Reynolds number effects on particle agglomeration in turbulent channel flow

The work described in this paper employs large eddy simulation and a discrete element method to study particle-laden flows, including particle dispersion and agglomeration, in a horizontal channel. The particle-particle interaction model is based on the Hertz- Mindlin approach with Johnson-Kendall-Roberts cohesion to allow the simulation of Van der Waals forces in a dry air flow. The influence of different flow Reynolds numbers, and therefore the impact of turbulence, on particle agglomeration is investigated. The agglomeration rate is found to be strongly influenced by the flow Reynolds number, with most of the particle-particle interactions taking place at locations close to the channel walls, aided by the higher turbulence and concentration of particles in these regions

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

Large Eddy Simulation of Particle Agglomeration with Shear Breakup in Turbulent Channel Flow

A systematic technique is developed for studying particle dynamics as induced by a turbulent liquid flow, in which transport, agglomeration, and breakup are considered. An Eulerian description of the carrier phase obtained using large eddy simulation is adopted and fully coupled to a Lagrangian definition of the particle phase using a pointwise discrete particle simulation. An efficient hard-sphere interaction model with deterministic collision detection enhanced with an energy-balance agglomeration model was implemented in an existing computational fluid dynamic code for turbulent multiphase flow. The breakup model adopted allows instantaneous breakup to occur once the transmitted hydrodynamic stress within an agglomerate exceeds a critical value, characterised by a fractal dimension and the size of the agglomerate. The results from the developed technique support the conclusion that the local turbulence kinetic energy, its dissipation rate, and the agglomerate fractal dimension control the kinetics of the agglomeration and de-agglomeration processes, and as well as defining with time the morphology of the particles and their resultant transport. Overall, the results are credible and consistent with the expected physical behavior and with known theories

Particle Dispersion, Agglomeration and Deposition in Fully-Coupled Turbulent Channel Flow using Large Eddy Simulation and Discrete Element Method

The incentive for this research is to gain insight into fundamental aspects of turbulent fluid-particle flows. The project investigates the influence of inter-particle collisions on the particle and fluid phase variables in the context of particle agglomeration, dispersion and deposition for turbulent bounded flows laden with low particle numbers. The mathematical modelling technique used is large eddy simulation (LES), with flow solutions provided by this method coupled to a discrete element method (DEM) to predict particle motion and interaction. The results have been compared with single-phase bounded flows in order to investigate the effect of the particles on turbulence statistics. The four-way coupled simulations are also contrasted with one-way coupled (flow affects the particles only) results in which the inelastic collisions between particles are neglected. The influence of different particle surface energies, particle size, particle density, particle concentration and flow Reynolds numbers on particle agglomeration is investigated. The turbulent structure of the flow is found to dominate the motion of the particles, although the agglomeration rate is found to be strongly influenced by all of the variables noted above, with most of the particle-particle interactions taking place at locations close to the channel walls, aided by the higher turbulence levels and concentration of particles in these regions. The research proposed makes an original contribution to the literature in applying advanced predictive techniques which have not been coupled and applied to the problem of cohesive particle-interaction effects in turbulent flows before. It yields a fundamental understanding of how particles interact, and how these interactions result in the formation of agglomerates which affect the dispersion and deposition of particles within the flow. The overall results are relevant, and underpinning, to processes employed in a wide range of applications in the industrial and health sectors

Effect of Reynolds number on particle interaction and agglomeration in turbulent channel flow

The work described in this paper employs large eddy simulation and a discrete element method to study turbulent particle-laden channel flows at low concentrations (particle volume fraction 10−4–10−5), including particle dispersion, collision and agglomeration. Conventional understanding of such flows is that particle interactions are negligible, this work however demonstrates that such interactions are common at large Stokes numbers in turbulent flow. The particle-particle interaction model is based on the Hertz-Mindlin approach with Johnson-Kendall-Roberts cohesion to allow the simulation of cohesive forces in a dry air flow. The influence of different flow Reynolds numbers, and therefore the impact of fluid turbulence, on agglomeration behaviour is investigated. The agglomeration rate is found to be strongly influenced by the flow Reynolds number, with most of the particle-particle interactions taking place at locations close to the channel walls, aided by the higher turbulence levels and concentration of particles in these regions

Particle Agglomeration and Dispersion in Fully-coupled Turbulent Channel Flow Using Large Eddy Simulation and Discrete Element Method

