High Solid Volume Fraction Research Articles

Role of fluid heating in dense gas–solid flow as revealed by particle-resolved direct numerical simulation

Heat transfer is important in gas–solid flows that are encountered in many industrial applications such as energy generation. Computational fluid dynamics (CFD) simulations of heat transfer in gas–solid flow are based on statistical theories that result in averaged equations (e.g., the Eulerian–Eulerian two-fluid model). These averaged equations require accurate models for unclosed terms such as the average gas–solid heat transfer rate. The average gas–solid or interphase heat transfer rate is modeled in terms of the Nusselt number Nu, which is specified as a function of the solid volume fraction εs, mean slip Reynolds number Rem and Prandtl number Pr. In developing closure models for the average interphase heat transfer rate it is assumed that the gas–solid flow is locally homogeneous i.e., the effect of fluid heating (or cooling) on the average fluid temperature is negligible. However, continuous heating (or cooling) of the fluid along the flow direction causes the average fluid temperature to become inhomogeneous. In this work we develop a particle-resolved direct numerical simulation (PR-DNS) methodology to study heat transfer in steady flow past statistically homogeneous random assemblies of stationary particles. By using an analogy with thermally fully developed flow in pipes, we develop a thermal similarity condition that ensures a statistically homogeneous Nusselt number, even though the average fluid temperature field is inhomogeneous. From PR-DNS results we find that the effect of fluid heating (or cooling) cannot be neglected for gas–solid systems with high solid volume fractions and low mean slip Reynolds numbers. These results indicate that the assumption of scale separation implicit in two-fluid models is not always valid.

A numerical study of granular shear flows of rod-like particles using the discrete element method

AbstractThe effect of particle aspect ratio and surface geometry on granular flows is assessed by performing numerical simulations of rod-like particles in simple shear flows using the discrete element method (DEM). The effect of particle surface geometry is explored by adopting two types of particles: glued-spheres particles and true cylindrical particles. The particle aspect ratio varies from one to six. Compared to frictionless spherical particles, smaller stresses are obtained for the glued-spheres and cylindrical particle systems in dilute and moderately dense flows due to the loss of translational energy, which is partially converted to rotational energy, for the non-spherical particles. For dilute granular flows of non-spherical particles, stresses are primarily affected by the particle aspect ratio rather than the surface geometry. As the particle aspect ratio increases, the effective particle projected area in the plane perpendicular to the flow direction increases, so that the probability of the occurrence of the particle collisions increases, leading to a reduction in particle velocity fluctuation and therefore a decrease in the stresses. Hence, a simple modification is made to the kinetic theory for granular flows to describe the stress tensors for dilute flows of non-spherical particles by incorporating a normalized effective particle projected area to account for the effect of particle collision probability. For dense granular flows, the stresses depend on both the particle aspect ratio and the surface geometry. Sharp stress increases at high solid volume fractions are observed for the glued-spheres particles with large aspect ratios due to the bumpy surfaces, which impede the flow. However, smaller stresses are obtained for the true cylindrical particles with large aspect ratios at high solid volume fractions. This trend is attributed to the combined effects of the smooth particle surfaces and the particle alignments such that the major/long axes of particles are aligned in the flow direction. In addition, the apparent friction coefficient, defined as the ratio of shear to normal stresses, is found to decrease as the particle aspect ratio increases and/or the particle surface becomes smoother at high solid volume fractions.

A DEM-based analysis of the influence of aggregate structure on suspension shear yield stress

This study theoretically examined the effect of aggregate structure on the suspension shear yield stress. The aggregation process of colloidal particles was simulated using the discrete element model (DEM) combined with the well-known DLVO theory. The predicted aggregate structural characteristics, namely the coordination number and inter-particle forces were then used in a modified version of the Flatt and Bowen mechanistic model [6] to calculate the corresponding suspension yield stress. The effect of key parameters such as solid volume fraction, suspension pH and ionic strength on the aggregate structure and hence the yield stress of the suspension was investigated.The results showed that the yield stress increased significantly under conditions that were favourable for formation of complex net-like aggregate structures, such as high solid volume fractions, pH values near the iso-electric point, and high ionic strengths. In such cases, the mean coordination number reached a maximum value which was considered to be dependent on the particle size and size distribution. The suspension yield stress exhibited a power law dependency on the solid volume fraction. The interconnected network structure developed at high solid volume fractions was found to be the major contributing factor to the observed high suspension yield stress. As the particle–particle repulsion became significant, a decrease in both the number of bonds and the mechanical bonding strength of the aggregate structure was observed. That was considered to be responsible for the reduction in the suspension yield stress. The suspension yield stress became independent of the suspension ionic strength when the ionic strength exceeded the critical coagulation concentration. Satisfactory agreements were obtained between simulation results and the published experimental data.

