Determining a representative element volume for DEM simulations of samples with non-circular particles

Numerical studies on the number of particles or system size required to attain a representative element volume (REV) for discrete element method (DEM) simulations of granular materials have almost always considered samples with spherical or circular particles. This study considers how many particles are needed to attain a REV for 2D samples of 2-disc cluster particles where the particle aspect ratio (AR) was systematically varied. Dense and loose assemblies of particles were simulated. The minimum REV was assessed both by considering the repeatability of static packing characteristics and the shearing behaviour in biaxial compression tests, and by investigating the effect of sample size on the measured characteristics and observed shearing behaviour. The repeatability of the data considered generally improved with increasing sample size. The packing characteristics of the dense samples were more repeatable suggesting that the minimum REV reduces with increasing packing density. The minimum REV was observed to be sensitive to the characteristic measured. Although the overall responses of the samples during shear deformation were similar irrespective of the sample sizes, the smaller the sample size, the higher the fluctuations observed in the responses. Analysis of the coefficient of variation of the fluctuations around the critical state stress ratio can provide insight as to whether a REV is attained. The particle AR influences the effect of sample size on shearing characteristics and thus the minimum number of particles required to attain a REV; this can be explained by the influence of AR on the number of contacts within the samples.     

Evaluation of particle packing models by comparing with published test results

The existing particle packing density models each with two or more parameters accounting for certain particle interactions （the loosening effect parameter, wall effect parameter, wedging effect parameter, and compaction index, denoted by a, b, c, and K, respectively） may be classified into the 2-parameter model （with a and b incorporated）, the compressible model （with a, b, and 1（incorporated）, and the 3- parameter model （with a, b, and c incorporated）. This paper evaluates these models by comparing their respective packing density predictions with the test results published in the literature. It was found that their accuracy varies with both the size ratio and volumetric fractions of the binary mix. In general, when the size ratio is larger than 0.65, all the packing models are sufficiently accurate. However, when the size ratio is smaller than 0.65, some of them become inaccurate and the errors tend to be larger at around the volumetric fractions giving maximum packing density. Relatively, the 3-parameter model is the most accurate and widely applicable.     

Measurement of local size and velocity probability density distributions in two-phase suspension flows by Laser-Doppler technique

A technique is presented for the simultaneous measurement of the local number and velocity probability densities of a dilute two-phase suspension which has a distribution of particle sizes and a predominate direction of flow orientation such as in the cases of pipe and boundary-layer flows. It is shown that by a suitable scheme of discrimination on the amplitude as well as the residence time and frequency of the individual Laser-Doppler bursts, one can obtain the statistics on the size number density distribution and, for each size range, velocity distribution of the particulate phase together with the velocity probability distribution of the fluid phase.Results have been obtained for experiments conducted on a laminar uniform flow and a turbulent shear flow of a dilute glass particle-water suspension having a particle size distribution. Calibration needed for the scheme was accomplished by analyzing particle size and number density distribution data obtained from a Coulter particle sizing counter on a sample taken with an isokinetic probe.     

Evaluation of particle packing models by comparing with published test results

The existing particle packing density models each with two or more parameters accounting for certain particle interactions (the loosening effect parameter, wall effect parameter, wedging effect parameter, and compaction index, denoted by a, b, c, and K, respectively) may be classified into the 2-parameter model (with a and b incorporated), the compressible model (with a, b, and K incorporated), and the 3-parameter model (with a, b, and c incorporated). This paper evaluates these models by comparing their respective packing density predictions with the test results published in the literature. It was found that their accuracy varies with both the size ratio and volumetric fractions of the binary mix. In general, when the size ratio is larger than 0.65, all the packing models are sufficiently accurate. However, when the size ratio is smaller than 0.65, some of them become inaccurate and the errors tend to be larger at around the volumetric fractions giving maximum packing density. Relatively, the 3-parameter model is the most accurate and widely applicable.     

分子筛振动装填密度实验研究

实验研究了粒径分布范围在1.3mm～3.0mm之间的分子筛在振动装填过程中,各种振动参数包括振动频率、振动幅度和振动时间等对分子筛装填密度的影响,并得到最佳的振动装填工艺参数。结果表明,随着频率和振幅增大,装填密度先增加后减小,存在最佳值。在本文实验条件下,频率和振幅的最佳值分别为55Hz和0.6mm。装填密度随振动时间的增大逐渐增加并趋于定值。这些可以作为确定该粒径范围内分子筛振动装填工艺参数的基础。     

Viscoelasticity and diffusion in miscible blends of saturated hydrocarbon polymers

