Computational Fluid Dynamics Discrete Element Method Research Articles

Investigation on tee junction erosion caused by sand-carrying fracturing fluid

The high-speed flow of sand-carrying fracturing fluid through tee junctions causes severe erosion and affects the safety of the fracturing operation. This study improves the traditional computational fluid dynamics-discrete element method (CFD-DEM) to avoid the (dense particles) collision-leading abnormal penetration but also considers the influence of fracturing fluid viscosity on particle collisions. Then the tee junction erosion model under fracturing conditions is established and verified by experiments. The simulation analysis shows that with the increase of two-phase flow velocity and Stokes number, the dominant effect of particle motion and collision changes from viscous force to inertial force, and then the tee junction erosion position also changes.

Trans-level multi-scale simulation of porous catalytic systems: Bridging reaction kinetics and reactor performance

Multi-scale porous structures inside and/or between the catalyst pellets or particles are found in many chemical processes, where strong coupling of reaction and transport results in complex apparent reaction kinetics influential to the reactor performance. Traditional continuum-based porous media models and simulation methods can hardly describe such structures and their scale effects faithfully. A trans-level multi-scale discrete computational framework is hence proposed to address this complexity and implemented for an olefin catalytic cracking (OCC) process. The apparent reaction kinetics at the REV (representative elementary volume) scale is obtained by hard-sphere pseudo-particle modeling (HS-PPM), and coupled with computational fluid dynamics / discrete element method (CFD-DEM) for the reactor-level hydrodynamics via a one-dimensional (1D) finite difference scheme for particle-level diffusion. The mesoscales of the REVs and the flow networks between the particles are thus covered by the framework, which are previously described by simple average quantities in the continuum methods. The reactant conversion rate and target product selectivity obtained agree well with experimental results, while a continuum approach may give significantly different and unreasonable results. The multi-scale method is, therefore, demonstrated to be necessary and effective for bridging the intrinsic reaction kinetics with the performance of porous catalytic reactors.

Numerical investigation of inlet section structure effect on wear characteristic of particle condition in centrifugal slurry pump by CFD-DEM coupling

To improve the wear resistance of the centrifugal slurry pump, the inlet structure is optimized, and an equal-diameter inlet section structure is designed. Considering the particle-particle and particle-wall interactions, a two-phase flow simulation model based on CFD-DEM (computational fluid dynamics-discrete element method) coupling is established under the sparse particle condition. The Archard wear model and Relative wear model are used for quantitative and qualitative wear analysis. The flow field and particle trajectory analysis are carried out under an equal-diameter inlet section structure and a different-diameter inlet section structure. The wear characteristics of the flow-passing parts are carried out. The results show that the simulation model has good accuracy. The error between the numerical simulation results and the experimental results is less than 5%. Under the equal-diameter inlet section structure, the overall wear of the slurry pump is reduced by 15%, the wear of the volute chamber is reduced by 17.5%, and the wear of the inlet section is reduced by 93.8%. The equal-diameter inlet section structure can reduce the turbulent intensity and make the flow field stable. And the equal-diameter inlet section structure helps to reduce the wear degree and wear area of the pump.

Grain-scale computations of barchan dunes

Barchans are crescent-shaped dunes commonly found in diverse environments and scales: from the 10-cm-long barchans found under water to the 1-km-long barchans on Mars, passing by the 100-m-long dunes on Earth's deserts. Although ubiquitous in nature, there is a lack of grain-scale computations of the growth and evolution of those bedforms. In this paper, we investigate the values of grain properties (coefficients of sliding friction, rolling friction, and restitution) necessary to carry out numerical simulations of subaqueous barchans with CFD–DEM (computational fluid dynamics–discrete element method), and how the values of those coefficients change the barchan dynamics. We made use of LES (large eddy simulation) for the fluid, varied the coefficients of sliding friction, rolling friction, and restitution in the DEM, and compared the outputs with experiments. We show (i) for the case of glass spheres, the values of coefficients for correctly obtaining the dune morphology, timescales, trajectories of individual grains, and forces experienced by grains; (ii) the LES meshes allowing computations of bedload while capturing the main disturbances of the fluid flow; (iii) how different values of coefficients affect the morphology of barchans; and (iv) that spheres with higher coefficients of rolling friction can be used for simulating barchans consisting of angular grains. Our results represent a significant step for performing simulations that capture, at the same time, details of the fluid flow (large eddies) and grains' motion (individual particles).

