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Continuum description of particle-fluid flows is of fundamental importance. In the past, various averaging methods were developed to derive local quantities directly from particle scale information. However, it is not clear if the long-sought resolution-independence of the local quantities exists. Here, using a discrete element method, a statistical analysis is conducted for a gas-solid fluidized bed of monodispersed coarse particles under various conditions. The mean free path and time are first examined with the variation of inlet gas velocity. Then, the particle motion is analyzed according to the Langevin equation and it is revealed that the motion of individual particles in the driven system follows the fluctuation-dissipation relation. Finally, the particle collisional frequency and velocity distribution are analyzed. These distributions can achieve a steady state. The particle collisional frequency distribution deviates from the Gaussian distribution noticeably at high gas velocities. These findings lead to a better understanding of particle motion in a gas-solid fluidized bed and support the seeking of resolution-independent local quantities with averaging methods.

A void zone, the so-called raceway, may be formed when gas is laterally injected into a particle packed bed and the char particles may combust when the gas enters at a high temperature for example coke combustion in the raceway of ironmaking blast furnaces. Experimental and numerical studies of multiphase flow in the raceway have been conducted widely in recent years, however, the study of reacting flows in the raceway is still limited at a particle scale. This work is to study the gas-solid reacting flows in the raceway using a particle scale CFD-DEM approach, featuring heat and mass transfers and chemical reactions between the gas and solid phases. The simulation results are comparable with the measurements in the physical experiments and previously calculated results. The key phenomena of flow and thermochemical behaviour related to the raceway formation at the particle scale are obtained. Then the model is used to study the dependency of raceway formation and thermochemical behaviour on several key raceway variables including gas inlet velocity, particle size, bed height and particle discharge rate. The simulation results indicate that before generating a stable raceway, a larger blast inlet velocity or a larger discharge rate can form a larger raceway cavity, while the effect of packed bed level shows an opposite trend. The model provides fundamental insights into the complex reacting flows in the raceway zone for a better understanding and optimization in operations.

The packing of cohesive particles is of paramount importance in many industries because the packing structure is closely related to process performance. A general relation between packing density and interparticle force was previously proposed based on packing structures formed without dynamic fluid flows. Its universality is examined here in two different packings, formed in settling and defluidization of static and dynamic fluids, respectively. First, it is shown that the packings of the same particles formed by two different methods have different structures because of different impact-induced pressures. Nevertheless, a one-to-one relationship between packing density and structural properties still holds regardless of the different packing methods, and the force distribution in those packings obeys similar rules. Finally, the packing densities obtained by the different methods are demonstrated to be universally correlated with the ratio of the interparticle force to the effective gravity. These findings indicate that different phenomena of particulate systems at a macro- or meso-scale may share similar microscopic origins, with the interparticle force playing a crucial role.

This paper presents a method to establish explicit force models for identical superellipses. Currently for two superellipses in contact, the overlap parameters must be solved by using numerical iteration but cannot be explicitly calculated. Our method is firstly to numerically solve these solutions in the comprehensive range of the orientation angles with the angles varying discretely. Then using two-dimensional (2D) discrete Fourier transform (DFT), the discrete solutions as a function of two angles are transformed to the frequency domain. The transformed terms are sorted according to their magnitudes, and those below a certain value are filtered in the inverse discrete Fourier transform (IDFT). This finally results in simplified Fourier series that can approximate overlap parameters explicitly. The method was realized for a wide range of superellipse shapes and the errors between the approximate Fourier series and the original solutions were analyzed and shown to be small. In addition, the established explicit force model was applied to the simulation of the packing of superellipses. The results show that the explicit force model established by this method can be effectively used in DEM simulations. And the packing of superellipses is complicatedly dependent on the squareness, aspect ratio and friction. These findings provide a novel way to establish the explicit force models for non-spherical particles and also improve our understanding on the packing of non-spherical particles.

Screw feeders are widely used in industries to transfer granular materials at relatively precise rates. Often, granular materials can have a certain level of cohesiveness which can significantly affect the transport among other factors. The critical mechanism that can stop granular flow is the formation of bridge or arching. In this work, novel screw designs are proposed to promote cohesive solid flows in a screw feeder. First, through a numerical model based on the discrete element method, it is demonstrated that the new designs are effective. Then, the underlying mechanism is analyzed both macroscopically in terms of particle flow pattern, velocity field and the motion of screw and microscopically in terms of temporal and spatial variations of contact force between particles. It is revealed that the new screw designs can induce a bulk perturbation to the granular material in the bin in addition to the existing local perturbation by the screw blade. Thus, the formation of bridge can be deterred, and the cohesive solid flow is promoted. It is suggested that such designs could also be effective to non-cohesive granular materials. Further study should be conducted to optimize the designs.

