On the effect of static and dynamic particle size distribution on flow turbulence modulation

The effect of an evolving particle size distribution due to particle agglomeration and breakup, and the direction and absence of gravitational acceleration, on flow turbulence modulation is investigated using large eddy and discrete particle simulation of a turbulent channel flow. The results are compared with the case in which the particle size distribution is static and where only inter-particle collision is allowed. Due to the small particle Stokes number considered, inherent in a solid-liquid flow, and the small simulation time, only small effects were observed for the static versus dynamic particle size distribution on the fluid turbulence. For vertical channel flows, however, the influence of flow direction and gravity lead to different particle segregation patterns which, together with changes in wall shear stresses and mass flow rate due to buoyancy effects, do affect the flow turbulence and the evolution of inter-particle collisions, collision efficiency and agglomerate breakup

Preferential Concentration of Inertial Fibres in a Turbulent Channel Flow

Large eddy simulation (LES) is developed to study particle preferential concentration, in which an initially uniform distribution of inertial particles spontaneously segregates into clusters in a turbulent flow, driven primarily by the small-scale turbulent fluctuations and slip velocity. Dynamic modelling of sub-grid scale effects on the LES and Langevin-type sub-grid scale modelling of particle motion ensures particle clustering is well captured, which is computational cheap when compared to fully-resolved simulations. Results shows that prediction of particle clustering near walls due to turbophoresis is strongly depended on (i) the particle inertia, where particle inertia is parameterised by its Stokes number, (ii) particle shape, parameterised by its aspect ratio, (iii) binning method and (iv) simulation time

Large Eddy Simulation of Particle-Particle Interactions in Turbulent Flow: Collision, Agglomeration and Break-Up Events

A numerical study of particle-particle interactions in a turbulent flow is performed using an Eulerian-Lagrangian particle tracking code with a hard-sphere collision model extended to take into account coalescence between the colliding particles and break-up of agglomerates. The effect of the agglomerate fractal dimension on the break-up events, and eventually on collision and agglomeration, is presented. The computational domain was seeded with primary particles (calcite, a nuclear waste simulant) of size 60 micron and allowed to run until steady state before the particle-particle interactions were activated. Break-up events reduce as the agglomerate fractal dimension (df = 2.0, 2.5, 2.8 and 3.0) increases, and with no break-up event as the control, the effect of break-up on particle-particle interactions is presented. The results show an increase in the number of collisions, and the number of collisions leading to agglomeration, with a decrease in the agglomeration rate and agglomerate size with increasing hydrodynamic stress, as a consequence of break-up

The impact of coupling and particle volume fraction on fluid-particle interactions in a turbulent channel flow

Direct numerical simulation, facilitated by a spectral element method, is used to predict a multi-phase fluid flow through a channel at a shear Reynolds number of 300. Following validation of single-A nd multi-phase flow results against other DNS predictions available in the literature, a channel flow is simulated utilising a Lagrangian particle tracker to model 300,000 particles with a diameter of 100 'Ým, having a density ratio equivalent to that of water to glass, and a particle volume fraction of approximately 0.01%. This flow is calculated using multiple levels of coupling between the particles and the flow; one-way, two-way and four-way. The mean streamwise velocity of the fluid and the particles, along with the shear and normal stresses, are compared for the different coupling methods, with the differences between them analysed and, although small, they are found to be consistent across the channel. A second set of runs is performed using in excess of 2 million particles in order to facilitate a tenfold increase in the particle volume fraction, to 0.1%, with the particles expected in this case to have a greater impact upon the properties of the fluid. The statistics of the fluid and particles in these simulations are then compared with those from the simulation with a lower concentration of particles in order to determine the magnitude of the effect the particles have on the fluid in this flow. The effects of the different couplings on the flow are much greater in this case due to the increased number of particles affecting the flow. Also, the presence of the particles is seen to increase the turbulence levels of the fluid, especially in the streamwise direction. The accuracy of the simulations clearly increases with the level of coupling. However, the speed of the simulations decreases. One way of achieving decreased run times, for both volume fraction cases, is to use a faster stochastic version of the particle tracking code for four-way coupling. This is tested, replacing the Lagrangian collision mechanics in the four-way coupled simulations with a probabilistically-determined mechanistic model. For the lower volume fraction, the normal stresses of the particles are exaggerated somewhat using the stochastic method. The simulation time is decreased compared to the Lagrangian approach, although the results presented suggest that the stochastic method requires further refinement

