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We investigate creeping flow of a viscoelastic fluid through a three dimensional random porous medium using computational fluid dynamics. The simulations are performed using a finite volume methodology with a staggered grid. The no slip boundary condition on the fluid-solid interface is implemented using a second order finite volume immersed boundary (FVM-IBM) methodology [1] . The viscoelastic fluid is modeled using a FENE-P type model. The simulations reveal a transition from a laminar regime to a nonstationary regime with increasing viscoelasticity. We find an increased flow resistance with increase in Deborah number even though shear rheology is shear thinning nature of the fluid. By choosing a length scale based on the permeability of the porous media, a Deborah number can be defined, such that a universal curve for the flow transition is obtained. A study of the flow topology shows how in such disordered porous media shear, extensional and rotational contributions to the flow evolve with increased viscoelasticity. We correlate the flow topology with the dissipation function distribution across the porous domain, and find that most of the mechanical energy is dissipated in shear dominated regimes instead, even at high viscoelasticity. [ABSTRACT FROM AUTHOR]

Gas-liquid-solid flows are frequently encountered in chemical, petrochemical and biochemical industries. To overcome the heat and mass transfer limitations in trickle bed reactors and bubble slurry columns, respectively, a micro-structured bubble column (MSBC) can serve as an attractive alternative. In a MSBC, wire meshes are introduced to cut the bubbles in smaller bubbles and enhance the surface renewal (and hence gas-liquid mass transfer) rates, by deformation of the bubbles. Earlier (Jain et al., 2013) modeling efforts using the Euler-Lagrange approach to simulate a micro-structured bubble column employed a bubble cutting closure based on purely geometrical considerations. To improve on this ad hoc procedure in this paper we explore the possibilities of Direct Numerical Simulations to gain more insight in this complex phenomenon with the ultimate aim to develop improved closures. A combined Volume of Fluid-Immersed Boundary method was applied to simulate the interactions between bubbles and wire meshes. When the bubbles are aligned with the opening of the wire mesh, cutting of the bubbles is not observed in our simulations, while cutting was expected based solely on geometrical considerations. When the Eötvös number, Eo , is larger than 4, the bubbles are highly deformable and squeeze themselves through the opening of the wire mesh. In addition, the bubble gets stuck underneath the mesh when the bubbles are small ( Eo ⩽ 4 ) and/or the opening is in the wire mesh is small. Almost all bubbles that hit the intersection of two crossing wires get stuck underneath the mesh, except for large bubbles ( Eo = 15 ), which get cut by the mesh. Based on these results, it is concluded that the cutting of bubbles depends on the Eötvös number, the opening of the wire mesh and geometrical considerations. However, the results also seem to indicate that the diameter of the wire mesh will also influence the cutting behavior. [ABSTRACT FROM AUTHOR]

Gas–liquid–solid three phase flows are commonly found in (bio-)chemical processes, e.g. in bubble slurry columns in the Fischer–Tropsch process. In order to facilitate the rational scale-up and the design of such columns, a detailed understanding of the complex phase interactions is required. In this work, we focus on the effective drag acting on particles and bubbles, using Direct Numerical Simulations. We combined the Front Tracking method of Roghair et al. (2013b) and the second order implicit Immersed Boundary method of Deen et al. (2012) . The resulting hybrid Front Tracking Immersed Boundary method is able to simulate dense three phase flows and is used to study swarm effects in terms of drag. A correlation has been obtained for the drag coefficient for a system consisting of 2 mm bubbles and 1 mm particles at several phase volume fractions. In future research, the developed methodology can be applied to study the effect of the bubble and particle size and their ratio. [ABSTRACT FROM AUTHOR]

In this work we study heat transfer from a complex geometry consisting of crossing cylinders subject to forced convection. For several sizes of a wire-mesh inserts direct numerical simulations (DNS) are performed using an implicit implementation of the immersed boundary method (IBM). The local heat flux is studied and compared to the total heat flux. A heat transfer correlation is derived describing the Nusselt number as a function of the Reynolds number and the ratio of the pitch to the cylinder diameter, p / D . [ABSTRACT FROM AUTHOR]

