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Computational Fluid Dynamics - Discrete Element Method (CFD-DEM) is a numerical tool used for detailed fluidized bed studies. However, CFD-DEM is computationally expensive, leading to a restriction regarding the number of simulated particles. Therefore, coarse-graining techniques have been developed to increase the simulation scale or reduce computational requirements for CFD-DEM simulations in fluidized beds. In this work, we critically compared the coarse-graining scaling laws of Mu et al. [2020, Chemical Engineering Science: X 6.] and Sakai and Koshizuka [2009, Chemical Engineering Science: 64, 533–539.] for their effectiveness in characterizing the original system. The first mentioned approach was not able to accurately characterize the original system, while the latter methodology showed good correspondence. We also demonstrated that applying a continuous two-way smoothing function to the coarse-graining process can yield grid-independent solutions, particularly when the particle diameter is much larger than the grid cell size, which is often the case in CFD-DEM simulations.

In this paper the influence of the inlet and outlet configurations of a pseudo-2D fluidized riser on the hydrodynamics (i.e. flow pattern and cluster characteristics) are studied. A detailed comparison between experimental data and full 3D CFD-DEM simulation results is performed. Solids volume fraction and mass flux are characterized and particle clusters are detected in our CFD-DEM simulations and compared to experimental data. It is shown that a correct representation of the outlet is very important for a correct simulation of the solids holdup in the top section of the riser. The core-annulus flow behavior is generally well predicted by the model and cluster characteristics such as cluster occurance and velocity are also in good agreement with experimental data. Finally, an outlook is given for the future use of CFD-DEM simulation approaches to study riser flows.

Fluidized bed drying is an unsteady process where particle and gas properties evolve in time. The bed hydrodynamics and its interplay with mass and heat transfer changes in time. In this study, experiments in a pseudo-2D fluidized bed setup are performed to obtain insight in this complex behavior. A coupled particle image velocimetry infrared thermography technique provides solids velocity and temperature fields in the bed. Significant hydrodynamic changes due to the evaporation of water from the porous γ-Al2O3 material for three superficial velocities were observed. The 1.25 umf case resulted in an fixed bed state, but after drying for 580 s a bed inversion took place. The 1.50 and 1.75 umf experiments showed an increase in bed expansion over time. Furthermore, clear asymmetrical solids volume fluxes were observed for the wet material that became more symmetrical over time when the material dried.

A full‐field PIV‐DIA technique has been coupled to concentration measurements in order to characterize the riser performance at different gas superficial velocities and reaction rates for ozone decomposition. Experiments under riser flow conditions were performed with the use of a compact pseudo‐2D lab‐scale riser reactor. The purpose of this work is to generate reliable experimental data to validate computational models for mass transfer in riser flow conditions. The catalytic activity of the particles was characterized. To analyze the reaction rate effect over the performance of a riser reactor, two catalyst batches with different activities were used and their kinetics were measured. The hydrodynamics of the gas‐solid flow was determined using the PIV‐DIA technique. Particle clusters were identified and characterized. By correlating the cluster information and mass concentration measurements, it was shown that a higher frequency of local flow heterogeneities near the walls led to higher conversion rates than in the core of the riser. Reaction rate and gas superficial effects are analyzed.

Fluidized bed drying is an evolving process: The bed hydrodynamics, including the segregation and mixing tendencies, and its interplay with mass and heat transfer change in time. In this study, pseudo-2D fluidized bed experiments are performed using binary (50/50%) and ternary (33.3/33.3/33.3%) solids mixtures. A coupled particle image velocimetry infrared thermography technique provides local solids velocity and temperature fields. Furthermore, a machine learning algorithm is applied to characterize the bed segregation and mixing dynamics. Significant hydrodynamic changes and various segregation and mixing tendencies due to the evaporation of water from the porous γ-Al2O3 material were observed. A competition between size and density-based segregation and mixing dynamics resulted in changing bed configurations, which are correlated to the local solids drying behavior. Besides, it showed that drying of wider size distributions could lead to more complex efficient process operation.

Large scale simulation models can aid in improving the design of spray dryers. In this work an Eulerian-Lagrangian model with coupled gas phase and droplet heat and mass transfer balances is used to study airflow dynamics, temperature and humidity profiles at different positions in the spray. The turbulent gas flow is solved using large eddy simulation (LES). A turbulent dispersion model accounts for the stochastic subgrid fluid velocity fluctuations along the droplet trajectory. The dispersed phase is treated with Lagrangian transport of droplets, and collisions between droplets which are detected with a stochastic Direct Simulation Monte Carlo (DSMC) method. The outcome of a binary collision is described by collision boundary models for water and milk concentrates. The drying of droplets is modeled by the reaction engineering approach (REA). The effect of the inlet air conditions and of droplet viscosity on the temperature and humidity distributions are analyzed. Most of the heat and mass transfer occurs in the first 10-20 cm from the nozzle where the slip velocities and temperature and humidity driving forces are higher. The droplets size increases, both in the axial and radial direction, because of the dominance of coalescence over separation in the droplet spray studied here. Because the spray domain considered in this work is relatively small, the droplet residence time is small, and consequently the amount of evaporation is still low. The droplet size distributions of milk concentrates are affected by the predominance of coalescence over separation events. The coalescence dominated regime increases when the droplet viscosity is higher.

