Effect of high stirring speed on the agglomerate behaviors for cohesive SiO2 powders in gas fluidization

The effects of superficial gas velocity and mechanical stirring speed on the precise regulation of flow regimes for cohesive SiO₂ powders (mean diameter is 16 μm) were experimentally investigated in a stirring-assisted fluidized bed. The results showed that compared with the agglomerates formed in the non-assisted fluidization of cohesive SiO₂ powders, the introduction of mechanical stirring could effectively reduce the size of agglomerates and well disperse the agglomerates during fluidization. The best regulation range of agglomerate particulate fluidization can be achieved at 600 rpm when agglomerate sizes were reduced to below 200 μm. Further investigation based on the operational phase diagram revealed that transformations of flow regimes were dominated by both stirring speed and gas velocity. The stirring applied enlarges the operational range of agglomerate particulate fluidization (APF) with a delayed onset of bubbling for cohesive particles. However, the exorbitant speed increases the collision velocity and contact area between small agglomerates, which results in the formation of unstable agglomerates and the whirlpool of powder.     

Experimental and numerical study of the rotation and the erosion of fillers suspended in viscoelastic fluids under simple shear flow

When a porous agglomerate immersed in a fluid is submitted to a shear flow, hydrodynamic stresses acting on its surface may cause a size reduction if they exceed the cohesive stress of the agglomerate. The aggregates forming the agglomerate are slowly removed from the agglomerate surface. Such a behaviour is known when the suspending fluid is Newtonian but unknown if the fluid is viscoelastic. By using rheo-optical tools, model fluids, carbon black agglomerates and particles of various shapes, we found that the particles had a rotational motion around the vorticity axis with a period which is independent on shape (flat particles not considered), but which is exponentially increasing with the elasticity of the medium expressed by the Weissenberg number (We). Spherical particles are always rotating for We up to 2.6 (largest investigated We in this study) but elongated particles stop rotating for We>0.9 while orienting along the flow direction. Erosion is strongly reduced by elasticity. Since finite element numerical simulation shows that elasticity increases the local stress around a particle, the origin of the erosion reduction is interpreted as an increase of cohesiveness of the porous agglomerate due to the infiltration of a viscoelastic fluid.     

A novel preparation of nanosized hexagonal Mg(OH)2 as a flame retardant

Nanosized dispersive hexagonal magnesium hydroxide (Mg(OH)₂) has been prepared using an ammonia-hydrothermal method. Citric acid and monoethanolamine (MEA) were added to the reaction system during the ammonia precipitation and hydrothermal processes, respectively, to improve the crystallinity and dispersion of the (Mg(OH)₂) particles. The resulting Mg(OH)₂ samples obtained under the optimum preparation conditions were characterized by scanning electron microscopy, X-ray diffraction and thermal gravity analysis, which showed that this newly developed procedure afforded well-dispersed hexagonal nanoplates of Mg(OH)₂ with a mean diameter of 246 nm.     

Homogeneous nanoparticle dispersion prepared with impurity-free dispersant by the ball mill technique

The homogeneous dispersion of nanoparticles in solvents or polymer matrices is essential for practical application of nanocomposites. In this study, the planetary ball milling technique was used to de-agglomerate silica nanoparticles in butyl acetate. The size of the nanosilica aggregates was evaluated by TEM and SEM. With the addition of polyacrylate polymer to the organic solvent, the nanoparticle agglomerates were effectively broken up by planetary ball milling at the proper milling time; however, re-agglomeration occurred after a longer milling time. The results of TGA and FTIR indicated that the polyacrylate molecules could be adsorbed in situ onto the nanoparticles. Behaving similar to a dispersant, the adsorbed polyacrylate reduced the blend viscosity significantly and prevented re-agglomeration of the nanoparticles. Utilizing the polyacrylate polymer both as the dispersant and the polymer matrix, the polyacrylate-based nanocoatings were further prepared. The optical transmittance and haze value of the nanocoatings were found to be sensitive to the dispersion level of the nanoparticles, and the elastic modulus and hardness of the nanocoatings were improved in comparison with those of the neat polymer coating.     

