT provides the most accurate particle velocity field followed by HSI, whilst ERT yields the most accurate concentration field, followed by HSI. The enhanced HSI emerges as a simple cost-effective option compared to PEPT and ERT. A combined measurement approach using PEPT for local particle velocity and ERT for local concentration, however, delivers the best comprehensive two-phase flow characterization, highlighting potential synergies between these methods for complex flow studies.

Multiscale analysis of turbulence in horizontal pipes: Liquid and particle-liquid flow investigation

An experimental–theoretical methodology is developed to investigate the characteristics of turbulence in horizontal particle-liquid pipe flows. Using a discrete wavelet transform, the three-dimensional Lagrangian trajectories of the liquid phase experimentally determined by positron emission particle tracking are decomposed into their deterministic and stochastic sub-trajectories, which are then utilized to construct profiles of local fluctuating velocity components and turbulent kinetic energy. The results for a single-phase flow are independently validated using computational fluid dynamic simulation and the analysis parameters are fine-tuned using direct numerical simulation data from the literature. In a particle-liquid flow, the investigation explores the influence of various factors including particle size, density, and concentration on turbulence intensity. Remarkably, the results demonstrate significant effects of the particle size and density on liquid turbulence. The enhanced understanding gained regarding turbulence intensity helps to advance our fundamental interpretation of the dynamics of particle-liquid flows, thus potentially aiding the rational design of such complex flows and associated equipment.

Evaluating the effectiveness of CFD-DEM and SPH-DEM for complex pipe flow simulations with and without particles

We investigate the effectiveness of two computational fluid dynamics (CFD) approaches: mesh-based CFD and meshfree particle-based smoothed particle hydrodynamics (SPH) for simulating pipe flows of varying complexity. The study covers laminar and turbulent flows, different fluid rheologies (Newtonian, power-law, Bingham plastic, Herschel-Bulkley), and different particle-laden scenarios, validated using experimental Lagrangian measurements obtained by positron emission particle tracking or available theoretical solutions, as appropriate. We assess these methods based on their ability to predict radial profiles of local phase velocity and concentration, as well as computational cost. In single-phase flows, CFD aligns well with experimental data and theoretical models. SPH exhibits boundary discrepancies due to no-slip condition approximations and limitations in turbulent flow simulation which need further development. Integrating the discrete element method (DEM) significantly enhances both techniques for particle-liquid flows. Mesh-based CFD is computationally efficient, while particle-based SPH can offer more insights into Lagrangian fluid dynamics.

In vivo real-time positron emission particle tracking (PEPT) and single particle PET

Positron emission particle tracking (PEPT) enables 3D localization and tracking of single positron-emitting radiolabelled particles with high spatiotemporal resolution. The translation of PEPT to the biomedical imaging field has been limited due to the lack of methods to radiolabel biocompatible particles with sufficient specific activity and protocols to isolate a single particle in the sub-micrometre size range, below the threshold for capillary embolization. Here we report two key developments: the synthesis and 68Ga-radiolabelling of homogeneous silica particles of 950 nm diameter with unprecedented specific activities (2.1 ± 1.4 kBq per particle), and the isolation and manipulation of a single particle. We have combined these developments to perform in vivo PEPT and dynamic positron emission tomography (PET) imaging of a single radiolabelled sub-micrometre size particle using a pre-clinical positron emission tomography/computed tomography scanner. This work opens possibilities for quantitative assessment of haemodynamics in vivo in real time, at the whole-body level using minimal amounts of injected radioactive dose and material.