To improve the sedimentation efficiency of iron droplets during iron extraction process from converter slag by smelting reduction, a mechanical stirring is proposed and the coalescence and sedimentation behavior of iron droplets under experimental crucible scale are studied based on the established coalescence model. Coalescence mechanism of droplets is elucidated and the effects of fluid flow, rotating speed and structure of the impeller are examined. The model is validated by the high-temperature experiment results. The results show that with rotating flow in radial direction and “double roll” pattern in axial direction, the settling mode of droplets is no longer a monotonous vertical uniform sedimentation, but a spiral sedimentation with acceleration, deceleration and eventual uniform velocity stages. The mechanical stirring improves the sedimentation efficiency by 3.4% and reduces the total settling time by 6.8%. The coalescence mode with mechanical stirring is mainly due to the oblique collision, instead of the chase collision of the large droplet chasing the small one without stirring. Although the coalescence of the colliding droplet pairs becomes difficult due to the increased relative velocities of droplets, the mechanical stirring greatly improves the coalescence number of iron droplets by a factor of 5.0 because of the increased collision probability. With the impeller rotating speed increased from 50 to 80 rpm, the coalescence of colliding iron droplet pairs is more difficult but the cumulative coalescence number and sedimentation efficiency increase. The same trend is also obtained for the three different impeller structures of ‘Y’ shape, straight shape and cross shape. Therefore, higher rotating speed (80 rpm) and cross-shape impeller are more favored during smelting reduction process.

Spouted beds are commonly simulated through the Computational Fluid Dynamics – Discrete Element Method approach. The choice of the drag model is still a matter of debate, as they feature peculiar operative conditions. In this work, we simulated two spouted beds containing Geldart-D particles. We tested seven drag models: three are classic models, while four are developed through advanced computational techniques. The results indicate that the key variable is the ratio between the operative and the minimum spouting gas velocity (u/ums). At u = ums only the Gidaspow model can always predict fluidisation, but at low u/ums values the Beetstra model is the best compromise. For higher values, the Rong and Di Felice models behave better, while the others overestimate the particles' velocity. These results can be useful to identify the best performing model and show there is a need for more appropriate models for spouted beds.

A better understanding of feeding operations is pressing for value-added processing of waste biomass powders. This paper examines the feeding of Spent Coffee Grounds (SCGs) using a non-mechanical L-valve both experimentally and numerically. L-valve provides stable solids feeding, showing different flow regimes. Powders' height in the standpipe must be monitored to guarantee smooth operations with the valve, and the data for SCGs differ significantly from those reported for glass and sand powders. A new correlation to predict the solids flowrate from simple pressure measurements was proposed for valve operating under high air flowrates. For low to medium air flowrates, a two-fluid model (TFM) was proposed and validated. The SCGs' flowrate in the feeder was accurately predicted by the TFM and the correlation. Furthermore, key information for the design of L-valves was obtained from the TFM simulation. The findings are useful for producing renewable thermal energy and fuels with biomass SCGs.

Benefiting from wide solar energy access and high energy conversion efficiency, solar-driven interfacial water evaporation has shown great potential in critical separation processes such as desalination and waste brine treatment. However, salt accumulation on the evaporation surfaces of these evaporators still greatly limits their continuous and efficient operation, particularly in hypersaline brine desalination. Current methods suffer from either low salt tolerance or low energy efficiency. Here we show a novel self-rotating solar evaporator, which achieves ultrahigh salt tolerance (300 g L−1) and energy efficiency (around 90%) simultaneously by periodic self-regeneration. This study advances the development of solar-thermal desalination technologies for practical applications.