Effect of heterogeneity on interphase heat transfer for gas–solid flow: A particle-resolved direct numerical simulation

Particle-resolved direct numerical simulation (PR-DNS) of flow past a particle cluster is conducted to analyze the influence of heterogeneous particle distribution on the gas–solid heat transfer calculation. Then, the heat transfer rates calculated using Gunn's correlation are systematically compared with the DNS results for virtual computational fluid dynamics-discrete element method (CFD-DEM) grids with different levels of heterogeneity. The results show that, for a grid located at the interface between the dense cluster region and dilute region, Gunn's correlation significantly overestimates the heat transfer rate, especially at small Reynolds numbers. This is caused by the large temperature difference between the dense and dilute regions in the heterogeneous CFD-DEM grid. The value calculated by Gunn's correlation can be up to ten times the DNS result. For a homogeneous grid inside a dense region, the conventional Nusselt correlation fails to capture the rapid increase in the fluid temperature gradient around the near-interface particles when the grid approaches the cluster–fluid interface. Furthermore, even if the size of the CFD-DEM grid is reduced to twice the particle diameter, the heterogeneous particle distribution still leads to a remarkable error in the heat transfer calculation. Finally, modifications to Gunn's correlation are proposed for three typical cross-interface cases, which can well reflect the influence of the heterogeneous distribution of particles and yield a heat transfer rate close to the PR-DNS results. The mean relative deviations of the three fitted correlations are 5.8%, 14.3%, and 22.4%, respectively.

An improved CFD-DEM modelling of raceway dynamics and coke combustion in an industrial-scale blast furnace

The raceway dynamics and combustion by lateral injection of high-temperature gas into a packed bed of solid fuel are widely practised in various industries such as ironmaking blast furnace (BF), yet the particle-scale simulation and understanding were hindered by low resolution and huge computational cost. In this work, an advanced reactive computational fluid dynamics-discrete element method (rCFD-DEM) featuring two state-of-the-art techniques - coarse-grained method and smoothing method is developed to describe raceway dynamics and coke combustion in a three-dimensional (3D) industrial-scale BF. The model is validated against the measurements in the open literature. The simulation results show that the smoothing method can more reliably capture raceway morphology and the coarse-grained treatment reduces 78.14 % computational costs under the given conditions. The typical phenomena are illustrated in terms of raceway shape and size, temperature field and gas species distributions. Then, the effect of the key operating parameters is quantified in terms of raceway size, gas temperature and species distributions. The comparisons indicate increasing the blast rate from 2.78 Nm3/s to 3.05 Nm3/s and oxygen mass fraction from 0.17 to 0.27, the raceway volume expands by 61.11 % and 47.62 %, respectively; moreover, lower blast rate, higher bed temperature and higher oxygen mass fraction lead to higher temperature distribution and higher CO concentration. The present work provides a cost-effective tool for understanding and optimizing raceway operation in practical BFs.

Lagrangian Numerical Simulation of Proppant Transport in Channel Fracturing

This paper focuses on the numerical simulation of particle (“proppant”) transport in channel fracturing, in which fiber-proppant fluid is pumped intermittently into hydraulic fractures, alternated with clean fluid pulses. The pulsed pumping protocol leads to heterogeneous/channelized proppant distribution in fractures, generating open flow channels with high conductivity. To understand the evolution of the channelized proppant distribution, we develop an efficient pseudo-3D multiphase particle-in-cell (P3D MP-PIC) method to simulate the proppant transport during the pumping process. Compared with the Eulerian-Eulerian (EE) models, the MP-PIC approach has higher accuracy in modeling particle-fluid and particle-particle interactions by tracing the particles in a Lagrangian fashion. Compared with the computational fluid dynamics-discrete element method (CFD-DEM), the MP-PIC method is more computationally efficient due to a parcel feature and the fast calculation of particle forces. Reduced from the 3D MP-PIC method, the P3D MP-PIC method has better computational efficiency and flexibility to couple with other subsurface processes (e.g., fracture propagation and fluid leakoff) while also achieving sufficient accuracy for engineering purposes. Due to the Eulerian-Lagrangian (EL) nature, the P3D MP-PIC method can track the trajectories of the fiber-proppant clusters whose effective viscosity and the settling velocity are determined based on the laboratory results. With an accurate description of fluid and particle dynamics, this work reveals the critical physical mechanisms of the proppant transport in channel fracturing: stable displacement and Saffman-Taylor (ST) instability. The alternate occurrence of these two mechanisms gives rise to the channelized proppant distribution. To investigate the influential factors of the channel patterns, we conduct parametric studies on the operational parameters, including injection rate, fiber concentration, and pulsing mode. The effect of fracture propagation and gravitational settling is also discussed in detail.