A kinetics theory of catalyst deactivation is presented of the solid acid-catalyzed alkylation reaction of isobutane with propylene or butene that gives alkylate, a high octane fuel, as product. The intimate relation between the kinetics network of the reaction, catalyst deactivation kinetics, and residence time distribution is analyzed. The question is addressed why the deactivation of the alkylation reaction in a continuously stirred tank reactor (CSTR) is slow compared to that in a tubular plug flow reactor (PFR). Conditions are derived where such differences will be minimum and maximum. In the reaction regime of high alkylate selectivity, linear and quadratic power law kinetics equations in propylene concentration can be deduced from microkinetics. They are used to derive analytical expressions of deactivation times for CSTR and PFR. The theoretical power law kinetics equations can be related to previously established empirical rate equations of catalyst deactivation. We show that, in the CSTR, the self-alkylation reaction path contributes substantially to the deactivation time. In the self-alkylation reaction, alkylate is formed by reaction of the isobutene reaction intermediate and isobutane. Catalysts of high proton strength can benefit catalyst deactivation times by suppressing the carbenium ion deprotonation reaction that produces alkenes as isobutene. In the PFR, selective alkylate formation occurs only when the reaction occurs in a reaction zone of the catalyst bed. Deactivation is faster than in CSTR because of the reactant profile in the reaction zone. This reaction zone has restricted mobility due to the fast deactivation of reactive protons located behind the reaction zone by alkenes formed by nonselective reactions in the reaction zone. In PFR, as long as the reaction is limited to an immobile reaction zone, deactivation time is independent of reaction site density and contact time. Contact time dependence arises when the reaction zone is mobile. Overall deactivation time then depends strongly on the degree of deactivation of the protons behind the reaction zone.

Recently, we have extended the kinetic theory of granular flow (KTGF) to include friction between the spherical particles and tested it in rectangular geometries. In this study, the extended KTGF implemented in cylindrical coordinates is used to model the more-commonly employed cylindrical bubbling fluidized beds. Special attention is paid to the anti-symmetric part of the velocity gradient in the solids stress tensor. For verification of the implementation, a comparison of the present model in the limit of zero friction with the original (frictionless) KTGF model was made. Subsequently, simulations of bubbling fluidized beds of inelastic particles were performed using our extended KTGF and an effective KTGF model for inelastic particles of Jenkins and Zhang. The simulation results show good agreement for the time-averaged solids volume fraction distribution and solids circulation patterns. Finally, our model is validated by predicting the individual bubble behavior in dense bubbling fluidized beds containing different granular materials in a comparison with experimental data from Verma et al. (2014). The extended KTGF leads to an improved agreement with experimental bubble data. Compared to previous work (Yang et al., 2016b, 2017c), and by introducing cylindrical coordinates, the current work demonstrates that the extended KTGF improves predictions for the temporal bubble behavior of cylindrical fluidized beds.

In this paper, an efficient ghost-cell based immersed boundary method (IBM) is used to perform direct numerical simulation (DNS) of reactive fluid-particle systems. With an exothermic first order reaction proceeding at the exterior particle surface, the solid temperature rises and consequently increases the reaction rate via an Arrhenius temperature dependence. In other words, the heat and mass transport is dependently coupled through the particle thermal energy equation and the Arrhenius equation, and they offer dynamic boundary conditions for the fluid phase thermal energy equation and species equation respectively. The fluid-solid coupling is enforced at the exact position of the particle surface by implicit incorporation of the boundary conditions into the discretized momentum, species and thermal energy conservation equations of the fluid phase. Different fluid-particle systems are studied with increasing complexity: a single sphere, three spheres and a dense array consisting of hundreds of randomly generated particles. In these systems the mutual impacts between heat and mass transport processes are investigated.

In this work we investigate droplet-droplet collision interactions in a spray system using an Eulerian-Lagrangian model with subgrid turbulence dispersion. The effect of different droplet viscosities on the type and frequency of droplet collision is investigated, knowledge of which is essential for industrial processes such as spray drying for production of milk powder. The dispersed phase is treated with Lagrangian transport of droplets and the turbulent self-induced gas flow using large eddy simulation (LES). A stochastic Direct Simulation Monte Carlo (DSMC) method is used to detect collisions between droplets. The outcome of a binary collision is described by a collision boundary models for water and milk concentrates. A turbulence dispersion model, based on the Langevin equation, accounts for the stochastic subgrid fluid velocity fluctuations along the droplet trajectory. We compare the spray dynamics with and without droplet interactions and turbulence dispersion. For a spray with typical droplet size of 50 µm, we find that the turbulence dispersion model enhances the total collision frequencies by approximately 25%. The performance of the turbulent dispersion model is tested by investigating the rate of collisions for different milk concentrates. The evolution of size distributions inside the spray is strongly influenced by the complementary effects of collision boundary models and turbulence dispersion.

In this paper two immersed boundary methods (IBM), specifically a continuous forcing method (CFM) and a discrete forcing method (DFM), are applied to perform direct numerical simulations (DNSs) of heat transfer problems in tubular fluid-particle systems. Both IBM models are built on the well-developed models utilized in momentum transfer studies, and have the capability to handle mixed boundary conditions at the particle surface as encountered in industrial applications with both active and passive particles. Following a thorough verification of both models for the classical Graetz-Nusselt problem, we subsequently apply them to study a much more advanced Graetz-Nusselt problem of more practical importance with a dense stationary array consisting of hundreds of particles randomly positioned inside a tube with adiabatic wall. The influence of particle sizes and fractional amount of passive particles is analyzed at varying Reynolds numbers, and the simulation results are compared between the two IBM models, finding good agreement. Our results thus qualify the two employed IBM modules for more complex applications.