Drying is the most important post-harvest processing method for the long-term storage of food grains. Drying and related particle shrinkage are usually determined by various operational conditions and grain properties like grain size, density and initial moisture content. The impact of these food grain properties on drying and shrinkage is investigated by the recently developed computational fluid dynamics-discrete element method drying and shrinkage model. First, particle mixing in a fluidised bed and general drying characteristics are discussed followed by the contribution of different heat transfer modes on fluidised bed drying. The particle scale investigation found that the convective heat transfer is dominant, but the conductive heat transfer becomes important at low air velocities. Then the effect of grain size, density and initial moisture content on drying rate are quantified in terms of drying rate. The drying rate increases exponentially with decreasing grain size, but a slightly smaller drying rate is observed with decreasing initial moisture content and grain density. The shrinkage rate, resembling the drying rate, increases with decreasing grain size or increasing initial moisture content and grain density. Finally, the effects of these food grain properties on the drying quality, quantified by moisture and grain size distributions, are evaluated in this study. The grain scale results revealed that the uniformity of moisture and grain size distributions increases with increasing grain size, decreasing initial moisture content or decreasing grain density. The findings should be useful for a better understanding and control of the drying process in the fluidised bed.

A discrete element method (DEM) model of the crushing process of a toothed double-roll crusher (TDRC) is established using the bonded particle model. DEM results and experimental data are compared quantitatively and a relatively good agreement is observed. The effects of rotation speed and structure of crushing rolls on the performance of TDRC are investigated numerically. The results show that when the rolls' speed is relatively high, the nipping condition would be improved, and more cracks could be created to release the increasing strain energy, generating more fractions of small sizes in the products. But, when rolls' speed exceeds 150 rpm, the crushing performance would not be significantly improved. A reasonable working gap and better nipping behaviour are obtained using the spiral-tooth-roll or the staggered-tooth-roll. The validated DEM model could be applied to gain a fundamental understanding of the crushing mechanisms of TDRC.

Solid descending and residence properties in COREX shaft furnace with a center gas supply device (CGSD) were studied using the discrete element method. Solid flow pattern, residence time distribution, and local solid residence time (SRT) were analyzed. Moreover, the effects of the rotation speed of screws, the radius, and height of CGSD were investigated. The results show that the solid flow is dominantly a plug flow in the shaft furnace with the CGSD. In the upper region of the shaft furnace for gas–solid reduction, uniform distribution of local SRT is observed, which is preferable to match the even gas distribution expected due to the CGSD and slots. For different rotation speeds of the screws, the distribution of solid flow patterns is similar. The averaged residence time, dispersed plug volume fraction, and dead volume fraction decrease with increasing the rotation speed of the screws. The radius of the CGSD affects the solid residence in the shaft furnace. Local SRT above the CGSD increases with the increase in the radius of CGSD, possibly leading to the formation of more agglomerates. Hence, the radius of CGSD should be chosen carefully. The height of CGSD has a minor effect on solid residence in the shaft furnace.

DEM study of the size segregation of spherical and cubical wet particles was conducted with a linear cohesion contact model. The effects of cohesion force on segregation pattern and speed were examined. A hill-shaped segregation mode was observed and analysed in terms of total normal force between small-sized particles, tangential force along the Z-axis by container wall and torque acting on the particles. Then, the effect of vibration parameters on the segregation process was investigated. The results show that the cohesion force has a greater influence on the segregation pattern and speed of spherical particles than on cubical ones. The cubical particles can segregate more easily than spherical particles when the cohesion force is relatively high. With the increase of the cohesion force, both the normal force between small-sized particles, the tangential force along the Z-axis and the torque acting on the particles increase. The average forces and torque of the spherical particles are generally larger than that of the cubical particles under the same conditions. The segregation speed and the volume concentration of large particles are generally increasing with the increase of vibration frequency or amplitude. Moreover, the vibration parameter combination with a relatively low vibration frequency and a relatively high vibration amplitude is good for the segregation of wet particles for a given vibration intensity.

Understanding the relationship between a fluid flow and the structural properties of a bed is important for various industrial applications and a proper description of particle–fluid flow. This wor...

Food grains naturally undergo physical and structural changes during a drying process. The volumetric change of particles or particle shrinkage is one of the important and complicated physical changes in drying. In this work, a shrinkage model for particle diameter reduction is incorporated into the computational fluid dynamics- discrete element method (CFD-DEM) drying model for food grains. First, mixing, general drying and shrinkage characteristics including particle and air moisture content, and particle diameter variation are reproduced. Then, the model is tested by comparing the predicted moisture reduction and volume shrinkage curve with the experimental data of wheat from the literature. The results demonstrate the capability of the current model in predicting drying and particle shrinkage characteristics. Finally, the effects of inlet air temperature and velocity on drying and particle shrinkage are studied. It is revealed that the shrinkage rate increases significantly with increasing air temperature but increases slightly with increasing inlet air velocity. The uniformity of grain size, quantified here by the standard deviation of the particle diameter distribution, increases with decreasing air temperature or increasing air velocity. This grain scale drying model with particle shrinkage should be useful for the design and control of many drying processes.