Coarse-grained CFD-DEM simulation of coal and biomass co-gasification process in a fluidized bed reactor: Effects of particle size distribution and operating pressure

The co-gasification of coal and biomass through fluidized bed gasifier is a promising technology for the improvement of the utilization efficiency of fossil fuels and the development of renewable energy. In this work, a coarse-grained computational fluid dynamics-discrete element method (CFD-DEM) simulation is performed to investigate the co-gasification process in a bubbling fluidized bed gasifier. The effects of particle size distribution and operating pressure on the hydrodynamics, heat transfer, and co-gasification performance are analyzed. The results show that the increase of sand particle size distribution (PSD) width slightly affects the particle concentration distribution, particle residence time distribution, and co-gasification performance. The heat transfer rate of convection is an order of magnitude higher than those of conduction and radiation. At higher PSD width, the formation of the vertical solid temperature gradient at the bed bottom is attributed to the deteriorated solid mixing. As the operating pressure increases, the bed temperature decreases apparently. In addition, the variation of the convective heat transfer rate dominates the heat transfer efficiency as the operating pressure changes. The application of elevated operating pressure benefits to the thermal-chemical conversion of blend fuel since the char conversion reaction is accelerated.

Local Scour Mechanism of Offshore Wind Power Pile Foundation Based on CFD-DEM

The local scour around offshore pile foundations often seriously affects the normal operation of offshore wind power. The most widely used numerical simulation method in the study of local scour is the Euler two-fluid model (TFM). However, the contact effect between sediment particles is neglected in this model. Thus, the momentum and energy transfer between sediment particles and the fluid is not realistically reflected, which limits its significance in revealing the mesoscopic mechanism of local scour. Therefore, the computational fluid dynamics-discrete element method (CFD-DEM) numerical model was applied in this study, which fully considers the contact between solid particles and momentum transfer between two phases. The model was first verified by experimental data of a local scour test under clear water scour. Then, the mechanism of local scour was further discussed from macro and micro perspectives. The results showed that CFD-DEM could be effectively used to study the local scour around a pile foundation. The local scour was comprehensively affected by flow velocity, gravity, fluid force, drag force, and interaction between particles, etc. Although the maximum average drag force happened in the area about 90° from the direction of incoming flow, the maximum scour depth always occurred at about 45°. Corresponding findings and conclusions can be used for future reference when designing and protecting the offshore wind power pile.

Memory access optimization for particle operations in computational fluid dynamics-discrete element method simulations

Computational Fluid Dynamics – Discrete Element Method is used to model gas-solid systems in several applications in energy, pharmaceutical and petrochemical industries. Computational performance bottlenecks often limit the problem sizes that can be simulated at industrial scale. The data structures used to store several millions of particles in such large-scale simulations have a large memory footprint that does not fit into the processor cache hierarchies on current high-performance-computing platforms, leading to reduced computational performance. This paper specifically addresses this aspect of memory access bottlenecks in industrial scale simulations. The use of space-filling curves to improve memory access patterns is described and their impact on computational performance is quantified in both shared and distributed memory parallelization paradigms. The Morton space filling curve applied to uniform grids and k-dimensional tree partitions are used to reorder the particle data-structure thus improving spatial and temporal locality in memory. The performance impact of these techniques when applied to two benchmark problems, namely the homogeneous-cooling-system and a fluidized-bed, are presented. These optimization techniques lead to approximately two-fold performance improvement in particle focused operations such as neighbor-list creation and data-exchange, with ∼ 1.5 times overall improvement in a fluidization simulation with 1.27 million particles.