The hydrodynamic interaction of two bubbles released from submerged orifices was simulated using a 3D front-tracking type technique called the Local Front Reconstruction Method (LFRM). The effects of fluid properties and orifice spacing on the bubble trajectory, the interaction between two bubbles, and the vortex shedding pattern were investigated. Different types of interaction behaviors, which were observed in experiments by Kong et al. (2019), could be replicated by varying the liquid viscosity. The simulation results revealed that bubbles at low Reynolds number repel each other because the diffusion of vorticity at the bubble surface is blocked and becomes asymmetric due to the presence of a neighboring bubble. In contrast, at medium and high Reynolds numbers, the bubbles attract each other after their detachment from the orifices, and their subsequent possible bouncing depends on the orifice spacing. The bubble pair will feature a zigzagging motion while rising when double-threaded vortices are generated behind the bubble.

ThermoCatalytic Decomposition of methane (TCD) is studied as a method to convert natural gas into hydrogen and functional carbon. In these processes the carbon typically formed on top of a catalyst phase leading to particle growth. Therefore, the development of a particle growth model is necessary to understand the limitations of thermocatalytic decomposition of methane and to assess optimal parameters and process conditions. In this paper, a particle growth model is presented to describe the growth of functional carbon on the catalyst particle. This coupled model requires kinetic equations and information on deactivation rates which have been studied from literature. The morphology of the particle changes due to carbon formation, which leads to eventual deactivation. Therefore, these kinetic expressions are coupled to a particle growth model based on the analogy with the growth of particles in polyolefin production. To combine the effects of particle growth, kinetics, and internal heat and mass transfer, the Multi-Grain Model (MGM) was used. Results confirm that with the currently available catalysts the carbon yield is not affected by heat and mass transfer limitations, however, with the availability of more active catalysts these limitations will become important. Temperature, however, has a significant role in that it regulates the kinetic rate and thus growth rate, which in turn influences the catalyst deactivation. The optimum temperature for the production of nano-carbon, within a reasonable process time, therefore sensitively depends on the choice of catalyst.

Liquid-solid fluidisation is frequently encountered in drinking water treatment processes, for instance in seeded crystallisation softening processes. For modest superficial fluid velocities, liquid–solid fluidisation systems are generally considered to be homogeneous, as reported in literature. However, during fluidisation experiments with calcite grains, open spaces of water can be observed between the fluidised particles, even at relatively low fluid velocities. Moreover, significant heterogeneous particle–fluid patterns are detected at higher fluid velocities. Such heterogeneous behaviour can beneficially or adversely affect the chemical crystallisation efficiency. To obtain information about voids in bulk regions, complementary Computational Fluid Dynamics - Discrete Element Method (CFD-DEM) simulations were performed and compared with the experimental results for validation. Simulations were performed using different water inlet velocities and fractionised calcite granules obtained from full-scale reactors. Here, the results are analysed using the bed height, voidage and pressure drop of the system. Furthermore, images of the experiments and simulations are visually compared for the formation of voids. The simulations showed distinct differences in void fraction in the cross-section of the column. It is shown that throughout the range of considered water velocities, heterogeneous behaviour exists and cannot be neglected. The heterogeneity and onset of fluidisation behaviour obtained from the simulations and experimental observations were compared and found to agree reasonably well.

High heat treatment, using Direct Contact Condensation (DCC), is applied in the production of dairy products to ensure a high level of food safety. During this process a protein rich deposit layer can be formed causing fouling of the system resulting in a loss of effective production time and an increase in cleaning cycles. The dynamics of the heat treatment process is modelled, including the description of the pre-heating step with two laminar flow tubular heat-exchangers followed by a DCC. In this system the bulk reactions for whey proteins are modelled using unfolding, re-folding and aggregation kinetics. The formation of the deposit layer is described in the segment after the DCC using a flow averaged model incorporating diffusion of species through the mass boundary layer as well as adsorption to the wall and desorption from the wall. The desorption kinetics are key as the protein deposit layer releases from the wall once the wall friction increases due to the decreasing area available for throughflow. The model shows good agreement with experimental data and is capable of capturing the process dynamics, both for the heat exchanger and the DCC. The deposit layer formation model is in agreement with the trend observed in the experimental data but is not fully capturing the process dynamics.

In this paper, the influence of the free surface on hydrodynamics is investigated numerically for a pilot-scale bubble column. The free surface is modelled by means of the Volume of Fluid method (VOF), whereas the gas entities are represented by means of the Direct Simulation Monte Carlo (DSMC) method. The VOF method is parallelized using MPI and incorporated in already parallel in-house numerical code for DSMC, FoxBerry. The results obtained with the modeled free surface are compared with the reference simulation, in which the free surface is mimicked using the outlet slits, placed at the top of the column. The results show a significant impact of the free surface on generation of the small-scale eddies. Moreover, the presence of the free surface reduces the gas-holdup in the domain.

