Assessing mixing characteristics of particle-mixing and granulation devices

The mixing of particulates such as powders is an important process in many industries including pharmaceuticals, plastics, household products （such as detergents） and food processing. The quality of products depends on the degree of mixing of their constituent materials which in turn depends on both geometric design and operating conditions. Unfortunately, due to lack of understanding of the interaction between mixer geometry and the granular material, limited progress has been made in optimizing mixer design. The discrete element method （DEM） is a computational technique that allows particle systems to be simulated and mixing to be predicted. Simulation is an effective way of acquiring information on the performance of different mixers that is difficult and/or expensive to obtain using traditional experimental approaches. Here we demonstrate how DEM can be used to unravel flow dynamics and assess mixing in several different types of devices. These devices used for mixing and/or granulation of particulates, are classified broadly as gravity controlled, bladed and high shear. We also explore the role of particle shape in mixing performance and use DEM to test whether Froude number scaling is suitable for predicting scale performance of rotating mixers.     

Drying of suspensions and solutions on inert particle surface in mechanically spouted bed dryer

To eliminate some disadvantages of the conventional spouted bed dryers the mechanically spouted bed （MSB） system was developed. This dryer type is convenient to use inert particles providing an increased surface area for drying of materials of high-moisture content and heat sensitive materials. On three different drying tasks are demonstrated the experimental optimization of process parameters to obtain products of demanded quality. The main object was at drying of AIO（OH） suspension to preserve the particle size under 2.5μm and to obtain product with a moisture content of about 0.05 kg/kg （d.b.）. For this reason a very thin particle coating and intensive abrasion had to be assured. At drying of tomato concentrates the thermoplasticity makes the process very difficult. To jump over the deliquescent and sticky state developed at the critical temperature-moisture content values a very short drying time （8-10 s） must be provided. The third task was to form powder-like product from bovine serum albumin （BSA） solution having very low solid content （2-4%）. The selected process parameters given in this paper resulted in a mean particle size of less than 20 μm while the soluble oreserved orotein content was higher than 90%.     

Correlation of powder flow properties to interparticle interactions at ambient and high temperatures

A combination of a continuum approach and a particle–particle approach to describe the multi-scale nature of the mechanical properties of bulk solids may be beneficial to scientific and engineering applications. In this paper, a procedure is proposed to estimate the interparticle forces beginning with the bulk flow properties as measured with standardized techniques. In particular, the relationship between interparticle forces and bulk solid tensile strength is adopted based on the microscale approaches of Rumpf(1970) and Molerus(1975). The flow properties of fluid cracking catalyst(FCC), corundum and glass bead powders were all characterized with a modified Schulze ring shear cell capable of operating at temperatures up to 500℃. The powder test conditions were selected such that the van der Waals forces were the most significant particle–particle interactions. The model equations describe two cases, in which either elastic or plastic deformation of the contact points is assumed. The results indicate that the model provides the correct order of magnitude for the values of the tensile strength when proper values for the mean curvature radius at the contact points are taken into account. A sensitivity analysis for the main parameters in the model was performed. This analysis indicated that the assumption of plastic deformation at contact surfaces coupled with a decrease in porosity justified an increase of the tensile strength with consolidation stress. Furthermore, the effect of temperature on the measured flow behavior can be explained as a change in the strength of the material.     

Variation of granule mass fraction with coordination number in wet granulation process

In granulation, fine particles combine to form a coarse granule in the form of a particle matrix partially or fully saturated with a binder liquid. The final product of granulation possesses a wide variety of granule size distributions with surface mean diameters which differ with operating conditions. The final granule size depends on the operating conditions, e.g. operating gas velocity, inlet air temperature, initial feed particle size, and viscosity of the binder. The objective of this paper is to find out the uniformity in the relation between the granule mass fraction in the final granule size distribution and the number of feed particles present in the granules. The total number of granules obtained depends on the experimental conditions but the granule mass fraction and the number of feed particles forming a single granule are independent of operating variables, feed material and method of granulation. The paper purports further to compare the uniform nature of mass fraction of the granules in final granule size distribution and the primary particles required to form that particular granule size irrespective of experimental conditions of granulation.     