The Influence of Gravity on Particle Collision and Agglomeration in Turbulent Channel Flows

The study described in this paper concerns the simulation of a particle-laden turbulent channel flow at high mass loadings, with and without the presence of gravity. Large eddy simulation (LES) is used to simulate the fluid phase, with solutions combined with a Lagrangian particle tracker to model the particle phase. Particle-particle interactions are detected using an algorithm based on a deterministic collision treatment (hard-sphere collision model), and particle agglomeration is based on the use of a particle restitution coefficient, energy balance and the sum of the van der Waals’ force on each colliding particle. In order to establish the validity of the treatment, results are compared with those based on a DNS, with good agreement being found. Subsequent runs for colliding and agglomerating particles in a channel flow demonstrate that the rate of particle agglomeration peaks towards the channel walls due to increased particle concentrations and turbulence levels in these regions. Agglomeration is also greatly influenced by the presence of gravity, with this effect accentuated on the lower wall of the channel

Particle concentration and stokes number effects in multi-phase turbulent channel flows

This investigation examines the effect that particle concentration has on the dynamics of two-phase turbulent channel flows at low and high density ratios. In the literature, little explanation is offered for the existence of high particle turbulence intensities in the buffer layer and viscous sublayer for particles with high Stokes number. The present study aims to explore particle dynamics in those regions. The spectral element method DNS solver, Nek5000, is used to model the fluid phase at a shear Reynolds number Re=, Particles are tracked using a Lagrangian approach with inter-phase momentum exchange (two-way coupling). Mean fluid and particle velocity statistics are gathered and analysed to determine the effect of increasing both Stokes number and concentration. Results indicate that the system with the greater Stokes number (air) has a much larger impact on the mean streamwise velocity and turbulence intensity profiles. As the concentration is increased, the mean flow velocity and turbulence intensity are reduced in the bulk and increased very close to the wall. For the low Stokes system, there is negligible effect on the flow statistics at low concentration. One-way coupled solid-phase statistics indicate that particles in water follow the flow very closely. At the higher densityratio, particles lag behind the flow in the bulk, but overtake the flow in the near-wall region, where the existence of increased streamwise turbulence intensities is also observed. To elucidate the dynamics, concentrations and fluxes are analysed. Particles are observed to be distributed more densely close to the wall in air, compared to a reasonably uniform distribution in water. Finally, contour plots indicate that particles in air tend to congregate in regions of low streamwise fluid velocity, and the extent to which this differs between the two systems is then quantitatively measured

Effect of Four-Way Coupling on the Turbulence Field in Multi-Phase Channel Flows

This paper investigates and compares the effect of a solid, spherical particle phase on surrounding carrier fluids (air and water) in a turbulent channel flow. The fluid phase properties are chosen to represent a flow typical of the nuclear waste industry, with the flow modelled using the direct numerical simulation (DNS) code, Nek5000, at a shear Reynolds number of 180. A Lagrangian particle tracker is developed and implemented to simulate the dispersed phase, capable of accommodating two-way coupling between the fluid and discrete phase and inter-particle collisions (four-way coupling). In order to investigate the effect that the four-way coupled particulate phase has on the turbulence field, mean fluid velocities and turbulence intensity statistics are recorded. The work demonstrates that the introduction of two-way coupling does indeed impact slightly on the turbulence field. Specifically, it reduces the mean velocity profile and increases the streamwise turbulence intensity in the near-wall region. Upon the introduction of inter-particle collisions, the flow statistics studied show a negligible response. Collision density distributions are studied and a temporal migration to the near-wall region is observed. Along-side this, to investigate particle density-ratio effects, water-based results are contrasted with simulations in air. The way in which the flow statistics are modified are shown to differ in air and water. Finally, a DLVO agglomeration model is demonstrated, whereby particles colliding with enough energy to overcome the potential barrier are considered bound. This is applied to the four-way coupled flow with temporal distributions of agglomerate counts presented