Break-Up of Aerosol Agglomerates in Highly Turbulent Gas Flow

Agglomerate aerosols in a turbulent flow may be subjected to very high turbulent shear rates which through the generation of lift and drag can overcome the adhesive forces binding the constituents of an agglomerate together and cause it to break-up. This paper presents an analysis of the experimental measurements of the breakup of agglomerates between 0.1?C10???m in size, in a turbulent pipe flow followed by an expansion zone with a Reynolds numbers in the range 10⁵ to 10⁷. The analysis shows that even in wall bounded turbulence, the high turbulent shear stresses associated with the small scales of turbulence in the core can be the main source of breakup preceding any break-up that may occur by impaction at the wall. More importantly from these results, a computationally fast and efficient solution is obtained for the General Dynamic Equation (GDE) for agglomerate transport and breakup in highly turbulent flow. Furthermore the solution for the evolution of the aerosol size distribution is consistent with the experimental results. In the turbulent pipe flow section, the agglomerates are exposed continuously to turbulent shear stresses and experience more longer term breakup than in the expansion zone (following the pipe flow) where the exposure time is much less and break-up occurs instantaneously under the action of very high local turbulent shear stresses. The validity of certain approximations made in the model is considered. In particular, the inertia of the agglomerates characterised by a Stokes Number from 0.001 for the smallest particles up to 10 for 10???m particles and the fluctuations of the turbulent shear stresses are important physical phenomena which are not accounted for in the model.     

Dynamic mechanical and fracture properties of an infiltrated TiC-1080 steel cermet

The dynamic mechanical and fracture properties of a TiC porous network infiltrated with1080 steel are reported. Following infiltration, the cermet is subjected to various heat treatments that affect essentially the steel matrix. Dynamic compression tests show that the heat treatments increase the fracture strength of the cermet. The quasi-static fracture toughness (K_Ic) is also increased by the heat treatments. The dynamic (initiation) fracture toughness (K_Id) is substantially higher (by about a factor of 3) than its static counterpart. Failure mechanisms consist mainly of cleavage of the TiC and matrix grains, along with minor interfacial decohesion. However, dynamic loading induces substantial damage around the crack tip, consisting essentially of cleavage of TiC grains. Microcrak toughnening is believed to be responsible for the high dynamic toughness of the material. The critical microstructural fracture event is thus identified as the spreading of TiC cleavage microcracks into the neighboring steel grains.     

MULTI-SCALE AGGREGATION OF PARTICLES IN GAS-SOLIDS FLUIDIZED BEDS

The multi-scale characteristics of clusters in a fast fluidized bed and of agglomerates in a fluidized bed of cohesive particles are discussed on the basis of large amounts of experiments.The cluster size and concentration are dominated by the local voidage of the bed.A cluster consists of many sub-clusters with different sizes and discrete par-ticles,and the sub-cluster size probability density distribution appears as a negative exponential function.The agglom-erates in a fluidized bed of cohesive particles also possess the multi-scale nature.The large agglomerates form a fixed bed at the bottom,the medium agglomerates are fluidized in the middle,and the small agglomerates and discrete parti-cles become the dilute-phase region in the upper part of the bed.The agglomerate size is mainly affected by cohesive forces and gas velocity.The present models for prediction the size of clusters and agglomerates can not tackle the in-trinsic mechanism of the multi-scale aggregation,and a challenging problem for establishing mechanistic model is put forward.     

颗粒聚合体碰撞破损的细观力学仿真研究

粉末和颗粒材料常常以聚合体(agglomerate,还可译成结块, 聚团等)的形式存在.无论是自然环境中还是工业处理过程中, 微小颗粒聚合体碰撞破损是一常见物理现象.近十几年来, 对颗粒聚合体碰撞研究在试验和数值模拟方面均取得了很大的进展.特别是利用颗粒离散元方法, 结合经典接触力学理论, 对微米颗粒聚合体碰撞破损的细观力学机理进行的研究, 取得了很多重要成果.基本形成了较为完善的模拟分析方法, 提出了许多新概念, 形成了目前适于分析研究的专用分析程序.本文介绍了目前国际上颗粒聚合体碰撞破损模拟研究的一般方法和理论, 总结了现有的主要研究内容及成果, 并提出一些研究展望.      