As a low-cost green technology, solar steam generation using nanostructured photothermal materials has been drawing increasing attention in various applications, e.g. seawater desalination, and zero liquid discharge of industrial wastewater. However, the crystallisation of salts on the surface of photothermal materials during steam generation leads to a gradual decline in the water evaporation rate. Herein, this challenge was overcome by a novel design involving controlled water transport, edge-preferential crystallisation and gravity-assisted salt harvesting. The crystallisation sites of the salt were spatially isolated from the water evaporation surface, achieving continuous steam generation and salt harvesting in over 600 hours of non-stop operation. The study provides new insights into the design of solar steam generators and advances their applications in sustainable seawater desalination and wastewater management.

The raceway is a zone of considerable significance in a blast furnace (BF) because it supplies energy and reducing agents that ensure successful and stable BF operation. Recently, experimental and numerical studies of BF have been widely conducted to examine the inner multiphase flow transport phenomena; however, the study of reacting flows at the particle level in BF is limited. In this study, a multiscale method that couples computational fluid dynamics (CFD) with discrete element method (DEM) is employed to examine the dynamic evolution of the raceway and inner thermo-chemical behaviours in a BF. Raceway evolution and formation, microscale characteristics, and coke temperature and combustion are comprehensively explored and analysed under various operating conditions. The predicted results show that the distributions of coke temperature, carbon loss, and diameter variation are consistent. More burning coke particles occurs in the vicinity of the region next to the tuyere, where there is a stronger high-temperature circular gas flow than in other regions. The increase in oxygen concentration indirectly increased the carbon monoxide concentration, but changes in the inlet gas temperature and flow rate yielded no effect on the carbon monoxide level in the studied ranges. These new perceptions of the complicated reacting flows inside the raceway area are beneficial for the fundamental understanding of energy utilisation and process optimisation.

Due to the increasing coffee production, Spent Coffee Grounds’ (SCGs) generation has grown dramatically, hence appropriate management of this solid biomass waste is imperative. SCGs can be used as feedstocks for renewable energy and fuel generation provided that a stable feeding of powders to reactors is maintained. Recently, a non-mechanical spouted bed feeder proved itself an excellent alternative in feeding SCGs to a pilot-scale circulating fluidized bed reactor. Nonetheless, further studies are necessary for the feeder’s implementation in commercial applications. Here the feeding of SCGs with the spouted bed feeder is addressed by using Computational Fluid Dynamics. Firstly, a Two-Fluid Model (TFM) is validated against experimental data, and then the effects of five operating and design parameters were analyzed aiming at improving the handling of SCGs. The solids flowrate (WS) in the reactor could be stably controlled from 4 to 30 g/s depending on the settings. The feeder performance is enhanced by operating it under high gas flowrate (Q), high entrainment length (z), and high mass of solids in the feeder (HS). Using feeders with low cone angle (γ) or reactors with large diameter (DR) increases WS, which is appealing for the operation of medium-to large-scale units. The proposed TFM is a cost-effective tool for implementing spouted bed feeders in commercial applications. With the feeder coupled to the process, SCGs are treated continuously in the reactor for energy generation, thus reducing the disposal problems associated with this waste and improving the management of SCGs globally.

The fluidization of quartz in the fluidized bed has great influence on the combustion and gasification of refuse-derived fuel (RDF). The combined computational fluid dynamics (CFD) and discrete element method (DEM) approach was used to explore the gas-solid hydrodynamics and mixing characteristics in a three-dimensional fluidized bed. All numerical analyses were performed referring to the experiments (Goldschmidt, Beetstra, and Kuipers 2004). The simulation results indicated that the quartz volume fraction agrees well with the experimental data. Furthermore, the cylinder-shaped RDF particles can mix well with the quartz particles as they were added from upside. For binary systems, it is necessary to investigate solid flow characteristics as well as pressure drops and examine the influence of superficial gas velocity on the solid mixing. Two main parameters are discussed: mixing degree and the time required to reach the steady state. It is also found that inlet gas velocity and particle properties ...

