Discrete Element Computational Fluid Dynamics Research Articles

Investigation of Interphase Drag Force Affected by Clouded Bubble via a Computational Fluid Dynamics–Discrete Element Method Approach

Bubbles as mesoscale structures play an important role in the development of the inhomogeneous drag force model, which is key to the prediction of fluidized beds. A coupled computational fluid dynamics–discrete element method is employed in this work to investigate the distribution of interphase drag force around a clouded bubble in an incipiently fluidized bed. A method for the determination of the bubble boundary is proposed. The results reveal that there is a stronger positive effect of drag force around the clouded bubble because of the presence of bubble crown and wake, which is different from the cloudless bubble. The bubble-induced local inhomogeneous coefficient is also proposed to evaluate the ensemble inhomogeneous impact around the bubble.

Effects of dose loading conditions and device geometry on the transport and aerosolization in dry powder inhalers: A simulation study

The transport and aerosolization of particles are studied in several different dry powder inhaler geometries via Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) simulations. These simulations combine Large Eddy Simulation of gas with Discrete Element Model simulation of all the carrier particles and a representative subset of the active pharmaceutical ingredient (API) particles. The purpose of the study is to probe the dominant mechanism leading to the release of the API particles and to demonstrate the value of the CFD-DEM simulations where one tracks the motion of all the carrier and API particles. Simulations are performed at different inhalation rates and initial dose loading conditions for the screen-haler geometry, a simple cylindrical tube inhaler, and five different geometry modifications that took the form of bumpy walls and baffles. These geometry modifications alter the residence time of the powder sample in the inhaler, pressure drop across the inhaler, the severity of gas-carrier interactions, and the number of collisions experienced by the carrier particles, all of which are quantified. The quality of aerosolization is found to correlate with the average air-carrier slip velocity, while collisions played only a secondary role. Some geometry modifications improved aerosolization quality with very little increase in the pressure drop across the device.

Mixing and dispersion behaviours of ellipsoid particles in a bubbling fluidized bed

Fluidization of non-spherical particles is widely practised in chemical engineering processes. In this work, the mixing and dispersion behaviours of non-spherical particles in a bubbling fluidized bed (BFB) are explored by a computational fluid dynamics-discrete element method (CFD-DEM) featuring a super-quadric model to describe particle morphology. After model validation, the general flow patterns, solid mixing, and solid dispersion of ellipsoid particles are studied, together with the discussion of the effect of aspect ratio (AR) on fluidization behaviours. The results show that minimum fluidization velocity increases with the aspect ratio. Bed permeability depends on bed porosity and particle shape. Bubbles place a significant role in determining flow patterns, which induce a double-recirculation pattern where particles mainly move from the wall region to the central region in the lower bed and oppositely in the upper bed. Ellipsoid particles with higher sphericity (e.g., AR = 0.75 and 1.5) show a smaller mixing index in the bed. The magnitudes of solid dispersion coefficients in x, y, and z directions are about ~10−3 m2/s, ~10−5 m2/s, and ~ 10−2 m2/s, respectively. Ellipsoid particles with higher sphericity show larger solid dispersion intensity. These findings shed light on the fundamental understanding of the particle shape on fluidization dynamics.

CFD-DEM Modeling of Dense Sub-Aerial and Submerged Granular Collapses

Sub-aerial (dry) and submerged dense granular collapses are studied by means of a three-phase unresolved computational fluid dynamics-discrete element method (CFD-DEM) numerical model. Physical experiments are also performed to provide data for validation and further analysis. Validations show good compatibility between the numerical and experimental results. Collapse mechanism as well as post-collapse morphological parameters, such as granular surface profile and runout distance, are analyzed. The spatiotemporal variation of solid volume fraction is also investigated. The effect granular column aspect ratio is studied and found to be a key factor in granular morphology for both submerged and dry conditions. The volume fraction analysis evolution shows an expansion and re-compaction trend, correlated with the granular movement.