In this work, two powerful methods are combined for bubble column simulations namely, the volume of fluid and the discrete bubble model. While the former method takes care of the free surface, the discrete bubble model tracks and handles the dynamics of the dispersed bubbles. The hybrid model presented in this work is verified and validated with existing established experimental results. A model parameter study for bubble break-up and coalescence is performed to find the optimum values of the model parameters, i.e. the critical Weber number and the coalescence calibration factor. [ABSTRACT FROM AUTHOR]

Abstract: In this paper a simulation model is presented for the direct numerical simulation (DNS) of wall-to-liquid heat transfer in dispersed gas–liquid two-phase flow using a volume of fluid (VOF) approach. Our model extends the VOF model developed by van Sint Annaland et al. (2005) to non-isothermal conditions. Our VOF method involves an interface reconstruction technique based on piecewise linear interface representation. The surface tension is incorporated using a three-dimensional version of the continuous surface force (CSF) model of Brackbill et al. (1992). The model is applied to predict the heat exchange rate between the liquid and a hot wall kept at a fixed temperature. It is found that, due to bubble induced agitation in the liquid phase, the heat transfer rate between the wall and the liquid is considerably enhanced especially when bubble coalescence prevails in the vicinity of the hot wall. Although this finding is obvious from an intuitive point here we present quantitative results to describe this phenomenon. [Copyright &y& Elsevier]

Abstract: Bubble column reactors suffer from bubble coalescence resulting in decreasing efficiency. In order to increase the mass transfer rate in bubble column reactors, a novel reactor is proposed, the micro-structured bubble column (MSBC). The new reactor makes use of a wire mesh serving the purpose of cutting bubbles into smaller bubbles. This increases bubble surface area and enhancing the mass transfer coefficient, due to the enhanced surface dynamics. The first step is understanding of fluid–structure interaction in the limiting case of single-phase flow past crossing cylinders not found in literature. To this end Direct Numerical Simulations (DNS) are performed, where the presence of the wire mesh consisting of intersecting cylinders is accounted for using an Immersed Boundary Method (IBM). The IBM is verified with existing literature data for flow past a single cylinder. Finally a new set of simulations is done for intersecting cylinders varying in cylinder diameter and Reynolds numbers. From these simulations a new drag relation is derived describing the drag as function of the Reynolds number and the pitch to cylinder diameter, p/D, ratio. [Copyright &y& Elsevier]

The effect of elevating the spout on the dynamics of a spout-fluidized bed, both numerically and experimentally is studied. The experiments were conducted in a pseudo-two-dimensional (2-D) and a cylindrical three dimensional (3-D) spout-fluidized bed, where positron emission particle tracking (PEPT) and particle image velocimetry (PIV) were applied to the pseudo-2-D bed, and PEPT and electrical capacitance tomography (ECT) to the cylindrical 3-D bed. A discrete particle model (DPM) was used to perform full 3-D simulations of the bed dynamics. Several cases were studied, that is, beds with spout heights of 0, 2, and 4 cm. In the pseudo-2-D bed, the spout-fluidization and jet-in-fluidized-bed regime, were considered first, and it was shown that in the spout-fluidization regime, the expected dead zones appear in the annulus near the bottom of the bed as the spout is elevated. However, in the jet-in-fluidized-bed regime, the circulation pattern of the particles is affected, without the development of stagnant zones. The jet-in-fluidized-bed regime was further investigated, and additionally the experimental results obtained with PIV and PEPT were compared with the DPM simulation results. The experimental results obtained with PIV and PEPT agreed mutually very well, and in addition agreed well wtih the DPM results, although the velocities in the annulus region were slightly over predicted. The latter is probably due to the particle-wall effects that are more dominant in pseudo-2-D systems compared with 3-D systems. In the jet-in-fluidized-bed regime, the background gas velocity is relatively high, producing bubbles in the annulus that interact with the spout channel. In the case of a non elevated spout, this interaction occurs near the bottom of the bed. As the spout is elevated, this interaction is shifted upwards in the bed, which allows the bubbles to remain undisturbed providing the motion of the particles in the annulus near the bottom of the bed. As a result, no dead zones are created and additionally, circulation patterns are vertically stretched. These findings were also obtained for the cylindrical 3-D bed; although, the effects were less pronounced. In the cylindrical 3-D bed the PEPT results show that the effect on the bed dynamics starts at hspout =1 4 cm, which is confirmed by the ECT results. Additionally, ECT measurements were conducted for hspout =1 6 cm to verify if indeed the effect happens at larger spout heights. The root mean square of the particle volume fraction slightly increased at hspout =1 2 cm, whereas a larger increase is found at hspout = 4 and 6 cm, showing that indeed more bubbles are formed. The presented results have not been reported so far and form valuable input information for improving industrial granulators. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2524-2535, 2012 [ABSTRACT FROM AUTHOR]

