Query: CFD DEM PFC

title: Semi resolved CFD DEM modeling of gas particle two phase flow in the micro abrasive air jet machining
abstract: Abrasive air jet (AAJ) machining is attractive for micromachining hard and brittle materials, while it is usually a big challenge to numerically investigate the particle velocity and concentration distribution in the particle erosion process in the AAJ. In this work, a recently developed semi resolved CFD DEM approach which bridges the simulation gap between the resolved and unresolved CFD DEM, is further improved to reconstruct the background information with a double cosine kernel function. The semi resolved CFD DEM is then employed to numerically investigate the gas particle two phase flow in the AAJ and numerical results show that this semi resolved CFD DEM is more accurate in modeling particulate flow in the fine AAJ nozzle than the conventional unresolved CFD DEM. We further conduct mechanism investigations on the AAJ micromachining process including particle flow characteristics inside the cylindrical nozzle, velocity, and concentration distribution over the nozzle exit, which are essential jet characteristic features. We identified the particle flow patterns, analyzed the particle distribution, and its correlation with air pressure, abrasive mass flow rate, and turbulence effects. The present simulation results and analyses can be great helpful in understanding the erosion mechanism and optimizing the setting parameters to improve the cutting performance of AAJ.

title: A CFD DEM model for the simulation of direct reduction of iron ore in fluidized beds
abstract: The aim of this work is the investigation of the indirect reduction of hematite in fluidized beds and the effects of different kinetic parameters on the reduction process at a numerical level, which are validated against experimental results. We utilize a CFD DEM coupling method, where gas phase is represented by classic CFD and iron ores by DEM. Both phases are strongly coupled by heat and mass transfer. The particle reduction is modeled using unreacted shrinking core model taking into account the morphological changes of the ore. The model is validated and critical reaction parameters such as the activation energies, pre exponenital factors and equilibrium constants are investigated. It sheds light on how much an influence the kinetic parameters have on the reduction degree and becomes evident that the rate determining step is reduction from wustite to iron. The model is then applied to a lab scale fluidized bed reactor.

title: Un resolved CFD DEM method: An insight into its limitations in the modelling of suffusion in gap graded soils
abstract: The un resolved CFD DEM method has been applied to the modelling of suffusion in gap graded soil columns recently. However, a fundamental question that remains unanswered is whether the un resolved CFD DEM simulations can reproduce the reasonable critical hydraulic gradient and seepage failure pattern of suffusion in gap graded soil columns packed under gravity. This research reveals that the un resolved CFD DEM models predict unreasonably greater critical hydraulic gradients for suffusion in the gap graded soil columns packed under gravity. Meanwhile, the seepage failure presents an unreasonable pattern, i.e. the overall instability of fine and coarse particles, which is confirmed by variations of fine and coarse particle energies. It is distinct from the experimental observations. Further investigations with the aid of the authors' recently developed semi resolved CFD DEM method [Cheng et al. Compt Geotech, 100: 30 51(2018) show that the above discrepancies are caused by the excessive locally averaging and the relatively low resolution of the fluid flow in the un resolved CFD DEM method.

title: Assessing debris flow impact on flexible ring net barrier: A coupled CFD DEM study
abstract: Flexible ring net barriers have become increasingly popular in practical mitigation of debris flow worldwide. Systematic assessments of their response and performance subjected to the impact of realistic debris flows remain challenging. This study presents a novel computational approach based on coupled CFD DEM to model the impact of debris flow on a flexible ring net barrier in a unified framework. The debris flow is treated as a solid fluid mixture, where the solid phase and the fluid phase are modeled by DEM and CFD, respectively. The barrier is simulated as a system of different deformable components, including rings, cables and energy dissipators, and is modeled with DEM. The proposed method expedites a convenient, unified consideration of multi way interactions among the debris solid, the debris fluid, and the barrier. The simulation of a flexible barrier is calibrated against existing experimental data and past numerical results, by examining the quasi static responses of different barrier components and the dynamic reactions of the entire barrier. The barrier system is further subjected to the impact of debris flows with different Froude numbers to examine its performance, in terms of its retaining capacity of debris mass and peak sustained forces in the barrier. Two energy related indices, energy dissipation ratio and energy absorption ratio, are estimated for design reference. The study provides a novel, physically based predictive computational tool for future design and analysis of flexible ring net barriers in debris flow mitigation.