Modeling of the flue gas desulfurization in a CFB riser using the Eulerian approach with heterogeneous drag coefficient

Flue gas desulfurization (FGD) in a CFB at moderate temperatures with the T–T sorbent, a hydrated calcium-based sorbent was numerically simulated using the Eulerian approach with two drag models for heterogeneous gas–solid flow, one is an EMMS-based theoretical model (QL-EMMS), the other is an experimental model (O-S). The QL-EMMS model was analyzed in terms of the cluster parameters, the drag reduction and CFB modeling by comparing with the experimental data and results of the O-S model. Results using a heterogeneity index show that the QL-EMMS model is more accurate than other theoretical drag models, but there are still some differences from experimental data due to an inaccurate description of the solid volume fractions in clusters. The factors influencing the moderate temperature desulfurization were then investigated based on the accurate O-S drag model. Predictions of the desulfurization efficiency and reaction rate indicate that higher solid volume fractions and more homogeneous solid distributions lead to higher desulfurization efficiencies. The reaction time also has an important effect on the desulfurization.

Stability analysis of unbounded uniform dense granular shear flow based on a viscoplastic constitutive law

Asymptotic and transient stability analyses of unbounded uniform granular shear flow at high solids volume fractions were carried out in the paper, based on a model composed of the viscoplastic constitutive law [P. Jop, Y. Forterre, and O. Pouliquen, Nature (London) 441, 727 (2006)] and the dilatancy law [O. Pouliquen et al., J. Stat. Mech.: Theory Exp. (2006) P07020]. We refer to this model as the VPDL (meaning of the “viscoplastic and dilatancy laws”) thereinafter. In this model, dense granular flows were treated as a viscoplastic fluid with a Drucker–Prager-like yielding criterion. We compared our results to those obtained using the frictional-kinetic model (FKM) [M. Alam and P. R. Nott, J. Fluid Mech. 343, 267 (1997)]. Our main result is that unbounded uniform dense granular shear flows are always asymptotically stable at large time based on the VPDL model, at least for two-dimensional perturbations. This is valid for disturbances of layering modes (i.e., the perturbations whose wavenumber vectors are aligned along the transverse coordinate) as well as for nonlayering modes (the streamwise component of the wavenumber vector is nonzero). By contrast, layering modes can be unstable based on the FKM constitutive laws. Interestingly, in the framework of the VPDL, the analysis shows that significant transient growth may occur owing to the non-normality of the linear system, although disturbances eventually decay at large time.

DEM simulation of aggregation of suspended nanoparticles

A DEM-based model was developed and examined for simulation of aggregation in suspensions of α-alumina nanoparticles. In the model, the random Brownian diffusion and the externally induced dielectrophoresis (DEP) motion were considered as the driving mechanisms for the transport of particles in colloidal suspension. To simulate particle interactions, the non-contact surface force and the contact force were taken into account using the well-known Derjaguin-Landau-Verway-Overbeek (DLVO) theory and the soft-sphere model, respectively. Specifically, the model was used to study the effects of pH, solid volume fraction and external AC electric field on α-alumina aggregate growth which was expressed in terms of coordination number, longest dimension, and fractal dimension. The simulations were carried out over a pH range of 4–10, solid volume fraction of 0.02–0.4, and a variety of AC electric fields. In relatively dilute suspensions, the aggregates predominantly exhibited chainlike structures, whereas at high solid volume fraction, aggregates with complex netlike structures were formed. It was also evident that, in concentrated colloidal suspensions, DEP had a negligible influence on aggregate growth over the examined conditions. The effect of DEP however, was found to be more noticeable on aggregate structure leading to the formation of more compact aggregates with a greater particle number density. The break-up and reattachment of sub-aggregates as well as the rearrangement of nanoparticles in the particle assemblies and subsequent curling of the loose network promoted by a strong AC electric field was deemed to be responsible for this structural transformation. Finally, the DEM-based model was used to predict the size of α-alumina aggregates over a range of pH. The predictions were found to be in good agreement with the published experimental data, particularly around the isoelectric point.