Resolved CFD-DEM simulation of blood flow with a reduced-order RBC model

Mathematical modeling of the blood flow with a resolved description of biological cells mechanics such as red blood cell (RBC) has been a challenge in the past decades as it involves physical complexities and demands high computational costs. In the present study, we propose an approach for efficient simulation of blood flow with several suspended RBCs. In this approach, we employ our previously proposed reduced-order model for deformable particles (Nair et al. in Comput Part Mech 7:593–601, 2020) to mimic the mechanical behavior of an individual RBC as a cluster of overlapping spheres interconnected by flexible mathematical bonds. This discrete element method-based model is then coupled with a fluid flow solver using the immersed boundary method with continuous forcing in the context of computational fluid dynamics-discrete element method (CFD-DEM) coupling. The present computational method is tested with a couple of validation cases in which the single RBC dynamics, as well as the blood flow with several RBCs, were tested in comparison with existing literature date. First, the RBC deformation index in shear flow at different shear rates is studied with a good accuracy. Then, the blood flow in micro-tubes of different diameters and hematocrits was simulated. The key characteristics of blood flow such as cell-free layer (CFL) thickness, Fahraeus effect and the relative apparent viscosity are used as the validation metrics. The proposed approach can predict the formation of the migration of RBC toward the tube center-line and the CFL thickness in good agreement with previous measurement and simulations. Furthermore, the model is employed to study the CFL enhancement for plasma separation based on channel constriction. The simulation results compute the CFL thickness downstream of the channel constriction in good agreement with the experiments in a wide range of flow rates and constriction lengths. The original contribution of this study lies in proposing an efficient resolved CFD-DEM simulation method for blood flows with many RBCs which can be employed for numerical investigation of bio-microfluidic applications.

Liquid–Solid Flow Characteristics in Vertical Swirling Hydraulic Transportation with Tangential Jet Inlet

In order to improve the efficiency and safety of vertical hydraulic transport systems for non-spherical particles, a new pipeline transport system with a tangential jet inlet is adopted in this study, and a modified non-spherical drag coefficient model is used to analyze the liquid–solid flow characteristics based on the CFD-DEM (Computational Fluid Dynamics-Discrete Element Method) coupling method. The focus of the study is on the influence of different tangential flow proportions in terms of the velocity distribution, the vorticity, the total pressure, the concentration and drag force of particles of various shapes. The conveying efficiency is measured according to the fluid velocity distribution and the particle concentration, and the safety of conveying is evaluated according to the flow structure and drag force of the particles. The result shows that the velocity of the swirling pipes is significantly higher than the straight pipe. With the increase of the tangential flow proportion, the swirling number and the vorticity magnitude increase, and the vortex core is broken and merged more quickly. Furthermore, the concentration gap and axial drag force gap between particles of various shapes are reduced with the effect of swirling flow, the particle concentration increases, and the particles of each component are uniformly mixed and transported.

Particle behaviours of biomass gasification in a bubbling fluidized bed

Biomass gasification in a bubbling fluidized bed (BFB) reactor is numerically studied based on a particle-scale computational fluid dynamics-discrete element method (CFD-DEM), with thermochemical and polydispersity effects featuring. After model validation, the particle-scale information (e.g., particle motions, particle mixing, solid dispersion, and heat transfer contribution) are thoroughly explored with the discussion of the effects of several critical operating parameters on particle behaviours. The results show that the middle dense region has the highest biomass pyrolysis reaction rate due to the vigorous particle motion. Sand and biomass particles show synchronous horizontal motions, and the solid vertical dispersion coefficients are much higher than the solid horizontal ones, denoting that the vertically introduced gas flow dominates bed hydrodynamics. A higher operating temperature causes a higher solid dispersion coefficient. Elevating temperature and steam to biomass ratio (S/B) first increases and then decreases the particle mixing index. Convection plays a dominant role during the biomass gasification process, followed by the radiation and heat of reaction. The conduction accounts for the smallest proportion and can be neglected. Increasing operating temperature promotes chemical reactions, biomass temperature, and all heat transfer modes. Increasing S/B promotes biomass motions and gasification reactions, leading to more heat consumed and biomass temperature decrease. Decreasing biomass temperature results in a larger temperature difference between biomass particles and bed material, which enhances the conduction, convection, and radiation.