Using Gaussian process for velocity reconstruction after coronary stenosis applicable in positron emission particle tracking: An in-silico study

Accurate velocity reconstruction is essential for assessing coronary artery disease. We propose a Gaussian process method to reconstruct the velocity profile using the sparse data of the positron emission particle tracking (PEPT) in a biological environment, which allows the measurement of tracer particle velocity to infer fluid velocity fields. We investigated the influence of tracer particle quantity and detection time interval on flow reconstruction accuracy. Three models were used to represent different levels of stenosis and anatomical complexity: a narrowed straight tube, an idealized coronary bifurcation with stenosis, and patient-specific coronary arteries with a stenotic left circumflex artery. Computational fluid dynamics (CFD), particle tracking, and the Gaussian process of kriging were employed to simulate and reconstruct the pulsatile flow field. The study examined the error and uncertainty in velocity profile reconstruction after stenosis by comparing particle-derived flow velocity with the CFD solution. Using 600 particles (15 batches of 40 particles) released in the main coronary artery, the time-averaged error in velocity reconstruction ranged from 13.4% (no occlusion) to 161% (70% occlusion) in patient-specific anatomy. The error in maximum cross-sectional velocity at peak flow was consistently below 10% in all cases. PEPT and kriging tended to overestimate area-averaged velocity in higher occlusion cases but accurately predicted maximum cross-sectional velocity, particularly at peak flow. Kriging was shown to be useful to estimate the maximum velocity after the stenosis in the absence of negative near-wall velocity.

Flowpattern in hydrocyclones, numerical simulations with experimental verification

This paper reports a detailed study of the flow in cyclone separators, with the use of most up to date computational fluid dynamics simulations, which are validated with positron emission particle tracking (PEPT) experiments tracing the movement of particles through the cyclone. The parameters varied were the viscosity of the carrier liquid, the flowrate and, in the numerical simulations, the inlet configurations of the cyclone, namely one and two inlets and, with the two inlets, a) both at right angles to the cyclone axis and b) angled downwards. The study reveals features of the flow, which have not been seen till now, but are necessary for the understanding and modelling of the separation and purification efficiency of cyclones. The results of the simulations and the close agreement with experiment are a testament to the reliability and accuracy of large eddy simulation (LES), even for flow features as difficult to simulate as the confined strongly swirling flows in cyclone separators. The results show that a contiguous, smooth surface of zero axial velocity does exist and has approximately the shape that has been assumed by modellers. The significant effects of fluid viscosity, underflow and modifications to the inlet are also shown.

A coupled CFD–DEM model for tumbling mill dynamics—Effect of lifter profile

In comminution circuits, grinding is the most energy-intensive process. Although significant work was focused on the charge particle dynamics via only DEM in the past, but lifter effects and slurry viscosity have often been overlooked. This work aims to develop a coupled CFD–DEM model to precisely predict mill dynamics involving slurry, charge, and air as free surface. The model uses the VOF method for the slurry–air interface and DEM for charge motion, connecting them through momentum exchange. Suitable slurry rheology and viscous drag sub-models are incorporated. Tumbling mill slurry and charge dynamics are simulated for varied lifter profiles, mill speeds, and slurry pulp conditions. Coupled CFD–DEM and only DEM models are validated against slurry-based Positron Emission Particle Tracking (PEPT) based data. Further, this coupled model is used to assess the effect of lifter face angles and slurry wt% solids on charge dynamics in terms of the mill’s power consumption and the kinetic energy spectra.

The performance of rotating membrane emulsification in the presence of baffles: Insights from flux experiments, microstructural analysis, and positron emission particle tracking

Rotating Membrane Emulsification (RME) is a bottom-up emulsification technique developed to circumvent the significant energy requirements of conventional methods; however, its implementation has been hindered by low emulsion throughputs. This work presents a novel baffled-RME setup and investigates the potential improvement to emulsion throughput and droplet microstructure, whilst employing both surface-active and Pickering particle emulsifiers to assess whether any advantages are emulsifier-specific. Overall, baffle addition improves emulsion throughputs, however the droplet microstructure was positively influenced only when using surfactant-based emulsifiers. Positron Emission Particle Tracking (PEPT) was utilised to demonstrate that these advantages result from improved hydrodynamic conditions instigated by the break-up of streamlines, inducing higher turbulence near the membrane surface, and by increasing the transmembrane pressure drop and drag force through flow restrictions. Overall, by detailing the first baffled-RME setup and first application PEPT analysis to such equipment, this work lays the foundation for further optimisation of bottom-up emulsification approaches.