title: CFD DEM simulation of proppant transport by supercritical CO2 in a vertical planar fracture
abstract: The great potential of supercritical CO2 fracturing technique in the stimulation of unconventional reservoirs requires further insight into the mechanism of proppant transport in the fracture. In this paper, we adopted the computational fluid dynamics (CFD) coupled discrete element method (DEM) scheme to study the proppant transport in supercritical CO2 fluid, and verified this method via previous experiment data. Based on the visual analysis at the particle level, transport mechanisms of proppant in the fracture were revealed. The difference in the proppant transport capacity between supercritical CO2 and commonly used fracturing fluids were demonstrated, and effects of slurry injection velocity, proppant concentration, injection temperature, and proppant diameter were discussed. Results indicate that the transport and placement process of the proppant can be divided into three stages, i.e. flat laying stage, proppant dune development stage, and proppant bank development stage. The supercritical CO2 slurry is characterized by obvious stratified flow in the fracture: the static bed at the bottom, the fluidization layer above it, and the pure fluid flow region at the top. Viscous drag transport, lifting transport, and fluidization transport are three proppant transport mechanisms for supercritical CO2 slurry flow, among which the fluidization transport plays a more dominant role. This work is expected to provide an in depth understanding of the transport mechanism of supercritical CO2 slurry in a vertical planar fracture.

title: CFD DEM bidirectional coupling simulation and experimental investigation of particle ejections and energy conversion in a spouted bed
abstract: Fluidized bed has gained popularity in the renewable energy industry due to its superior thermochemical conversion efficiency. Investigating the flow patterns inside the fluidized bed is of great importance to obtain detailed knowledge of the energy conversion of biofuel and to further optimize the combustion performance. In this paper, the effect of different inlet gas flow rates on bed hydrodynamics parameters such as bed height, equivalent bubble diameter, and particle ejection were studied by the high speed photography technology and numerical simulation methods. The results show that changes in the particle bed height and bubble area can be accurately predicted by numerical simulation. The mechanism of particle ejection is varied under different inlet flow conditions. At the low inlet flow rate of 300 L/min, the particles ejected into the freeboard region are only from the bursting bubbles. As the inlet flow rate increases to 400 L/min, the ejected particles are not only from the bursting bubble but also from the bubble wakes. Particle ejection is mainly caused by the bubble expansion rate. These findings could serve as the reference for the optimal design of fluidized beds to achieve higher combustion efficiency.

title: CFD DEM simulation of fluidized bed with an immersed tube using a coarse grain model
abstract: The computational fluid dynamic combining the discrete element method (CFD DEM) is widely used to comprehend the mechanism of gas solid flow. However, as a Lagrangian method, DEM suffers from the huge cost for tracking each particle in the systems when applied to industrial applications with billions of fine particles, regardless of the great advances in computing capacity of computers. To address this issue, a coarse grain (CG) model is proposed in this paper, where the original fine particles are represented by a small number of large sized CG particles. In the current study, the new developed CG model combining CFD DEM (CG CFD DEM) is applied to investigate the fluidized bed with an immersed tube. The applicability and accuracy of the proposed CG CFD DEM are validated by the good agreement of the macroscopic behaviors predicted by simulations and those obtained from experiments. For the first time, the CG CFD DEM is used to predict the erosion around the tube based on the shear impact energy model (SIEM) and the computational results calculated by CG CFD DEM and original CFD DEM agree well, which demonstrates that the proposed model can predict the erosion of the tube in the fluidized bed accurately and efficiently. Besides, as expected, the calculation time is drastically reduced due to the CG model, which makes the simulations of large scale industrial applications accessible.