Toward two‐phase flow modeling of nondilute sediment transport in open channels

In this paper, we generalize several models based upon the multiphase flow theory to address the nondilute transport of suspended sediment in open channels. These models naturally include the dilute condition as a special case. We assess the range of validity of models through simulations of the experimental tests by Vanoni (1946), Einstein and Chien (1955), Taggart et al. (1972), Coleman (1986), and Wang and Qian (1992). The K − ɛ model is used to represent the turbulence in the carrier phase, and the kinetic theory of gases is employed for the closure of stresses due to inter‐particle collisions. We test various closures for the eddy viscosity of the disperse phase; we show the effects of increasing sediment concentration on the interaction forces between the two phases and on the stresses developed due to inter‐particle collisions. We determine the values of the Schmidt number required to approximate the experimental profiles of the volumetric concentration of sediment, suggesting that while the Schmidt number is smaller than one for dilute mixtures, it is larger than one for nondilute flows. Results indicate that nondilute models of increasing complexity are indeed necessary for flows having high solid volume fractions. Our results also suggest a more tested range of values for the threshold of sediment concentration of nondilute flows; this threshold can be located between 2 and 5%. It is observed that the virtual mass force becomes important in the simulation of nondilute flows. The eddy viscosity of the carrier phase is found to decrease with the increasing volumetric concentration in the flow.

Vibro-extrusion: a new forming process for cement-based materials

Extrusion is a common forming process for a wide range of materials (food, polymer, clay, metal), although it is not yet a common way to form cement-based materials on an industrial scale. This is both explained by the heterogeneity (large range of grain size) and the specific rheological behaviour (frictional behaviour due to high solid volume fraction) of such materials. Recent studies have highlighted the predominance of the filtration phenomenon during the extrusion flow of firm cement-based materials. Such a filtration leads to process blockage and prohibits its utilisation in industrial settings. In the present study the introduction of a vibration system to the extruder in order to inprove the extrusion flow was investigated. The effects of such external excitation on the rheological behaviour of a firm extrudible cement-based paste (yield stress reduction without losing retaining shape properties) were examined and some extrusion tests (ram and screw extruders) were performed to show the potential of a vibration/extrusion coupling.

Shear Rheology of Molten Crumb Chocolate

The shear rheology of fresh molten chocolate produced from crumb was studied over 5 decades of shear rate using controlled stress devices. The Carreau model was found to be a more accurate description than the traditional Casson model, especially at shear rates between 0.1 and 1 s(-1). At shear rates around 0.1 s(-1) (shear stress approximately 7 Pa) the material exhibited a transition to a solid regime, similar to the behavior reported by Coussot (2005) for other granular suspensions. The nature of the suspension was explored by investigating the effect of solids concentration (0.20 < phi < 0.75) and the nature of the particles. The rheology of the chocolate was then compared with the rheology of (1) a synthetic chocolate, which contained sunflower oil in place of cocoa butter, and (2) a suspension of sugar of a similar size distribution (volume mean 15 mum) in cocoa butter and emulsifier. The chocolate and synthetic chocolate showed very similar rheological profiles under both steady shear and oscillatory shear. The chocolate and the sugar suspension showed similar Krieger-Dougherty dependency on volume fraction, and a noticeable transition to a stiff state at solids volume fractions above approximately 0.5. Similar behavior has been reported by Citerne and others (2001) for a smooth peanut butter, which had a similar particle size distribution and solids loading to chocolate. The results indicate that the melt rheology of the chocolate is dominated by hydrodynamic interactions, although at high solids volume fractions the emulsifier may contribute to the departure of the apparent viscosity from the predicted trend.

Potentials and challenges in jetting microdroplets onto nonwoven fabrics

Nonwoven fabrics are very diverse in their structural properties. This paper discusses potential opportunities and challenges involved in jetting and depositing microdroplets on such materials. This study reports on the interaction of controlled droplets with the nonwoven substrates. Droplets used had velocities of about 1.8 m/s and diameters of about 90 μm and were produced by using a drop-on-demand (DOD) inkjet printhead. Nonwovens used consisted of two groups of high and low solid volume fraction (SVF) substrates. The results indicate that in the case of low-SVF nonwovens, the local spacing and orientation of the fibres plays a significant role in determining the outcomes of the jetting process. Drops were seen to penetrate deep into a low-SVF nonwoven and deposit on a single fibre or bundle of fibres. Low-SVF nonwovens, therefore, can hold the fluid within their structures—a case of interest in printing electric circuits. The case of jetting on high-SVF nonwovens was found to be primarily dependent on the fibres' surface properties. The drops were found to stay above the surface in the case of hydrophobic fibres and below the surface in the case hydrophilic ones.