Drying is a common practice for post-harvest processing of food grains. Fluidised beds are often adopted for this purpose. It is of importance to understand the fluidised bed drying process for improving its energy efficiency. This work establishes a numerical drying model based on the combined approach of computational fluid dynamics and discrete element method for describing heat and mass transfer in the gas-solid flow system. Water evaporation is modelled in resemblance to a chemical reaction, thereby requiring fewer model parameters. The model is first described in detail. Then it is tested by comparing model predictions with those experimental data of corn kernel from the literature. General drying characteristics including grain and air moisture contents are reproduced qualitatively. The predicted drying rate curves are quantitatively comparable with those of experimental data. Finally, the effects of inlet air velocity and temperature are examined. The model predictions confirm that the drying rate increases with both the inlet air velocity and temperature. However, the drying product quality, here represented by the standard deviation of grain moisture distribution, increases with increasing air velocity or decreasing air temperature. This grain scale model would be useful to the design and control of the drying process.

Rotating drums are widely used in industries to handle and process particulate materials and heat transfer is often involved in these processes. This work numerically investigated heat transfer inside a drum mixer by coupling the discrete element method (DEM) with a conductive heat transfer model. The mixer was fitted with different types of lifters and rotated at various speeds so their effects on heat transfer characterised by heat transfer coefficient (HTC) could be investigated. The simulation results showed that the specific HTC of the particle flow increased with increasing rotating speed but decreased with the number and height of the lifters. While all types of lifters sped up heat transfer, the straight lifers were more efficient than the arc shape lifters. To better understand the mechanisms of heat transfer, the specific HTC was linked to the particle-wall contact area and dynamic behaviour of particles. The change of lifter configuration, i.e. lifter height, number and shape, mainly affected the particle-wall contact area while the change of mixer rotating speed affected particle mixing behaviour.

Intensive heat and mass transfer between continuum fluids and discrete particulate materials is quite common in many chemical processes. To understand and improve the operation of these processes, discrete particle models are very helpful when they are combined with the flow, heat transfer and chemical reaction models. Here, a quasi-steady state model for investigating thermo-chemical behaviors is established and tested for an experimental blast furnace (BF). First, the new treatments and assumptions are discussed in detail. Then, the model is tested against available experimental data under comparable conditions in terms of in-furnace flow state, temperature distribution, and the characteristics of the cohesive zone. Finally, a discussion of further development is presented. Such a model can be used to study the effects of burden distribution, inlet gas composition and material properties on the operation and energy efficiency of a BF. Such particle scale modeling can be extended to other chemical processes such as fluidized beds and rotary kilns not only for better fundamental understanding but also for better process design and control.

Bed structure is important to heat and mass transfer in fluidization. This study investigates solid structural transition for cohesive particles by the combined approach of computational fluid dynamics and discrete element method, supported by Voronoi tessellation. Macroscopic behaviors of fluidized beds are first discussed in connection with microscopic structural transitions. Then, the properties of Voronoi polyhedra are analyzed at both bed and particle scales. In addition, the correlation of coordination number and porosity is examined at a particle scale, and their combined probability distribution is analyzed to identify dominant structure of particles in different flow regimes. Finally, an index is proposed as the ratio of coordination number and porosity to differentiate microscopic states of a particle. These findings should be useful for the study of flow, heat, and mass transfer in fluidized beds and for the understanding of solid-like to fluid-like transition of granular materials.

Granular and particle-fluid flows are widely observed in nature and in industry. Their behaviors are determined by the injection of energy and the subsequent cascade of energy. This work presents a DEM-based approach for the analysis of energy transitions in such flows. The approach can consider the energy injection, dissipation and conversion due to driving forces such as particle-fluid interactions and the gravitational force, particle-particle/wall collisions and frictions. The effectiveness is demonstrated in two systems respectively and the transient and averaged energy transitions are discussed. The results reveal the connection between the formation of a cluster and the variation of energy in fluidized beds. From the viewpoint of energy transition, the results also illustrate the minimization of energy dissipation, the startup of fluidization and the particle flow distribution. Furthermore, the correlation of the contact between particles and the energy transition is demonstrated. The findings should be useful for the understanding of structure formation in fluidized beds and for industrial applications.