In the second wind-induced regime, the jet deformations are influenced by aerodynamic instabilities acting on the jet surface. In addition, velocity relaxation in the liquid is an important mechanism with a strong influence on the violent break-up of laminar jets (i.e., the bursting). The exact interplay between the aerodynamic forces and the velocity profile relaxation is insufficiently described in the literature. Thus, we investigate numerically cylindrical liquid jets with fully developed and uniform velocity inlet profiles for different liquid and gas properties to elucidate the physical mechanisms governing the bursting of the jet. For this study, the gas-liquid interface is tracked using a front tracking technique: the Local Front Reconstruction Method (LFRM). In contrast with traditional front tracking techniques, LFRM allows for complex topological changes (merging and break-up) while keeping a sharp representation of the interface. The comparison of the obtained results with literature suggests that LFRM is a suitable technique for capturing the complex morphological deformations of a bursting jet. Additionally, we analyzed the influence of air and liquid properties on the jet deformations, which evidenced that velocity profile relaxation plays an important role in the complex process of break-up of the jet, that in combination with the destabilizing effect of the aerodynamic forces lead to the bursting of the jet. • A cylindrical jet is simulated using LFRM allowing for a sharper definition of the interface than front capturing methods. • The capabilities of LFRM at capturing the cylindrical jet in the second-wind induced regime are evaluated. • The results suggest that LFRM is a well-suited method for simulating the jet deformations in the second wind-induced regime. • The effect of velocity profile relaxation is investigated by studying fully developed and flat velocity profiles. • The effect of air and liquid properties (i.e., density, viscosity, and inertia) on the jet deformations are studied. [ABSTRACT FROM AUTHOR]

Direct Numerical Simulations have been performed to study the droplet spreading behaviour on a spherical surface. A coupled immersed boundary and volume of fluid method is used to represent the gas-liquid-solid interactions. The contact area of the droplet on the surface is recorded in order to fit the initial spreading with a power-law representation, using the contact-angle and interface curvature as fitting parameters. Small viscous droplets are used to reduce interfacial oscillations as well as low drop velocities to reduce impact forces. A decrease of spreading area with increasing curvature is observed. Moreover, the model shows good agreement compared to equilibrium states. A strong contact-angle dependence is found for the pre-factor of the power law, which is expected, and a linear decrease was found in the exponent for increasing curvature of the surface. Unlabelled Image • The spreading dynamics change with increasing surface curvature. • Two spreading regimes become apparent at increased curvatures and make the fitting of the power law impossible. • Due to the reduced spreading area for increased curvatures, droplet oscillations are more pronounced. • Less viscous spreading occurs at higher surface curvatures. [ABSTRACT FROM AUTHOR]

• Fully resolved simulations of fluid-particle systems. • Surface reaction with significant heat effects. • Temperature-dependent reaction rates determined by the Arrhenius equation. • Simulation parameters adopted from realistic POX reaction. 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. [ABSTRACT FROM AUTHOR]

Extensive direct numerical simulations were performed to obtain the heat transfer coefficients (HTC) of bidisperse random arrays of spheres. We have calculated the HTC for a range of compositions and solids volume fractions for mixtures of spheres with a size ratio of 1:2. The Reynolds numbers are in the range of 30–100. It was found that the correlation of the monodisperse HTC can estimate the average HTC of bidisperse systems well if the Reynolds and Nusselt numbers are based on the Sauter mean diameter. We report the difference between the HTC for each particle type and the average HTC of the bed in the bidisperse system as function of solids volume fraction, Sauter mean diameter of the mixture, Reynolds number and investigate the heterogeneity of the individual particle HTCs. [ABSTRACT FROM AUTHOR]