Orientation simulation and image experiment for flexible biomass particle in wedge fluidization channel

Flexible biomass particles have a large proportion in food production and processing, and particle orientation is related to the quality and efficiency of food production. Thus far, the particle orientation control mechanism has rarely been revealed. In this work, Computational Fluid Dynamics–Discrete Element Method (CFD–DEM) model is presented to study the flow characteristics of tea in a wedge channel. The biomass particles are represented by a double chain bonded-sphere model. The CFD–DEM model for flexible particles is verified by image experiments. The effects of gas velocity, particle length, feeding rate, and wedge outlet diameter on particle orientation distribution are investigated and discussed. Results show that the particle orientation is slightly affected by gas superficial velocity. The proportion of particles oriented at 0–30° increases as the particle length and feeding rate increase and decreases as channel outlet diameter increases. The particles with the orientation angles of 0–10° and 0–30° are about 25% and 80%, respectively, when the channel outlet diameter is 40 mm. These findings provide great significance for the processing, storage, and transportation of tea and other biomass. • A double-chain bonded-sphere model of flexible particles is proposed. • Image experiment is designed to verify the established numerical model. • An orientation control method of flexible biomass particles is proposed. • Particle orientation is influenced by gas velocity gradient and particle length.

CFD-DEM simulation of sand erosion into defective gravity pipes under constant groundwater table

As sewer pipes age, cracks and other forms of structural damage may develop leading to soil infiltration into the pipes that can cause sinkholes and damage the overlying roads and moving vehicles. The fluctuations in the groundwater table following snow melt or heavy rainfall events can exacerbate soil erosion posing risk to the surrounding infrastructure. To understand and adequately characterize erosion in such cases, microscale analysis of both sand and water is needed to resolve the complex interactions around the pipe defects. In this study, a detailed numerical investigation is carried out using a series of coupled Computational Fluid Dynamics-Discrete Element method (CFD-DEM) simulations to evaluate the effects of groundwater table, crack size, defect angle, and the height of sand fill above the pipe on erosion. It is found that raising the groundwater table accelerates erosion only to a certain limit, above which, raising the groundwater level does not have a tangible effect on erosion. This helps explain some of the misunderstandings reported in previous investigations regarding the effect of water level on the rate of erosion. With respect to the location of defects, it was found that, when groundwater is involved, the erosion rate is fastest when the defect is located at the pipe crown. However, inclined defects ultimately resulted in a larger spatial extent of eroded mass. In addition, the thickness of the sand fill above the pipe was found to have little effect on the rate of erosion as a freefall arch regime was dominant in all simulations. Finally, simplified relationships were developed for estimating the total eroded mass for given sand properties, defect geometry, and location of the groundwater table.

Benchmarking a DEM‐CFD Model of an Optical Belt Sorter by Experimental Comparison

AbstractA DEM‐CFD (discrete element method ‐ computational fluid dynamics) model of an optical belt sorter was extensively compared with experiments of a laboratory‐scale sorter to assess the model's accuracy. Brick and sand‐lime brick were considered as materials. First, the transport characteristics on the conveyor belt, involving mass flow, lateral particle distribution and proximity, were compared. Second, sorting results were benchmarked for varying mixture proportions at differing mass flows. It was found that the numerical model is able to reproduce the experimental results with high accuracy.

Characteristic simulation and working parameter optimization of the aggregate dryer

The aggregate dryer is generally used in an asphalt mixing plant for road construction equipment and serves to heat the aggregates. In this work, the coupled method of Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) is used to study the gas-solid coupling of aggregate dryers to improve the heat exchange efficiency between flue gas and aggregate. The heating process of aggregates in an aggregate dryer is visualized by analyzing the particle discrete field and the fluid temperature field during drying. The motion process and distribution characteristics of aggregate particles are established for different outputs and rotational speeds, and the hot flue gas distribution and temperature characteristics of the aggregate dryer are also obtained. The results show that the rotational speed affects the distribution of the aggregate veiling zone and pressure, which in turn affects the stability of the flame. Reducing the rotational speed results in a more uniform distribution of the aggregate veiling zone and thus an increase in heat utilization. In addition, when the output of the aggregate dryer is 80 t/h and the rotation speed is 6.2 rpm, the aggregate discharge temperature is the highest and the thermal efficiency of the aggregate dryer is the best.

