分形粒径分布对颗粒流粒径分选的影响规律

高速远程滑坡反粒序及其形成机制是目前研究的热点问题,为进一步探讨该问题,以高速远程滑坡分形粒径分布特征为依据,通过物理模型实验与数值模拟方法,研究不同分形维数颗粒流堆积体的粒径分布特征及其对应的颗粒分选过程,结果表明:随着颗粒流分形维数的增大,细颗粒含量的增加,颗粒流运动过程中不同粒径颗粒的分选作用减弱,导致堆积体中反粒序的发育程度降低;在不同粒径组成条件下,颗粒流前缘及尾部主要呈离散态运动,中部则以密集态为主,仅表面局部颗粒发生离散,颗粒流堆积体中反粒序的形成是多种分选机制综合作用的结果。基于结果分析,高速远程滑坡运动过程中整体不具备颗粒分选的条件,仅在局部范围内可能存在颗粒分选作用。     

Branched pore kinetic model analysis of geosmin adsorption on super-powdered activated carbon

Super-powdered activated carbon (S-PAC) is activated carbon of much finer particle size than powdered activated carbon (PAC). Geosmin is a naturally occurring taste and odor compound that impairs aesthetic quality in drinking water. Experiments on geosmin adsorption on S-PAC and PAC were conducted, and the results using adsorption kinetic models were analyzed. PAC pulverization, which produced the S-PAC, did not change geosmin adsorption capacity, and geosmin adsorption capacities did not differ between S-PAC and PAC. Geosmin adsorption kinetics, however, were much higher on S-PAC than on PAC. A solution to the branched pore kinetic model (BPKM) was developed, and experimental adsorption kinetic data were analyzed by BPKM and by a homogeneous surface diffusion model (HSDM). The HSDM describing the adsorption behavior of geosmin required different surface diffusivity values for S-PAC and PAC, which indicated a decrease in surface diffusivity apparently associated with activated carbon particle size. The BPKM, consisting of macropore diffusion followed by mass transfer from macropore to micropore, successfully described the batch adsorption kinetics on S-PAC and PAC with the same set of model parameter values, including surface diffusivity. The BPKM simulation clearly showed geosmin removal was improved as activated carbon particle size decreased. The simulation also implied that the rate-determining step in overall mass transfer shifted from intraparticle radial diffusion in macropores to local mass transfer from macropore to micropore. Sensitivity analysis showed that adsorptive removal of geosmin improved with decrease in activated carbon particle size down to 1 μm, but further particle size reduction produced little improvement.     

次轻混凝土匀质性影响因素研究

本文提出了用分层度评价次轻混凝土匀质性的方法。并研究了混凝土水胶比、轻集料体积掺量、砂率、轻集料粒径、纤维掺量、硅类掺量、增粘剂掺量对次轻混凝土分层度的影响。研究结果表明,轻集料的粒径是影响次轻混凝土分层度的主要因素,轻集的粒径应控制在4．75mm～9．5mm的范围内,采用低的水胶比并同时掺加适量的硅灰、聚合物纤维和增粘剂可以显著提高轻混凝土的匀质性。采用0．31的水胶比．掺加30％粒径为4．75mm～9．5mm的轻集料、8％的硅灰和0．3％的增粘剂,制备出分层度小于0．1的C50次轻混凝土。     

On the stochastic nature of compact debris flight

The stochastic nature of debris flight is investigated through a series of Monte Carlo simulations based on the debris flight equations for compact debris presented by Holmes (2004). Any given debris flight situation presents a number of uncertainties such as the size of the piece of debris and the time-varying turbulent wind flow. Current debris flight models are largely deterministic and do not account for such uncertainty in input parameters. The simulations presented model the flight of a single spherical particle whose diameter is given by a probability distribution function, driven by a turbulent wind with velocity fluctuations appropriate to the atmospheric boundary layer. The model predicts the mean and standard deviation of the particle flight distance and impact kinetic energy. Results show that introducing uncertainty in particle diameter, horizontal turbulence intensity, or vertical turbulence intensity leads to larger mean values for flight distance and impact kinetic energy, compared to the condition where there is no variability in input parameters. Introducing input parameter variability also leads to variability in flight distance and impact kinetic energy that is quantified in this study. While the simulations presented do not realistically characterize the complex flow within an urban canopy, the results provide significant physical insight into the influence of particle size variability and turbulence on the mean and standard deviation of the flight distance and impact kinetic energy.     