The process of bubble formation from submerged orifices is encountered in various industrial applications. It is therefore essential to understand the dynamics of bubble formation under such conditions. In the present work, the process of bubble formation in a steel-argon system is studied using the Local Front Reconstruction Method (LFRM), a Front Tracking method that enables the simulation of interface merging and breakup. The numerical simulations are performed over a wide range of gas injection rates to study the bubble formation dynamics under quasi-static and dynamic regimes. The simulation results show that the detached bubbles in a steel-argon system are generally bigger compared to the bubbles detached in a water-air system due to higher surface tension and lower wettability. In liquid cross-flow, the bubble at the orifice mouth becomes asymmetric due to the drag force created by the liquid flow. Under non-wetting conditions, the bubble can slide over the orifice without forming a bubble neck when the orifice plate is sufficiently large. On the other hand, under wetting conditions, the detached bubble volume decreases when the orifice plate is gradually tilted from a horizontal to vertical orientation at lower shear rates. However, this trend reverses at higher shear rates because the drag force exerted by the flowing liquid becomes dominant.

Heat transfer between solid particles and liquid droplets is a key phenomenon encountered in many industrial applications. We investigate the fluid dynamics and heat transfer between a liquid droplet and a static solid particle by means of direct numerical simulations. The presence of the solid particle is represented through a sharp interface immersed boundary method (IBM) on a Cartesian computational grid, whereas the motion of the gas-liquid interface is tracked by a mass conservative volume of fluid method (VOF). To account for the thermal coupling between the gas and the liquid, and more specifically the conjugate heat transfer between the fluid and the solid, a second order accurate diffused interface formulation is proposed for the conduction term in the energy equation. A detailed validation of the method is also presented. We have studied the effect of wettability and solid conductivity both of which have been found to have significant effect on the total heat transfer rate.

A parallel and scalable stochastic Direct Simulation Monte Carlo (DSMC) method applied to large-scale dense bubbly flows is reported in this paper. The DSMC method is applied to speed up the bubble-bubble collision handling relative to the Discrete Bubble Model proposed by Darmana et al. (2006) [1]. The DSMC algorithm has been modified and extended to account for bubble-bubble interactions arising due to uncorrelated and correlated bubble velocities. The algorithm is fully coupled with an in-house CFD code and parallelized using the MPI framework. The model is verified and validated on multiple cores with different test cases, ranging from impinging particle streams to laboratory-scale bubble columns. The parallel performance is shown using two different large scale systems: with an uniform and a non-uniform distribution of bubbles. The hydrodynamics of a pilot-scale bubble column is analyzed and the effect of the column scale is reported via the comparison of bubble columns at three different scales.

With increasing the computational resources, the number of publications about coupled computational fluid dynamics – discrete element method is in the rise in the recent years. This technique is very useful, especially in simulation of fluid-solid flows in process engineering. This paper provides an introduction to CFD-DEM modeling in process engineering systems, including heat and mass transfer and long range forces, and reviews the major researches in simulation of two-phase processes such as drying, coating, granulation, crystallization, chemical reactions (including combustion, gasification and pyrolysis) and mixing. Details of implementing unresolved CFD-DEM in these applications are explained in details and major assumptions and findings are discussed.

In this work, the analysis of the mass transfer phenomena in catalytic open-cell foams is carried out through the combination of computational fluid dynamics (CFD) simulations and experiments, using the CO oxidation on Pt(1%)/γ-Al2O3/foam as a model reaction. The influence of the local hydrodynamic effects on the diffusion-reaction phenomena occurring at the gas-solid interface of the open-cell solid foams are investigated by Direct Numerical Simulations assuming an infinitely fast reaction. A correlation for the Sherwood number as function of Reynolds -for low Re- is proposed. To validate this experimentally, aluminum foams coated with Pt(1%)/γ-Al2O3 are tested at different reaction conditions for the CO oxidation. The obtained reaction rates and apparent activation energy show the presence of external mass transfer limitations. An analysis of the diffusion-reaction phenomena taking place in the wash-coated layer is presented in terms of dimensionless numbers. A practical criterion is developed in terms of the Thiele modulus (ϕw) and the Biot number (Bim) for the identification of the reaction regimes: kinetic control (ϕw2/Bimw2/Bimw2/Bim>10).

Fully resolved simulations of flow and mass transfer in a unit cell of structured open-cell foam catalysts are presented. Numerical studies are conducted on a uniform three-dimensional Cartesian grid where the fluid-solid interface coupling is enforced via a sharp interface Immersed Boundary technique. Several validation cases for the numerical method are presented followed by extensive calculations to quantify hydrodynamic dispersion in open-cell foams. In our study five different porosities of the idealized foam structure, represented by the spatially periodic Kelvin’s unit cell, were considered. Dimensionless dispersion coefficients were calculated for varying Peclet numbers and flow directions using volume-averaging theory. Our numerical studies indicate that Taylor dispersion is the dominant mechanism for structured porous media in the Darcy-Brinkman flow regime.

In the present study the authors compare two different simulation models for the modeling of droplet collisions. The simulation models are: the Local Front Reconstruction Method (LFRM) and the Diffuse Interface Model (DIM). Results for fully three-dimensional simulations of droplet collisions at relatively high Weber number simulated with both models are presented and compared. Additionally, a detailed analysis of the dissipation and energy transfer processes of the collision is presented. An overall good agreement is seen in the collision outcomes. Some differences are observed in the interface evolution and the energy transfer/dissipation process during the droplet collision. A significant portion of these differences can be attributed to the differences in the configuration of the initial velocity field. Therefore, for the initial configuration a divergence-free vortical velocity field is introduced to achieve a better match between the simulation models. This improves the agreement of the simulation results.