On the effect of enduring contact on the flow and thermal characteristics in powder lubrication

The effect of the enduring contact and the true temperature are investigated for a granular lubricant sheared between two parallel plates. The appropriate equations are derived including both the enduring contact as well as the inelastic collision between the granules. The characteristics of the flow are investigated in a ‘transitional regime’ where both kinetic-collision effect and enduring contact exist concurrently. The results of this paper reveal that the effect of enduring contact between granules is very important and must be considered especially at low operating speeds and when dealing with high solid volume fractions, which is typically the case when dealing with granular lubrication. Moreover, the enduring contact shows a definite effect on the true temperature.

Simulating permeability of 3-D calendered fibrous structures

Many fibrous materials such as nonwoven materials are often consolidated by means of hot calenders, i.e., hot compaction rolls. Hot calendering compresses the fiber assembly and can cause changes in the structure. In nonwovens, calendering has an added function of thermally bonding the fibers at their respective crossovers to form a strong but yet somewhat porous material. Calendering causes a significant increase in the solid volume fraction (SVF) of the media and therefore, affects their permeability. To our knowledge, no work in the literature has been dedicated to modeling the permeability of calendered nonwovens. In this study, virtual nonwoven structures are generated and compressed from top and bottom to resemble the hot calendering process. In agreement with our experimental observations, it was found that the average SVF profile across the material's thickness turns into a U-shape profile after the calendering. In this work, the dimensionless permeability of the calendered media is computed using CFD tools and reported for different compaction ratios. Results of our simulations are compared with the experiment as well as the available empirical and/or analytical permeability models in the literature and good agreement, depending upon the SVF, is observed. We also studied the influence of orientation distribution of the fibers on the dimensionless permeability of the fabric and noticed that permeability decreases by increasing the directionality of the fibers. This is found to be primarily due to the fact that highly oriented uncompressed fiber-webs tend to have a higher SVF. Fiber-webs of identical SVF, however, exhibited almost identical permeabilities regardless of their fiber orientations.

Effects of Solid Volume Fraction on the Gelcasting of Alumina Suspensions

The effects of solid volume fraction (SVF) on the gelation of alumina suspensions for gelcasting, debonding and sintering of the green body were studied. It was found that with SVF rising, the gelation of alumina suspension delayed; and the strength of green body decreased. On the other hand, high SVF resulted in that polymerized acrylamide split at a relative low temperature. These phenomena manifest that the fast polymerization of monomers in high SVF alumina suspension was inhibited, and the flexibility of the gelcasting was improved. However, Excessive solid volume fraction was prone to a bad rheological behavior of alumina suspension, and deteriorated the microstructure and properties of sintered body.

Population balance modeling of aggregation and breakage in turbulent Taylor–Couette flow

An experimental and computational study of aggregation and breakage processes for fully destabilized polystyrene latex particles under turbulent-flow conditions in a Taylor–Couette apparatus is presented. To monitor the aggregation and breakage processes, an in situ optical imaging technique was used. Consequently, a computational study using a population balance model was carried out to test the various parameters in the aggregation and breakage models. Very good agreement was found between the time evolution of the cluster size distribution (CSD) calculated with the model and that obtained from experiment. In order to correctly model the left-hand side of the CSD (small clusters), it was necessary to use a highly unsymmetric fragment-distribution function for breakage. As another test of the model, measurements with different solid volume fractions were performed. Within the range of the solid volume fractions considered here, the steady-state CSD was not significantly affected. In order to correctly capture the right-hand side of the CSD (large aggregates) at the higher solid volume fraction, a modified aggregation rate prefactor was used in the population balance model.

Numerical Simulation on Thixoforming of Wrought Magnesium Alloy

The thixoforming of wrought magnesium alloy was analyzed with computer numerical simulation based on rigid plastic/rigid viscoplastic finite element method. The simulated parameters were chosen and depended on the constitutive model of semi-solid AZ61 alloy that was established using the multiple nonlinear regression method in our prior literature. Thixo mechanical properties of semi-solid AZ61 alloy in high solid volume fraction were also analyzed. Behaviors of metal flow and temperature field were obtained, which were also compared with experimental data in literature. Research showed that the hard deformation magnesium alloy had great filling ability in semi-solid state, and the stress and strain distributions in workpiece after thixoforming were uniform. The semi-solid thixoforming technology has taken priority of the traditional processing.

