Coupled DEM-CFD Model to Predict the Tumbling Mill Dynamics

The charge motion in a tumbling mill has been mostly described by empirical, mechanistic and computational models. The computational model presented in this work is a three phase approach for the tumbling mill that combines a particle description for the solids modelled by the Discrete Element Method, and the continuum description for the fluid by CFD approach. In the present work a phase coupled approach is developed using C++ subroutines to model the charge and slurry dynamics inside a tumbling mill by mapping the particles on the CFD mesh and resolving the particle volume and velocities on per cell basis. The coupled DEM-CFD approach is implemented and the effect of drag force on the slurry by the particle motion. The set of coupled simulation were run varying the slurry rheology and results were validated with equivalent PEPT experiment of lab scale mills and a very good agreement is found in some cases. The Beeststra drag correlation was used to calculate the drag force between the fluid and the particles. The free surface profile of the charge as well as the slurry is calculated as well as the axial center of mass profile of the mil

Prediction of solid recirculation rate and solid volume fraction in an internally Circulating Fluidized bed (ICFB)

Numerical & experimental study of gas and solid flow in an internally circulating fluidized bed (ICFB). The gas and solid hydrodynamics have been simulated by using the commercially available computational fluid dynamics (CFD) softwar e package, ANSYS’s Fluent. A three dimensional geometry was used to represent key parts of a laboratory ICFB. In 3D ICFB, the two - fluid Eulerian model with kinetic theory of granular flow option and the various drag laws like Gidaspow, Syamlal - Obrien, Gibl aro and Arastoopour drag models used to predict the hydrodynamic behavior of ICFB. The simulation results by four drag laws show that the Gidaspow and Arastoopour drag models predict the fluidization dynamics in terms of flow patterns, void fractions and a xial velocity fields were compared with experimental data. With the Arastoopour drag model the simulations are giving the best fits to the experimental data. The effect of superficial gas velocity, presence of draft tube on solid hold - up distribution, soli d circulation pattern, and variations in gas bypassing fraction for the 3D ICFB investigated through CFD simulations. The mechanism governing the solid circulation in an ICFB has been explained based on gas and solid dynamics obtained from the simulations

DENSE SLURRY CFD MODEL FOR HYDROCYCLONE PERFORMANCE EVALUATION INCORPORATING RHEOLOGY, PARTICLE DRAG AND LIFT FORCES

Common applications of hydrocyclones include classification, thickening, de-sliming and de-gritting. In all the operations cyclone separators usually operate under high solid loading conditions. Computational Fluid Dynamic (CFD) models developed so far are unable to predict the behavior at high percentage of solids. Therefore, present paper is aimed to develop the CFD model corrected with suitable rheological model, particle drag and lift forces to account particle fluid interactions at high solid volume fractions. Turbulence is resolved using large eddy simulation. Drag is corrected with solids loading; rheology is modelled using Newtonian model corrected with fines. CFD predicted two-phase water split and air-core data is validated against Electrical Resistance Tomography and High Speed Video camera measured data. The influence of feed solids on the air core size also investigated. Multiphase simulations ran with 0-50% feed solid loadings are analyzed and validated in terms of cut size and efficiency. Modified CFD model is able to predict the experimental data with reasonable accuracy. Additional validation in terms of cut size in 250 mm Krebs cyclone (Devulapalli, 1997) is also provided in comparison with discrete phase model and standard mixture model

Two Way Coupled CFD-DEM Model to Predict Tumbling Mill Dynamics

The high energy inefficiency of tumbling mills has been the focus of much of modern day comminution research, Harsh milling conditions limit experimental measurement so the mill designers have to rely on empirical relations. The scope of the present work is to prepare a computational model that can accurately predict the complex multiphase dynamics of charge, air and slurry inside the mill using a coupling between computational fluid dynamics and discrete element method. The slurry air system is modelled using Volume of fluid (VOF) method and the charge motion is modelled by discrete element method. The phases are coupled using the interphase momentum exchange between the charge and the fluids at each fluid flow time step. The coupled model is compared with experimental results from the positron emitting particle tracking (PEPT) camera. The results show good qualitative agreement with equivalent PEPT findings. It is observed that out of the four modelling approaches investigated the Two-way coupled model gives the best agreement with the PEPT findings. The model can be useful in better understanding of the mill dynamics which can lead to more efficient design