Numerical simulation of the penetration capability of sprinkler sprays

Computational models have been utilized to investigate the penetration capability of sprinkler sprays directly above a fire source with respect to water flow rate, spray drop size, and spray momentum. The spray models are generated by assigning a representative drop size, mass flow rate, discharge speed, and discharge angle for each of 275 trajectories in such a way that they produce computed results which match the measured water flux distribution and spray momentum in the absence of a fire. The spray/fire plume interaction models are created by combining the spray models using a Lagrangian particle tracking scheme with free-burn fire plume models. Actual delivered densities and penetration ratios are computed through the interaction simulations at six flow rates, three fire sizes, and two ceiling heights. Drop sizes and spray momentum at two flow rates are increased by 25 and 50% from the original values without changing the other spray characteristics in order to investigate the effects of each parameter on penetration capability independently. The study indicates that there is an optimal flow rate for a given sprinkler that gives the highest penetration ratio within a practical flow range. It is also shown that increasing drop size is a much more effective way for obtaining a higher penetration ratio compared to increasing spray momentum.     

Particle breakage evolution of coral sand using triaxial compression tests

Particle breakage continuously changes the grading of granular materials and has a significant effect on their mechanical behaviors.Revealing the evolution pattern of particle breakage is valuable for development and validation of constitutive models for crushable materials.A series of parallel triaxial compression tests along the same loading paths but stopped at different axial strains were conducted on two coral sands with different particle sizes under drained and undrained conditions.The tested specimens were carefully sieved to investigate the intermediate accumulation of particle breakage during the loading process.The test results showed that under both drained and undrained conditions,particle breakage increases continuously with increasing axial strain but exhibits different accumulating patterns,and higher confining pressures lead to greater particle breakage.Based on the test results,the correlations between particle breakage and the stress state as well as the input energy were examined.The results demonstrated that either the stress state or input energy alone is inadequate for describing the intermediate process of particle breakage evolution.Then,based on experimental observation,a path-dependent model was proposed for particle breakage evolution,which was formulated in an incremental form and reasonably considers the effects of the past breakage history and current stress state on the breakage rate.The path-dependent model successfully reproduced the development of particle breakage during undrained triaxial compression using the parameters calibrated from the drained tests,preliminarily demonstrating its effectiveness for different stress paths.     

缩尺效应对粗粒料压实密度影响的试验研究

堆石料的级配缩尺后的试验替代料与原级配料的试验结果之间存在一定的差异,弄清这种差异对准确把握原级配堆石料的力学性质十分重要。将缩尺效应归结为最大粒径和级配结构两部分变化产生的影响。利用土的连续级配方程结合相似级配法缩尺的思路,设计了大量最大粒径分别为20,40和60 mm的试验级配。对各级配粗粒料进行了表面振动压实试验,研究了最大粒径和级配结构对粗粒料压实密度的影响。最大粒径和级配结构对粗粒料压实密度的影响可以用一定的函数形式定量表述,建立了考虑缩尺效应的粗粒料压实密度的预测模型,探讨了该模型对不同性质粗粒料的适用性,并用多组室内最大干密度试验数据进行了验证。此外,提出了一种能够定量研究粗粒料缩尺效应的思路。     

A critical state based viscoplastic model for crushable granular materials

?Experimental studies have confirmed that the critical state of a granular material varies with alteration in granular fabric, particle shape and grain size. On the other hand, granular materials demonstrate significant strain rate dependency in the presence of particle crushing. While the first feature is well explored, the strain rate effects on the crushability of granular material and consequent critical state alteration are less ventured. This study highlights the strain rate dependence of the critical state of crushable granular materials like sand. A rate-dependent model is proposed bridging the macro and microscopic understanding. The model follows a consistent viscoplastic formulation without using any overstress function. The proposed model considers various loading rate effects at different porosities, confinements and pore water drainage conditions. Further, it can predict the strain rate-dependent particle crushing and dilation features that affect the critical state of granular materials. The model has been validated by comparing its responses with both the experimental and discrete particle simulations for drained and undrained triaxial conditions. An implicit stress return integration scheme is devised to enable accurate numerical response from the model. ? Finally, a parametric study is presented that envisages the evolution of critical state due to coupled strain rate and particle crushing effect.     