Linear viscoelasticity and tracer diffusion were investigated as functions of temperature, component molecular weight and blend composition for entangled, single-phase blends of nearly monodisperse poly(ethylene-alt-propylene) (PEP) and head-to-head polypropylene (HHPP). Both components are non-polar and, despite evidence for slight differences of component glass temperatures in their blends, the viscoelastic data obey time-temperature superposition rather well. The properties of the blends were compared at constant T-T _g (blend) with predictions of the tube-model theories. The composition dependence of viscosity agrees best with the double-reptation prediction, as had been found earlier for molecular weight blends. The variation in plateau modulus with composition is consistent with reptation, but the changes are too small to provide a definitive test. The tracer diffusion coefficients, D ^* _PEP and D ^* _HHPP are nearly independent of composition, consistent with the reptation prediction and in sharp contrast with tracer diffusion for blends with specific associations. Results for the recoverable compliance depart from this pattern, varying differently and much less strongly with composition than the predictions of either single or double reptation. It thus seems that microstructural blends may behave in significantly more complex ways than molecular weight blends even for components with only weak dispersive interactions and rather modest differences in glass temperature and plateau modulus.Dedicated to Prof. John D. Ferry on the occasion of his 85th birthday.     

Aerosol formation in an isothermal stagnation flow

An experiment has been performed in a laminar stagnation point flow in which two non-premixed reactants produced an aerosol of sub-micron particles. The reactants were NH₃ and HCl. The rate of mixing of the reactants was determined by the velocity gradient or strain rate of the flow; the response of the aerosol dynamics to the flow field was measured with a laser light scattering technique. Laser Doppler Spectroscopy was used to measure the particle size. It was found that the particle size was independent of the strain rate of the flow. On the other hand, the particle number density decreased as the strain rate increased. It is argued that the intensity of light scattered from the aerosol is, therefore, a measure of the amount of product of the relatively slow NH₃-HCl reaction.     

Shear stability of metallic glasses

The effect of sample size on the shear deformation and compressive plasticity of different metallic glasses were investigated. The experimental results showed that the deformation and fracture behaviors of samples prepared from chemically identical Zr-, Ti-, Fe-, or Mg-based metallic glass ingots were strongly dependent on the sample size and machine stiffness, and a super-high compressive plasticity was achieved in the Zr-based metallic glasses with sample size of 1.0 mm in width. It is also found that the sample size can significantly influence the density of elastic energy dissipated in the shear band: with sample size decreasing and machine stiffness increasing, the density of the elastic energy dissipated in the shear band of metallic glasses is prominently decreased, thus the shear deformation turns to be more stable, resulting in the improvement of plasticity in ductile metallic glasses and the transition from fragmentation fracture to shear fracture in brittle metallic glasses. This finding is important for the potential applications of the present metallic glasses and for designing new metallic glasses with better mechanical properties.     

The Effect of Reservoir Wettability on the Production Characteristics of the VAPEX Process: An Experimental Study

The impact of fractional wettability on the production characteristics of a VAPEX process at the macroscale was investigated. Conventional VAPEX experiments were conducted in a 220 Darcy random packing of glass beads in a rectangular physical model and n-pentane was used to recover the Cold Lake bitumen from the oil-saturated model in the absence of connate water. The composition of oil-wet beads in the packed bed was altered from completely water-wet beads to completely oil-wet beads at different proportions of oil-wet beads mixed with water-wet beads. A substantial increase (about 40%) in the production rate of live oil was observed during the VAPEX process when the wettability of the porous packing was entirely oil-wet beads. A critical oil-wet fraction of 0.66 was found for the heterogeneous packing of water-wet and oil-wet beads of similar size distribution. Above this critical composition, the live oil production rate was not affected by further increase in the proportion of the oil-wet beads. It is believed that above this critical composition of the oil-wet beads, the crevice flow process is dominated by the continuity of higher conductivity live oil films between particles through the oil-wet regions. Below this critical composition, the live oil production rate increased linearly with the fraction of the oil-wet beads in the packing. The oil-wet regions favor the live oil drainage compared to that of the water-wet regions as they enhance the rate of imbibition of the live oil from the oil-filled pores to the vacated pores near the nominal VAPEX interface. These two factors enhance the live oil production rate during the VAPEX process. The solvent content of the live oil, the solvent-to-oil ratio (SOR), and the residual oil saturation did not correlate strongly with the proportion of the oil-wet beads in the packing. The average solvent content of the live oil and the residual oil saturation were measured to be 48% by weight and 7% by volume respectively.     

