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A population balance model appropriate for the shear flow of thixotropic colloidal suspensions with yield stress is derived and tested against experimental data on a model system available in the literature. Modifications are made to account for dynamic arrest at the onset of the yield stress, in addition to enforcing a minimum particle size below which breakage is not feasible. The resulting constitutive model also incorporates a structural based relaxation time, unlike existing phenomenological models that use the inverse of the material shear rate as the relaxation time. The model provides a reasonable representation of experimental data for a model thixotropic suspension in the literature, capturing the important thixotropic timescales. When compared to prevalent structure kinetics models, the coarse-grained population balance equation is shown to be distinct, emphasizing the novelty of utilizing population balances as a basis for thixotropic suspension modeling. © 2016 American Institute of Chemical Engineers AIChE J, 63: 517-531, 2017 [ABSTRACT FROM AUTHOR]

If a particle is in contact with another solid electric charge may be exchanged. Previous experimental observations clearly prove that the amount of exchanged charge does not only depend on the charge carried by the particle prior to the contact but also on its local distribution on the particles surface. However, most existing modeling approaches utilize the assumption of a uniformly charged particle. In the present paper we propose a new model which accurately accounts for the charge distribution on the particles surface during particle-wall and inter-particle collisions. Further, the results obtained with the new model are compared to data of single particle charging experiments. Moreover, we implemented the model in a Computational Fluid Dynamics (CFD) simulation of powder pneumatically transported in a pipe. We show that the charging behavior of the powder is quantitatively and qualitatively different if a non-uniform charge distribution is taken into account. [ABSTRACT FROM AUTHOR]

We propose a novel scheme to handle moving boundaries, both straight and curved, within the lattice Boltzmann method (LBM). In this scheme, which is broadly based on the Chapman-Enskog expansion, the fictitious distributions are constructed exactly on the moving boundary. This is in contrast to existing methods where such distributions are constructed on neighboring nodes which may not lie on the moving boundary. The post-collisional distributions on the fluid nodes near the moving boundary are then constructed using first- or second-order interpolations. The proposed scheme also overcomes the requirement to have separate interpolation formulations for different values of the intersection parameter. Several validation tests presented here indicate improved accuracy and numerical stability, compliance with Galilean invariance principle, an ability to preserve the geometric fidelity of curved surfaces. [ABSTRACT FROM AUTHOR]

The triboelectric charging of powders and hoses during pneumatic transport can cause hazardous spark discharges. In this work we employ numerical simulations to gain a deeper understanding in the physical processes involved in the build-up of electrostatic charge. To this end, Large Eddy Simulations are performed to simulate the turbulent flow of the gaseous phase while the particulate phase is described in Lagrangian framework. Dynamic models accounting for the particle-wall and particle-particle charge exchange are implemented and a model describing the charge distribution in an insulating hose is also developed. Our numerical results are in good agreement with available experimental data, while our simulations provide additional physical insight and information about the charging process. The results presented herein can be used to improve the safety of operation of many pneumatic components in process engineering. [ABSTRACT FROM AUTHOR]

One of the challenges in the numerical modelling of chemically reacting multi-phase flows, e.g. including solid particles and a gas phase, is the complexity of transport processes occurring on the particle surface. This complexity is the result of coupling between the gas flow in the void spaces, which is three-dimensional a priori, and heterogeneous reactions on the particle surface. This complexity and three-dimensionality of the flow requires the use of computationally expensive numerical grids to resolve the particle surface and the void space between particles. Thus, as of yet it is not feasible to simulate the heat and mass transfer in different fixed-bed reactors (with as many as hundreds of thousands of particles) resolving all particles. At the same time, the accuracy of existing 1-D models depends on the empirical closure relations describing the heat and mass transfer between particles and the fluid flow. In this view, one way to solve this problem applied to a structured packed bed is to approximate the particulate beds using 2-D geometry, which will still allow us to calculate the heat and mass transfer directly without using any empirical relations. Thus, the main subject of this work is the development and validation of an axisymmetric representative element approximating the heat and species transport, and gas flow in a 3-D structured fixed bed with different porosities. Comparative analysis of 3-D and 2-D simulations shows acceptable agreement. Finally, we explore the influence of the void fraction on the gas-phase temperature behaviour between particles. [ABSTRACT FROM AUTHOR]

Low concentration particle transport in multiphase horizontal pipes in the presence of a viscous liquid is experimentally investigated. The experiments were conducted for a wide range of liquid and gas flow rates in both intermittent and stratified flows. Critical flow rates (velocity) is defined as the minimum required liquid and gas flow rates (velocities) to keep particles constantly moving in the pipe. The effects of physical parameters such as sand concentration, sand size, pipe size, and liquid viscosity are also experimentally investigated. It is observed that that critical velocity is a function of sand concentration and sand size and increases by increasing either within the ranges of particle concentrations and sizes examined. Regarding the effect of carrier liquid viscosity, the experimental data reveal that by increasing viscosity the minimum required flow rates to constantly move sand along the pipe increases within the range examined. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1821-1833, 2016 [ABSTRACT FROM AUTHOR]