Effect of face velocity and the nature of aerosol on the collection of submicrometer particles by electrostatic precipitator

Despite the electrostatic collection of aerosol particles as one of the most widely used air cleaning methods, there has not been sufficient amount of effort devoted to investigate its performance in the full range of operating conditions. This paper reports results of the tests of a two-stage electrostatic precipitator (ESP) conducted in the particle size range of 0.018-1.2 microns over a range of flow rates using NaCl and Environmental Tobacco Smoke (ETS) test aerosols. The total collection efficiency of the precipitator was found to increase with an increase in the count median diameter (CMD) of the particles, to have polynomial dependence on flow rate and no significant dependence on the type of test aerosol. The fractional efficiency of the precipitator was found to be dependent on flow rate. However, the 'critical' particle size of about 1.2 microns was found to exist when the fractional collection efficiency becomes independent of flow rate. For submicrometer particles, the collection efficiency was found to be independent of particle size at flow rates below 560 l/s. A minimum in the efficiency was observed in the 0.1-0.45 micron particle size range and for particles smaller than about 0.02 micron.     

砂土流滑运动的模型试验研究

砂土流滑发生时,大面积土体高速流动将导致巨大的人员伤亡和财产损失。流滑砂土运动距离、破坏范围的研究可以为抗震减灾提供依据。文章通过砂土流动的室内模型试验,记录了砂土在不同初始状态下的流态化运动过程。然后,选择砂土模型中的特征质点,标定其在构形图中的位置,计算并分析其应变变化情况。研究表明:砂土的流动过程可分为整体流动阶段和局部流动阶段两个阶段。随着砂土流动时间的增长,总应变逐渐增长,应变率先增加后减少。     

Experimental study on the stability of asphalt emulsion for CA mortar by laser diffraction technique

A laser diffraction particle size analyzer was employed to measure the particle size variations of two asphalt emulsions under stress conditions to evaluate their storage stability, freeze-thaw stability, and mechanical stability. Results showed that when the dosage of non-ionic emulsifier was increased from 0.4% to 0.8%, the particle size and its increasing magnitude were both reduced, indicating a more stable emulsion. This new method can provide the desired information for fast and continuous evaluation on the dynamical changes of emulsion stability under stress conditions, compared to the traditional storage stability tests.     

Concentration and size distribution of particles in abstracted groundwater

Particle number concentrations have been counted and particle size distributions calculated in groundwater derived by abstraction wells. Both concentration and size distribution are governed by the discharge rate: the higher this rate the higher the concentration and the higher the proportion of larger particles. However, the particle concentration in groundwater derived from abstraction wells, with high groundwater flow velocities, is much lower than in groundwater from monitor wells, with minimal flow velocities. This inconsistency points to exhaustion of the particle supply in the aquifer around wells due to groundwater abstraction for many years. The particle size distribution can be described with the help of a power law or Pareto distribution. Comparing the measured particle size distribution with the Pareto distribution shows that particles with a diameter >7 μm are under-represented. As the particle size distribution is dependent on the flow velocity, so is the value of the “Pareto” slope β.     

Effect of ventilation design on removal of particles in woodturning workstations

Wood processing tasks such as circular sawing and turning that is associated with woodturning operators produce particularly high exposure levels. A computational model including a humanoid and lathe within a test chamber was simulated with monodisperse particles under five different ventilation designs with the aim of reducing the particle suspension within the breathing zone. A commercial CFD code was used to solve the governing equations of motion with a k–ε RNG turbulence model. A discrete Lagrangian model was used to track the particles individually. Measurements to evaluate the efficiency of each ventilation design included total particle clearance and the percentage of particles crossing through the breathing plane. It was concluded that the percentage of particles that cross the breathing plane is of greater significance than the other measurements as it provides a better determination of exposure levels. Ventilation that emanated from the roof and had an angled outlet provided greatest total particle clearance and a low number of particles in the breathing plane. It was also found that the obstruction from the local roof ventilations caused separation of the air that flowed along the ceiling to produce a complex flow region. This study provides a basis for further investigation into the effects of particle size and density on the particle flow patterns and potential inhalation conditions for a given ventilation design.     