Study on tribocharging of CaCO3 particles based on CFD-DEM gas-solid two-phase flow coupling

Unsuccessful prediction of particle tribocharging in tribocharger is partly due to the complexity of tribocharging process. In this work, computational fluid dynamics-discrete element method (CFD-DEM) modeling approach has been used to investigate particle charge in a cyclone tribocharger. Then particle image velocimetry (PIV) method is used to calculate the particle charge-to-mass ratio. The CaCO3 particle charge in a cyclone tribocharger is simulated and predicted under different gas velocities, particle sizes and moving distances. The simulation results carry out the quantitative and qualitative relationship between different parameters. At the same time, the charge-to-mass ratio between the particle simulation and the experimental results under different gas velocities is compared, and the results are all in the magnitude of 10−4. Experimental results in terms of particle charge-to-mass ratio are thus obtained. The CFD-DEM model is verified by comparison with the experimental results

Numerical simulation of clogging in a microchannel with planar contraction

Clogging is the mechanism that interrupts the flow in confined geometries due to the complete blockage of the channel cross section. It represents a critical issue in the processing of particle suspensions for both industrial and biological applications, and it is particularly relevant in microfluidics and membrane technology due to the high particle confinement and the difficult device cleaning. Although numerous experimental and numerical studies have been carried out to understand the mechanism governing this complex multiscale phenomenon, the picture is not yet clear and many questions still remain, especially at the particle level. In this regard, the numerical simulations may represent a useful investigation tool since they provide a direct insight to quantities not easily accessible from experiments. In this work, a detailed computational fluid dynamics-discrete element method simulation study on the clogging mechanism in a microchannel with planar contraction is carried out. Both constant flow rate and constant pressure drop conditions are investigated, highlighting the effect of flow conditions, particle volume fraction, cohesion forces, and contraction angle. The onset of clogging conditions is discussed.

Exploring mechanism of spout deflection in a spout fluidized bed

Spout deflection has been widely encountered in spout fluidized beds and can seriously narrow the operating range of normal spouting. However, the underlying mechanism of spout deflection phenomenon remains unknown. In this study, two hypotheses are proposed to explain the spout deflection mechanisms in spout fluidized beds, and two virtual experiments are designed to verify these two hypotheses respectively using the Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) approach. To verify Hypothesis I that the unsymmetrical particle fountaining, including particle spurting and particle falling, is one main reason for the spout deflection, virtual experiment I is designed where the particles are manually collected from the spurting surface and then symmetrically released to the free space above the two annulus regions to allow them to fall freely. Three symmetrical release patterns are compared, and the results confirm that the spout deflection disappears when the symmetrical particle fountaining is deployed. Moreover, two univariate tests are conducted to check the role of unsymmetrical particle falling and unsymmetrical particle spurting, respectively. The results further confirm the Hypothesis I. To verify Hypothesis II that the rheological properties of the annulus region also play a key role in determining whether the spout deflection happens, virtual experiment II is designed, in which the rheological properties of the annulus region are controlled by changing particle restitution coefficient, particle friction coefficient, background gas velocity, and particle diameter. The simulation results show that spout deflection tends to happen when the rheological properties in the annulus regions get better (i.e., larger particle restitution coefficient, smaller particle friction coefficient, larger particle diameter and larger background gas velocity). This work unveils the mechanism of spout deflection and is useful for process design and control in spout fluidized beds.