In this work, the combined effects of vibration parameters on the circularly screening processes are analysed based on the Taguchi orthogonal experiment method. First, numerical investigations of the screening process including the average particle velocity and screening efficiency of different particle size fractions are presented. The variations of the average velocity and the screening efficiency of different particle size fractions for both spherical and non-spherical particle models are examined. The effect of vibration intensity on the screening process is discussed and the potential error induced by the analysis of a single factor on the screening performance is demonstrated. Then, the orthogonal test data of average particle velocity and screening efficiency for 16 cases are analysed by using the direct analysis method. The combined effects of amplitude, frequency and the inclination angle of the screen deck on the screening performance are discussed based on the calculated average values of each level of the factors. The corresponding optimal combinations of the three factors for two screening performance indexes are obtained, respectively. The effect weights of the vibration parameters on the screening performance indexes are also obtained by comparing the ranges of the factors. This work should be helpful for the understanding of the combined effects of operation parameters and provides essential references for efficient operation and optimal design of a vibrating screen.

In this work, DEM results and experimental data from a specially designed circularly vibrating screening model are compared quantitatively under a range of operating conditions. The validation is carried out in two aspects. First, the motion of tracked particles is recorded using high-speed dynamic analysis system and their velocities are compared with DEM results. Second, the total screening performance indexes are compared to those predicted by DEM. Overall agreements for both spherical and non-spherical particle models are observed in both the motion of individual particles and the screening performance quantities. Generally, better agreements are found for non-spherical particle model with the particle shape reasonably considered. Quantitatively, spherical particle model is less accurate because the simplification of particle shape leads to good mobility and over-prediction of velocity and screening efficiency. The validity of the DEM model of the screening process is hence approved. The validated DEM model can be applied to perform extensive numerical studies to achieve optimal performance and better operation of such complicated and important processes in mineral processing.

A coupled approach of discrete element method and pore network model (DEM-PNM) is developed for simulating particle–fluid flow. By this approach, the particle/solid flow is described at a particle scale by DEM and the fluid flow is described at a pore scale by PNM. The coupling scheme between DEM and PNM and the boundary conditions for realizing particle–fluid flows under different conditions are explicitly introduced. The capability is examined in three systems: a dynamic cell, a gas-fluidized bed, and a packed bed with lateral gas injection. The simulated results by the DEM-PNM approach agree with those results by either DEM-LBM (lattice Boltzmann method) approach or experiments. In particular, the simulated reasonable flow patterns in gas fluidization and lateral gas injection demonstrate the applicability of the proposed approach to complex particle–fluid flow. However, the current model presents less accuracy in dilute flow region and does not consider some important factors, which should be further improved in the future.

The effects of material properties and tube array settings on gas–solid flow and heat transfer characteristics in fluidized beds with tubes are investigated by the combined approach of computational fluid dynamics and discrete element method, incorporated with heat transfer models. First, the effect of material properties is illustrated by considering cohesive and non-cohesive powders with different particle sizes. The contributions of different heat transfer mechanisms are discussed at two tube temperatures. Significant differences of gas–solid flow between cohesive and non-cohesive powders are observed. The results reveal that conductive heat transfer between a fluidized bed and a tube is dominant for small cohesive particles while convective heat transfer is dominant for large non-cohesive particles. Then, the uniformity of particle velocity and temperature fields is analyzed. It is shown that material properties and gas velocity affect the uniformity of particle velocity and temperature in a complicated manner. Finally, the effect of tube array settings is examined in terms of two geometrical parameters for both in-line and staggered settings. Complicated gas–solid flow and heat transfer characteristics are observed. An effort is made to link macroscopic observations to microscopic information such as local porosity and contact number between fluidized particles and tubes. The findings should be helpful for the optimization of operation and design of fluidized systems with tubes.

COREX is one of the commercialized smelting reduction ironmaking processes. It mainly includes two reactors: a reduction shaft and a melter gasifier (MG). Lump coal is used in the MG to generate reducing agents, and hence its distribution plays an important role in determining the temperature distribution and the reduction of iron ores. To understand the formation of burden profiles, a three-dimensional discrete element method (DEM) model is established and validated against experimental tests. The effects of key operational parameters such as the chute angle and the rotational speed are investigated, and the underlying mechanisms in the formation of an asymmetrical pile are analyzed. The results show that the asymmetrical pile is intrinsic to the discharge system due to the outward motion of lump coal. The results also reveal a significant size segregation of lump coal along the radial direction. Small particles tend to accumulate in the inner side of the pile while the large ones have large fractions in the outer side. The effect of the rotational speed is minor within the range from 3 to 12 rpm. These findings are helpful for the control and optimization of the MG to improve its energy efficiency.