Simulation of Particle-Laden Pipe Flows with a Homogeneous Stationary Sediment Bed

The characteristics of particle-laden turbulent pipe flows with stationary variable homogeneous bed heights are studied using large eddy simulation coupled to discrete particle simulation, and compared to a reference full pipe flow. Homogeneous stationary sediment beds of various heights are investigated to mimic geometries with the presence of deposited particles that have formed a bed at the bottom of the pipe which are relevant to nuclear waste processing operations. The flat boundary formed by the bed is found to induce different degrees of secondary flows and modifications to the fluid turbulence statistics and particle dynamics. The results obtained are of direct relevance to the behaviour of the two-phase flows encountered during nuclear waste processing, and have implications for particle dispersion, agglomeration, deposition and bed formation, as well as particle re-suspension from beds, in pipe flows transporting such waste

Simulation of Inertial Fibre Orientation in Turbulent Flow

The spatial and orientational behaviour of fibres within a suspension influences the rheological and mechanical properties of that suspension. An Eulerian-Lagrangian framework to simulate the behaviour of fibres in turbulent flows is presented. The framework is intended for use in simulations of non-spherical particles with high Reynolds numbers, beyond the Stokesian regime, and is a computationally efficient alternative to existing Stokesian models for fibre suspensions in turbulent flow. It is based on modifying available empirical drag correlations for the translation of non-spherical particles to be orientation dependent, accounting for the departure in shape from a sphere. The orientational dynamics of a particle is based on the framework of quaternions, while its rotational dynamics is obtained from the solution of the Euler equation of rotation subject to external torques on the particle. The fluid velocity and turbulence quantities are obtained using a very high-resolution large eddy simulation with dynamic calibration of the sub-grid scale energy containing fluid motions. The simulation matrix consists of four different fibre Stokes numbers (St = 1, 5, 25, and 125) and five different fibre aspect ratios (λ = 1.001, 3, 10, 30, and 50), with results considered at four distances from a channel wall (in the viscous sub-layer, buffer, and fully turbulent regions), which are taken as a measure of the flow velocity gradient, all at a constant fibre to fluid density ratio (ρp/ρ = 760) and shear Reynolds number Reτ = 150. The simulated fibre orientation, concentration, and streakiness confirm previous experimentally observed characteristics of fibre behaviour in turbulence, and that of direct numerical simulations of fibres in Stokesian, or creeping flow, regimes. The fibres exhibit translational motion similar to spheres, where they tend to accumulate in the near-wall (viscous sub-layer and buffer) region and preferentially concentrate in regions of low-speed streaks. The current results further demonstrate that the fibres’ translational dynamics, in terms of preferential concentration, is strongly dependent on their inertia and less so on their aspect ratio. However, the contrary is the case for the fibre alignment distribution as this is strongly dependent on the fibre aspect ratio and velocity gradient, and only moderately dependent on particle inertia. The fibre alignment with the flow direction is found to be mostly anisotropic where the velocity gradient is large (i.e., viscous sub-layer and buffer regions), but is virtually non-existent and isotropic where the turbulence is near-isotropic (i.e., channel centre). The present investigation highlights that the level of fibre alignment with the flow direction reduces as a fibre’s inertia decreases, and as the shape of the fibre approaches that of a sphere. Short fibres, and especially near-spherical λ = 1.001 particles, are found to exhibit isotropic orientation with respect to all directions, whilst sufficiently long fibres align themselves parallel to the flow direction, and orthogonal to the other two co-ordinate directions, and the vorticity and flow velocity gradient directions