The effects of varying the elastic modulus, coefficient of restitution, and coefficient of friction of adhesive particles on fluidized bed dynamics have been investigated via numerical simulations. It is found that lower values of the elastic modulus and coefficient of restitution lead to a greater degree of particle clustering, and the formation of smaller bubbles. Coordination numbers are found to initially increase, and then fall, with increasing coefficient of friction, while bubble velocities follow the opposite trend. It is concluded that artificially reducing the elastic modulus of adhesive particles has a significant impact on the fluidization behavior. The change in dynamics of the fluidized bed due to varying the coefficient of friction is more complex: particle clustering increases up to a point, beyond which clusters become increasingly rigid. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1467-1477, 2016 [ABSTRACT FROM AUTHOR]

Solid particles of nonuniform sizes exhibit behaviors not seen in monodisperse distributions. Following a previous study of monodisperse particles, the goal of this study is to investigate effects of a binary-size distribution on erosion of lunar soil from a rocket landing. The discrete element method is used to examine near-field (near impingement) and far-field behavior. For near-field simulations, small particles preferentially collide with large particles from below, due to a difference in velocity caused by gas forces, which is analogous to behavior for entrainment in gas-solid fluidized beds. These collisions cause vertical momentum transfer from small to large particles that affects species and total erosion flux. For far-field simulations, collisions result in an increase in vertical dispersion (binary) and a reduction in maximum horizontal velocity (binary and monodisperse). These results have implications on the most appropriate modeling framework to use for predictions over large distances utilizing wide size distributions. © 2015 American Institute of Chemical Engineers AIChE J, 62: 984-995, 2016 [ABSTRACT FROM AUTHOR]

The rigid motions of powders widely exist in powder-based laser additive manufacturing, like powder deposition, entrainment and spattering. These phenomena simultaneously happen as the laser melts these powders, and can have significant impacts on the quality of the printed products. However, existing models can hardly efficiently reproduce the co-existence of melting and rigid motion which greatly influences the molten pool and molten track evolution. The bottleneck therein lies in the demand that unresolved Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) has on the CFD grid size, that is, larger than 3 times of the particle diameter. However, this large size ratio (CFD grid cell vs particle diameter) handicaps the description of the particle deformation during melting. Hence in this paper, a kernel approximation-based semi-resolved CFD-DEM numerical model that allows the refinement of CFD grid is employed, and then coupled with the Volume of Fluid (VOF) method to reproduce the molten metal-gas interface. On this basis, the rigid motions of the powders, like entrainment and spattering, are finally simultaneously realized as the molten pool evolves at high efficiency. Powders remained in the unclosed molten pool trail defect are well explained with the proposed model as well. This model can also be applied to simulate the particle-fluid interactions in Directed Energy Deposition. It is believed that this semi-resolved VOF-DEM Additive Manufacturing model will serve as a good tool for future investigations into the particle behaviors in powder-based additive manufacturing. [Display omitted] • Semi-resolved VOF-DEM Additive Manufacturing model is developed. • Particle rigid motion is simultaneously modeled with molten pool evolution. • Spattered powders left in the defect are well explained. • Modeling of complex particulate flows in Directed Laser Deposition is realized. [ABSTRACT FROM AUTHOR]

We present an adaptive high-order accurate time-stepping numerical scheme for the flow of vesicles suspended in Stokesian fluids. Our scheme can be summarized as an approximate implicit spectral deferred correction (SDC) method. Applying a fully-implicit SDC scheme to vesicle flows is prohibitively expensive. For this reason we introduce several approximations. Our scheme is based on a semi-implicit linearized low-order time stepping method. (Our discretization is spectrally accurate in space.) We expect that the accuracy can be arbitrary-order, but our examples suffer from order reduction which limits the observed accuracy to third-order. We also use invariant properties of vesicle flows, constant area and boundary length in two dimensions, to reduce the computational cost of error estimation for adaptive time stepping. We present results in two dimensions for single-vesicle flows, constricted geometry flows, converging flows, and flows in a Couette apparatus. We experimentally demonstrate that the proposed scheme enables automatic selection of the time step size and high-order accuracy. [ABSTRACT FROM AUTHOR]

There is large uncertainty in the radiative forcing induced by aircraft contrails, particularly after they transform to cirrus. It has recently become possible to simulate contrail evolution for long periods after their formation. We review the main physical processes and simulation efforts in the four phases of contrail evolution, namely the jet, vortex, vortex dissipation, and diffusion phases. Recommendations for further work are given. [ABSTRACT FROM AUTHOR]