A study of ellipsoidal and spherical particle flow, clogging and unclogging dynamics

To study ellipsoidal and spherical particle flow, clogging behavior and unclogging dynamics, a series of numerical simulations were carried out using our validated 3-D coupled computational fluid dynamics–discrete element method (CFD–DEM) model. For the first time, the strength of clogging arches is assessed in terms of fluid pressure gradient, which can effectively break up the clog. The main findings are as follows. In a clogged assembly, only when the flushing fluid reaches the unclogging pressure gradient, the ellipsoidal and spherical particles can flow in a continuous state. For the ellipsoids and spheres with the same equivalent volume, the unclogging pressure gradient of an ellipsoidal assembly is 11.8 times higher than that of a spherical assembly. Clogging arches at the outlet show the highest strength when the longitudinal orientations of the ellipsoids are perpendicular to the outlet plane. In contrast, the formed arch has the lowest stability when the longitudinal orientations of the ellipsoids are in the same direction as the long side of the outlet, as shown in the graphical abstract. The unclogging pressure gradient of the former case is 1.9 times greater than that of the latter case. The granular arches observed in our 3-D model are categorized as complex 3-D backbone arches. In the flowing region, particles rotate nearly perpendicular to the outlet plane, especially those approaching the outlet. Moreover, triangular stagnant zones are formed on both sides of the bottom of the rectangular box. The model presented here offers a great promise for future investigation of various granular systems at the particle scale.

Thermochemical heat storage performances of fluidized black CaCO3 pellets under direct concentrated solar irradiation

Conventional solar thermochemical heat storage based on indirect surface-heating usually suffers from high heat losses and low solar-chemical efficiency. Here, a different solar thermochemical heat storage system based on direct solar illumination on fluidized black CaCO3 pellets is proposed. A Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) model considering irradiation ray tracing, granular flow, heat and mass transfer, and chemical reaction, is built. Black CaCO3 pellets are fabricated via a facile template mixing method, and the solar absorptance is enhanced to 63.9% from 27.9% of traditional pure CaCO3. Effects of gas velocity and irradiative flux on thermochemical heat storage performance in a fluidized volumetric bed are investigated by incorporating measured kinetic and solar absorptance properties of designed black CaCO3 pellets. The peak solar-chemical efficiency reaches a value higher than 43% benefiting from enhanced solar absorptance, higher gas velocity and irradiative flux. This work guides the design of the high-efficiency direct solar thermochemical heat storage system.

Role of melt flow dynamics on track surface morphology in the L-PBF additive manufacturing process

Laser powder bed fusion (L-PBF) is an additive manufacturing process used to fabricate intricate metallic parts by melting successive layers of metallic powder. However, the fabricated part's poor surface roughness limits the extent to which the process can be used. The process involves rapid melting and solidification, and the unsteady flows within the melt pool dictate the surface morphology of the solidified tracks, which governs the surface smoothness of the end product. This work reports an experimental and computational investigation on the role of melt flow dynamics on the track's surface morphology. Single tracks were experimentally built by varying laser spot radius and scanning speed, and the corresponding melt pool and track surface morphology were characterized using metallographic and 3D optical profilometry techniques. Track formation is numerically simulated using an integrated 3D Discrete Element- Computational Fluid Dynamics (DEM-CFD) model. DEM is used to determine the spatial arrangement of the particles in the powder layer. In the CFD model, a free surface (VOF) approach is employed to precisely capture the interface between the gas and the metal phase. The interaction of laser with the powder bed and substrate, the physics of melting, solidification, and evaporation, and the melt flow due to interfacial forces (surface tension and recoil pressure) are accounted. With the help of simulation results, the role of melt flow dynamics on track morphology is described. Track roughness in the experimental samples is predicted with the help of multivariate polynomial regression analysis. The experimental and numerical data of track surface roughness and melt pool characteristics show decent agreement. Based on computational and experimental results, it was found that there exists an optimum scanning speed and laser spot radius which provide the minimum track surface roughness.