Interfacially active particles in droplet/matrix blends of model immiscible homopolymers: Particles can increase or decrease drop size

Particles have been shown to adsorb at the interface between immiscible homopolymer melts and to affect the morphology of blends of those homopolymers. We examined the effect of such interfacially active particles on the morphology of droplet/matrix blends of model immiscible homopolymers. Experiments were conducted on blends of polydimethylsiloxane and 1,4-polyisoprene blended in either a 20:80 or 80:20 weight ratio. The effects of three different particle types, fluoropolymer particles, iron particles, and iron oxyhydroxide particles, all at a loading of 0.5 vol.%, were examined by rheology and by direct flow visualization. Particles were found to significantly affect the strain recovery behavior of polymer blends, increasing or decreasing the ultimate recovery, slowing down or accelerating the recovery kinetics, and changing the dependence of these parameters on the applied stress prior to cessation of shear. These rheological observations were found to correlate reasonably well with particle-induced changes in drop size. The particles can both increase as well as decrease the drop size, depending on the particle type, as well as on which phase is continuous. The cases in which particles cause a decrease in drop size are analogous to the particle stabilization of “Pickering emulsions” well-known from the literature on oil/water systems. We hypothesize that cases in which particles increase drop size are analogous to the “bridging–dewetting” mechanism known in the aqueous foam literature.     

Particle-turbulence interaction in a boundary layer

Particle-turbulence interaction in wall turbulent flows has been studied. A series of experiments varying particle size, particle density, particle loading and flow Re has been conducted. The results show that the larger polystyrene particles (1100 μm) cause an increase in the number of wall ejections, giving rise to an increase in the measured values of the turbulence intensities and Reynolds stresses. On the other hand, the smaller polystyrene particles (120 μm) bring about a decrease in the number of wall ejections, causing a decrease in the measured intensities and Reynolds stresses. These effects are enhanced as the particle loading is increased. It was also found that the heavier glass particles (88 μm) do not bring about any significant modulation of turbulence. In addition, measurements of the burst frequency and the mean streak-spacing show no significant change with increase in particle loading. Based on these observations, a mechanism of particle transport in wall turbulent flows has been proposed, in which the particles are transported (depending on their size, density and flow Re) by the bursting events of the wall regions.     

Representative elementary volume analysis of polydisperse granular packings using discrete element method

The representative elementary volume (REV) for three-dimensional polydisperse granular packings was determined using discrete element method simulations. Granular mixtures of various sizes and particle size distributions were poured into a cuboid chamber and subjected to uniaxial compression. Findings showed that the minimum REV for porosity was larger compared with the REV for parameters such as coordination number, effective elastic modulus, and pressure ratio. The minimum REV for porosity and other parameters was found to equal 15, 10, and 5 times the average grain diameter, respectively. A study of the influence of sample size on energy dissipation in random packing of spheres has also confirmed that the REV size is about 15 times the average grain diameter. The heterogeneity of systems was found to have no effect on the REV for the parameters of interest for the narrow range of coefficient of uniformity analyzed in this paper. As the REV approach is commonly applied in both experimental and numerical studies, determining minimum REV size for polydisperse granular packings remains a crucial issue.     

A phenomenological description of moisture capacity of iron ores

Moisture capacity of iron ore, defined as its ability to hold water, has been found to be affected by its particle size and chemical composition. To explain this phenomenon, mathematical model was developed on the basis of particle size, specific surface area, pore properties and contact angle between the iron ore and water. The specific surface area and the pore volume of iron ores were measured to validate the model, and parametric studies were carried out to investigate the effects of these factors, showing that particle size and surface area of the iron ore and thickness of the water film are the principal factors determining the moisture capacity. The model gave reasonable explanation of the phenomenon that moisture capacity increases with decreasing particle size. A correlation developed can well predict the moisture capacity of iron ores. Deviations between measurements and calculations were analyzed.     

Elastic constants for granular materials modeled as first-order strain-gradient continua

Formulation of a stress–strain relationship is presented for a granular medium, which is modeled as a first-order strain-gradient continuum. The elastic constants used in the stress–strain relationship are derived as an explicit function of inter-particle stiffness, particle size, and packing density. It can be demonstrated that couple-stress continuum is a subclass of strain-gradient continua. The derived stress–strain relationship is simplified to obtain the expressions of elastic constants for a couple-stress continuum. The derived stress–strain relationship is compared with that of existing theories on strain- gradient models. The effects of inter-particle stiffness and particle size on material constants are discussed.     

Scale-up of a high shear wet granulation process using a nucleation regime map approach

Scale-up of the high shear wet granulation (HSWG) process is considered a challenge because HSWG is complex and influenced by numerous factors, including equipment, formulation, and process variables. For a system of microcrystalline cellulose and water, HSWG experiments at three scales (1, 2, and 4 L working vessel) were conducted with a granulator. Scale-up was implemented on the basis of a nucleation regime map approach. To keep dimensionless spray flux and drop penetration time constant, water addition time at three processing scales were 300, 442, and 700 s, respectively. The other process parameters were kept unchanged. Granule size distributions were plotted and compared, and scanning electron microscopy was used to analyze granule surface morphology. Physical characterization was undertaken using a modified SeDeM method. At nearly all scales, granule yield was greater than 85% and all the cosine values were larger than 0.89. At the same experiment points, granules at all scales had similar surface morphology and similar physical characteristics. The results demonstrate that a rational scaling-up of the HSWG process is feasible using a regime map approach.     