Effects of particle size on flame spread over magnesium powder layer

The spreading of a flame over a layer of magnesium powder has been examined to clarify the mechanisms of flame spread over metal powder layers and to establish effective ways to extinguish metal fires. Four grades of magnesium powder were used, with average grain diameters of 60, 170, 360 and 500 μm. The flame spread rate for larger particle sizes (D>170 μm) increased slightly with increasing particle size. However, the flame spread rate for smaller particle sizes (D<63 μm) increased sharply. The detailed surface temperature history of the magnesium powder layer was measured using infrared thermography (IR). Based on the temperature distribution in the pre-heat zone, the characteristic length l, characteristic depth δ, and characteristic time τ were calculated. The characteristic scale ratio, L/δ, for the larger particle sizes (D>170 μm) is almost unity, which suggests that the dominant heat transfer mechanism is heat conduction through the magnesium powder layer (solid phase). However, the L/δ for the smaller particle sizes (D<63 μm) is about 2.7, suggesting that the dominant heat transfer mechanism is convection. The flame spread rate over the magnesium powder layer was calculated by the de Ris model, a one-dimensional flame spread model, and a surface flash model. For larger particle sizes (D>170 μm), there is good agreement between the experimental flame spread rate and the flame spread rate estimated by the one-dimensional model. However, the flame spread rate was underestimated by the de Ris model, apparently because the de Ris model only considers heat feedback from the gas phase. For small-particle sizes (D<63 μm), there is good agreement between the experimental flame spread rate and the flame spread rate estimated by the surface flash model. This suggests that the flame spread over a small-particle layer can be described by a mechanism rather similar to that of gas phase flame propagation.     

Particle breakage of sand subjected to friction and collision in drum tests

This paper presents a laboratory experimental study on particle breakage of sand subjected to friction and collision,by a number of drum tests on granular materials(silica sand No.3 and ceramic balls)to investigate the characteristics of particle breakage and its effect on the characteristics of grain size distribution of sand.Particle breakage increased in up convexity with increasing duration of drum tests,but increased linearly with increasing number of balls.Particle breakage showed an increase,followed by a decrease while increasing the amount of sand.There may be existence of a characteristic amount of sand causing a maximum particle breakage.Friction tests caused much less particle breakage than collision tests did.Friction and collision resulted in different mechanisms of particle breakage,mainly by abrasion for friction and by splitting for collision.The fines content increased with increasing relative breakage.Particle breakage in the friction tests(abrasion)resulted in a sharper increase but with a smaller total amount of fines content in comparison with that in the collision tests(splitting).For the collision tests,the fines content showed a decrease followed by an increase as the amount of sand increased,whereas it increased in up convexity with increasing number of balls.The characteristic grain sizes D₁₀ and D₃₀ decreased in down convexity with increasing relative breakage,which could be described by a natural exponential function.However,the characteristic grain sizes D50 and D60 decreased linearly while increasing the relative breakage.In addition,the coefficients of uniformity and curvature of sand showed an increase followed by a decrease while increasing the relative breakage.     

Modelling of the Indoor Environment – Particle Dispersion and Deposition

Abstract A three-dimensional drift-flux model for particle movements in turbulent airflows in buildings is presented. The interaction between the carrier air and the particles has been treated as a one-way coupling, assuming the effect of particles on air turbulence is negligible due to low solid loadings and comparatively small particle settling velocities. Turbulence effects are modelled with a standard κ-? model. Wall functions are applied at near-wall grid points. Aerosol measurements carried out under turbulent room flow conditions are used to validate the numerical calculations. Several particle size distributions are considered in the simulations. The model is then applied to mixed flow conditions in a room, as well as to homogeneous air supply conditions around a human body. The flow fields and particle distributions are analysed. Close to a standing person, the particle distribution pattern from a downstream point source is strongly dependent on the ventilation air supply rate. This has been confirmed by experiments reported in the literature.     

约束条件下粗细混合颗粒动力机理分析

混合颗粒的堆积问题一直是自然界以及工业界一个悬而未决的问题。研究的对象是相同密度不同尺寸的两相混合颗粒。通过三维离散元软件PFC3D建立高仿真数值模型并进行离散元数值试验,通过数值模拟和试验对比确定本次研究所用的参数。然后对不同条件斜槽下的颗粒堆积形态进行观察并总结其堆积规律;紧接着进一步测定相同体积条件下均匀颗粒和混合颗粒分别对结构体的动力荷载变化;最终通过研究不同斜槽约束条件下混合粒径条件下的颗粒流的运动情况并得到相应规律性的结果。通过研究发现,混合颗粒粒径条件下的颗粒流运动情况和均匀颗粒粒径条件下的颗粒流运动情况差异较大,颗粒分选对其动能的影响,有约束条件下的斜槽会增加颗粒流的滑程并提高其龙头的速度。该结论对于揭示碎屑流远程特性有一定的参考价值。     