The dynamics of the loop seals in a large-scale dual fluidized bed (DFB) system is investigated as a function of variations in the flux of the bed material through the seal and changes in the bed material density. These investigations are performed numerically with a computational fluid dynamics (CFD) model and experimentally for the loop seals of the Chalmers 2-4 MWth DFB gasifier. Both experiments and simulations show that more of the aeration gas leaves the loop seal in the direction of the solids when a low-density bed material (silica) is used rather than a high-density one (bauxite). The simulations also reveal homogeneous fluidization in a vertical connection to the loop seal, whereas an inclined connection yields heterogeneous fluidization. The minor discrepancies between the experiments and simulations with silica are attributed to particle agglomeration, and it is proposed that CFD models applied to loop seals should account for this phenomenon. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3580-3593, 2015 [ABSTRACT FROM AUTHOR]

A numerical approach is presented for the direct numerical simulation of particle motion that combines the MacCormack scheme and the immersed boundary method. It exhibits the advantageous features of the explicit MacCormack scheme which is second-order accurate in time and space with simplicity in programing. The approach solves the compressible Navier–Stokes equations and uses the immersed boundary method to tackle the interactions between the fluid and the suspended particles. The force due to the interaction of two phases is computed via an elastic forcing method. The numerical approach is validated using uniform flow past a stationary circular cylinder, sedimentation of circular discs, and particle motion (orientation and translation) in unidirectional flows. Results are also compared to simulation obtained from a mixture model for solid particles for the same flow conditions. [ABSTRACT FROM AUTHOR]

A new model is proposed to predict minimum flow rates required to constantly move particles in both intermittent and stratified flow regimes. The new model consists of a single-phase flow model along with an appropriate length scale to be extended to multiphase flow regime. A comparison of the new model with experimental data in a multiphase air-water flow shows that the new model is able to predict minimum flow rates well for a wide range of operating conditions. The new model can capture the effects of particle size, particle concentration, and pipe size as confirmed by experimental data. A comparison of the new model with previously proposed models in the literature shows that the new model improves critical velocity predictions significantly. Moreover, the new model is the only model that takes into account the effect of particle concentration and can predict critical velocity in both intermittent and stratified flow regimes. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2634-2646, 2015 [ABSTRACT FROM AUTHOR]

Quantitatively predicting segregation of size-disperse granular materials is of potential value in many industrial applications. We consider granular segregation of size-bidisperse particles in quasi-2D bounded heaps, a canonical granular flow, using an advection-diffusion transport equation with an additional term to account for particle segregation. The equation is characterized by two dimensionless parameters that are functions of control parameters (flow rate, system size, and particle sizes) and kinematic parameters (flowing layer depth, diffusion coefficient, and percolation length scale). As the kinematic parameters are usually difficult to measure in practice, their dependence on the control parameters is determined directly from discrete element method simulations. Using these relationships, it is possible to determine which values of the control parameters result in a mixed or segregated heap. The approach used here is broadly applicable to a wide range of other flow geometries and particle systems. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1524-1534, 2015 [ABSTRACT FROM AUTHOR]

The unresolved model is widely applied in the CFD-DEM coupling simulation for particulate flows due to its lower computational cost compared with the resolved model. However, it requires that the size ratio of a particle to the located CFD cell is small enough, usually less than 1/3, otherwise the precision and stability of the simulation cannot be guaranteed. In this work, a semi-resolved model was developed using a two-way domain expansion method, in which both the dependent domain and influential domain of a considering particle were expanded from the particle-located CFD cell. A weighting factor using the Gauss kernel function was applied to interpolate the ambient fluid variables of a considering particle from all the CFD cells in the dependent domain. The interphase forces and the volume of particles were allocated to the CFD cells in the influential domain. A hierarchical searching algorithm was established to efficiently index the CFD cells located in the expanded domain. The developed model was first employed to simulate the settling of a single spherical particle in liquid driven by gravity. The results were compared with the experiments to validate the two-way domain expansion method and verify the model performance on eliminating the limitation of the unresolved model. The model performance was further investigated and validated in the test cases of the granular group settling. The analysis of the vortex structures for cases with different fluid volume fractions focused on validating the capability of the updated model to simulate the complex fluid flow induced by the granular group settling. [ABSTRACT FROM AUTHOR]