In this work, a detailed three dimensional model is employed for the quantitative description of flow and coupled heat and mass transfer in a gas channel coated with porous adsorbent layer. The flow field of the gas stream is obtained by solving the Navier-Stokes equation. The highly coupled mass and heat transfer in both the gas channel and the adsorbent layer are locally described, and the accompanying adsorption/desorption dynamics in the adsorbent layer are modelled as well. A parametric study has been carried out, in which the influences of thermal conductivity of the adsorbent layer, specific heat, porosity, tortuosity, layer thickness and geometrical shape on the performance of moisture adsorption processes are investigated in an exhaustive way. The heat and mass transfer mechanisms in the investigated cases are thoroughly analysed taking advantage of the rigorous 3D model, which deepens our understanding on the intricately coupled transport processes. The results reported in this work are useful for rational design and optimization of adsorption processes in adsorbent coated gas channels. [ABSTRACT FROM AUTHOR]

A fully resolved direct numerical simulation of flow and heat transfer is presented for slender randomly packed bed reactors. The flow and temperature field are solved over a non-body fitted, non-conformal Cartesian computational domain. The coupling between fluid and solid (both spherical particles and cylindrical wall) is enforced by a second order accurate, sharp interface immersed boundary method (IBM). The present numerical technique neither requires any challenging volumetric mesh generation process nor demands manipulation of the geometry near the particle-particle and particle-wall contact points. Conjugate heat transfer has been considered where the temperature field is calculated both inside the solid particles and in the fluid. A discrete element method (DEM) is used to generate the random packings of spherical particles in the cylindrical column, and a methodology is proposed to calculate the radial porosity profile of the bed. The column-to-particle diameter ratio ( N ) is varied from 4 to 8, and two separate cases have been considered where N → ∞ . The particle Reynolds number ( Re d ) is varied from 1 to 500. The numerically obtained pressure drop and overall wall-to-bed heat transfer coefficient for different simulation cases are critically compared with empirical correlations and a good agreement is reported. Moreover, based on the current numerical results, correlations are proposed for the pressure drop and the wall-to-bed heat transfer coefficient. The effect of the column-to-particle diameter ratio ( N ) on both the flow and heat transfer, as-well-as the effect of the solid to fluid thermal conductivity ratio on the conjugate heat transfer are discussed. Furthermore, the fully resolved accurate numerical simulations have helped to elucidate the detailed pore-scale flow and heat transfer feature in the packed beds. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Direct numerical simulation results of mass transfer in dense bubble swarms using a Front-Tracking (FT) model will be presented, where the effect of the gas hold-up has been investigated. The FT method is particularly suited for bubble swarm simulations, since bubbles do not coalesce artificially, but traditional FT techniques often suffer from artificial volume loss of the bubbles. For this reason, a specialized remeshing technique is presented to counteract any occurring volume defects, while keeping all physical undulations on the bubble surfaces unharmed. For the simulation of gas-to-liquid mass transfer, a species transport equation (convection–diffusion–reaction) was coupled to the FT hydrodynamics solver, which was solved on a superimposed refined mesh for higher accuracy. The velocity components have been interpolated to the refined grid using a higher-order solenoidal method. Enforcement of the Dirichlet condition for the concentration at the gas–liquid interface is achieved with an immersed boundary method, enabling the description of gas to liquid mass transfer. Careful validation of the newly implemented model shows satisfactory results. The liquid side mass transfer coefficient in dense bubble swarms, with gas fractions between 4% and 40%, has been investigated using the new model. The simulations have been performed in a 3D domain with periodic boundaries, mimicking an infinite swarm of bubbles. The results indicate that the liquid-side mass transfer coefficient rises only slightly with increasing gas fraction. [ABSTRACT FROM AUTHOR]

In the present contribution a sharp interface Immersed Boundary Method (IBM) for flow and heat transfer has been developed to fully resolve highly complex random solid structure on a non-body fitted Cartesian computational grid. A 3D image data set from a Micro-CT (computed tomography) scan of an actual foam geometry is usually converted into a surface mesh of unstructured triangular elements and the current frame work can embed it as an immersed boundary in the CFD domain. A second-order implicit (semi-implicit) method is used to incorporate fluid-solid coupling for flow (heat transfer) at the non-conforming immersed boundary in a structured grid. Both temperature Dirichlet and Neumann boundary conditions, as well as a conjugate heat transfer condition are incorporated at the fluid–solid interface. The proposed method is validated rigorously with existing literature results. The use of a Cartesian grid for flow makes this method robust, computational friendly, and at the same time it avoids the tedious volumetric mesh generation process for very complex shapes. To demonstrate the capability of the current method, it is applied to study flow and heat transfer of a typical foam sample with different flow properties. A closure for pressure drop and heat transfer coefficient are derived for the typical foam sample. [ABSTRACT FROM AUTHOR]