title: CFD DEM Simulation of Biomass Pyrolysis in Fluidized Bed Reactor with a Multistep Kinetic Scheme
abstract: The pyrolysis of biomass in a fluidized bed reactor is studied by a combination of a CFD DEM algorithm and a multistep kinetic scheme, where fluid dynamics, heat and mass transfer, particle collisions, and the detailed thermochemical conversion of biomass are all resolved. The integrated method is validated by experimental results available in literature and a considerable improvement in predicting the pyrolysis product yields is obtained as compared to previous works using a two fluid model, especially the relative error in the predicted tar yield is reduced by more than 50% Furthermore, the evolution of light gas, char and tar, as well as the particle conversion, which cannot easily be measured in experiments, are also revealed. Based on the proposed model, the influences of pyrolysis temperature and biomass particle size on the pyrolysis behavior in a fluidized bed reactor are comprehensively studied. Numerical results show that the new algorithm clearly captures the dependence of char yield on pyrolysis temperature and the influence of heating rate on light gas and tar yields, which is not possible in simulations based on a simplified global pyrolysis model. It is found that, as the temperature rises from 500 to 700 degC, the light gas yield increases from 17% to 25% and char yield decreases from 22% to 14% In addition, within the tested range of particle sizes <1 mm) the impact on pyrolysis products from particle size is relatively small compared with that of the operating temperature. The simulations demonstrate the ability of a combined Lagrangian description of biomass particles and a multistep kinetic scheme to improve the prediction accuracy of fluidized bed pyrolysis.

title: Semi resolved CFD DEM for thermal particulate flows with applications to fluidized beds
abstract: Particulate flows with heat transfer widely exist in industry including fluidized bed reactor and powder based 3D printing, and are frequently modeled with CFD DEM coupled approaches. Recently, we developed a semi resolved CFD DEM approach, which bridges the simulation gap between the resolved and unresolved CFD DEM (Wang et al. 2019) The semi resolved CFD DEM is as efficient as the conventional unresolved CFD DEM and as accurate as the resolved CFD DEM, while it is validated for particulate flows with momentum exchange with Model B without heat exchange. In this paper, we further extend the semi resolved CFD DEM approach to model particulate flows with momentum exchange and thermal convection. Firstly, similar to background fluid velocity, background fluid temperature is corrected through kernel based approximations on the neighboring fluid cells rather than simply taking values in the local cell containing the concerned particle. Secondly, Model A is adopted in the semi resolved CFD DEM, leading to more powerful capabilities in handling complicated flows. Force models like the Magnus force, virtual mass force are also included. Thirdly, turbulence model is incorporated into the semi resolved CFD DEM approach to address possible turbulence effects. Finally, the developed semi resolved CFD DEM is applied to modeling the complex particulate flows in fluidized bed. The obtained numerical results are compared with experimental data and results from other sources. It is demonstrated that the present semi resolved CFD DEM is effective in modeling particulate flows with thermal convection. Compared to the conventional unresolved CFD DEM, this semi resolved CFD DEM can provide better predictions and more details on particle temperature distribution, mean temperature, solid/fluid fraction, velocity autocorrelation and vorticity field.

title: CFD DEM simulation of gas solid flow of wet particles in a fluidized bed with immersed tubes
abstract: In an intensified fluidized bed processing of wet particles, it is often to use heat transfer immersed tubes to maintain reasonable temperature distribution. The wet particles could adhere to the immersed tubes, which hinders the normal fluidization. We studied the gas solid flow in a pseudo 2D fluidized bed of wet particles with the presence of immersed tubes, by using CFD DEM simulation with the hysteretic contact model. The gas solid flow characteristics around the immersed tubes and the effects of immersed tubes, adhesive force and fluidization velocity on the fluidization of wet particles are analyzed. With the increase of the adhesive force the agglomeration mainly occurs near the bed wall and the channeling mainly appears in the middle of the bed. The immersed tubes can cause the particles to accumulate on the upper surface of the tubes. Also, it can break the bubbles and agglomerates when the fluidization velocity is high. As the fluidization becomes poor, the particle agglomeration becomes more obvious and coordination number increases. The local poor fluidization zone is observed mainly in the gaps between the wall and the immersed tubes.