A number of studies have modeled flow and mixing of granular materials using the discrete element method (DEM). In an attempt to reduce computational costs, many of these DEM studies model particles larger than the actual particle size without investigating the implications of this assumption. Using DEM, the influence of the modeled particle size on flow and mixing in a bladed granular mixer is studied. The predicted flow microdynamics, including mixing rates, are strongly dependent on the particle diameter. The effect of particle size on macroscopic advective flow also is significant, particularly for dilute flow regions. These results suggest that the influence of particle size needs to be taken into consideration when using larger particles in DEM mixing simulations. To guide scale-up efforts, particle-size-based scaling relationships for several key flow measurements are presented. © 2014 American Institute of Chemical Engineers AIChE J, 61: 46-57, 2015 [ABSTRACT FROM AUTHOR]

We consider numerical algorithms for the simulation of the rheology of two-dimensional vesicles suspended in a viscous Stokesian fluid. The vesicle evolution dynamics is governed by hydrodynamic and elastic forces. The elastic forces are due to local inextensibility of the vesicle membrane and resistance to bending. Numerically resolving vesicle flows poses several challenges. For example, we need to resolve moving interfaces, address stiffness due to bending, enforce the inextensibility constraint, and efficiently compute the (non-negligible) long-range hydrodynamic interactions. Our method is based on the work of Rahimian et al. (2010) [33]. It is a boundary integral formulation of the Stokes equations coupled to the interface mass continuity and force balance. We extend the algorithms presented in that paper to increase the robustness of the method and enable simulations with concentrated suspensions. In particular, we propose a scheme in which both intra-vesicle and inter-vesicle interactions are treated semi-implicitly. In addition we use special integration for near-singular integrals and we introduce a spectrally accurate collision detection scheme. We test the proposed methodologies on both unconfined and confined flows for vesicles whose internal fluid may have a viscosity contrast with the bulk medium. Our experiments demonstrate the importance of treating both intra-vesicle and inter-vesicle interactions accurately. [ABSTRACT FROM AUTHOR]

The correct calculation of cell void fraction is pivotal in accurate simulation of two-phase flows using a computational fluid dynamics-discrete element method (CFD-DEM) approach. Two classical approaches for void fraction calculations (i.e., particle centroid method or PCM and analytical approach) were examined, and the accuracy of these methodologies in predicting the particle-fluid flow characteristics of bubbling fluidized beds was investigated. It was found that there is a critical cell size (3.82 particle diameters) beyond which the PCM can achieve the same numerical stability and prediction accuracy as those of the analytical approach. There is also a critical cell size (1/19.3 domain size) below which meso-scale flow structures are resolved. Moreover, a lower limit of cell size (1.63 particle diameters) was identified to satisfy the assumptions of CFD-DEM governing equations. A reference map for selecting the ideal computational cell size and the suitable approach for void fraction calculation was subsequently developed. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2000-2018, 2014 [ABSTRACT FROM AUTHOR]

The laminar forced convection of a heated rotating sphere in air has been studied using a three-dimensional immersed boundary based direct numerical simulation method. A regular Eulerian grid is used to solve the modified momentum and energy equations for the entire flow region simultaneously. In the region that is occupied by the rotating sphere, a moving Lagrangian grid is used, which tracks the rotational motion of the parti-cle. A force density function or an energy density function is introduced to represent the momentum interaction or thermal interaction between the sphere and fluid. This numeri-cal method is validated by comparing simulation results with analytical solutions of heat diffusion problem and other published experimental data. The flow structures and the mean Nusselt numbers for flow Reynolds number ranging from 0 to 1000 are obtained. We compared our simulation results of the mean Nusselt numbers with the correlations from the literature and found a good agreement for flow Reynolds number greater than 500; however, a significant discrepancy arises at flow Reynolds number below 500. This leads us to develop a new equation that correlates the mean Nusselt number of a heated rotating sphere for flows of 0 < Re < 500. [ABSTRACT FROM AUTHOR]

Highlights: [•] Simulation of flows with suspended particles. [•] Fictitious Domain/Finite Elements method. [•] Topological data structure for mixed triangular/quadrangular meshes. [•] Object oriented programming. [ABSTRACT FROM AUTHOR]

As a gift to Professor Bird, hydrodynamic interactions between two slender toroidal particles immersed in a viscous fluid are derived to extend the Rotne-Prager-Yamakawa (RPY) tensor formulation of bead-spring and bead-rod models of polymer kinetic theory to toroidal beads. As a relatively simple example of a multiply connected domain, this model geometry exhibits the special 'role of the hole' and features new tensorial constructs beyond the Oseen-Burgers and Stokes quadrupoles that are encountered in the classical RPY theory. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1517-1522, 2014 [ABSTRACT FROM AUTHOR]