SIMULATING MULTI-COMPONENT PARTICLES BEHAVIOUR DURING THE CLASSIFICATION PROCESS IN A HYDROCYCLONE USING MULTIPHASE CFD MODEL

Numerical simulation of the hydrocyclone is known for its complexity and non-trivial solving strategies. The flow inside the hydrocyclone is highly turbulent and intricate in nature. Most of the mathematical model reflects the single average mineral density for the hydrocyclone multiphase classification performance. The behaviour of multicomponent particles in a hydrocyclone is superficially understood and the component interactions are unaccounted for most of the available mathematical models .In this work, multi size and density simulation of hydrocyclone are carried out using CFD approach. The turbulence is solved using the large eddy simulation (LES) model. The multiphase is modelled using the volume of fluid (VOF) and algebraic slip mixture (ASM) model. The multi-phase numerical simulation contains 10 phases at an instant i.e. water, air, 4 phases of magnetite and silica each, having different sizes and volume fractions. The mixture of magnetite and silica ratios i.e. 1:9, 2:8, 1:1 is considered for the understanding of interaction between components and sizes in complex flow system at optimized hydrocyclone conditions. The CFD model is able to predict the salient features of the cyclone flow fields in great detail, thus providing a better understanding of the solid recovery to the underflow, where authors have observed high Rs for the heavier particle i.e. magnetite. Separation characteristics of the silica and magnetite particles are explained using locus of zero vertical velocities (LZVV) and equilibrium radius

Computational Fluid Dynamic study on the effect of near gravity material on dense medium cyclone treating coal using Discrete Phase Model and Algebraic Slip mixture multiphase model

In this paper, the effect of near gravity material at desired separation density during the coal washing is studied. It is believed that the Dense Medium Separation of coal particles in the presence of high percentage of near gravity material, results in a significant misplacement of coal particles to wrong products. However the performance of dense medium cyclone does not merely depend on the total amount of near gravity materials but also on their distribution as well as on their quality. This paper deals with numerical simulation of magnetite medium segregation and coal partitioning handled in a 350 mm dense medium cyclone. Volume of Fluid coupled with Reynolds Stress Model is used to resolve the two-phase air-core and turbulence. Algebraic Slip mixture multiphase model with the granular options are considered to predict magnetite medium segregation. Medium segregation results are validated against Gamma Ray Tomography measurements. Further, Discrete Phase Model is used to track the coal particles. Residence Time Distribution of different size and density coal particles are also estimated using Discrete Phase Model. Additionally, Algebraic Slip mixture model is also utilised to simulate magnetite and coal particle segregation at different near gravity material proportions. Discrepancies in the coal particle behaviour at different near gravity material content are explained using locus of zero vertical velocities, mixture density, coal volume fractions

Hydrodynamics Study of Gas-Solid Flow Pattern in an Internally Circulating Fluidized Bed with a Draft Tube

Circulating fluidized beds (CFB) are widely used in coal combustion and gasification processes. These CFBs suffer from having very tall column as a solids raiser and accompanying additional external circulation of solids through cyclone. To avoid the external circulation accessories a compact internally circulating fluidized bed (IFCB) is developed. ICFB represent a modified spouted fluidized bed with a draft tube inside to avoid problem of gas bypassing. The hydrodynamics of ICFB have studied using combination of experiments and CFD simulations. Solids circulation rate and bed pressure drop were measured using high speed camera and pressure manometers for a wide range of particle sizes and bed heights. 3D Twofluid Eulerian granular flow model was adopted to predict the hydrodynamic behavior of ICFB. Effect of gas velocity, presence of draft tube on solid hold-up distribution, solid circulation pattern and variations in gas bypassing fraction in the ICFB have been investigated with the help of CFD simulations. CFD validation against the experimental data is mad