Dynamic release process of pollutants during suspended sediment transport in aquatic system

Pollutants release is highly consistent with suspended sediment concentration （SSC） in water column, especially during re-suspension and transport events. The present research focuses on pollutant dynamic release from re-suspended sediment, especially the vertical distribution relationship between them. The sediment erosion experiments on a series of uniform flow are conducted in a circulate flume. Reactive tracer （phosphorus） is used as the contaminant in fine-grained sediments to identify the release characteristic length and time. Experimental results show that the flow condition near-bed depends on the sediment surface roughness. The region with high turbulent intensities corresponds to a high concentration sediment layer. In addition, the SSC decreases with the distance, water depth, and particle grain size. The sediment in a smaller grain size takes much more time to reach equilibrium concentration. Total phosphorus （TP） concentration changes along the water depth as SSC in the initial re-suspension stage, appearing in two obvious concentration regimes： the upper low-concentration layer and the high-concentration near-bottom layer. This layered phenomenon remains for about 3 hours until SSC distri- bution tends to be uniform. Longitudinal desorption plays an important role in long-way transport to reduce the amount of suspended sediment in water column.     

Research on particle dispersion in a plane mixing layer with coherent structures

The numerical simulation with two-way coupling was performed in a liquid -particle mixing layer and the corresponding experiment study was made. In the process of vortex rolling up and vortices pairing, the particles with different St number have a very different pattern of dispersion. The mean velocity of particle with St = 1 is higher than that of the fluid phase on the low-speed side, and lower than that of the fluid phase on the high-speed side. The RMS of particle approaches that of the fluid phase with decreasing particle St number. The RMS in the transverse direction is smaller than that in the streamwise direction. The velocity fluctuation correlation of particle is smaller than the Reynolds shear stress, the “overshoot“ phenomenon that the velocity fluctuation correlation of particle is larger than the Reynolds shear stress does not appear. The larger the St number of particle is, the wider the range of the particle dispersion will be. The computed results are in agreement with the experimental ones.     

PREDICTION OF PARTICLE TRANSPORT IN ENCLOSED ENVIRONMENT

Prediction of particle transport in enclosed environment is crucial to the welfare of its occupants. The prediction requires not only a reliable particle model but also an accurate flow model. This paper introduces two categories of flow models-Reynolds Averaged Navier-Stokes equation modeling （RANS modeling） and Large Eddy Simulation （LES）; as well as two popular particle models-Lagrangian and Eulerian methods. The computed distributions of air velocity, air temperature, and tracer-gas concentration in a ventilated room by the RANS modeling and LES agreed reasonably with the experimental data from the literature. The two flow models gave similar prediction accuracy. Both the Lagrangian and Eulerian methods were applied to predict particle transport in a room. Again, the computed results were in reasonable agreement with the experimental data obtained in an environmental chamber. The performance of the two methods was nearly identical. Finally the flow and particle models were applied to study particle dispersion in a Boeing 767 cabin and in a small building with six rooms. The computed results look plausible.     

Evaluation of the effect of wall boundary conditions on numerical simulations of circulating fluidized beds

A computational fluid dynamics （CFD） modeling of the gas-solids two-phase flow in a circulating fluidized bed （CFB） riser is carried out. The Eularian-Eularian method with the kinetic theory of granular flow is used to solve the gas-solids two-phase flow in the CFB riser. The wall boundary condition of the riser is defined based on the Johnson and Jackson wall boundary theory （Johnson ＆ Jackson, 1987） with specularity coefficient and particle-wall restitution coefficient.The numerical results show that these two coefficients in the wall boundary condition play a major role in the predicted solids lateral velocity, which affects the solid particle distribution in the CFB riser. And the effect of each of the two coefficients on the solids distribution also depends on the other one. The generality of the CFD model is further validated under different operatin~ conditions of the CFB riser.     

ENERGY AND MASS TRANSPORT PROCESSES IN THE GRANULAR BED OF AN INDIRECTLY HEATED ROTARY KILN

The transport mechanisms of momentum,mass,species,and energy are invertigated in detail for the ro-tary kiln process.The residence time prediction of the granular bed is well improved by considerng differet flow patterns in the drum.Introducing a mixed flow pattern of the basic slipping and slumping behaviour has the most important effect on the improvement of the residence time prediction.The granular bed is assumed to hehave as a Bingham fluid in the active layer of the bed.The transport mechanisms of momentum,species,and energy are modelled on the basis of this assumption and using the kinetic gas theory.Additionally,a mathematical transformation is presented to save comput a-tional time.The model results of the temperature field are in very good agreement with experimental data.     