• A hybrid smoothed particle hydrodynamics coupling fictitious domain method is developed for the particulate flows. • A higher-order interpolation scheme is applied to preserve the consistency and accuracy of the kernel function. • Several benchmarks are validated, indicating the robustness of our SPH-FD method. • This method is successfully applied in simulating the dynamical behaviors of a fiber in a 3D printing process. A hybrid smoothed particle hydrodynamics (SPH) coupled to a direct-forcing fictitious domain (FD) scheme is proposed for the simulation of particulate flows. The new method is effective in calculating the particle-fluid interaction forces and uses a kernel interpolation function to realize the momentum exchange between fluids and particles. A higher-order interpolation scheme is applied to preserve the consistency and accuracy of the kernel function. We first test our method by simulating the benchmark of flow past a cylinder at Reynolds number ranging from 20 to 200 and obtain the convergent results, including accurate lift and drag coefficients, Strouhal number, length of the recirculation bubble, and separation angle. Subsequently, we verify our new method for the typical particulate flows, including the motion of a neutrally-buoyant cylinder in Couette flow, the motion of a neutrally-buoyant cylinder in a planar Poiseuille flow, the sedimentation of a circular cylinder in a channel, and the motion of a neutrally-buoyant slender particle in Couette flow. A good agreement with the existing experimental data, with other numerical results, and with available theoretical solutions is obtained indicating the accuracy and the robustness of the new method. Subsequently, our SPH-FD is applied to simulate a slender fiber in a three-dimensional (3D) printing process, showing a good potential and advantages in simulating particulate flows of some industrial processes. [ABSTRACT FROM AUTHOR]

We determined and fine-tuned the solids transport models appropriate for predicting the single-phase carrier fluid velocity to transport solid particles in conduits for horizontal, low concentration flow. A database with 538 experimental data points was compiled. A literature review was performed to determine the data ranges, forces, and mechanisms used to develop 44 models, and their velocity predictions were compared against the database using statistics. Using the dimensionless forms of the models and the data, the model parameters were adjusted to improve their accuracy and identify the dominant forces. At low concentrations: for liquid/solid flow from a bed of solids and gas/solid flow from the bottom of pipelines, the particle weight, and inertial and viscous forces dominate; for gas/solid flow from a bed of solids, the particle weight, and inertial, viscous, and adhesive forces play a role; and gaps exist in the data for large-diameter pipes and high-density gases. © 2013 American Institute of Chemical Engineers AIChE J, 60: 76-122, 2014 [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a development of the direction splitting algorithm for flows in complex geometries proposed in [2] to the case of flows containing rigid particles. The main novelty of this method is that the grid is fit to the time-dependent domain at each time step which allows for an optimal spatial approximation of the Navier–Stokes equations. In general, this is a very hard task for multidimensional problems. However, it is significantly simplified if the momentum equations are discretized by a direction splitting scheme. Such a scheme requires only solving a set of one-dimensional problems, and the grid can be easily fit to the boundary in the direction of each of these problems. Here we use a MAC discretization stencil but the same idea can be applied to other discretizations. The equations of motion of each particle are discretized explicitly and the computed particle velocities are imposed as Dirichlet boundary conditions for the Navier–Stokes equations on the adapted grid. The pressure is extended within the particles in a fictitious domain fashion, but the divergence stencil is fit to the particle boundaries. In this paper, the direction splitting algorithm with boundary fitting is compared to pure fictitious domain methods. [Copyright &y& Elsevier]

Neal Amundson's seminal contributions in chemical engineering stem from the mathematically framed elegance of his research and the consequences of rigorous analysis. However, many significant problems in chemical engineering have resisted couching in elegant formalisms, the flow of granular materials being a particularly important example. There are valuable lessons that come sharply into focus by examining the nature of the difficulties. Here, we focus on questions of framing and analysis-determining a theoretical approach, choosing an appropriate level of description, and selecting from multiple quantitative tools-using examples spanning vibration, mixing, surface flow, and segregation and pattern formation. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3237-3246, 2013 [ABSTRACT FROM AUTHOR]

Rigid particles transported through a pinched-flow fractionation (PFF) device are simulated using boundary-integral methods (BIM). The PFF device separates particles by size using a bifurcated microfluidic channel. The critical flow ratio of the two input channels required to achieve complete separation of large and small particles decreases with increasing diameter of the larger particles relative to the pinch height, and is nearly independent of the smaller particle size. A narrow pinch with a square exit was shown to have the lowest critical flow ratio and was selected as the model device to be fabricated. Experiments conducted using this device confirm that the larger particles exit further from the top wall than do the smaller particles, due to steric exclusion, and the final exit positions are within a few percent of the simulation results. It is shown that BIM is a valuable tool in the design of microfluidic devices. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3444-3457, 2013 [ABSTRACT FROM AUTHOR]

