Positron Emission Particle Tracking Measurements Research Articles

Evaluation of coarse-grained CFD-DEM models with the validation of PEPT measurements

Computational Fluid Dynamics coupled with Discrete Element Method (CFD-DEM) is a commonly used numerical method to model gas-solid flow in fluidised beds and other multiphase systems. A significant limitation of CFD-DEM is the feasibility of the realistic simulation of large numbers of particles. Coarse-graining (CG) approaches, through which groups of multiple individual particles are represented by single, larger particles, can substantially reduce the total number of particles while maintaining similar system dynamics. As these three CG models have not previously been compared, there remains some debate, however, about the best practice in the application of CG in CFD-DEM simulations. In this paper, we evaluate the performance of three typical CG methods based on simulations of a bubbling fluidised bed. This is achieved through the use of a numerical validation framework, which makes full use of the high-resolution 3D positron emission particle tracking (PEPT) measurements to rigorously validate the outputs of CFD-DEM simulations conducted using various different coarse-graining models, and various different degrees of coarse-graining. The particle flow behaviours in terms of the particle occupancy field, velocity field, circulation time, and bubble size and velocity, are comprehensively analysed. It is shown that the CG simulation starts to fail when the size ratio between the bed chamber and the particles decreases to approximately 20. It is also observed, somewhat surprisingly, that the specific CG approach applied to interparticle contact parameters does not have a substantial effect on the simulation results for the bubbling bed simulations across a wide range of CG factors.

DEM2GATE: Combining discrete element method simulation with virtual positron emission particle tracking experiments

Positron Emission Particle Tracking (PEPT) is a Lagrangian particle tracking technique useful for imaging flow in opaque granular media. The scientific and industrial systems studied using PEPT and the PEPT technique can also be simulated: Discrete Element Method (DEM) for granular systems and Monte Carlo for PEPT. Using the open-source DEM2GATE library, we quantify the reconstruction of particle trajectories and overall system behaviour by integrating particle trajectories from a rotating drum simulation with a PEPT detector simulation. The results show the main drivers of PEPT spatial error are particle acceleration and gamma-ray scattering. The accuracy of reconstructed flow fields increases logarithmically with measurement time. Measurements of higher-order, derived quantities require more time to reach a desired level of accuracy. Surprisingly, our results imply PEPT measurements relying on fluctuating velocity deviate substantially from the system's true behaviour. However, using DEM2GATE, these errors are predictable and can be corrected.

Validation of a Discrete Element Method (DEM) Model of the Grinding Media Dynamics within an Attritor Mill Using Positron Emission Particle Tracking (PEPT) Measurements

Positron emission particle tracking (PEPT) was used to investigate the grinding media dynamics in a laboratory-scale attritor mill in the absence of powder. The grinding media motion was analysed as a function of the equipment’s typical operating parameters: impeller speed, impeller clearance and bead fill level. It was observed that the impeller speed had the strongest influence on the media motion. An increase of the impeller speed from 300 rpm to 600 rpm led to a change in the bead recirculation patterns with the increasing formation of well segregated upper and lower recirculation loops that fully developed at the maximum speed of 600 rpm. For a constant impeller speed, an increase of the bead loading did not majorly affect the bead velocity as remarked by minor changes on the flow field. For all the impeller clearance values, the occupancy plots revealed an inefficient dead region at the bottom of the attritor where the beads were moving at very low velocity. In this region the beads were tightly packed under their own weight and, furthermore, there was an absence of direct contact with the impeller arms. The depth of this region increased proportionally to the distance between the bottom of the impeller and the vessel base indicating that a minimum value of clearance should be set to optimise the lower recirculation pattern. For two experimental conditions, the data generated by PEPT measurements were utilised to set-up a friction-adjusted discrete element method (DEM) model. Here, the simulation results were qualitatively and quantitatively compared against the PEPT data by assessing the averaged velocity flow fields and the average velocity profiles at different radial locations inside the vessel. Given the intrinsic uncertainty of the PEPT measurements, the DEM model results were in considerably good agreement with the experimental results. The major discrepancy was observed close to the vessel wall where the simulations overpredicted the velocity by about 10%.