Particle modulations to turbulence in two-phase round jets

The particle modulations to turbulence in round jets were experimentally studied by means of two-phase velocity measurements with Phase Doppler Anemometer （PDA）. Laden with very large particles, no significant attenuations of turbulence intensities were measured in the farfields, due to small two-phase slip velocities and particle Reynolds number. The gas-phase turbulence is enhanced by particles in the near-fields, but it is significantly attenuated by the small particles in the far-fields. The smaller particles have a more profound effect on the attenuation of turbulence intensities. The enhancements or attenuations of turbulence intensities in the far-fields depends on the energy production, transport and dissipation mechanisms between the two phases, which are determined by the particle prop- erties and two-phase velocity slips. The non-dimensional parameter CTI is introduced to represent the change of turbulence intensity.     

Solids flow diagram of a CFB riser using Geldart B-type powders

Riser operating modes are vital to designing a circulating fluidized bed (CFB) reactor for a required process of either a gas-solid or a gas-catalytic nature. Different operating modes provide different solids’ residence times and mixing behaviors, which define the reactions’ efficiency and yield. The literature demonstrates distinct operating modes resulting from observed differences in slip factors and the range of particle velocities and their associated residence time distribution. The present research uses positron emission particle tracking (PEPT) in a riser of B-type bed material to determine the different operating modes by measuring (i) particle velocities and residence time distribution, (ii) population densities of these particles in the cross-sectional area of the riser, and (iii) solids flow pattern at the bottom of the riser. Data treatment defines four distinct solids hold-up regimes in the riser and proposes a "phase diagram" depicting the existence of the different operating modes (dilute, dense, core-annulus and combined) as a function of the superficial gas velocity and solids circulation flux in the riser. The delineated regimes have good agreement with available literature data and known industrial operations. Comparison with literature data for risers using A-type powders is also fair. The diagram enables CFB designers to better delineate operating characteristics.     

Calibration of granular material parameters for DEM modelling and numerical verification by blade-granular material interaction

The discrete element method (DEM) is a promising approach to model blade-granular material interactions. The accuracy of DEM models depends on the model parameters. In this study, a calibration process was developed to determine the parameter values. The particle size was the same as the real material and the particle shape was modelled using two spherical particles rigidly clumped together to form a single grain. Laboratory shear tests and compressions tests were used to determine the material internal friction angle and stiffness, respectively. These tests were replicated numerically using DEM models with different sets of particle friction coefficients and particle stiffness values. The shear test results are found to be dependent on both the particle friction coefficient and the particle stiffness. The compression test results show that it is only dependent on the particle stiffness. The combination of shear test and compression test results can be used to determine a unique set of particle friction and particle stiffness values. The calibration process was validated experimentally and numerically by modelling a blade moving through granular material. Results show that the forces acting on the blade can be accurately modelled with DEM and the maximum error is found to be 26%. The relative particle-blade displacements were used to predict the position and shape of the shear lines in front of the blade. A good qualitative correlation was achieved between the experiments and the DEM simulations.     

GPU-enhanced DEM analysis of flow behaviour of irregularly shaped particles in a full-scale twin screw granulator

During twin screw granulation (TSG), small particles, which generally have irregular shapes, agglomerate together to form larger granules with improved properties. However, how particle shape impacts the conveying characteristics during TSG is not explored nor well understood. In this study, a graphic processor units (GPUs) enhanced discrete element method (DEM) is adopted to examine the effect of particle shape on the conveying characteristics in a full scale twin screw granulator for the first time. It is found that TSG with spherical particles has the smallest particle retention number, mean residence time, and power consumption; while for TSG with hexagonal prism (Hexp) shaped particles the largest particle retention number is obtained, and TSG with cubic particles requires the highest power consumption. Furthermore, spherical particles exhibit a flow pattern closer to an ideal plug flow, while cubic particles present a flow pattern approaching a perfect mixing. It is demonstrated that the GPU-enhanced DEM is capable of simulating the complex TSG process in a full-scale twin screw granulator with non-spherical particles.     

Explosive dispersal of solid particles

Abstract. The rapid dispersal of inert solid particles due to the detonation of a heterogeneous explosive, consisting of a packed bed of steel beads saturated with a liquid explosive, has been investigated experimentally and numerically. Detonation of the spherical charge generates a blast wave followed by a complex supersonic gas-solid flow in which, in some cases, the beads catch up to and penetrate the leading shock front. The interplay between the particle dynamics and the blast wave propagation was investigated experimentally as a function of the particle size (100–925 m) and charge diameter (8.9–21.2 cm) with flash X-ray radiography and blast wave instrumentation. The flow topology during the dispersal process ranges from a dense granular flow to a dilute gas-solid flow. Difficulties in the modeling of the high-speed gas-solid flow are discussed, and a heuristic model for the equation of state for the solid flow is developed. This model is incorporated into the Eulerian two-phase fluid model of Baer and Nunziato (1986) and simulations are carried out. The results of this investigation indicate that the crossing of the particles through the shock front strongly depends on the charge geometry, the charge size and the material density of the particles. Moreover, there exists a particle size limit below which the particles cannot penetrate the shock for the range of charge sizes considered. Above this limit, the distance required for the particles to overtake the shock is not very sensitive to the particle size but remains sensitive to the particle material density. Overall, excellent agreement was observed between the experimental and computational results. Received 16 August 1999 / Accepted 26 June 2000     