Large-scale transportation and storage of wood pellets:Investigation of the change in physical properties

The change in physical properties of wood pellets,with a focus on particle size distributions due to pellet breakage and attrition,was studied in a large-scale(...     

基于均匀化理论的页岩微观多孔黏土强度特性

页岩强度是页岩油气开发所必需的基础技术参数之一,对页岩强度的研究贯穿于钻完井、压裂工艺施工的全过程.常规宏观室内实验存在试样获取困难、耗时较长,受井下工矿制约,地球物理方法获取资料品质欠佳且增加了井下设备卡、埋风险.因此,提出基于均匀化理论评价页岩微观多孔黏土强度的方法,进行多孔黏土组成与力学分析.基于耗散能原理和Drucker-Prager准则,开展了微观多孔黏土的强度与$\pi$函数的应变求解分析;讨论黏土颗粒与粒间孔隙的力学特性,建立多孔黏土的均匀化应变能;采用强度均匀化理论构建微观非线性函数模型,建立与多孔黏土组成、摩擦系数、内聚系数等参数相关的均匀化函数模型;基于纳米力学实验、量纲分析和有限元模拟,分析多孔黏土硬度、强度与组成的内在关系.研究结果表明,页岩微观多孔黏土的弹性模量和硬度与黏土堆积密度正相关,当黏土堆积密度一定时,硬度与内聚系数的比值受摩擦系数影响较大,为非线性递增;通过量纲分析和有限元模拟,求解页岩微观多孔黏土关于硬度--强度--堆积密度的$\pi$函数,揭示页岩微观黏土矿物的组成与力学性质的关系,为进一步深入研究页岩细观强度参数和宏观强度预测奠定基础.      

Attenuation technique for measuring sediment displacement levels

A technique for obtaining accurate, high (spatial) resolution measurements of sediment redeposition levels is described. In certain regimes, the method may also be employed to provide measurements of sediment layer thickness as a function of time. The method uses a uniform light source placed beneath the layer, consisting of transparent particles, so that the intensity of light at a point on the surface of the layer can be related to the depth of particles at that point. A set of experiments, using the impact of a vortex ring with a glass ballotini particle layer as the resuspension mechanism, are described to test and illustrate the technique.

---

MONODISPERSED AND NANOSIZED DENDRIMER／POLYSTYRENE LATEX PARTICLES

Emulsion polymerization of styrene was carried out using dendrimer DAB-dendr-(NH2)64 as seed. The size and size distribution of the emulsion particles were characterized by transmission electron microscopy (TEM) and dynamic light scattering (DLS), and the effects o.f emulsion polymerization conditions on the preparation of emulsion particle were investigated. It has been found that the nanosized dendrimer/polystyrene polymer emulsion particles obtained were in the range of 26～64nm in diameter, and were monodisperse; the size and size distribution of emulsion particles were influenced by the contents of dendrimer DAB-dendr-(NH2)64, emulsifier and initiator, as well as the pH value.     

Nano-particle sizing by laser-induced-incandescence (LII) in a shock wave reactor

Abstract. Laser-Induced Incandescence (LII) is a relatively new optical diagnostic for particle sizing which is currently used in combustion science. Its advantage against light extinction and light scattering methods is the possibility of getting size information with high time and space resolution even for nano-particles. LII is mostly applied to particle formation or particle removal in reactive stationary flows, but it can also be used in shock-induced reactive flows. This is demonstrated in three examples: soot particle formation during high temperature pyrolysis of benzene, iron particle formation from iron pentacarbonyl, and formation of carbon-coated iron particles. From the principles of LII, it is not possible to obtain a complete particle growth curve from one individual shock tube experiment. Therefore, the kinetics of particle growth evolution must be determined from several “identical” shock tube experiments with a delayed triggering of the heat-up laser. The principles of LII, the in-situ measurement of particle size, and the comparison to beam-collected particles, which were visualized by a high resolution transmission electron microscope (HRTEM), are demonstrated. It was found that the energy accommodation coefficient during the particle cooling is for a soot surface but is significantly lower e.g. for an iron surface. Received 30 April 2002 / Accepted 9 December 2002 Published online 4 February 2003 Correspondence to: R. Starke (e-mail: starke@ivg.uni-duisburg.de)