Direct numerical simulations are conducted to characterize the fluid–particle heat transfer coefficient in fixed random arrays of non-spherical particles. The objective of this study is to examine the applicability of well-known heat transfer correlations, that are proposed for spherical particles, to systems with non-spherical particles. In this study the spherocylinders are used to pack the beds and the non-isothermal flows are simulated by employing the Immersed Boundary Method (IBM). The simulations are performed for different solids volume fractions and particle sizes and low to moderate Reynolds numbers. Using the detailed heat flow pattern, the average heat transfer coefficient is calculated for the different operating conditions. The numerical results show that the heat-transfer correlation of spherical particles can be applied to all test beds of spherocylinders by choosing a proper effective diameter. Our results reveal that the diameter of a spherocylinder is the proper effective diameter for characterizing particle–fluid heat transfer. [ABSTRACT FROM AUTHOR]

Abstract: Gas–liquid–solid three phase flows are encountered in for example the Fischer–Tropsch process for the production of synthetic fuels in bubble slurry columns. To predict the hydrodynamics in large slurry bubble columns a multi-scale modeling approach can be used, which accounts for the large variation in time and length scales. In this paper, the smallest scale model has been developed using the Front Tracking model of Dijkhuizen et al. (2010b) and the Immersed Boundary model of Kriebitzsch (2011). In the Front Tracking model, each bubble is tracked separately. Furthermore, the Immersed Boundary method introduces the particle–fluid and the particle–particle (via a hard sphere model) interactions in the model. The resulting hybrid Front Tracking Immersed Boundary model is able to simulate dense three-phase flows and accounts for swarm effects in a fundamental manner. From our simulations we found that the relative drag coefficient of bubbles in three-phase flows seems to increase with increasing solids volume fraction. However, longer averaging periods are needed to derive a fully predictive correlation for the relative drag coefficient with respect to the solid volume fraction. [Copyright &y& Elsevier]

Abstract: We performed Direct Numerical Simulation (DNS) of a gas-fluidized bed using the Immersed Boundary method combined with traditional Computational Fluid Dynamics. We have compared the individual fluid–particle force with the force as evaluated in an unresolved Discrete Element Model (DEM) simulation using closures for the gas–solid force for one fluidisation condition. We find that the average DEM gas–solid force is about 33% smaller than the “true” value which follows from the DNS model. For more realistic DEM simulations, a modification of the gas–solid force calculation is required, for instance by adding fluctuations or including the effect of the granular temperature. [Copyright &y& Elsevier]

Abstract: Among many research studies on fluidized bed membrane reactors, only very few focus on the details of the effects of gas permeation through the membranes on the hydrodynamic properties of the membrane-assisted fluidized bed. For this reason, in the first part of this paper, a novel hybrid Immersed Boundary Method (IBM) Discrete Particle Model (DPM) has been developed. In the second part of the paper, this model is employed to investigate fluidized bed membrane reactors in more detail. Simulations without membrane inserts (having vertical membranes at the side walls instead), with membrane tubes in an in-line arrangement and membranes in a staggered arrangement are compared with each other. The time-averaged particle fraction maps display a lower bed height for the simulations with inserts. A very important aspect of the simulation with gas extraction/addition via the side walls is the solids circulation; an inversion of the solids circulation is observed when gas is added via the side walls with altered bubble properties. Because of this phenomenon, the bubble size counter-intuitively decreases when gas is added, and increases slightly when gas is extracted. The latter is a result of the existence of compacted zones near the membranes. For the simulation cases with membrane inserts, the solids circulation and bubble properties are different; in these cases they mainly depend on the local fluidization velocity, and not so much on the extent of permeation itself. Although the arrangement of the membrane tubes plays only a minor role in this matter, their presence has a pronounced influence on the bubble size. The system''s energy and energy dissipation are both smaller for the simulations with inserts compared to those with permeation via the side walls. Also in this respect, the local fluidization velocity is found to be more important than the extent of permeation. [Copyright &y& Elsevier]