Assessing mixing characteristics of particle-mixing and granulation devices

The mixing of particulates such as powders is an important process in many industries including pharmaceuticals, plastics, household products (such as detergents) and food processing. The quality of products depends on the degree of mixing of their constituent materials which in turn depends on both geometric design and operating conditions. Unfortunately, due to lack of understanding of the interaction between mixer geometry and the granular material, limited progress has been made in optimizing mixer design. The discrete element method (DEM) is a computational technique that allows particle systems to be simulated and mixing to be predicted. Simulation is an effective way of acquiring information on the performance of different mixers that is difficult and/or expensive to obtain using traditional experimental approaches. Here we demonstrate how DEM can be used to unravel flow dynamics and assess mixing in several different types of devices. These devices used for mixing and/or granulation of particulates, are classified broadly as gravity controlled, bladed and high shear. We also explore the role of particle shape in mixing performance and use DEM to test whether Froude number scaling is suitable for predicting scale performance of rotating mixers.     

含颗粒污染物的油液均质流的数值分析

含颗粒污染物的油液是浓度很稀、粒径极小的伪均质流,为了掌握颗粒污染物在输送过程中的浓度分布,利用一维扩散方程构建了污染油液的数学模型;通过特征线法数值求解,获得了污染油液中各相的动态特征。结果表明,油液压力和速度沿管长呈脉动规律运动,且随着时间的延长逐渐衰减;颗粒污染物对油液速度具有极好的跟随特性;颗粒污染物的浓度分布也随着油液流速的变化而呈现规律性的变化;在不同运行时间内油液压力沿管长的衰减趋势不同,油液速度沿管长的变化趋势与压力的趋势相反;颗粒污染物速度和浓度分布沿管长与油液速度具有紧密的联系。     

Three-dimensional PIV measurement of flow around an arbitrarily moving body

A three-dimensional (3D) particle image velocimetry measurement technique capable of simultaneously monitoring 3D fluid flows and the structure of an arbitrarily moving surface embedded in the flow was proposed with a heavy emphasis on image processing methods. The costs associated with the experimental apparatus were reduced by recording the surface and the trace particles at one image plane without the use of additional cameras or illumination devices. An optimal exposure time for surface and particle imaging was identified using red fluorescent tracer particles in conjunction with a long-pass glass filter. The particle image and surface image were then separated using an image separation process that relied on the feature scaling differences between the particles and the surface texture. A feature detection process and a matching process facilitated estimation of the 3D surface points, and the 3D surface structure was modeled by Delaunay triangulation. The particle volume reconstruction algorithm constrained the voxels inside the surface structure to zero values to minimize ghost particle generation. Volume self-calibration was employed to improve the reconstruction quality and the triangulation accuracy. To conserve computing resources in the presence of numerous zero voxels, the MLOS-SMART reconstruction and the direct non-zero voxel cross-correlation method were applied. Three-dimensional experiments that modeled the flows around an eccentric rotating cylinder and a flapping flag were conducted to validate the present technique.     

Wavefront sensing for single view three-component three-dimensional flow velocimetry

We present the application of wavefront sensing to particle image velocimetry for three-component (3C), three-dimensional (3D) flow measurement from a single view. The technique is based upon measuring the wavefront scattered by a tracer particle and from that wavefront the 3D tracer location can be determined. Hence, from a temporally resolved sequence of 3D particle locations the velocity vector field is obtained. Two approaches to capture the data required to measure the wavefronts are described: multi-planar imaging using a distorted diffraction grating and an anamorphic technique. Both techniques are optically efficient, robust and compatible with coherent and incoherent scattering from flow tracers. The depth (range) resolution and repeatability have been quantified experimentally using a single mode fiber source representing a tracer particle. The anamorphic approach is shown to have the greatest measurement range and hence was selected for the first proof of principle experiments using this technique for 3D particle imaging velocimetry (PIV) on a sparsely seeded gas phase flow.