Numerical Investigation of the Two-Fluid Model and Computational Fluid Dynamics–Discrete Element Method in Supercritical Methanol Fluidized Beds

This work focuses on the comparison of simulated methanol fluid–particle fluidization behavior using the two-fluid model (TFM) and computational fluid dynamics–discrete element method (CFD-DEM) in subcritical methanol (SbM) and supercritical methanol (SCM) fluidized beds (FBs). The solid-phase constitutive correlations are modeled using the low density ratio-based kinetic theory of granular flow. The fluidization states of methanol fluid–particle mixtures using the TFM and CFD-DEM are compared with four thresholds reported in the literature. The outcome of the comparison shows that simulations using the TFM and CFD-DEM capture the coexistence of wave-like flow and churn-like flow along bed height in SbM FBs, and particle aggregates and fluid voids flow in SCM FBs. Agreement in methanol fluid pressure is observed, while discrepancy in volume fractions and velocities is found from the TFM and CFD-DEM in SbM and SCM FBs. Predicted methanol fluid fluctuating quantities of turbulent kinetic energy, dissipation rate, and granular temperatures show sensitivity to the two-equation turbulence model and collisional properties. The predicted expansion bed height and volume fractions agree well with experimental data in superficial carbon dioxide fluid and ambient water FBs.

Application of coarse-graining for large scale simulation of fluid and particle motion in spiral jet mill by CFD-DEM

Spiral jet milling is commonly used for size reduction of high value particulate solids, such as pharmaceutical ingredients, for which low contamination is critical. The mill utilises high-pressure gas nozzles to form an internal vortex and induce particle breakage through inter-particle collisions. Due to the centrifugal flow field, the particles can move radially inwards towards the mill centre and escape the mill only when their size is sufficiently reduced for the fluid drag to exceed the centrifugal force. Large particles move radially towards the outer wall and form a dense particle bed. The bed itself circulates in the milling chamber due to the induced effect of the inclined gas jets. In this study, we analyse the implementation of a coarse-graining (CG) approach on a coupled Computational Fluid Dynamic-Discrete Element Method simulation. Along with the actual particle size, four CG scale cases are compared (CG-2, CG-4, CG-8 and CG-10). To analyse the success of the approach in predicting the dynamics of fluid and particle motion, the characteristic features of the particle bed and fluid field, i.e. the fluid and particle velocity distributions and dissipated energy through particle collisions are analysed. There is good agreement between the original particle size and the two smallest scaled cases (CG-2, CG-4) for the above characteristics. However, modelling the lean phase is less successful, as there are fewer particles that reside there at any given time. There is also good agreement between these three cases in terms of dissipated energy through particle collisions. An average value for dissipated energy of 0.6 mJ is recorded for each of the three lowest cases. This proves beneficial for simulation time required, as the particle number is reduced 2-fold and 4-fold, respectively, and interparticle collision rate is reduced 60% each time. Application of the Coarse Grain method in a spiral jet mill using coupled Computational Fluid Dynamics and Discrete Element Method Modelling. Top: Original particle size system and four scaled systems. Bottom: Resulting temporal average dissipated energy through interparticle collisions and particle wall collisions. • Coarse graining method is applied in the spiral jet mill using a CFD-DEM framework. • Particle and fluid motion is similar in the bed at smaller scales. • Insufficient particles were present in the lean phase to model the original system. • Temporal dissipated energy remained constant for low coarse grain numbers.

Experimental and numerical study on the development process and flow characteristics of powder fuel jet in the powder fuel scramjet

The injection process of powder fuel directly affects the scramjet operating performance. This study analyzes gas-particle two-phase flow during powder fuel injection. The macroscopic behavior of powder fuel jets under different injection pressure conditions is characterized using the high-speed shadow imaging technology. Simultaneously, the coupled model of computational fluid dynamics-discrete element method is employed to determine the experimental conditions, and the flow field structure and gas-particle interactions are analyzed. The results show that the development of powder fuel jets mainly occurs in three stages: the low-speed development stage, the high-speed development stage, and the stable stages. A long transition period exists between different stages, and the transition time decreases with increasing injection pressure. In the low-speed development stage, the powder fuel jet initially develops slowly and then develops rapidly. In the high-speed development stage, the jet initially develops rapidly and then develops slowly. The jet in the stable stage is classified into the powder particle, fluidized gas, and free flow regions. The distribution range of the particles decreases with increasing injection pressure. The fluidized gas velocity and drag force curve on the axis experience many fluctuations, and the particle velocity is almost always accelerated. Fluidized gas velocity, drag force, particle velocity, and particle acceleration distance increase with the injection pressure.