Intensive heat and mass transfer between continuum fluids and discrete particulate materials plays a critical role in many chemical reactors. The residence and chemical reactions of particulate materials could span over hours. To understand and improve the operation of these reactors, discrete particle models are very helpful and computationally demanding. Different to the previous coarse grain model in reducing computational cost by reducing total particle number with large representative particles, a scaling approach by changing process parameters and thus the time scale is established to significantly reduce computational cost for the combined computational fluid dynamics (CFD) and discrete element method (DEM) modelling of moving bed reactors. The scaled model is first derived based on the governing equations of mass, momentum and energy for two-phase flow and then applied to a moving bed reactor. The results in terms of flow, heat and mass transfer and chemical reactions with different time scaling factors demonstrate that two-order acceleration in terms of computational time can be achieved while reliably representing the same physical process. The possible use of the scaling approach to other systems is discussed. The scaling approach represents a critical step forward towards establishing virtual real-time thermochemical reactors with discrete particle models.

Corex has a great potential to produce hot metal economically and environmentally friendly. Reduction shaft (RS) for the direct reduction of iron ore is one of the two major reactors in Corex. This work uses a two-dimensional slot model to investigate the thermal behavior of the gas–solid flow in the RS by a combined computational fluid dynamics and discrete element method approach.The general heat transfer characteristics are firstly examined. The complicated transient gas–solid and heat transfer phenomena are demonstrated in the RS process. Then, the effects of discharge rate and two common burden profiles are investigated qualitatively. The results show a linear relation between the steady state bed temperature and discharge rate, which could be helpful for the construction of a reliable scaled down discrete particle model of the RS. The difference between the temperature distributions for two burden profiles is illustrated, which indicates that the control of burden profile could be an effective method to improve energy efficiency. These findings should be useful for the control and optimization of the RS operation.

Screw feeders are widely used in various industries to transfer granular materials at relatively precise rates. The performance can be affected by many factors such as the shape and size of particles, and the design of a screw and associated charging container. Some of these factors are difficult to investigate experimentally, including, for example, the cohesion between particles. In this work, a numerical model is developed by means of the discrete element method to study the flow of cohesive particles in screw feeders. In the model, the magnitude of the cohesive force is assumed, but it can be related to the van der Waals attraction for fine particles or the capillary force for wet particles. Based on the simulated results, a correlation for the prediction of solid flowrate is formulated as a function of the magnitude of cohesive force and the rotational speed of a screw. The mechanisms are then depicted in terms of contact forces and their spatial and temporal distributions. Three flow regimes, namely, continuous, intermittent and stable arch, are identified based on the standard deviation of solid flowrate in a screw feeder. Possible methods to reduce the effect of cohesive force on solid flow are discussed, in the study of the effects of the screw length in the associated charging container as well as the container design. The findings should be useful for the design and operation of screw feeders.

Perforated plates are used in many applications such as electrostatic precipitators in environmental control systems as a method of fluid flow control. The detailed structure of perforated plates causes difficulties in the design and optimization of systems by mathematical modelling. Hence, a simplified model for the perforated plates is important. In the current work, numerical experiments based on Computational Fluid Dynamics (CFD) are carried out at a microscopic, unit cell scale. The effects of a series of parameters on the flow pattern and pressure loss are investigated, including Reynolds number, open porosity, orifice diameter, plate thickness, surface roughness and plate inclination angle. Good agreement is observed between the current predictions and empirical equations/experimental data in the literature. The pressure loss is found to be determined by the large flow structures in the expansion behind the plate. The results can be potentially used for the modelling of flow distribution in electrostatic precipitators in particular.

The micromechanics of different particle–fluid flow regimes, such as fixed, expanded and fluidized beds, in gas fluidization is investigated for group A and B powders. To establish the connection between macroscopic and microscopic descriptions of complex particle–fluid flows, focus is given to the following two aspects: the formation of a stable expanded bed in relation to the interparticle cohesive, sliding and rolling frictional forces, and the correlation between coordination number (CN) and porosity ( e ). The method employed is the combined approach of three-dimensional (3D) discrete element method (DEM) and two-dimensional (2D) computational fluid dynamics. The results show that compared to 2D DEM, 3D DEM is more reliable in investigating the micromechanics of granular media, although both can capture key features of different flow regimes. The roles of various forces between particles and between particles and fluid are examined, and the origin of different flow regimes is discussed. It is shown that the cohesive force is critical to the formation of a static expanded bed, while the sliding and rolling frictional forces also play a role here. The criterion for bed expansion is analyzed at bulk and particle scales, and the deficiency at a bulk scale is identified. CN, as a key measure of local structure, is analyzed. It is found that the CN–e relationship for group A powders has a transitional point between the expanded and fluidized bed flow regimes at a bulk scale, unlike group B powders. A new phase diagram is established in terms of CN – e relationship that has two branches representing expanded and fluidized (bed) states, which corresponds to the one in terms of interparticle forces.