The work described in this paper employs large eddy simulation and the discrete element method to study particle-laden flows, including particle dispersion and agglomeration, in a horizontal channel. The particle-particle interaction model used is based on the Hertz-Mindlin approach with Johnson-Kendall-Roberts cohesion to allow the simulation of Van der Waals forces in the dry air flow considered. The influence of different particle surface energies, particle size, particle concentration and flow Reynolds numbers on particle agglomeration is investigated. The turbulent structure of the flow is found to dominate the motion of the particles, although the agglomeration rate is found to be strongly influenced by all of the variables noted above, with most of the particle-particle interactions taking place at locations close to the channel walls, aided by the higher turbulence levels and concentration of particles in these regions

Numerical Modeling Of Collision And Agglomeration Of Adhesive Particles In Turbulent Flows

Particle motion, clustering and agglomeration play an important role in natural phenomena and industrial processes. In classical computational fluid dynamics (CFD), there are three major methods which can be used to predict the flow field and consequently the behavior of particles in flow-fields: 1) direct numerical simulation (DNS) which is very expensive and time consuming, 2) large eddy simulation (LES) which resolves the large scale but not the small scale fluctuations, and 3) Reynolds-Averaged Navier-Stokes (RANS) which can only predict the mean flow. In order to make LES and RANS usable for studying the behavior of small suspended particles, we need to introduce small scale fluctuations to these models, since these small scales have a huge impact on the particle behavior. The first part of this dissertation both extends and critically examines a new method for the generation of small scale fluctuations for use with RANS simulations. This method, called the stochastic vortex structure (SVS) method, uses a series of randomly positioned and oriented vortex tubes to induce the small-scale fluctuating flow. We first use SVS in isotropic homogenous turbulence and validate the predicted flow characteristics and collision and agglomeration of particles from the SVS model with full DNS computations. The calculation speed for the induced velocity from the vortex structures is improved by about two orders of magnitude using a combination of the fast multiple method and a local Taylor series expansion. Next we turn to the problem of extension of the SVS method to more general turbulent flows. We propose an inverse method by which the initial vortex orientation can be specified to generate a specific anisotropic Reynolds stress field. The proposed method is validated for turbulence measures and colliding particle transport in comparison to DNS for turbulent jet flow. The second part of the dissertation uses DNS to examine in more detail two issues raised during developing the SVS model. The first issue concerns the effect of two-way coupling on the agglomeration of adhesive particles. The SVS model as developed to date does not account for the effect of particles on the flow-field (one-way coupling). We focused on examination of the local flow around agglomerates and the effect of agglomeration on modulation of the turbulence. The second issue examines the microphysics of turbulent agglomeration by examining breakup and collision of agglomerates in a shear flow. DNS results are reported both for one agglomerate in shear and for collision of two agglomerates, with a focus on the physics and role of the particle-induced flow field on the particle dynamics

Numerical Simulation of Particle Collision and Agglomeration in Turbulent Channel Flows

The study described in this thesis concerns the simulation of dispersed and dense particle-laden turbulent channel flows. The research primarily investigates the role of gravity; in terms of its contribution to particle collision, agglomeration and re-distribution. Large eddy simulation is employed to predict the fluid-phase, with solutions coupled with a Lagrangian particle tracking routine to model the particle-phase. In order to establish the validity of the preferred numerical method, results generated from the single-phase and the dilute particle-phase predictions were compared with those based on DNS, with good agreements found. Results obtained for horizontal zero gravity channel flows, show effects of particle size, particle concentration and turbulence on colliding and agglomerating particles. All variables were shown to strongly impact on collision and agglomeration, with the number of events reaching maximum towards the channel walls due to increased particle concentrations and turbulence levels in these regions. The collision and agglomeration is, however, shown to enhance exponentially with the inclusion of gravity and accentuated on the lower wall of the channel. An extension of the investigation into vertical channels of upward and downward flow configurations, also demonstrated the significance of the gravity force on particle collision and agglomeration. The effect of the particles on the flow is small, owing to the low mass-loading. Agglomeration is found to be most favourable for flows of low turbulence; and unlike collisions, dominantly forms in the channel centre. The investigation presented is a novel contribution to literature that provides a fundamental improvement on the understanding of turbulent fluid-particle flows. Particularly, it extends the existing knowledge on cohesive particle behaviours in turbulent flows by examining the effect of gravity on such flows. The contribution finds relevance in many engineering and industrial flow processes and should aide the design of better flow processes