A note on permeability simulation of multifilament woven fabrics

A conventional approach for modeling permeability of multifilament fabrics is to consider their warps and wefts to be individual thick filament made of homogeneous porous media and solve the flow equations for such monofilament fabrics. In this work, for the first time, the full 3-D geometry of an idealized multifilament woven fabric, wherein the filaments are packed in hexagonal arrangement, is generated to compute its permeability and compare with the homogeneous anisotropic lumped model of Gebart (1992. Permeability of unidirectional reinforcements for RTM. Journal of Composite Materials 26(8), 1100–1133). While a relatively good agreement is obtained, our results indicate that Gebart's model slightly underestimate the permeability of multifilament fabrics even at high yarn's solid volume fractions.

Simulation of Ultrasound Propagation Through Three-Dimensional Trabecular Bone Structures: Comparison with Experimental Data

We present a direct comparison between numerical simulation of wave propagation, performed through 28 volumes of trabecular bone, and the corresponding experimental data obtained on the same specimens. The volumes were reconstructed from high resolution synchrotron microtomography experiments and were used as the input geometry in a three-dimensional (3D) finite-difference simulation tool developed in our laboratory. The version of the simulation algorithm that was used accounts for propagation in both the saturating fluid and bone, and does not take absorption into account. This algorithm has been validated in a previous paper [Bossy et al.: Med. Biol. 50 (2005) 5545] for simulation of wave propagation through trabecular bone. Two quantitative ultrasound parameters were studied at 1 MHz for both simulated and experimental signals: the normalized slope of the frequency dependent attenuation coefficient (also called normalized broadband ultrasound attenuation (nBUA) in the medical field), and the phase velocity at the center frequency. We show that the simulated and experimental nBUA are in close agreement, especially for the high porosity specimens. For specimens with a low porosity (or a high solid volume fraction), the simulation systematically underestimate the experimentally observed nBUA. This result suggests that the relative contribution of scattering and absorption to nBUA may vary with the bone volume fraction. A linear relationship is found between experimental and simulated phase velocity. Simulated phase velocity is found to be slightly higher than the experimental one, but this may be explained by the choice of material properties used for the simulation.

Viscosity measuring technique for gas-solid suspensions

Abstract It is important that we clarify the interaction between particles in particle processing. The interaction between particles under shear flow is considered to be evaluated by energy dissipation, i.e. viscosity, in a powder layer. By measuring the torque exerted on the mixing blade of a powder rheometer, the viscosity of the powder layer could be calculated. The viscosity decreased with the blade velocity and such a shear thinning rheological property is similar to that observed for solid-liquid suspensions. By applying the suspension rheology model, which was developed for the evaluation of the agglomeration state of solid-liquid mixtures under shear flow, particle agglomeration around the blade under dry powder flow conditions was evaluated. As the shear rate increases, the size of agglomerated particle clusters is proved to decrease. At high shear rate or low solid volume fraction, the inter-particle bonding energy is revealed to be constant independent of the shear rate and solid volume fraction.

Pore metamorphosis during liquid phase sintering in microgravity

Liquid Phase Sintering (LPS) experiments have been conducted on several sounding rocket flights, Space Shuttle missions and aboard Mir Station, where more than 100 samples were processed for various times. Analysis of those samples revealed considerable pore formation and metamorphosis. Pore filling and coarsening was found in most samples while pore breakup was also found in low liquid volume samples. Pores showed bifurcated behaviors based on their liquid volume fractions. These behaviors resulted from particle rearrangement, particle growth and different diffusion patterns (surface diffusion and volume diffusion) that associated with interfacial energy differences, instabilities, and grain coarsening along the interface between phases. Low liquid volume fraction and the presence of the agglomeration, which results in high local solid volume fraction, enhances the volume diffusion during the process which causes the pore breakup. This paper attempts to show pore bifurcation behavior, which produces either smaller pore by a breakup mechanism or coarsened pore of large size. An initiation mechanism induced by grain growth, capillary force and other weak forces is proposed and the results from theoretical analysis and CFD numerical simulation are presented.

Theoretical Analysis of Plastic Forming Process for Semi-Solid Material

The plastic forming of magnesium alloy is difficult, but the semi-solid material forming is a good method solved this problem. The mechanical model of the semi-solid materials was treated as that of the continuous porous materials in the high solid volume fraction. The upper bound theory applied for semi-solid metal plastic forming process was developed. The velocity discontinuities exist not only in the tangential component but also in normal component for the kinematically admissible displacement increment filed. The latter one was responsible for a change in solid volume fraction when the material passes the discontinuity. An upper bound analytical model and theoretical method of plastic forming process for semi-solid material has been proposed. The calculating formulas of deformed power were derived. It is theoretical basement to apply further for the practice technology analysis such as the plastic forming of magnesium alloy.