Computational fluid dynamics—Discrete element method simulation of the contact forces between microcapsules using a speed-up method with reduced Young's modulus

The small time-step due to the small size of microcapsules using the discrete element method (DEM) requires a significant number of computations when analyzing the force condition of microcapsules for mixing self-healing concrete. In this study, a speed-up method with a reduced Young's modulus is proposed to correct the particle contact force after establishing the relationship between the maximum contact force and the Young's modulus of particles in the collision process by using theoretical derivations and data fitting. The proposed method was verified using a stirred box model with the DEM and a stirred tank model with computational fluid dynamics–DEM. The effects of different particle numbers, impeller speeds, and time-step sizes on the accuracy of the proposed method were investigated. The results demonstrated the feasibility of analyzing the maximum contact force between particles using the speed-up method with a reduced Young's modulus.

CFD-DEM modeling of filtered fluid-particle drag and heat transfer in bidisperse gas-solid flows

This work extends filtered models to investigate the influence of particle polydispersity on the filtered fluid-particle drag and heat transfer behaviors based on well-resolved computational fluid dynamics-discrete element model (CFD-DEM) simulations. By systematically filtering the CFD-DEM simulation data, it is found that the markers used for the monodisperse systems, (i.e., the filtered slip velocity and the gas phase pressure gradient for the drag correction; the filtered interphase temperature difference for the heat transfer correction) are indispensable in the bidisperse systems. Furthermore, the dependence of the filtered corrections on the particle size ratio and volume fraction ratio is identified. The increase in the proportion of small particles generally corresponds to the reduction of drag and interphase heat transfer corrections. The Sauter mean diameter is applicable to quantify the effect of particle bidispersity. Eventually filtered models of fluid-particle drag and heat transfer corrections for the bidisperse systems are developed.

RESEARCH ON PARTICLE SWARM COLLISION SEARCH AND ADVANCEMENT ALGORITHM FOR CFD-DEM COUPLING DOMAIN SOLVING

When the computational fluid dynamics discrete element method (CFD-DEM) is used for solid-liquid two-phase coupling analysis, the selection of particle calculation time step directly affects the accuracy and efficiency of the coupling calculation. For this reason, each target particle is selected as the research object, and interpolation function is introduced to calculate the motion displacement of the time step, and a variable spatial search grid is constructed. An improved particle collision search algorithm (modified discrete element method, MDEM) was proposed by selecting possible collision particles to build a search list and using reverse search Method to judge collision particles. The algorithm in particle group and fluid coupling calculation, the particle counting the initial time step selection particle collision time without limit, realization of automatic adjustment and correction by large step, calculated by the real-time update of fluid particles and fluid coupling conditions, time step, the granular computing time step selection, as a result of low computational efficiency, selection is too large too small to solve the problem of false negatives, particle collision of particles and fluid coupling numerical simulation provides a effective calculation method. Through the numerical simulation of two particles and multiple particles, the relative errors of the collision forces, collision positions and times between particles obtained are all less than 2% compared with the theoretical calculation results. Compared with the traditional DEM collision search algorithm, the three calculation time steps selected do not affect the calculation accuracy, and the calculation efficiency is higher. Through the coupling numerical simulation of multiple particles and fluid, using the traditional CFD-DEM method, the precise solution can be obtained only when the particle calculation time step is 10$^{-6}$ s or smaller, while the precise solution can be obtained by using the proposed method to take 10$^{-4}$ s, which avoids the problem of missed decision caused by particle collision with the increase of time step, and the calculation time is reduced by 16.7%.

Centrifugal force caused high-density rotating downward quasi-plug flow in cyclone reactors

Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) model is used to simulate particle motions in the cyclone reactor, which reveals that the particles accumulate near the reactor wall due to the centrifugal effect, and with the increase in particle flux, their distribution pattern evolves from a uniform circumferential distribution to a local accumulation state and eventually develops into a particle belt pattern. No matter how the particles are distributed, they rotate downward in a form of “quasi-plug flow”. Most extant literatures report that the movement of particles always leads to the formation of an “accelerated quasi-plug” under the free-fall with the action of gravity. However, in this study, it is found that the rotating downflow exhibits “decelerated quasi-plug” or “constant velocity quasi-plug” behavior. Given similar particle flux and residence time in the reactor, the particle residence time distribution under the rotating downflow is narrower than that under the free-fall, and closer to an ideal plug flow. Meanwhile, centrifugal effects also significantly increase the solid holdup, permitting high-density operation of cyclone reactors even under the condition of lower inlet particle flux.