In this work, we investigate the influence of channel structure and fluid rheology on non-inertial migration of non-Brownian polystyrene beads. Particle migration in this regime can be found in biomedical, chemical, environmental and geological applications. However, the effect of fluid rheology on particle migration in porous media remains to be clearly understood. Here, we isolate the effects of elasticity and shear thinning by comparing a Newtonian fluid, a purely elastic (Boger) fluid, and a shear-thinning elastic fluid. To mimic the complexity of geometries in real-world application, a random porous structure is created through a disordered arrangement of cylindrical pillars in the microchannel. Experiments are repeated in an empty channel and in channels with an ordered arrangement of pillars, and the similarities and differences in the observed particle focusing are analyzed. It is found that elasticity drives the particles away from the channel walls in an empty microchannel. Notably, particle focusing is unaffected by curved streamlines in an ordered porous microchannel and particles stay away from pillars in elastic fluids. Shear-thinning is found to reduce the effect of focusing and a broader region of particle concentration is observed. It is also noteworthy that the rheological characteristics of the fluid are not important for the particle distribution in a randomly arranged pillared microchannel and particles have a uniform distribution for all suspending fluids. Moreover, discussion on the current discrepancy in the literature about the equilibrium positions of the particles in a channel is extended by analyzing the results obtained in the current experiments.

In this paper we quantify the effect of capillary number ( Ca ) , contact angle ( θ ) and viscosity ratio ( M ) on the mobility of multiphase flow through porous media. The focus is mainly on oil-water flows through porous rocks observed during the water flooding process. Simulations are performed using a finite volume method employing a staggered grid formulation. Interactions between fluids and complex solid boundaries are resolved by a direct forcing, implicit and sharp interface immersed boundary method (IBM). The fluid-fluid interface is tracked by a mass conservative sharp interface volume of fluid (VOF) method. IBM and VOF are coupled by imposing the contact angle as a boundary condition at the three phase contact line. Our methodology has been verified/validated for several test cases including multiphase Poiseuille flow in a channel, a viscous finger in a channel and mesh convergence of the contact force. Two types of porous structures are considered: (i) a repeated single pore and (ii) a random multi-pore arrangement. Temporal evolution of phase pressure difference and oil saturation have been studied as viscous fingers penetrate the pores. We observed that the residual oil saturation for different capillary numbers shows exactly the opposite trend for the single and multi pore arrangement. The residual oil saturation for multi-pore shows a well defined linear trend with log Ca , θ and log M .

The formation of gas bubbles by submerged orifices in a cross-flowing liquid is encountered in many industrial applications. It is therefore important to understand the dynamics of bubble formation and to accurately predict the bubble detachment characteristics under such situations. In the present work, the process is numerically studied using the Local Front Reconstruction Method (LFRM), a Front Tracking Direct Numerical Simulation method that enables the simulation of interface merging and breakup. Experiments of bubble formation subjected to cross-flow induced shear are also performed to provide data for the validation of the numerical simulations. The predictions of the bubble shape and the detached bubble volume obtained by the numerical model show good agreement with the experimental results. The validated numerical model is then used to study the effects of volumetric gas flow rate and fluid physical properties on the bubble detachment characteristics under various shear rates in the quasi-static bubble growth regime. The simulation results show that the shear flow advances the bubble detachment and decreases the bubble size. Consequently, the bubble formation frequency and the detached bubble size can be controlled by exerting different shear rates. At higher shear rates, the simulated bubbles are highly deformed due to the drag force created by the tangential liquid flow and the influence of liquid properties on the bubble detachment characteristics becomes less significant.

A numerical simulation study was carried out to study the combined thermal behavior and hydrodynamics of a pseudo-2D fluidized bed using a computational fluid dynamics&ndash, discrete element method (CFD-DEM). To mimic the effect of heterogeneous exothermic reactions, a constant heat source was implemented in the particle energy equation. The effects of superficial gas velocity, bed height and heat source distribution were analyzed with the aid of averaged volume fraction and temperature distributions and velocity profiles. It was found that both the gas superficial velocity and the bed aspect ratio have a profound influence on fluidization behavior and temperature distributions.

In biomass processing fluidized beds are used to process granular materials where particles typically possess elongated shapes. However, for simplicity, in computer simulations particles are often considered spherical, even though elongated particles experience more complex particle–particle interactions as well as different hydrodynamic forces. The exact effect of these more complex interactions in dense fluidized suspensions is still not well understood. In this study we use the magnetic particle tracking technique to compare the fluidization behavior of spherical particles to that of elongated particles. We found a considerable difference between fluidization behavior of spherical versus elongated particles in the time-averaged particle velocity field as well as in the time-averaged particle rotational velocity profile. Moreover, we studied the effect of fluid velocity and the particle's aspect ratio on the particle's preferred orientation in different parts of the bed, which provides new insight in the fluidization behavior of elongated particles.