Abstract: A numerical scheme for fully resolved simulation of particle–fluid systems with freely moving rigid particles is developed. The approach is based on a fictitious domain method wherein the entire particle–fluid domain is assumed to be an incompressible fluid but with variable density. The flow inside the particle domain is constrained to be a rigid body motion using an additional rigidity constraint in a fractional step scheme. The rigidity constraint force is obtained based on the fast computation technique proposed by Sharma and Patankar (2005) [1]. The particle is assumed to be made up of material points moving on a fixed background mesh where the fluid flow equations are solved. The basic finite-volume solver is based on a co-located grid incompressible but variable density flow. The incompressibility constraint is imposed by solving a variable-coefficient pressure equation. Use of density-weighted reconstruction of the pressure gradients was found to give a stable scheme for high density ratio particle–fluid systems. Various verification and validation test cases on fixed and freely moving particles are performed to show that the numerical approach is accurate and stable for a wide range ( – ) of particle–fluid density ratios. [Copyright &y& Elsevier]

We consider coupled models for particulate flows, where the disperse phase is made of particles subject to size variations. We are thus led to kinetic equations with coagulation and breakup operators, coupled to fluid mechanics equations. We discuss the existence and stability of stationary solutions. We also derive macroscopic models through asymptotic hydrodynamic regimes, once relevant scaling parameters have been identified. [ABSTRACT FROM AUTHOR]

Unresolved discrete particle ( DP) models, where a Lagrangian method is used for the solid phase, and a Eulerian method for the fluid phase, have become increasingly popular. The fluctuations of the drag force on individual particles in a homogeneous random array obtained from fully resolved simulations using a lattice Boltzmann method were analysed; such fluctuations are by construction ignored in the unresolved DP model. The drag on individual particles in the array can differ up to 40% with the drag that would be used in DP type simulations was found. A detailed analysis shows that the drag on an individual particle depends strongly on all its surrounding neighbors within a distance of at least two particle diameters. As in DP models, the local flow field is unresolved, the conclusion is that this root mean square ( RMS) deviation in the drag force is inherent to the model. © 2012 American Institute of Chemical Engineers AIChE J, 59: 316-324, 2013 [ABSTRACT FROM AUTHOR]

To investigate the dielectrophoretic motions of multiple particles, we have performed the so-called direct numerical simulations on two-dimensional flows involving inertialess dielectric particles of two to five suspended in a viscous fluid under a uniform external electric field and then compared the results with those of the corresponding magnetophoretic counterparts. For the simulations, the electric field (or the force acting on each particle) is described by the numerical solution of the Maxwell equation (or Gauss's law), where the smoothed representation technique is employed to tackle the jump of electric conductivity across the particle-fluid interface. The flow field, on the other hand, is described by the solution of the continuity and momentum equations, where one-stage smoothed profile method is employed to satisfy the no-slip condition at the interface. In all the simulations, the particles are initially equi-spaced on a circle with an origin at the center while a uniform electric field is externally imposed. Results show that all particles move with repelling or attracting one another depending on the locally nonuniform electric field formed due to the presence of multiple particles. Consequently, they become clustered largely into two groups, then revolve in the clockwise or counterclockwise direction, and finally get aligned in a line with the field direction. One exception is their initial configuration which is symmetric with respect to the axis perpendicular to the electric-field direction, where all the particles move eternally far away from one another with keeping the symmetry. In addition, it is found that the two-dimensional relative motions of dielectric particles under a uniform external electric field are qualitatively in fairly exact agreement with those of paramagnetic particles suspended in a nonmagnetic fluid under a uniform external magnetic field. [ABSTRACT FROM AUTHOR]

Thermal spray coatings produced from a liquid feedstock are receiving an increasing level of interest due to the advanced, nanostructured coatings which are obtainable by these processes. In this article, a high-velocity oxy-fuel (HVOF) thermal spray system is computationally investigated to make a scientific assessment of the liquid droplet behavior on injection. An existing liquid-fuelled HVOF thermal spray gun is simulated using the computational fluid dynamic approach. The steady-state gas-phase dynamics are initialized by the introduction of liquid kerosene and oxygen which react within the combustion chamber producing a realistic compressible, turbulent jet. Discrete-phase water droplets are injected at the powder injection port. On injection, the water droplets breakup and vaporize, while being entrained through the acceleration barrel of the HVOF system. The results obtained give an insight to the mechanism which control the water droplet sizes and disintegration process, and serve as a fundamental reference for future development of liquid feedstock devices. © 2012 American Institute of Chemical Engineers AIChE J, 2012 [ABSTRACT FROM AUTHOR]

A two-dimensional model of methane thermal decomposition reactors is developed which accounts for coupled radiative heat and polydisperse carbon particle nucleation, growth, and transport. The model uses the Navier-Stokes equations for the fluid dynamics, the radiative transfer equation for methane and particle species radiation absorption, the advection-diffusion equation for gas and particle species transport, and a sectional method for particle species nucleation, heterogenous growth, and coagulation. The model is applied to a tubular laminar flow reactor. The simulation results indicate the development of a reaction boundary layer inside the reactor, which results in significant variation of the local particle size distribution across the reactor. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2545-2556, 2012 [ABSTRACT FROM AUTHOR]