Abstract: Many novel reactor concepts based on the integration of membranes in fluidized beds have been proposed. For the detailed numerical investigation of such systems, a method is required describing the motion of the particles while accounting for (i) the gas motion and (ii) the encounters between particles and confining walls/membranes. In this paper a novel hybrid computational method is presented, combining the Discrete Particle Model (DPM) and the Immersed Boundary Method (IBM). The first part focuses on the implementation and validation of these models. The IBM component of the model has been verified by comparing the computed drag coefficients for Simple Cubic (SC) arrays of cylinders with data of Sangani and Acrivos (1982) at creeping flow conditions. For higher Reynolds numbers the code has been compared to Ferziger and Peric (2002) for a single off-center cylinder, after which simulations with addition and extraction of gas through the membrane surface have been conducted. In the second part of this work we will focus on actual fluidized bed membrane systems, and will report results of simulations for several membrane geometries as well as permeation ratios. [Copyright &y& Elsevier]

Abstract: Gas–solid flows involving wet particles are very frequently encountered in a variety of applications in industries, which includes live problems in the field of spray granulation, coking, gas phase polymerization, etc. These processes can be studied with discrete element models. The predictive capabilities of these models depend on the accuracy of the particle–particle contact dynamics. Although this dynamics is well understood for dry particles, this is not the case for wet particles. The current study focuses on a particle colliding with a thin liquid film to mimic the wet particle collisions and understand the effect of system parameters on the (apparent) restitution coefficient. Our model combines the VOF model developed by and the Immersed Boundary (IB) model reported by . The Volume of Fluid (VOF) part features (i) an interface reconstruction technique based on piecewise linear interface representation (ii) a three-dimensional version of the continuum surface force (CSF) model of . The Immersed Boundary (IB) part incorporates the particle–fluid interaction via a Direct Forcing Method (DFM). The hybrid VOF-IB model results are demonstrated and verified for a wide spectrum of parameters, from the experimental findings presented by . [Copyright &y& Elsevier]

• A novel numerical technique to simulate exothermic catalytic packed bed reactors. • Fluid phase and solid phase events are intrinsically coupled. • Ignition/Extinction phenomena due to the multiplicity of steady states are captured. • The hot-spot formation in a wall cooled reactor is analyzed. • The model is free of empiricism with results dependent only on transport and kinetic values. A fundamental continuum-based numerical model was developed to simulate a non-isothermal non-adiabatic reactor which does not employ any empirical closures. The model was able to capture unique features of an exothermic catalytic reactor such as parametric sensitivity, hot-spot formations and multiplicity of steady states. Furthermore, the model inherently accounts for the various aspects of classical phenomenological models such as axial and radial dispersion of heat and mass and the intrinsic coupling of heat and mass transport between the fluid phase and the solid phase. The numerical procedure was validated with existing literature data before moving on to the simulation of a bed consisting of 340 spherical particles packed using the Discrete Element Method. Five simulations were performed by varying the rate of reaction and keeping all other parameters constant to capture the ignition/extinction phenomena exhibited by exothermic packed bed reactors. [ABSTRACT FROM AUTHOR]

Droplet-droplet interactions of highly viscous liquids or suspensions are relevant in a broad range of industrial applications, such as spray drying, fuel injection or, coating and granulation. The efficiency of these processes is heavily determined by the surface area and volume of the droplets. To operate these processes efficiently, it is critical to accurately describe the physical mechanisms dominating the collision. Nonetheless, a complete description of the forces governing the collision is still missing, particularly when the liquids have a high viscosity. Front tracking techniques represent a promising alternative for studying droplet-droplet interactions as they ensure a sharper representation of the liquid-gas interface. In this work, the Local Front Reconstruction Method (LFRM) is the front tracking technique chosen as it allows merging and break-up. The validation of this method is extended for different Weber numbers, impact parameters, and liquid properties by determining the capabilities of LFRM to reproduce the collision of glycerol solution droplets. The simulation results are validated with dedicated experiments performed at the same Weber and impact parameter conditions. In the majority of the studied collisions, LFRM shows a good correspondence with experiments and literature, which proves the ability of LFRM to capture droplet collisions under the studied conditions. • Binary droplet collisions simulated using the Local Front Reconstruction Method. • Sharp definition of the interface compared to front capturing methods. • Validation with glycerol droplet collision with Weber number up to 100. • Deformations of the interfaces are well captured even at high Weber numbers. • Energy conservation graphs are studied for the high Weber impact. [ABSTRACT FROM AUTHOR]