Development of the front-end electronics for a cost-effective PET-like detector system

Most detector systems used for positron emission particle tracking (PEPT) are very expensive due to the use of inorganic plastic scintillators combined with a high number of readout electronic channels. This work aims to reduce the overall cost of a PEPT-capable detector system by using large and cost-effective plastic scintillators and developing custom 2 × 2 silicon photomultiplier (SiPM) arrays, preamplifiers, and discriminators. The use of long (20mm × 20mm × 1000 mm) plastic scintillator bars read out with photodetectors only at their respective ends allows an overall smaller number of photodetectors and associated readout electronics, which in turn reduces the overall cost of the system. In addition, the development of a custom SiPM array and preamplifier allows a free selection of interconnection and readout, as most commercial producers only offer specific types of interconnections and therefore lack other connections such as serial or hybrid. Thus, several common circuit types for SiPMs and preamplifiers were tested and compared in this work, and it was found that a serial connection implemented in a hybrid interconnection for the SiPMs and an inverting preamplifier based on a high-frequency operational amplifier provided the best results for the proposed detector system. Measured with a 22Na source, the combination of SiPM array and preamplifier led to a rise time of 3.7ns and a signal amplitude of 175 mV.

A simulation study on spatial and time resolution for a cost-effective positron emission particle tracking system

This work is the second part of a simulation study investigating the processing of densely packed and moving granular assemblies by positron emission particle tracking (PEPT). Since medical positron emission tomography (PET) scanners commonly used for PEPT are very expensive, a PET-like detector system based on cost-effective organic plastic scintillator bars is being developed and tested for its capabilities. In this context, the spatial resolution of a resting positron source, a source moving on a freely designed model path, and a particle motion given by a discrete element method (DEM) simulation is studied using Monte Carlo simulations and the software toolkit Geant4. This not only extended the simulation and reconstruction to a moving source but also significantly improved the spatial resolution compared to previous work by adding oversampling and iteration to the reconstruction algorithm. Furthermore, in the case of a source following a trajectory developed from DEM simulations, a very good resolution of about 1 mm in all three directions and an average 3D deviation between simulated and reconstructed events of 2.3 mm could be determined. Thus, the resolution for realistic particle motion within the generic grate system (which is the test rig for further experimental studies) is well below the smallest particle size. The simulation of the dependence of the reconstruction accuracy on tracer particle location revealed a nearly constant efficiency within the entire detector system, which demonstrates that boundary effects can be neglected.

Application of Positron Emission Particle Tracking (PEPT) for the evaluation of powder behaviour in an incline linear blender for Continuous Direct Compression (CDC)

Positron Emission Particle Tracking (PEPT) is a non-invasive measurement technique which offers the ability to track the motion of individual particles with high temporal and spatial resolution, and thus build up an understanding of the bulk behaviour of a system from its microscopic (particle level) dynamics. Using this measurement technique, we have developed a series of novel metrics to better understand the behaviours of powders during the steady-state operation of a continuous blender system. Results are presented concerning the response of particle motion to processing parameters (mixing blade configuration and RPM), quantifying the motion in terms of predicted mixing performance. It was found that both increasing rpm and increasing hold-up mass (by selecting fewer transport blades and more mixing blades) provided improved mixing conditions. Interestingly, under specific conditions, there is evidence of convection-like mixing occurring at the interface of the transport and mixing region. This suggests the existence of a potential ’folding region’ whereby powder is transported up the barrel (and away from the powder bulk bed) before being reconstituted back into the bulk mass. The results also provide valuable experimental data for the development, calibration and validation of future Discrete Element Method (DEM) simulations.