Numerical study of the solid flow behavior in a rotating drum based on a multiphase CFD model accounting for solid frictional viscosity and wall friction

In continuum-based models, the kinetic theory of granular flow (KTGF) provides a model for calculating solids stresses but has limitations in densely packed regions such as mills, kilns and rotating drums. The Eulerian-Eulerian multiphase model coupled with KTGF is evaluated in the present work. An additional frictional stress models were added, and their suitability evaluated. For the evaluation, a rotating drum at three rotational speeds (20rpm, 42rpm and 65rpm) was analyzed. Compared with Positron Emission Particle Tracking (PEPT) measurement data from literature, Johnson and Jackson's model and Schaeffer's model for the frictional stress both showed a lower angle of repose regardless of the wall boundary condition used. Thus, a new frictional viscosity model based on granular pressure was proposed. By adjusting the specularity coefficient of wall boundary condition, results of the present model agreed well with the PEPT measurements in terms of angle of repose and spatial velocity fields. In addition, considering that the actual Johnson and Jackson model for boundary condition includes two parts, collisional and frictional part, a discussion was made about boundary condition. The results showed that the validation of the proposed frictional viscosity model with experimental results could be completed at different rotational speeds by considering only the collisional part of the boundary condition with different specularity coefficients or only the frictional part of the boundary condition with different angles of friction or both parts with the same specularity coefficient and angle of friction. Nevertheless, when the complete Johnson and Jackson model of boundary condition was applied, the same specularity coefficient and angle of friction are used at different rotational speeds which is more physically meaningful. Moreover, it is found that the frictional contribution has greater influence on dynamic angle of repose than collisional contribution of the boundary condition in the current rotating drum.

Simulated time-dependent data to estimate uncertainty in fluid flow measurements

Abstract Positron Emission Particle Tracking (PEPT) is an emerging measurement technique for Lagrangian data collection in experimental fluid flows. The nature of the measurement extends the study of turbulent flows to applications lacking optical access. A simulated pipe flow PEPT measurement is used in this work to verify reconstruction algorithm and identify novel sources of uncertainty. Simulated measurement occurs in a 38 mm diameter pipe with 0.55 m/s mean flow velocity and Reynolds number equal 21,000. Flow is simulated using computational fluid dynamics (CFD), generating 1000 time-dependent trajectories of tracers from CFD to prescribe movements of 40 µCi, fluorine-18-point sources. Geant4 Application for Tomographic Emission (GATE) simulates the response of the Inveon Pre-Clinical scanner model and produces an array of coincidence lines for the prescribed sources. The array is provided to a multiple PEPT (mPEPT) code reconstructing the position of simulated tracers and assigns positions to coherent trajectories. The mPEPT reconstructed trajectories are compared to prescribed positions. Reconstruction of 754 trajectories is achieved with measurement bias centered at 0.0 mm in all directions and standard deviation of 0.29 mm, 0.29 mm, and 0.30 mm in the x, y, and z component respectively using 2.0 ms time-steps. Two instances of position misassignments induced by the close-approach of two tracers in measurement volume are identified. This is a novel reconstruction error to PEPT measurements, unable to be identified without prior knowledge of tracer trajectories provided by these simulation tools.

Dense gas-particle suspension upward flow used as heat transfer fluid in solar receiver: PEPT experiments and 3D numerical simulations

A dense particle suspension, also called an upflow bubbling fluidized bed, is an innovative alternative to the heat transfer fluids commonly used in concentrated solar power plants. An additional advantage of this technology is that it allows for direct thermal storage due to the large heat capacity and maximum temperature of the particle suspension. The key to the proposed process is the effective heat transfer from the solar heated surfaces to the heat transfer fluid, i.e. the circulating solid suspension. In order to better understand the process and to optimise the design of the solar receiver, it is of paramount importance to know how particles behave inside the bundle of small tubes. To access to the particle motion in the solar receiver, two different techniques are carried out: experimental using positron emission particle tracking (PEPT) and 3D numerical simulation via an Eulerian n-fluid approach with NEPTUNE_CFD code. Both numerical predictions and PEPT measurements describe an upward flow at the centre of the transport tube with a back-mixing flow near the wall which influences the heat transfer mechanism. Comparisons between experiment and computation were carried out for the radial profiles of the solid volume fraction, and vertical and radial time-averaged and variance velocities of solid, and demonstrating the capability of NEPTUNE_CFD code to simulate this peculiar upflow bubbling fluidized bed.