Abstract: The alignment of particles in a confined shear flow of a viscoelastic fluid is quantitatively analyzed using an extended finite element method (XFEM) with a temporary arbitrary Lagrangian–Eulerian (ALE) scheme. The no-slip boundary condition on the particle surface is realized by using a newly proposed weak boundary condition, which is equivalent to adding a positive definite stabilizing term in the momentum balance. Once particles form a string-like structure, the final state is independent of the initial particle positions and the histories to reach the steady-state. For a certain fluid rheology, the maximum obtainable length of a string of particles is limited. As the Weissenberg number increases, particles can form longer strings. Moderate wall confinement promotes the alignment of particles, however, too strong confinement hinders the alignment by enhancing repulsive interaction between particles. The steady-state angular velocities of particles are compared with respect to the length of strings. If particles can form sufficiently long strings, the steady-state angular velocities of the two end-particles do not change significantly, and those of the non end-particles increase, as the string length increases. In a given string, the angular velocity of the two end-particles is faster than those of the particles in between. We have also presented the steady-state interparticle distance between two neighboring particles in a string. As the string length increases, the interparticle distance increases. [Copyright &y& Elsevier]

This work is devoted to the development of a numerical model for particulate flows during phase-change processes. The model is based on the Direct Numerical Simulation (DNS) approach. The particle collisions are modelled using the hard sphere approach. The interface velocity of the melting/solidification is calculated by means of the Stefan condition for each particle. In this work we consider a two-dimensional system consisting of 40 circular ice dendrites moving up owing to the density difference in a two-dimensional channel filled with hot water (first case) and undercooled water (second case). The results of the simulations are discussed. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, recent advances in the discrete-element method (DEM) for describing motion, deposition, agglomeration or aggregation of a large number of adhesive spherical particles immersed in fluid flows, termed as adhesive particulate flow, are reviewed. The constitutive equations together with the length and time scales of DEM are compared with those of other similar Lagrangian particle methods, e.g., molecular dynamics (MD), Brownian dynamics (BD), dissipative particle dynamics (DPD). The adhesive contact force and torque models in the presence of different adhesive effects are examined, including van der Waals force, ligand-receptor binding, liquid bridging force, interface adhesion, and sintering forces, all of which play an important role in DEM formulations for different types of adhesive particulate flow problems of interest in energy, combustion and environmental fluid mechanics problems. A summary of various kinds of particle-field interactions is presented, including fluid forces, electric field forces, acoustic force, and thermophoretic force. The computational method is illustrated by application to a series of examples involving capture of spherical particles by a fiber in a uniform upstream flow, examining the deposition/aggregation patterns of both mono-size and binary-size particles on the cylinder with and without the presence of electric field effects, which may be due either to charging of the cylinder or polarization of the particles. Particle capture problems of this sort are commonly encountered in filtration problems and ash-removal problems experienced in environmental and combustion applications, respectively. The article concludes with a discussion of remaining modeling challenges in development of discrete-element methods for adhesive particulate flow fields. [Copyright &y& Elsevier]

Abstract: The so-called smoothed profile method, originally suggested by Nakayama and Yamamoto and further improved by Luo et al. in 2005 and 2009, respectively, is an efficient numerical solver for fluid–structure interaction problems, which represents the particles by a certain smoothed profile on a fixed grid and constructs some form of body force added into the momentum (Navier–Stokes) equation by ensuring the rigidity of particles. For numerical simulations, the method first advances the flow and pressure fields by integrating the momentum equation except the body-force (momentum impulse) term in time and next updates them by separately taking temporal integration of the body-force term, thus requiring one more Poisson-equation solver for the extra pressure field due to the rigidity of particles to ensure the divergence-free constraint of the total velocity field. In the present study, we propose a simplified version of the smoothed profile method or the one-stage method, which combines the two stages of velocity update (temporal integration) into one to eliminate the necessity for the additional solver and, thus, significantly save the computational cost. To validate the proposed one-stage method, we perform the so-called direct numerical simulations on the two-dimensional motion of multiple inertialess paramagnetic particles in a nonmagnetic fluid subjected to an external uniform magnetic field and compare their results with the existing benchmark solutions. For the validation, we develop the finite-volume version of the direct simulation method by employing the proposed one-stage method. Comparison shows that the proposed one-stage method is very accurate and efficient in direct simulations of such magnetic particulate flows. [Copyright &y& Elsevier]

The article presents information on a study which proposed an alternative approach of the quadrature method of moments (QMOM) for solving batch crystallization models describing crystals nucleation, size-dependent growth, aggregation, breakage and dissolution of small nuclei below certain critical size. It describes the application of orthogonal polynomials to determine the quadrature abscissas and weights. It compares the numerical results with analytical solutions and with the finite-volume scheme results.