A novel experimental technique is introduced to study fluid-particle heat transfer in dense arrays of stationary particle. First a Constant Temperature Anemometer is reconfigured to a heat transfer probe. The thermal driving force is defined as the difference between the probe temperature and the initial fluid temperature and well known in our system. Second an abacus-like structure is employed to accurately control the solids volume fraction. The solids fraction was varied between 0 and 0.6 with increments of 0.1. The Reynolds number varied between 0 and 800. This combination of approaches allows for a very well-defined system, that can be studied both experimentally and numerically, and as such can serve as a validation of heat transfer studies with Direct Numerical Simulations, in fluid-particle systems. A single particle in unbounded flow, the effect of inter-particle distance and shielding effects for an inline array of three spheres as well as semi-structured arrays of particles are studied. [ABSTRACT FROM AUTHOR]

This paper reports our study on fluidization of 5000 spherical particles in a pseudo-2D gas-fluidized bed by direct numerical simulations (DNS) and experiments as well. Simulations are performed using an immersed boundary method, together with the methodology developed in our earlier work for accurate prediction of gas–solid interactions at relatively low grid resolutions. This modelling approach provides detailed information on the gas flow and the motion of individual particles, which allows for a priori calculation of the bed hydrodynamics. Experimental measurements of solids mean motion are conducted using a combined technique of Particle Image Velocimetry (PIV) and Digital Image Analysis (DIA). Further, the PIV technique is extended and applied for instantaneous measurements of the particle granular temperature, which is the key characteristics of particle velocity fluctuations. For the first time, this paper reports a direct comparison in great detail between DNS results and experimental data for realistic gas fluidization. The detailed comparison reveals a reasonably good agreement with respect to the time-averaged solids motion and the pressure fluctuations. In addition, the granular temperatures calculated from the simulations agree well with the experimental data, but provide more details with respect to the variations corresponding to bubble formation and eruption. From our investigation, it also becomes clear that attention should be paid on the measurement and interpretation of the granular temperature. [ABSTRACT FROM AUTHOR]

Abstract: We numerically study the impact of a large sphere dropping into a prefluidized granular bed using a state-of-the-art hybrid discrete particle and immersed boundary (DP-IB) method. For the first time, both the gas-induced drag force and the contact force exerted on the intruder are investigated separately. Our results show that even for relatively large granular particles of 0.5mm diameter, namely Geldart B particles, and an intruder of 1cm, the drag exerted by the interstitial gas accounts for up to 5% of the total force experienced by the intruder. Our simulation results match well with existing experimental observations. This work shows that the current simulation scheme could become a tool to investigate the effect of interstitial gas on the dynamics of projectile impact cratering. More generally, the method allows for accurate simulation of the hydrodynamic effects of large internal objects moving through (pre-)fluidized granular beds. [Copyright &y& Elsevier]

Highlights: [•] Direct numerical simulation is used for non-isothermal gas–solid systems. [•] Extension of the immersed boundary method to heat transfer applications. [•] Nusselt numbers are computed for flows around arrays of spheres. [•] Results are in reasonable agreement with empirical heat correlations. [•] The numerical method is efficient for problems with complex geometries. [ABSTRACT FROM AUTHOR]

Abstract: In blast furnaces, particles like coke, sinter and pellets enter from a hopper and are distributed on the burden surface by a rotating chute. Such particulate flows suffer occasionally from particle segregation during transportation caused by differences in density or size. To get a more fundamental insight into these effects, we started an experimental study to investigate the effect of rotation on such particulate flows. Here, as a first step, we present an experimental study of granular flow of monodisperse 3mm spherical glass particles flowing with a constant mass rate through a rotating smooth rectangular chute, which is inclined at a fixed angle. Experiments are performed for a sufficiently long time to study steady (but streamwise accelerating) flow. Particle image velocimetry (PIV), electronic ultrasonic height sensors, and a dynamic weighing scale are used to measure the surface velocity of the particle stream, the particle bed height and mass flow rate in the chute, respectively. The influence of rotation speed and angle of inclination of the chute is studied. We find an interesting interplay between the Coriolis force, which pushes the granular flow to the sidewall of the chute and tends to diminish the acceleration of the flow, and centrifugal forces that accelerate the flow. The velocity components display a complex dependence on the spanwise and streamwise position in the chute. The bed height in the chute is also influenced by these effects of system rotation. These measurements provide a well-defined set of observations for refining and validating computer simulations of granular flows, and point out some important limitations of physical experiments. We present preliminary three-dimensional discrete particle simulations, which show that the experimental measurements of bed height and surface particle velocity in a chute inclined at 30° can be predicted nearly quantitatively both without and with rotation of the chute. [Copyright &y& Elsevier]