Abstract The formation of gas bubbles by submerged orifices is a fundamental process encountered in various industrial applications. The dynamics of the contact line and the contact angle may have a significant influence on the detached bubble size depending on the wettability of the system. In this study, the influence of wetting conditions on the dynamics of bubble formation from a submerged orifice is investigated experimentally and numerically. The experiments are performed using a hydrophobic orifice plate and a series of ethanol–water solutions to vary the wettability where the key characteristics of the bubbles are measured using a high-speed, high-resolution camera. An extensive analysis on the influence of wetting conditions on the bubble size, bubble growth mechanism and the behavior of the contact line is given. Bubble growth stages, termed (1) hemispherical spreading, (2) cylindrical spreading, (3) critical growth and (4) necking, are identified based on key geometrical parameters of the bubble and relevant forces acting on the bubble during the growth. The experimental results show that the apparent contact angle varies in a complicated manner as the bubble grows due to the surface roughness and heterogeneity. The experimental findings are finally used to validate the local front reconstruction method with a contact angle model to account for the contact angle hysteresis observed in the experiments. Graphic abstract

Fluidised beds are used in a variety of processes because of their favourable mass and heat transfer characteristics. In this and many other processes, non-spherical particles are commonplace, which can drastically affect the fluidisation behaviour. In this study, we use numerical models to study non-spherical fluidisation behaviour in detail. A crucial step in the development of the numerical model is a detailed validation with experimental data. The validated model can then be used with confidence for further investigations. In this study, the results obtained from CFD-DEM modelling are compared with detailed experiments (Mahajan et al., 2017). The particles used are of spherocylindrical shape with an aspect ratio 4. We discuss the numerical modelling strategy including the DEM contact detection algorithm and accurate voidage calculation algorithm. The non-spherical single particle drag model of Hölzer and Sommerfeld (2008) is compared with a DNS drag model for spherocylindrical particles developed in-house. We propose two new voidage correction models and compare results with the (Di Felice, 1994) model. The pressure drop, bed height, particle orientation, particle circulation, stacking of particles and coordination number obtained from simulations are compared with experiments. The numerical measurements show good agreement with experiments. Similar to experiments, simulations show that rod-like particles are prone to interlocking behaviour. At high gas flow rates above the minimum fluidisation velocity, vigorously bubbling fluidisation is observed, with gas bubbles moving up through the center and particles moving down at the side walls. The orientation of particles in the fluidised state do not match with the experiments when hydrodynamic torque is neglected. The importance of hydrodynamic torque and multi-particle drag in CFD-DEM modelling of non-spherical particles is demonstrated through these results.

A novel granular discrete element method (DEM) is introduced to simulate mixtures of particles of any convex shape. To quickly identify pairs of particles in contact, the method first uses a broad-phase and a narrow-phase contact detection strategy. After this, a contact resolution phase finds the contact normal and penetration depth. A new algorithm is introduced to effectively locate the contact point in the geometric center of flat faces in partial contact. This is important for a correct evaluation of the torque on each particle, leading to a much higher stability of stacks of particles than with previous algorithms. The granular DEM is used to generate random packings in a cylindrical vessel. The simulated shapes include non-spherical particles with different aspect ratio cuboids, cylinders and ellipsoids. More complex polyhedral shapes representing sand and woodchip particles are also used. The latter particles all have a unique shape and size, resembling real granular particle packings. All packings are analyzed extensively by investigating positional and orientational ordering.

In this paper, an accurate and stable sharp interface immersed boundary method(IBM) is presented for the direct numerical simulation of particle laden flows. The current IBM method is based on the direct-forcing method by incorporating the ghost-cell approach implicitly. An important feature of this IBM is the sharp representation of the solid surface, contrary to other variants of IBM for freely moving particles in which the solid surface is diffuse. Moreover, a correction of the diameter is not necessary for obtaining accurate results. The current ghost-cell IBM is stable because spurious oscillations incurred due to discontinuity in the pressure and velocity field in moving particle simulations is avoided. An algorithm for accurate torque computation is developed. The proposed algorithm is verified by comparison to an analytical expression and is shown to give a substantial improvement over the existing method. Finally, the present IBM is validated for various test cases of single and multi-particle systems and is shown to be accurate and robust for a wide range of flow conditions.

To accurately simulate industrial sized bubble columns, the accurate prediction of the turbulent structures is important. Currently used RANS and LES models cannot capture the dynamics of the bubble columns or require a high grid resolution, respectively. In this work the Detached Eddy Simulation (DES) was used to combine the advantages of RANS and LES approaches. The DES method was based on Spalart–Allmaras, k − ϵ and k − ω SST turbulence models and was used to simulate gas–liquid flow for a bubble column with a square cross-section. The results are compared with experimental data and Large Eddy Simulation (LES) results, based on the Vreman and Smagorinsky subgrid-scale models. Profiles of turbulence kinetic energy and average axial liquid and gas velocities are compared at three heights of the column: near the sparger, in the middle and at the top of the column. The obtained results are in a very good agreement with the experimental data and LES simulations, proving ability of DES to capture highly dynamic flow motion and accurately predict main liquid characteristics in gas–liquid systems.

In this work, a new method is presented to track discrete tracer particles in a two fluid model (TFM). This method is particularly useful for studying features of discrete particles, such as solids mixing. Following the implementation and verification of this method, its accuracy was studied. The results showed that the new method fulfills the continuity equation and it can represent individual solids motion very well. This method may suffer from false diffusion, which can be diminished by selecting a sufficiently small grid size. In addition, it has several advantages over other techniques, like simplicity, ease of implementation, straightforward processing and enabling the calculation of a mixing index based on the initial neighbor distance concept. Moreover, this method can open a new way to combine the TFM with Lagrangian approaches. After analyzing the strengths and drawbacks of our method and finding the proper simulation settings, the effects of superficial gas velocity and restitution coefficient on solids mixing were investigated. The results showed that the solids mixing is enhanced by increasing the gas velocity and/or decreasing the restitution coefficient. The observed trends can be attributed to altered bubble formation and dynamics. These results also confirm our earlier findings on the solids temperature distribution in fluidized beds for polyolefins production (Banaei et al., 2017).