The article provides information about the local unsteady flow characteristics indicated by a numerical model established on the kinetic theory of granular media. It is mentioned that the instantaneous solid fraction field, and the degree of fluctuating kinetic energy of the particles are controlled by a series of rotational structures with two dimensions that develops along the height of the solid-liquid fluidized beds. It is also inferred that these structures vanish to the benefit of a one-dimensional oscillating flow.

Abstract: We consider numerical algorithms for the simulation of hydrodynamics of two-dimensional vesicles suspended in a viscous Stokesian fluid. The motion of vesicles is governed by the interplay between hydrodynamic and elastic forces. Continuum models of vesicles use a two-phase fluid system with interfacial forces that include tension (to maintain local “surface” inextensibility) and bending. Vesicle flows are challenging to simulate. On the one hand, explicit time-stepping schemes suffer from a severe stability constraint due to the stiffness related to high-order spatial derivatives in the bending term. On the other hand, implicit time-stepping schemes can be expensive because they require the solution of a set of nonlinear equations at each time step. Our method is an extension of the work of Veerapaneni et al. [S.K. Veerapaneni, D. Gueyffier, D. Zorin, G. Biros, A boundary integral method for simulating the dynamics of inextensible vesicles suspended in a viscous fluid in 2D, Journal of Computational Physics 228(7) (2009) 2334–2353], in which a semi-implicit time-marching scheme based on a boundary integral formulation of the Stokes problem for vesicles in an unbounded medium was proposed. In this paper, we consider two important generalizations: (i) confined flows within arbitrary-shaped stationary/moving geometries; and (ii) flows in which the interior (to the vesicle) and exterior fluids have different viscosity. In the rest of the paper, we will refer to this as the “viscosity contrast”. These two problems require solving additional integral equations and cause nontrivial modifications to the previous numerical scheme. Our method does not have severe time-step stability constraints and its computational cost-per-time-step is comparable to that of an explicit scheme. The discretization is pseudo-spectral in space, and multistep BDF in time. We conduct numerical experiments to investigate the stability, accuracy and the computational cost of the algorithm. Overall, our method achieves several orders of magnitude speed-up compared to standard explicit schemes. As a preliminary validation of our scheme, we study the dependence of the inclination angle of a single vesicle in shear flow on the viscosity contrast and the reduced area of the vesicle, the lateral migration of vesicles in shear flow, the dispersion of two vesicles, and the effective viscosity of a dilute suspension of vesicles. [Copyright &y& Elsevier]

The article focuses on the use of particle image velocimetry to explore the velocity field and associated particle transport in an evaporating water droplet. It describes some experiments conducted where droplets with polystyrene particles were to be evaporated. Measures taken to avoid light refraction on the surface of the droplet are described, as well as the conditions leading to Marangoni flow. Outcomes relating to contact angle, droplet height, velocity field and the concentration of particles in the evaporating droplet were analyzed.

Abstract: In order to reduce power consumption and conveying velocity, a pneumatic conveying system where a dune model is mounted in a pipeline is proposed in this paper. The experimental study focuses on the effect of the mounted dune model in the horizontal pneumatic conveying system in terms of pressure drop, power consumption and conveying velocity. The test pipeline consisted of a horizontal smooth acrylic tube with an inside diameter of 80mm and a length of about 5m. Polyethylene spherical particles with a density of 952kg/m3 and diameters of 2.3 and 3.3mm are used as conveying materials. The mean air velocity is varied from 9 to 16m/s, and the solid mass flow rate is from 0.25 to 0.45kg/s. Firstly, the effect of the dune model location on pneumatic conveying is experimentally studied. It is found that in the lower air velocity range, the pressure drop of the pneumatic conveying with a mounted dune model is lower than that of a conventional pneumatic conveying system. A lower conveying velocity and energy-saving conveying can be realized by installing a dune model in the conveying pipe. Especially the case of fixing the dune model on the bottom of the pipe at the inlet of particle feed is more effective. The particle flow patterns also show that the dune model reduces the deposition of particles. Then, the effect of different surface materials of the dune model is examined. By using a surface material of the dune model with a large coefficient of restitution, the pressure drop of conveying large particles is the lowest. When conveying relatively small particles, however, the pressure drop becomes the lowest by a small coefficient of restitution. The maximum reduction rates of the minimum velocity and power consumption by the dune model are about 19% and 34%, respectively. [Copyright &y& Elsevier]