Modelling particle size distribution dynamics in marine waters

Numerical simulations were carried out to determine the particle size distribution (PSD) in marine waters by accounting for particle influx, coagulation, sedimentation and breakage. Instead of the conventional rectilinear model and Euclidean geometry, a curvilinear collision model and fractal scaling mathematics were used in the models. A steady-state PSD can be achieved after a period of simulation regardless of the initial conditions. The cumulative PSD in the steady state follows a power-law function, which has three linear regions after log-log transformation, with different slopes corresponding to the three collision mechanisms, Brownian motion, fluid shear and differential sedimentation. The PSD slope varies from -3.5 to -1.2 as a function of the size range and the fractal dimension of the particles concerned. The environmental conditions do not significantly alter the PSD slope, although they may change the position of the PSD and related particle concentrations. The simulation demonstrates a generality in the shape of the steady-state PSD in the ocean, which is in agreement with many field observations. Breakage does not affect the size distribution of small particles, while a strong shear may cause a notable change in the PSD for larger and fractal particles only. The simplified approach of previous works using dimensional analysis still offers valuable approximations for the PSD slopes, although the previous solutions do not always agree with the simulation results. The variation in the PSD slope observed in field investigations can be reproduced numerically. It is argued that non-steady-state conditions in natural waters could be the main reason for the deviation of PSD slopes. A change in the nature of the particles, such as stickiness, and environmental variables, such as particle input and shear intensity, could force the PSD to shift from one steady state to another. During such a transition, the PSD slope may vary to some extent with the particle population dynamics.     

砂土地基承载力离心模型试验中的粒径效应研究

通过对原型的模拟结果进行总结,得出对于承载力试验只要离心试验中的模型基础宽度与重力场试验中的不同,离心模型试验结果总大于重力场试验结果,即离心试验中总是存在粒径效应问题。随着模型基础宽度与地基材料平均粒径比值的增大,离心试验结果逐渐接近重力场试验结果;当比值足够大时,离心试验结果趋于稳定且与重力场试验结果之差可忽略不计。因此,可以利用模型的模拟的方法,讨论离心模型试验中的粒径效应问题。在一系列砂土地基上条形基础地基承载力的离心模型试验中,使模型基础宽度与地基材料平均粒径的比值在较大范围内变化,即最大比值为最小比值的5倍,且其变化范围的上限达291。基于试验结果导入评价粒径效应对试验结果影响程度的指标,定量地讨论了离心模型试验中的粒径效应问题。本试验结果表明,当模型基础宽度与地基材料平均粒径的比大于233时,粒径效应对试验结果的影响小于1%,基本可忽略不计。     

Extracellular polymeric substances (EPS) in upflow anaerobic sludge blanket (UASB) reactors operated under high salinity conditions

Considering the importance of stable and well-functioning granular sludge in anaerobic high-rate reactors, a series of experiments were conducted to determine the production and composition of EPS in high sodium concentration wastewaters pertaining to anaerobic granule properties. The UASB reactors were fed with either fully acidified substrate (FAS) consisting of an acetate medium (reactor R1) or partly acidified substrate (PAS) consisting of acetate, gelatine and starch medium (reactors R2, R3, and R4). For EPS extraction, the cation exchange resin (CER) method was used. Strength and particle size distribution were determined by assessing the formation of fines sludge under conditions of high shear rate and by laser diffraction, respectively. Batch tests were performed in 0.25 L bottles to study Ca²⁺ leaching from anaerobic granular sludge when incubated in 20 g Na⁺/L in the absence of feeding for 30 days. Results show a steady increase in the bulk liquid Ca²⁺ concentration during the incubation period. UASB reactor results show that the amounts of extracted proteins were higher from reactors R2 and R3, fed with PAS compared to the sludge samples from reactor R1, fed with FAS. Strikingly, the amount of extracted proteins also increased for all reactor sludges, irrespective of the Na⁺ concentration applied in the feed, i.e. 10 or 20 gNa⁺/L. PAS grown granular sludges showed an important increase in particle size during the operation of the UASB reactors. Results also show that, addition of 1 gCa²⁺/L to the high salinity wastewater increases the granules' strength.