Micro-scale particle tracking using hybrid detectors

Positron Emission Particle Tracking (PEPT) techniques allow the tracking of a radioactive tracer particle moving within a system of flow, enabling non-invasive study of dynamic systems. On the micro-scale, PEPT performance is limited by the achievable activity in radiolabelling a suitable tracer particle, and the fixed geometry of conventional detector systems. To enable application of PEPT towards these scales advanced instrumentation is required, and a hybrid detection system has been developed combining scintillator and semiconductor devices. A bismuth germanate oxide (BGO) scintillator array consisting of 1024 detector elements derived from CTI/Siemens PET scanners (512 pixels of 6.75 x 6.25 x 30 mm3 and 512 pixels of 4.1 x 4.0 x 30 mm3) forms a field of view of 150 x 196 x 101 mm3. A pair of pixelated cadmium zinc telluride room temperature semiconductors (9680 pixels of 1.8 x 1.8 x 0.5 mm3) form a high spatial resolution region of 62 x 42 x 20 mm3 placed within the larger field of view. The design choice maximizes absolute efficiency by merit of the scintillators and enhances spatial resolution through the semiconductors. Energy and timing resolutions of the BGO elements were determined, and sensitivity profiles of the system modelled numerically, enabling the characterization of the system absolute efficiency and spatial resolution. The results suggest the applicability of PEPT in the study of microscale flows for the first time, including investigating flows in capillaries and micro-fluidic devices.

Dynamics of physical flows measured by positron emission techniques

Short lived positron emitting species are used to produce flow-following tracer particles to study flow dynamics in a technique known as positron emission particle tracking (PEPT). The photon pairs produced by positron annihilation are detected in time coincidence by arrays of high-speed position sensitive detectors. Reconstruction of consecutive annihilations are used to determine the near-instantaneous position of the tracer particle. Hence, the resulting bulk flow dynamics are derived, including residence times, velocities, accelerations, and related kinematic properties. The Department of Physics at the University of Cape Town uses PEPT to study dynamic physical processes, turbulent, and multiphase flows. Studies aim to address global challenge topics including problems in water scarce environments, reducing industrial wastes, and enhancing developments towards sustainable economies through improved process efficiencies and design led approaches. The PEPT Cape Town enterprise is discussed, including the development of flow metrology systems and complementary nuclear measurement techniques. Research encompasses four key themes: radioisotope tracer production, instrumentation & detector development, data acquisition & processing, and flow metrology.

Density-modified tracers for positron emission particle tracking

Positron emission particle tracking (PEPT) is a powerful non-invasive technique used to study the underlying dynamics of granular and multiphase flowing systems. The essence of PEPT is to attach (chemically or otherwise) sufficient positron-emitting radioisotope to a particle chosen as representative of the bulk material under study, and hence used as a flow-following tracer. We report on standardised methods for creating density-modified tracer particles to expand the range of applications feasible to research with the PEPT technique.

Use of positron emission particle tracking to assess mixing of a graphite-based lithium-ion anode slurry in an Eirich mixer

The mixing of electrode slurries is a key processing step during the manufacturing of lithium-ion batteries, and poor mixing can have a profound effect on final cell electrochemical performance. This is complicated by the evolving non-Newtonian rheology of these slurries. In this study, Positron Emission Particle Tracking (PEPT) is used to determine the fluid dynamics and dispersion of a model graphite-based anode slurry in an Eirich EL1 mixer, equipped with a rotating pan, wall scraper and internal rotor. The main processes studied are the wetting of graphite particles and breakup of large graphite agglomerates. For experiments performed at different internal rotor tip speeds, with fixed outer pan speed, the dispersion dynamics change with the lowest rotor speed (2 m/s) leaving a large channel of poor mixing between the rotor region and the rotating pan, whilst the mixing within the vessel is mainly driven by the rotating pan. At higher mixing speeds (6 and 10 m/s) more of the vessel volume is active and there is no longer a poor mixing region observed, with 10 m/s tip speed producing the highest intensity of dispersion. An increasing mixer effectiveness (average dispersion across the mixing vessel) as well as an increasing dispersion co-efficient (rate of mixing) is also observed, which will be use to inform scale-up criteria.