Accurate direct numerical simulations are performed to determine the drag, lift and torque coefficients of non-spherical particles. The numerical simulations are performed using the lattice Boltzmann method with multi-relaxation time. The motivation for this work is the need for accurate drag, lift and torque correlations for high Re regimes, which are encountered in Euler-Lagrangian simulations of fluidization and pneumatic conveying of larger non-spherical particles. The simulations are performed in the Reynolds number range 0.1 ≤ Re ≤ 2000 for different incident angles ϕ. Different tests are performed to analyse the influence of grid resolution and confinement effects for different Re. The measured drag, lift and torque coefficients are utilized to derive accurate correlations for specific non-spherical particle shapes, which can be used in unresolved simulations. The functional forms for the correlations are chosen to agree with the expected physics at Stokes flow as well as the observed leveling off of the drag coefficient at high Re flows. Therefore the fits can be extended to regimes outside the Re regimes simulated. We observe sine-squared scaling of the drag coefficient for the particles tested even at R e = 2000 with C D , ϕ = C D , ϕ = 0 ∘ + ( C D , ϕ = 90 ∘ − C D , ϕ = 0 ∘ ) sin 2 ϕ . Furthermore, we also observe that the lift coefficient approximately scales as C L , ϕ = ( C D , ϕ = 90 ∘ − C D , ϕ = 0 ∘ ) sin ϕ cos ϕ for the elongated particles. The current work would greatly improve the accuracy of Euler-Lagrangian simulations of larger non-spherical particles considering the existing literature is mainly limited to steady flow regimes and lower Re.

In this paper, an efficient ghost-cell based immersed boundary method is applied to perform direct numerical simulation (DNS) of mass transfer problems in particle clusters. To be specific, a nine-sphere cuboid cluster and a random-generated spherical cluster consisting of 100 spheres are studied. In both cases, the cluster is composed of active catalysts and inert particles, and the mutual influence of particles on their mass transfer performance is studied. To simulate active catalysts the Dirichlet boundary condition is imposed at the external surface of spheres, while the zero-flux Neumann boundary condition is applied for inert particles. Through our studies, clustering is found to have negative influence on the mass transfer performance, which can be then improved by dilution with inert particles and higher Reynolds numbers. The distribution of active/inert particles may lead to large variations of the cluster mass transfer performance, and individual particle deep inside the cluster may possess a high Sherwood number.

Fluidization is widely used in industries and has been extensively studied, both experimentally and theoretically, in the past. However, most of these studies focus on spherical particles while in practice granules are rarely spherical. Particle shape can have a significant effect on fluidization characteristics. It is therefore important to study the effect of particle shape on fluidization behavior in detail. In this study, experiments in pseudo‐2D fluidized beds are used to characterize the fluidization of spherocylindrical (rod‐like) Geldart D particles of aspect ratio 4. Pressure drop and optical measurement methods (Digital Image Analysis, Particle Image Velocimetry, Particle Tracking Velocimetry) are employed to measure bed height, particle orientation, particle circulation, stacking, and coordination number. The commonly used correlations to determine the pressure drop across a bed of nonspherical particles are compared to experiments. Experimental observations and measurements have shown that rod‐like particles are prone to interlocking and channeling behavior. Well above the minimum fluidization velocity, vigorous bubbling fluidization is observed, with groups of interlocked particles moving upwards, breaking up, being thrown high in the freeboard region and slowly raining down as dispersed phase. At high flowrates, a circulation pattern develops with particles moving up through the center and down at the walls. Particles tend to orient themselves along the flow direction. © 2018 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 64: 1573–1590, 2018

Hypothesis Multiphase flow through porous media is important in a number of industrial, natural and biological processes. One application is enhanced oil recovery (EOR), where a resident oil phase is displaced by a Newtonian or polymeric fluid. In EOR, the two-phase immiscible displacement through heterogonous porous media is usually governed by competing viscous and capillary forces, expressed through a Capillary number Ca, and viscosity ratio of the displacing and displaced fluid. However, when viscoelastic displacement fluids are used, elastic forces in the displacement fluid also become significant. It is hypothesized that elastic instabilities are responsible for enhanced oil recovery through an elastic microsweep mechanism. Experiments In this work, we use a simplified geometry in the form of a pillared microchannel. We analyze the trapped residual oil size distribution after displacement by a Newtonian fluid, a nearly inelastic shear thinning fluid, and viscoelastic polymers and surfactant solutions. Findings We find that viscoelastic polymers and surfactant solutions can displace more oil compared to Newtonian fluids and nearly inelastic shear thinning polymers at similar Ca numbers. Beyond a critical Ca number, the size of residual oil blobs decreases significantly for viscoelastic fluids. This critical Ca number directly corresponds to flow rates where elastic instabilities occur in single phase flow, suggesting a close link between enhancement of oil recovery and appearance of elastic instabilities.