Shear and extensional rheology of carbon nanofiber suspensions

The morphology and rheology of carbon nanofiber (CNF) suspensions were studied. The CNFs, produced by decomposing organic vapors at elevated temperature in the presence of metal catalysts, have characteristic diameter and length of 100 nm and 20–100 m, respectively. The CNFs, as delivered, have a strong tendency to clump into mm-sized agglomerates. The efficacy of CNF/glycerol-water suspensions was studied vs. two processing parameters: mechanical sonication and chemical treatment. Experimental measurements revealed that sonication alone reduces the size of CNF clumps from millimeter to micrometer scale, but cannot achieve uniform dispersion. The chemically untreated sonicated suspensions contain clumps of nanofibers with a characteristic size of 20×50 m, together with smaller aggregations of partially dispersed nanofibers. In response to this unsatisfactory dispersion, the effect of acid treatment before dispersion was investigated. This acid treatment, which makes the surface of the CNFs more hydrophilic, greatly improves dispersion in the aqueous solution: treatment followed by sonication results in a uniform dispersion of individual nanofibers. At the same time, however, we observed that surface treatment and subsequent sonication greatly shorten the nanofibers.The rheology of CNF/glycerol-water suspensions is highly non-Newtonian both in shear and extensional flows, with strong dependence on the dispersion, particle length, and concentration of the CNFs. As the solvent is Newtonian, all of the elastic and strain-rate dependent behavior in the CNF/aqueous suspensions derives from the addition of nanofibers. The steady shear viscosity of the untreated-sonicated (poorly dispersed, with longer fibers) suspensions is highly shear thinning with a viscosity that increases three orders of magnitude as concentration varies from 0.5 wt% to 5 wt%. Beyond 5 wt% the suspensions are too viscous to be effectively mixed by sonication. When the CNFs are chemically treated and then sonicated (resulting in much better dispersion but shorter fibers), the viscosity exhibits little shear thinning, and only varies by a factor of two from pure solvent to 5 wt%. In small amplitude oscillatory shear measurements, we found strong indications of elastic behavior in both the treated and untreated suspensions, with elastic modulus G greater than loss modulus G. In particular, for both systems G exhibits a low-frequency plateau when nanofiber concentration is 3 wt% or above, a characteristic of elastic solidlike response. Again, there is a strong dependence on CNF dispersion and fiber length: At low frequencies, the elastic modulus of the 5 wt% untreated suspension (with agglomerates and longer fibers) is four orders of magnitude larger than that of the 5 wt% treated suspension (with uniformly dispersed, shorter fibers). In addition, G of untreated suspensions is a much stronger function of concentration than that of treated suspensions, indicative of network formation.The rheology and morphology of nanofiber suspensions were related by identifying morphology of the suspensions with the assumptions of the kinetic theory-based elastic and rigid dumbbell constitutive models; the approach is to specify the parameters in the kinetic theory models in terms of microscale morphological features measured in the SEM. Of those investigated, the elastic dumbbell model with anisotropic hydrodynamic drag is the most successful, effectively modeling the small amplitude oscillatory shear and steady shear behavior of the treated sonicated suspensions. As for the treated unsonicated and untreated sonicated suspensions, which contain mesoscale agglomerates not present in the underlying assumptions of the dumbbell models, it is discovered that the elastic dumbbell with parameters assigned from morphological measurements predicts the correct trends in the steady shear experiments, but fails to accurately predict the small amplitude oscillatory shear experiments.     

Study of material agglomeration during nozzle injection of multiviscosity liquid into a fluidized bed reactor by the electric conductance method

Fluidized bed agglomeration is an important and challenging problem for thermal cracking in fluid cokers. A low coker temperature can be problematic because the bitumen is injected into the fluidized bed with a different viscosity, resulting in formation of agglomerates of varying sizes, which slows the cracking reactions. In the present study, the bed material agglomeration process during nozzle injection of multiviscosity liquid was investigated in a fluidized bed operated at different mass ratios of the atomization gas to the liquid jets (GLR = 1%–3.5%) and gas velocities (3.9U_mf and 5.9U_mf) based on a conductance method using a water–sand system to simulate the hot bitumen–coke system at room temperature. During the tests of liquid-jet dispersion throughout the bed, different agglomeration stages are observed at both gas velocities. The critical amount of tert-butanol in the liquid jets that could lead to severe agglomeration of the bed materials (poor fluidization) at GLR = 1% is about 10 wt% at the low fluidizing gas velocity (3.9U_mf) and 18 wt% at the high gas velocity (5.9U_mf). This study provides a new approach for on-line monitoring of bed agglomeration during liquid injection to guarantee perfect contact between the atomized liquid and the bed particles.     