A comparison of magnetic resonance, X-ray and positron emission particle tracking measurements of a single jet of gas entering a bed of particles

Measurements of the lengths of a single jet of gas entering a packed bed were made using magnetic resonance imaging (MRI), positron emission particle tracking (PEPT) and X-ray radiography and the results compared. The experiments were performed using a Perspex bed (50mm i.d.) of poppy seeds: air at 298K was admitted to the base of the bed through a single, central orifice, 2mm in diameter. Poppy seeds (Geldart Group B, measured minimum fluidisation velocity with air at 298K and 1atm of 0.13m/s and particle density ~1060kg/m3) were used because of their high content of oil, which contains mobile protons and hence is suitable for MRI examination. The lengths of jet measured using the three techniques were in agreement between 50m/s<Uo<100m/s, where Uo is the superficial velocity through the orifice. Below Uo=50m/s, X-ray measurements of jet lengths were shorter than those measured using MRI. This was attributed to the minimum diameter of void, found to be 5mm, detectable in a 50mm bed using ultra-fast X-ray measurements. PEPT is most commonly used to calculate particle velocities, whilst jet lengths are usually calculated from determinations of voidage. However, the particle locations determined in this work by PEPT were used to calculate a fractional occupancy count, from which a jet length could be inferred.

Positron emission particle tracking measurements with 50 micron tracers

The trajectory of tracer particles of size 50±5μm has been measured with positron emission particle tracking (PEPT). Resin particles were labelled with 0.05–0.5MBq of 68Ga via ion exchange mechanisms. Their position was measured in a variety of PEPT experiments: at rest; rotating in fixed circular trajectories in air and in water, and moving freely in a Rushton stirred vessel. The uncertainties in the measured locations are of the order of 2.6mm for the measurements presented. The experiments were facilitated by new techniques in tracer labelling and handling, along with a high number of detector elements in the ECAT ‘EXACT3D’ PET scanner at PEPT Cape Town. This opens the way for PEPT studies to explore fine particle technologies, such as froth flotation and slurry flow.

Mixing of dense binary suspensions: Multi‐component hydrodynamics and spatial phase distribution by PEPT

AbstractThe three‐component flow field and spatial phase distribution of binary mixtures of glass particles suspended in water in a stirred vessel have been resolved using positron emission particle tracking (PEPT). The Lagrangian flow data provided by the technique have been converted to give a detailed Eulerian description of each of the three phase components of the flow generated by a pitched blade turbine. For the first time, it has been possible to determine the full 3D velocity and concentration fields of the liquid phase and both the solid components within opaque dense slurries of this type containing up to 40 wt % solids. Spatial distributions of local time‐averaged slip velocity have also been obtained for each solid component showing wide variations. The detailed PEPT measurements have enabled the solids mass balance and the mass continuity of the three phase components to be accurately verified throughout the vessel. © 2010 American Institute of Chemical Engineers AIChE J, 2011

Studies of the effect of tracer activity on time-averaged positron emission particle tracking measurements on tumbling mills at PEPT Cape Town

The statistical uncertainties associated with time-averaged positron emission particle tracking (PEPT) measurements of the position, velocity and acceleration of a small tracer particle have been investigated. Experiments were undertaken using the ECAT ‘EXACT3D’ PET scanner recently installed at the facilities of PEPT Cape Town at iThemba LABS, South Africa. Small “button” sources of 68Ga were attached to the outer shell of a small mill and PEPT data were recorded over several half-lives, for the cases of the mill being empty and entirely filled with steel balls. The effect of tracer activity on the statistical uncertainties in the positions, velocities and accelerations derived from the measurements was investigated, as a first step towards fully quantifying the uncertainties associated with tracking particles via PEPT in laboratory-scale tumbling mills.

Analysis of particle motion in a paddle mixer using Discrete Element Method (DEM)

The use of Discrete Element Method (DEM) modelling is rapidly expanding as a modelling tool to analyse particulate processes and to address technological needs in various aspects of pharmaceutical, food and detergent processing industries. Examples include estimation of parameters that are difficult to measure or quantify experimentally (e.g. internal flow fields and mixing patterns) and facilitating the scale-up of particulate processes from laboratory to pilot plant. This article compares DEM simulations of particle flow in a paddle mixer to experimental measurements using Positron Emission Particle Tracking (PEPT). Good qualitative agreement is shown between DEM and PEPT in terms of flow patterns inside the mixer. Quantitative comparison of PEPT measurements and DEM analyses shows reasonable agreement in terms of velocity distribution. In general, the DEM is shown to be capable of predicting the dynamics of the particulate flow inside the mixer.