Abstract: Numerical simulations of bubbly flows in industrial process equipment rely for an important part on accurate modeling of the drag force. The drag acting on bubbles determines the rise velocity to a very large extent and hence the inter-phase mass transfer characteristics. Recently, we have proposed a correlation (Roghair et al., 2011a) to describe the drag on bubbles in a mono-disperse bubble swarm up to high gas fractions, and in this work we have extended this work to simulations of dense bi-disperse bubble swarms. The influence of the gas hold-up is determined by simulating a bi-disperse swarm of 8 relatively small bubbles (3.5mm) and 8 relatively large bubbles (4.4mm) in a periodic domain. The resulting drag coefficient acting on the bubbles agrees very well with the earlier proposed correlation. The composition of the swarm has first been varied in different ways; first, the ratio of the number of large to small bubbles was varied. In this case, the resulting drag coefficient remained constant indicating that no influence of the number of large and small bubbles is expected, with a typical deviation from the mono-disperse correlation up to 15%. Second, the composition of the bi-disperse swarm has been changed by altering the diameter ratio. These simulations showed a good agreement with the mentioned correlation as well. In general, it can be said that the mono-disperse correlation describes the drag coefficient of the bubbles in a bi-disperse swarm well if an error margin in the order of 15% is allowed for. [Copyright &y& Elsevier]

Abstract: In this paper a novel simulation technique is presented to perform Direct Numerical Simulation (DNS) of fluid flow and heat transfer in dense fluid–particle systems. The unique feature of our fluid–solid coupling technique is the direct (i.e., implicit) incorporation of the boundary condition (with a second-order method) at the surface of the particles at the level of the discrete momentum and thermal energy equations of the fluid. Contrary to lattice Boltzmann or other commonly used immersed boundary implementations, our method does not require using any effective diameter. A fixed (Eulerian) grid is utilized to solve the Navier–Stokes equations for the entire computational domain. Dissipative particle–particle and/or particle-wall collisions are accounted via a hard sphere discrete particle approach using a three-parameter particle–particle interaction model accounting for normal and tangential restitution and tangential friction. Following the detailed verification of the method several dense multi-particle systems are studied in detail involving stationary arrays of particles and fluidized particles. [Copyright &y& Elsevier]

Abstract: The present paper describes an Euler–Lagrange model utilizing a drag closure derived from direct numerical simulations (front-tracking model) for (i) single isolated bubbles and (ii) bubbles rising in bubble swarms, expressed as a function of the local gas fraction. The model is applied to the prediction of an air/water system in a bubble column and for which experimental data is available. The effect of variation in size of the mapping window for the interphase coupling between the Eulerian and Lagrangian framework is investigated for both closure relations. It is found that the drag closure as a function of the local gas fraction is an improvement over the use of the drag closure for isolated single bubbles for the prediction of bubbly flow. [Copyright &y& Elsevier]

Abstract: In recent years CFD has proven to be a valuable and powerful tool for advancing our understanding of complex multiphase flow systems arising in industrial applications. However, the predictive capabilities of this tool are determined by many factors of physical and numerical origin but in particular by the quality of the closures adopted for the description of the interface forces. The objective of this study is to use direct numerical simulations to validate and improve these closures using an improved front tracking model. We have studied the drag force on single air bubbles rising in viscous liquids over a wide range of viscosities. Dedicated experiments were conducted to validate the model and to highlight the effect of contaminants. The results show an excellent agreement between the numerical simulations and available analytical theory, whereas existing drag force correlations and the in-house experiments (using liquids with the same physical properties) gave a much higher drag force. This demonstrates the important effect of contaminants on the drag force, which is an important subject for future research. [Copyright &y& Elsevier]