Numerical prediction on the minimum fluidization velocity of a supercritical water fluidized bed reactor: Effect of particle size distributions

The minimum fluidization velocity of a supercritical water fluidization bed reactor (SCWFBR) is numerically nvestigated based on coupled Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) simulations. The accuracy of the CFD-DEM model is firstly validated via previously published experimental data. Then, the model is adopted to study the effects of particle size distributions (PSD) on the minimum fluidization velocity in which four types of PSD including Gaussian-type, Mono-type, Flat-type and Binary-type are considered. Numerical results show that the minimum fluidization velocity for the Flat-type PSD is the smallest among the four while the one for the Mono-type PSD is the largest. The minimum fluidization velocities for the Mono-type and Gaussian-type PSD share quite similar values. However, it is conditionally valid when the largest amount of particle size in the Gaussian-type PSD is equal to its mean particle size which is used in the Mono-type PSD. The agreement of the minimum fluidization velocity between Mono-type and Gaussian-type PSD is also influenced by the number of small particles. The mechanism behind these phenomena is revealed by investigating the micro-structure of differently sized particles. Finally, predictive correlations for the minimum fluidization velocity of the SCWFBR are proposed based on the numerical results which demonstrates strong predictive capability for wide PSD.

Continuum-Based Approach to Model Particulate Soil–Water Interaction: Model Validation and Insight into Internal Erosion

Resolving the interaction between soil and water is critical to understanding a wide range of geotechnical applications. In cases when hydrodynamic forces are dominant and soil fluidization is expected, it is necessary to account for the microscale interactions between soil and water. Some of the existing models such as coupled Computational Fluid Dynamics–Discrete Element Method (CFD-DEM) can capture microscale interactions quite accurately. However, it is often computationally expensive and cannot be easily applied at a scale that would aid the design process. Contrastingly, continuum-based models such as the Two-Fluid Model (TFM) can be a computationally feasible and scalable alternative. In this study, we explored the potential of the TFM to simulate granular soil–water interactions. The model was validated by simulating the internal fluidization of a sand bed due to an upward water jet. Analogous to leakage from a pressurized pipe, the simulation was compared with the available experimental data to evaluate the model performance. The numerical results showed decent agreement with the experimental data in terms of excess pore water pressure, fluidization patterns, and physical deformations in violent flow regimes. Moreover, detailed soil characteristics such as particle size distribution could be implemented, which was previously considered a shortcoming of the model. Overall, the model’s performance indicates that TFM is a viable tool for the simulation of particulate soil–water mixtures.

CFD-DEM Simulation of Spouted Bed Dynamics under High Temperature with an Adhesive Model

Particle adhesion is of great importance to coating processes due to its effect on fluidization. Currently, Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) has become a powerful tool for the study of multiphase flows. Various contact force models have also been proposed. However, particle dynamics in high temperature will be changed with particle surface properties changing. In view of this, an adhesion model is developed based on approaching-loading-unloading-detaching idea and particle surface change under high temperature in this paper. Analyses of the adhesion model are given through two particle collision process and validated by experiment. Effects of inlet gas velocity and adhesion intensity on spouted bed dynamics are investigated. It is concluded that fluidization cycle will be accelerated by adhesion, and intensity of fluidization will be marginally enhanced by slight adhesion. Within a certain range, increasing inlet gas velocity will lead to strong intensity of particle motion. A parameter sensitivity comparison of linear spring-damping model and Hertz-Mindlin Model is given, which shows in case of small overlaps, forces calculated by both models have little distinction, diametrically opposed to that of large overlaps.