Influence of nozzle design on flow, mixing, and fluidisation in a bubbling bed fluidised by a single nozzle

In this paper we apply, for the first time, positron emission particle tracking (PEPT) to a fluidised bed in which gas is injected through a nozzle-type distributor. The detailed, three-dimensional data obtained provide direct insight into how the angle of the orifices through which gas is injected affects the fluidisation, mixing, and flow patterns observed within the bed. Our results show that the fastest and most consistent recirculation of material – an indicator of good mixing – as well as the most complete fluidisation may be achieved by using a nozzle with horizontal- or near-horizontal outlets.

On the Ability of Positron Emission Particle Tracking (PEPT) to Track Turbulent Flow Paths with Monte Carlo Simulations in GATE

Positron emission particle tracking (PEPT) has offered important insights into the internal dynamics of multiphase flows. High precision and frequency measurements of the location of the tracer particle are required to resolve individual eddies at the millimetre scale or smaller. To explore the potential of PEPT to perform these measurements, a model was developed of the Siemens ECAT “EXACT3D” HR++ positron emission tomography (PET) scanner at the PEPT Cape Town facility in South Africa with the software Geant4 Application for Tomographic Emission (GATE) and was used to generate Lagrangian tracks from simulations of moving tracer particles. The model was validated with measurements from both experiment and simulation and was extended to two virtual scenarios inspired by turbulent flows. The location data from the simulation accurately captured linear portions of an oscillating path up to high speeds of 25 m s−1; however, tracking tended to undercut the turning points due to the high tracer acceleration. For a particle moving on a spiral path of decreasing radius, the location data tracked the path above a radius of 2.0 mm with an uncertainty equivalent to the radius of the tracer particle, 300 μm. Improvements to the measurement are required to track sub-millimetre flow structures, such as the application of PET scanners with higher spatial resolution and upgrades to the sampling processes used in location algorithms.

Cohesive collisions of particles in liquid media studied by CFD-DEM, video tracking, and Positron Emission Particle Tracking

This paper investigates the cohesive collision of ice in an oil phase at temperatures ranging from −15.7 °C to −0.3 °C. The new information on the coefficient of restitution (COR) was obtained using three different velocity measurement methods: high-speed experimental video recording, Positron Emission Particle Tracking (PEPT), and numerical simulations. A new type of PEPT tracer was developed for the experiments. The COR values were in the interval 0.57...0.82, with a maximum at around −10 °C. The CFD-DEM coupled approach was applied to reproduce experiments with an ice particle drop and its collision with an inclined ice surface in a decane. The particle–wall interaction is modeled using commercial software, considering particle cohesion, particle size, and shape. CFD-DEM predicted the COR with an average deviation ∼10% from the experimental data. The numerical model’s results agree with the experiments, demonstrating that the CFD-DEM method is suitable for describing multiphase cohesive interactions.

Effect of Retrofit Design Modifications on the Macroturbulence of a Three-Phase Flotation Tank-Flow Characterization Using Positron Emission Particle Tracking (PEPT).

Turbulence in stirred tank flotation tanks impacts the bulk transport of particles and has an important role in particle-bubble collisions. These collisions are necessary for attachment, which is the main physicochemical mechanism enabling the separation of valuable minerals from ore in froth flotation. Modifications to the turbulence profile in a flotation tank, therefore, can result in improvements in flotation performance. This work characterized the effect of two retrofit design modifications, a stator system and a horizontal baffle, on the particle dynamics of a laboratory-scale flotation tank. The flow profiles, residence time distributions, and macroturbulent kinetic energy distributions were derived from positron emission particle tracking (PEPT) measurements of tracer particles representing valuable (hydrophobic) mineral particles in flotation. The results show that the use of both retrofit design modifications together improves recovery by increasing the rise velocity of valuable particles and decreasing turbulent kinetic energy in the quiescent zone and at the pulp-froth interface.