Composition and morphology of the thermal decomposition products of 3Mg(OH)2·MgCl2·8H2O nanowires

The thermal decomposition of 3Mg(OH)₂·MgCl₂·8H₂O (318MHCH) nanowires synthesized from agglomerated Mg(OH)₂ microspheres was investigated. The influence of heating rate and temperature on the composition and morphology of the products was investigated. Thermogravimetric-differential scanning calorimetry, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray diffraction showed that increasing the heating rate from 1 to 20 °C/min promoted the escape of crystalline water from the 318MHCH nanowires. 318MHCH nanowires were dehydrated stepwise to 310MHCH porous nanowires from room temperature to 320 °C, and then to MgO cubic nanoparticles from 420 to 700 °C. The nanowires retained their one-dimensional morphology, until the phase changed to MgO. The immediate collapse of the one-dimensional structure was attributed to the presence of Mg–O/Cl chains.     

Microstructure and fracture characteristic of Mg–Al–Zn–Si3N4 composites

Magnesium matrix composites reinforced by three dimensional (3-D) network structure were fabricated by pressure-less infiltration technology. The 3-D network structure reinforcement and its composites exhibited special topology structure and different fracture characteristic. Metal matrix fractured in a ductile mode is manifested by small dimples and craters on the fracture surface. When the volume fraction of reinforcement is not in excess of 6%, the composite had an improved fracture toughness. This is because of the relatively homogeneous Si₃N₄ particles distributed in the metal matrix and the occurrence of interface reaction product such as MgAlO₂ spinel phase. With the increases of volume fraction of reinforcement (>6%), the fracture toughness decreases slowly at the initial stages and then decreases rapidly towards the end. Therefore, the main fracture failure mechanisms consist of crack nucleation, growth, coalescence and crack propagation.     

First observations on break-up of particle agglomerates in shock waves

The destruction of solid agglomerates suspended in argon by means of shock waves (1 <M _s < 5) has been studied experimentally using a conventional shock tube apparatus. The change in particle size was measured by time-dependent in situ laser light scattering of the particle ensemble in the shock wave. The test facilities used are given in details, and the principle of the Mie-theory, necessary for complete understanding of the data reduction, is described. In this paper data reduction and first analysis employing two independent trial solutions involving the Mie-theory for polydisperse, spherical particle-ensembles are presented. Three types of agglomerates suspended in argon with a mean initial size of about 1 gm were exposed to the shock wave and different size reduction behaviours were observed. The deagglomeration effect depends largely on the wave intensity and on the physico-chemical structure of the particle aggregates.     

A critical review of dispersion in packed beds

The phenomenon of dispersion (transverse and longitudinal) in packed beds is summarized and reviewed for a great deal of information from the literature. Dispersion plays an important part, for example, in contaminant transport in ground water flows, in miscible displacement of oil and gas and in reactant and product transport in packed bed reactors. There are several variables that must be considered, in the analysis of dispersion in packed beds, like the length of the packed column, viscosity and density of the fluid, ratio of column diameter to particle diameter, ratio of column length to particle diameter, particle size distribution, particle shape, effect of fluid velocity and effect of temperature (or Schmidt number). Empirical correlations are presented for the prediction of the dispersion coefficients (D _T and D _L) over the entire range of practical values of Sc and Pe_m, and works on transverse and longitudinal dispersion of non-Newtonian fluids in packed beds are also considered.     

Non-passive Transport of Volatile Organic Compounds Infiltrated from a Surface Disk Source

An unsaturated flow and non-passive transport model for water-soluble organic compounds has been implemented in cylindrical coordinates, with a top boundary condition that accounts for different zones of the surface that can be under infiltration or volatilization independently. We simulated two-dimensional infiltration of aqueous mixtures of methanol from a disk source, its redistribution and volatilization in both homogeneous and heterogeneous soils. Simulations showed that the incoming composition significantly affects volumetric liquid content and concentration profiles, as well as the fraction of infiltrated mass of methanol that is released to the atmosphere. Concentration-dependent viscosity had the major impact on the liquid flow. The differences in volumetric liquid content and normalized concentration of methanol became more pronounced during transport through a soil composed of a clay lens embedded within a main matrix of sandy clay loam texture. Dispersion in the liquid-phase was the predominant transport mechanism when dispersivity at saturation was set to 7.8 cm. However, for dispersivity of 1.0 cm, changes in composition led to changes in surface tension inducing significantly higher liquid flow. In this case, liquid-phase advection was the most active transport mechanism for homogeneous soils and highly concentrated infiltrating mixtures.     