The effect of elevated pressure on gas-solid heat-transfer behavior in an olefin polymerization fluidized bed was numerically analyzed by using an in-house developed 3-D computational fluid dynamics model coupled with a discrete element model (CFD-DEM). To mimic the heat generation associated with the polymerization reaction, a constant volumetric heat production was incorporated in the particle thermal energy equation. Snapshots of the heat transfer driving force (the difference between single particle temperature and average gas temperature) are presented to provide insight into the particle temperature distribution in the fluidized bed. Furthermore, it was found from the Probability Distribution Function (PDF) of the particle temperature that with increasing operating pressure the average particle temperature drops and the bed becomes more isothermal. Moreover, the average particle-gas heat transfer coefficient, was found to increase with increasing operating pressure. However, it is independent from the superficial gas velocity when the bed was operated above the minimum fluidization condition at the same elevated pressure. Predictive based on a continuous stirred tank reactor (CSTR) approximation of the bed reveals that the average particle temperature at steady state is determined by the inlet gas temperature, the temperature difference of the two phases and the adiabatic temperature rise. In polymerization reactors the adiabatic temperature rise is relatively high, therefore the effect of the pressure on the average temperature of the particles can be entirely attributed to the increased gas density, whereas changes in the bed hydrodynamics have an effect on the spread of the particle temperature distribution.

A combined numerical and experimental study of riser hydrodynamics is reported for the fast fluidisation regime. The experimental data sets are used for validation of the CFD-DEM simulations. Special attention is paid to the formation of heterogeneities in the particle distribution, i.e., the formation of clusters. Application of Particle Image Velocimetry (PIV) in combination with Digital Image Analysis (DIA), provides a complete data set for the particle flow in a pseudo-2D riser. A CFD-DEM model is employed to simulate the co-current gas-particle flow. Particle clusters are detected and characterised in our CFD-DEM simulation results and compared to experimental data. Core-annulus flow is well predicted by the model and other hydrodynamic parameters such as cluster frequency, internal solids volume fraction and aspect ratio were also in good agreement with experimental data.

A novel micro-structured bubble column reactor (MSBC) is studied by a combined experimental and simulation approach, for the chemisorption of CO 2 into NaOH. To understand the reactor behavior different reactor configurations are tested for chemisorption by varying the internals (single wire mesh, staged wire meshes, Sulzer packing). Bubble size distribution, pH and holdup data are obtained from chemisorption experiments using an advanced digital image analysis technique. The effect of superficial gas velocities ranging from 5 to 25 mm/s and varying number of wire mesh stages (1, 6 and 10) is deduced from the experiments. The experimental results are used for the validation of the detailed VoF-DBM simulations.

The hydrodynamics and heat transfer of cylindrical gas-solid fluidized beds for polyolefin production was investigated with the two-fluid model (TFM) based on the kinetic theory of granular flow (KTGF). It was found that the fluidized bed becomes more isothermal with increasing superficial gas velocity. This is mainly due to the increase of solids circulation and improvement in gas solid contact. It was also found that the average Nusselt number weakly depends on the gas velocity. The TFM results were qualitatively compared with simulation results of computational fluid dynamics combined with the discrete element model (CFD-DEM). The TFM results were in very good agreement with the CFD-DEM outcomes, so the TFM can be a reliable source for further investigations of fluidized beds especially large lab-scale reactors.

The effect of normal restitution coefficient and friction coefficient on the hydrodynamics of a dense bubbling solid-gas fluidized bed is investigated using a two fluid model (TFM) based on our kinetic theory of granular flow (KTGF) for rotating frictional particles. A comparison between TFM simulations using the present KTGF model, and a simpler KTGF model for rapid flows of slightly frictional, nearly elastic spheres derived by Jenkins and Zhang [1], is carried out. The simulation results reveal that both the coefficient of normal restitution and friction coefficient play an important role in the homogeneity of the bubbling bed. The particle friction has a strong effect on the solids flow patterns and distribution, while the normal restitution coefficient has a relatively small effect on both. The present model also predicts a larger amount of energy dissipation caused by the inclusion of particle friction. The present KTGF model leads to better agreement with detailed discrete particle model (DPM) simulation results for the axial particle velocity profiles and solids volume fraction distribution.

In this work, the influences of non-spherical catalyst particles on the enhancement of the gas-liquid mass transfer are numerically studied. The 3D unsteady diffusion-reaction equation both in the liquid phase and the solid catalyst particles is solved. A pseudo-homogenous model is employed as well. It is found that the SV cylindrical particles increase enhancement factor while SRSV cylindrical particles decrease it in comparison with the corresponding spherical particles. The horizontal cylinder and the vertical cylinder show small differences. The enhancement factor increases with catalyst concentration, rate coefficient of reaction and decreasing particle size. The data of pseudo-homogenous model agree well with the corresponding results of the fully resolved simulations.

In this paper a scaling method is proposed for scaling down the prohibitively large number of particles in CFD-DEM simulations for modeling large systems such as circulating fluidized beds. Both the gas and the particle properties are scaled in this method, and a detailed comparison among alternative mapping strategies is performed by scaling both the computational grid size and the riser depth. A series of CFD-DEM simulations has been performed for a pseudo-2D CFB riser to enable a detailed comparison with experimental data. By applying the scaling method, the hydrodynamic flow behavior could be well predicted and cluster characteristics, such as cluster velocity and cluster holdups agreed well with the experimental data. For a full validation of the scaling method, four mapping conditions with different ratios of the grid size and particle volume and of modified ratio of riser depth to particle size were analyzed. The results show that in addition to hydrodynamic scaling of the particle and fluid properties, scaling of the dimensions for the interphase mapping is also necessary. Keywords: Scaling method, CFB riser, CFD-DEM, Hydrodynamics, Clusters