Experimental Investigation of solids concentration and velocity profiles in fluidized bed riser using optical fiber probe and validation using CFD

Study of hydrodynamics is very important for design and optimization of fluidized bed reactor. They are several intrusive and non-intrusive techniques available for characterizing hydrodynamics in multi-phase flow systems such as electrical capacitance tomography, Gamma ray tomography, sampling probe, capacitance probe and Gamma density probe etc. Out of these optical fiber probe stands out as a potential tool with its advantages over other measurement techniques. Optical fiber probe, an intrusive technique measures both the local solids concentration and velocity simultaneously. In this work, dual sensor optical probe is employed for hydrodynamic study of fluidized bed riser. The main objective of the study is to understand the effect of superficial velocity, bed height and particle size on the hydrodynamic behavior in different regimes of fluidization experimentally using optical fiber probe and validating these experimental results with the standard granular CFD Models. Experiments were conducted with fabricated riser having 105 mm internal diameter and 1000 mm height. A series of experiments were run for Geldart A and Geldart AB mixture at 123mm and 200mm bed height pertaining to bubbling and turbulent regimes. With Geldart AB particles, high solids concentration was found near the wall region, where as diluted bed observed in the central zone. Bubbling bed behavior was observed for both Geldart A and AB particles immediately after the minimum fluidization. 2D CFD granular model validation is initially pursued for the experimental results available in literature. A Eulerian-Eulerian two fluid model integrating kinetic theory of granular flow with Gidaspow drag correlation is employed for this purpose. After validating, the same CFD model is used for further validation of the current experimental results obtained. A series of CFD simulations run at which the optical probe measurements conducted for. Time average Solids concentration and particle velocity variation with radius and height from the CFD simulations are compared with experimental results found them in similar trends. Further fine tuning of CFD parameters and appropriate gas input strategies are required for improved simulations

HYDRODYNAMIC STUDY OF TWO PHASE FLOW OF COLUMN FLOTATION USING ELECTRICAL RESISTANCE TOMOGRAPHY AND CFD TECHNIQUES

The distribution characteristics of mean gas hold-up and flow field were studied using electrical resistance tomography (ERT) and CFD model in column flotation. The effect of superficial gas velocity and different types of spargers on mean gas hold-up and its radial distribution has been analysed. Experimental results show ERT is suitable as an online monitoring tool to provide useful information on the hydrodynamic parameters of column flotation. The ERT technique facilitates non-invasive and nonintrusive visualization of different parameters in a column flotation. The tomography images, which were generated using a modified sensitivity back projection algorithm, were employed to explore the influence of parameters in two phase flow in column flotation. The mean gas hold-up values from ERT have been validated against pressure transducer data. The measured data is validated against the two-fluid model based CFD data and found them in close agreement

Hydrodynamic Study of Gas–Solid Internally Circulating Fluidized Bed Using Multiphase CFD Model

In the present work, hydrodynamic study of gas and solid flow in an internally circulating fluidized bed (ICFB) was carried out using the CFD multiphase model. Two- (2D) and three-dimensional (3D) computational meshes were used to represent physical ICFB geometries of 0.186-m and 0.3-m diameter columns. The model approach uses the two-fluid Eulerian model with kinetic theory of granular flow options to account particle–particle and particle–wall interactions. The model also uses various drag laws to account the gas–solid phase interactions. The 2D simulation results by various drag laws show that the Arastoopour and Gibilaro drag models predict the fluidization dynamics in terms of flow patterns, void fractions, and axial velocity fields in close agreement with the Ahuja et al. (2008) experimental data. Three dimensional simulations were also carried out for a large-scale ICFB. The effects of superficial gas velocity and the presence of draft tube on solid holdup distribution, solid recirculation pattern, and gas bypassing dynamics for the 3D ICFB were investigated extensively. The mechanism governing the solid circulation and the pressure losses in an ICFB has been explained based on gas and solid dynamics obtained from these simulations. Predicted total granular temperature distributions in 3D ICFB draft tube and the annular zone are qualitatively in agreement with the experimental data. The total granular temperature tends to increase with the increase in solids concentration in the dilute region (ϵ 0.1)