Ultrasound-enhanced milling in the synthesis of phase-pure,highly crystalline ZnAl-layered double hydroxide of low Zn(II) content

Dry milling followed by ultrasonic irradiation in the presence of small amounts of aqueous NaOH, was used to synthesise ZnAl-layered double hydroxide (LDH). The synthetic conditions were optimised and the Zn(OH)₂, ZnO, and Al(OH)₃ contaminants were removed by rinsing with aqueous NH₃ solution from the sample prepared in the optimum conditions. Thus, phase-pure ZnAl-layered double hydroxide was obtained with high crystallinity. All materials produced during the syntheses were characterised by X-ray diffractometry, and a rose-like morphology was observed by scanning electron microscopy in the phase-pure sample. By changing the initial Zn/Al ratio, it was revealed that an LDH with low zinc content was always formed, probably through substitution of some of the Zn²⁺ ions by Al³⁺ ions in the gibbsite lattice.     

Viscoplasticity of agglomerated suspensions

This work is a theoretical study on the effects of agglomeration on the fluidity and plasticity of a suspension of neutrally buoyant particles in a Newtonian fluid. The dynamics of a cluster of permanently attached spherical particles in a simple shear field is analyzed. The viscous and plastic components of the drag force acting on each of the agglomerated particles is then calculated and found to depend on the size of the individual particle unit, its location being relative to the center of the cluster and the material properties of the engulfing fluid. This information in conjunction with the knowledge of the interparticle cohesive forces is used to establish criteria for the agglomerate size reduction during dispersive mixing. From the kinematics of the cluster movement and the forces acting on its particulate components the rate of energy dissipation is calculated and utilized to evaluate the viscosity and yield stress of the suspension. These rheological parameters depend on the volume fraction and architecture of the agglomerate, the number of fused particles per cluster, and the viscosity of the suspending fluid. The analysis is also extended to include the case of polydispersity of agglomerate sizes.     

基于光学诊断的某新型含铝推进剂燃烧特性分析

为探究某新型含铝固体推进剂燃烧特性和规律,在模拟固体发动机的高压条件下,采用可调功率激光器结合高速摄影、发射光谱等光学诊断技术对该新型含铝固体推进剂开展了系统的点火及燃烧过程研究。通过对该推进剂的点火延迟、退移速率、燃烧温度以及团聚物颗粒尺寸的定量测量和分析,明确了该推进剂的点火延迟量级;证实此推进剂的退移速率严格遵循Summerfield燃速公式;判断出其最高燃烧温度高于3 300 K,且随压力增大而升高;通过对燃烧过程中发光凝聚相产物面积的量化分析得出推进剂产物中团聚物粒径尺寸受环境参数的影响规律。     

Impact Behaviour of GLAREs with MWCNT Modified Epoxy Resins

In this study, GLAREs (Glass Reinforced Aluminum Laminates) with 3/2 configuration were fabricated in-house with multi-walled carbon nanotubes (MWCNTs) modified epoxy resins. Uniform dispersion of MWCNTs in epoxy resin was achieved via a two-step dispersion method with concentration up to 2.0 wt%. The influence of MWCNTs on the flexural property and the impact performance of GLAREs was investigated through the three-point bending and drop weight Dynatup impact testings, respectively. The incorporation of MWCNTs into epoxy evidently improved the flexural strength and modulus. In comparison with pure epoxy bonded GLARE, the modified GLAREs generally showed improvement of impact resistance. The improvement was more obvious at low concentration of MWCNT due to better dispersion of nanotubes in the resin and reasonable wettability of glass fibres and aluminium to the modified resin. Diversified failure mechanisms including plastic deformation and rupture of metal layers, breakage of fibres and matrix, delamination between composite and metal layers, and delamination between composite plies were observed. In addition, debonding, pull-out, and bridging effects of carbon nanotubes were observed, which proved the contribution of MWCNTs to the improved impact resistance of the modified GLAREs.     

Effects of bubble deformation on the viscosity of dilute suspensions

The relative viscosity (μ_rel=suspension viscosity/suspending fluid viscosity) of low Reynolds number, dilute and surfactant-free bubble suspensions in simple shear is studied with a rotating cylinder, Couette rheometer. The conditions of the experiments correspond to capillary numbers (Ca) of order 1 and bridge previous experimental, theoretical and numerical results that focused on either Ca⪡1 or Ca⪢1. The suspensions are shear thinning with μ_rel>1 for small Ca. At large Ca, μ_rel approaches a constant that is less than 1. These results are explained by a scaling analysis that considers how regions of viscous dissipation in and around bubbles change as bubbles are deformed by the flow.