Clogging effect of fines in seepage erosion by using CFD–DEM

The macroscopic phenomena and microscopic origin of penetrative migration and retention of fine particles in porous skeleton under the action of seepage are not yet clear. Migrating fine particles experience physical interactions with impermeable walls, carrier fluids, particle skeleton and other fines. Therefore, the simulations of suffusion based on the coupled computational fluid dynamics–discrete element method (CFD–DEM) are carried out with consideration of different coarse particle size distributions, flow velocities and inflow fines concentrations. Simulations are performed by releasing continuously produced fines into the coarse-grained matrix under hydraulic force. Macroscopic alterations (including the loss rate, concentration ratio, concentration region of cluster, velocity and size distribution of fine particle, pore throat diameter and permeability) and microscopic features (including interparticle force network, the spatial distribution and velocity field of fines) are quantified during suffusion. The results reveal that the pore size of porous media significantly contributes to the degree and position of clogging. Flow velocity affects interception efficiency and distribution of clusters. Additionally, the prominent development of particle clusters in the flow-avoidable areas is aided by the appearance of more inflow fines. With both macroscopic and microscopic responses, this study may increase the knowledge of clogging phenomena during suffusion.

Numerical simulation of proppant directly entering complex fractures in shale gas

Large-scale slickwater fracturing is an important technical method for the effective development of shale gas, which generates complex fractures with fracture width of millimeters in the reservoir. It is known that the transport law of proppant in complex fractures is the premise for realizing effective propping. Taking the behavior of 70/140 mesh proppant particles commonly used in shale gas fracturing as the object，a numerical model based on the computational fluid dynamics-discrete element method (CFD-DEM) and the geometric model of the intersection of the main and secondary fractures are established. It is used to study the two-phase flow law under the conditions of different fracture widths and angles, pump displacements, and fluid viscosities. The results show that the proppant enters the secondary fracture in two ways: carried by the fluid in a suspended manner and rolling into the fracture from the sand bank surface. Particles suspended in the fracture can be transported to the distal end of the secondary fracture. Owing to the influence of the inertia force of particles, the particle flow rate entering the secondary fracture is much smaller than the fluid flow rate in the secondary fracture. As the included angle between the secondary and main fractures decreases, the fluid and particle flow rate increase, and particles can easily enter the secondary fracture. As the displacement, secondary fracture width and fracturing fluid viscosity increase, proppant particles are easier to enter secondary fractures. The absolute values of the main and secondary fracture widths become smaller, and the relative value remains the same, making it more difficult for proppant particles to enter the fractures.

Improving Cleaning Performance of Rice Combine Harvesters by DEM–CFD Coupling Technology

The cleaning device is an important part of combine harvesters, as its superior or inferior performance directly affects the performance of the combine harvester greatly. With an increasing rice yield, the current single-duct cleaning performance declines greatly, and causes a large grain sieve loss level and a direct grain loss for the farms. To optimize the existing single-duct cleaning device to meet the large feeding rate requirement, firstly, the terminal velocity of rice grain and MOG (material other than grain) for different varieties was experimentally measured by the custom-made device. The effects of the moisture content of rice grains and the length of short straws on terminal velocity were studied in detail. Then, the gas–solid two-phase flow theory was comprehensively applied by utilizing the discrete element method (DEM) and computational fluid dynamics (CFD) to study the working mechanism of the existing single-duct cleaning unit, and the cleaning performance was evaluated from the view of the motion law of the threshing output within the cleaning shoe. At last, a multi-duct cleaning device was put forward, and a field experiment was performed to assess the performance of the newly developed cleaning device. The results showed that the grain sieve losses ratio and grain impurity ratio improved dramatically, proving that structure optimization of the cleaning device was feasible.