CFD–DEM modelling of particle flow in IsaMills – Comparison between simulations and PEPT measurements

The IsaMill™ is a high speed stirred mill with a horizontal configuration that offers advantages such as energy efficiency and an inert grinding environment. A combined computational fluid dynamics (CFD) and discrete element method (DEM) approach was developed to investigate the particle and fluid flows inside a simplified IsaMill™. The configuration of the mill was simpler than that of an actual IsaMill™ and no feed flow or rotor was considered. The CFD–DEM model is a progression from earlier DEM only models of “dry” systems which did not account for the fluid phase. The properties of flows at a macroscopically steady state, such as velocity field, distributions of particle velocity and acceleration in the radial direction and power draw, were analysed. Detailed comparisons were carried out between the simulation results and Positron Emission Particle Tracking (PEPT) measurements under similar conditions. The comparisons showed reasonable agreements, confirming that both techniques can capture the key features of the flow. The discrepancies between simulated and measured results were discussed. The findings indicated that the proposed model can be used to generate microdynamic information that is useful in leading to a better understanding of the underpinning physics of flow inside mills.

PEPT measurements of solid–liquid flow field and spatial phase distribution in concentrated monodisperse stirred suspensions

The technique of positron emission particle tracking (PEPT) has been used to determine the two-phase flow field and spatial phase distribution in a stirred vessel containing coarse glass particles suspended in water. The Lagrangian flow data provided by PEPT have been converted to give a detailed Eulerian description of the two-phase flow generated by a pitched blade turbine (PBT) operating in up-pumping or down-pumping mode. For the first time, it has been possible to determine the full 3D velocity and concentration fields of both the liquid and the solid phase within an opaque dense slurry of this type containing up to 40 wt% solids. The detailed PEPT measurements have also enabled the solids mass balance and the mass continuity of the two phases to be accurately verified throughout the vessel. The data show that the pumping effectiveness of the PBT is unaffected at moderate solid fractions, but is considerably reduced at high fractions and even more so in the up-pumping mode. A uniformity index based on the variance of the local solids concentration in the vessel shows that, overall, a down-pumping PBT achieves a significantly better homogeneity than an up-pumping PBT.

Solids mixing in the riser of a circulating fluidized bed

Gas/solid and catalytic gas phase reactions in CFBs use different operating conditions, with a strict control of the solids residence time and limited back-mixing only essential in the latter applications. Since conversion proceeds with residence time, this residence time is an essential parameter in reactor modelling. To determine the residence time and its distribution (RTD), previous studies used either stimulus response or single tracer particle studies. The experiments of the present research were conducted at ambient conditions and combine both stimulus response and particle tracking measurements. Positron emission particle tracking (PEPT) continuously tracks individual radioactive tracer particles, thus yielding data on particle movement in “real time”, defining particle velocities and population density plots. Pulse tracer injection measurements of the RTD were performed in a 0.1 m I.D. riser. PEPT experiments were performed in a small ( ∼ 0.05 m I.D.) riser, using 18F-labelled sand and radish seed. The operating conditions varied from 1 to 10 m/s as superficial velocity, and 25– 622 kg / m 2 s as solids circulation rate. Experimental results were compared with fittings from several models. Although the model evaluation shows that the residence time distribution (RTD) of the experiments shifts from near plug flow to perfect mixing (when the solids circulation rate decreases), none of the models fits the experimental results over the broad ( U , G ) -range. The particle slip velocity was found to be considerably below the theoretical value in core/annulus flow (due to cluster formation), but to be equal at high values of the solids circulation rate and superficial gas velocity. The transition from mixed to plug flow was further examined. At velocities near U tr the CFB-regime is either not fully developed and/or mixing occurs even at high solids circulation rates. This indicates the necessity of working at U > approx. ( U tr + 1 ) m / s to have a stable solids circulation, irrespective of the need to operate in either mixed or plug flow mode. At velocities above this limit, plug flow is achieved when the solids circulation rate G > approx . 200 kg / m 2 s . Solids back-mixing occurs at lower G and the operating mode can be described by the core/annulus approach. The relative sizes of core and annulus, as well as the downward particle velocity in the annulus ( ∼ U t ) are defined from PEPT measurements. Own and literature data were finally combined in a core/annulus vs. plug flow diagram. These limits of working conditions were developed from experiments at ambient conditions. Since commercial CFB reactors normally operate at a higher temperature and/or pressure, gas properties such as density and viscosity will be different and possibly influence the gas–solid flow and mixing. Further tests at higher temperatures and pressures are needed or scaling laws must be considered. At ambient conditions, reactors requiring pure plug flow must operate at U > approx . ( U tr + 1 ) m / s and G > approx . 200 kg / m 2 s . If back-mixing is required, as in gas/solid reactors, operation at U > approx . ( U tr + 1 ) m / s and G < approx . 150 kg / m 2 s is recommended.