Heat transfer characteristics of different powders in gas fluidization are investigated by means of a combined approach of discrete element method and computational fluid dynamics. First, the heat transfer characteristics in three flow regimes of group A powders are examined. Then, the effects of the Hamaker constant and particle size, both related to the van der Waals force, are investigated in detail. The results confirm that the convective heat transfer is dominant, and radiative heat transfer becomes important when the bed temperature is high. However, conductive heat transfer also plays a role depending on the flow regimes and material properties. Significant effects of the Hamaker constant and particle size are observed under certain conditions. Finally, an effort is made to quantify the effects of the Hamaker constant, particle size, and inlet gas velocity. The findings should be useful for better understanding and prediction of the heat transfer in gas fluidization.

Two approaches are widely used to describe particle systems: the continuum approach at macroscopic scale and the discrete approach at particle scale. Each has its own advantages and disadvantages in the modelling of particle systems. It is of paramount significance to develop a theory to overcome the disadvantages of the two approaches. Averaging method to link the discrete to continuum approach is a potential technique to develop such a theory. This paper introduces an averaging method, including the theory and its application to the particle flow in a hopper and the particle-fluid flow in an ironmaking blast furnace.

A combined approach of discrete particle simulation and computational fluid dynamics is used to study the heat transfer in a fluidized bed with a horizontal tube. The approach is first validated through the good agreement between the predicted distribution and magnitude of local heat transfer coefficient with those measured. Then, the effects of inlet fluid superficial velocity, tube temperature and main particle properties such as particle thermal conductivity and Young's modulus are investigated and explained mechanistically. The relative importance of various heat transfer mechanisms is analyzed. The convection is found to be an important heat transfer mode for all the studied conditions. A large convective heat flux corresponds to a large local porosity around the tube, and a large conductive heat flux corresponds to a large number of particle contacts with the tube. The heat transfer is enhanced by the increase of particle thermal conductivity while it is little affected by Young's modulus. Radiative heat transfer becomes increasingly important as the tube temperature is increased. The results are useful for temperature control and structural design of fluidized beds. © 2011 American Institute of Chemical Engineers AIChE J, 2012

The mechanism of inclusion aggregation in liquid steel in swirling flow tundish is analyzed by applying the theory of flocculation which was developed in the field of colloid engineering. The gas bridge forces due to the micro bubbles on hydrophobic inclusion surfaces were responsible for the inclusion collision and agglomeration, which can avoid the aggregation to breakup. The quantity of micro bubbles on hydrophobic inclusion particle is more than that on hydrophilic one. The trend of forming gas bridges between micro bubbles on particles is strong in the course of collision. The liquid film on hydrophobic particles is easy to break during collision process. Hydrophobic particles are liable to aggregate in collision. According to the analysis of forces on a nonmetallic inclusion particle in swirling chamber, the chance of inclusion collision and aggregation can be improved by the centripetal force. Hydrophobic particles in water are liable to aggregate in collision. Hydrophilic particles in water are dispersed although collision happens. The wettability can be changed by changing solid-liquid interface tension. The nonmetallic inclusion removal in swirling flow tundish is studied. The result shows that under certain turbulent conditions, the particle concentration and the wettability between particles and liquid steel are the main factors to induce collision and aggregation.

A particulate system can be described through the discrete approach at the microscopic level or through the continuum approach at the macroscopic level. It is very significant to develop the method to link the two approaches for the development of models allowing a better understanding of the fundamentals of particulate systems. Several averaging methods have been proposed for this purpose in the past, but they mainly focused on cohesionless particle systems. In this work, a more general averaging method is proposed by extending it for cohesionless particle systems. The application of the method to the particle-fluid flow in a gas fluidized bed is studied. The density, velocity and stress of this flow are examined. A detailed discussion has been conducted to understand the dependence of the averaged variables on sample size.