Thermal Conductivity Measurement of Bulk Solids

The coefficient of thermal conductivity of particulates and powders is of great importance in process engineering. The prediction of thermal properties of powders using empirical equations is still difficult due to the wide range of specific attributes. This article describes a new measurement methodology for a laboratory device that can be used to determine the thermal conductivity of bulk solids. The presented results show that the created device is highly applicable in industrial practice. It is possible to examine the coefficient of thermal conductivity depending on the sample temperature, the granulometry results and the morphological composition, the moisture content, the degree of consolidation, and other variables that may enter into the entire process and affect it significantly.     

Flow Properties of Highly Dispersed Powders at Very Small Consolidation Stresses

With a modified standard ring shear tester yield loci of highly dispersed, dry powders were measured at preshear normal stresses down to 32 Pa and shear stresses down to 10 Pa. At small consolidation stresses stress, σ₁, (< 500 Pa) the values obtained for the unconfined yield strength, σ_c, are proportional to the consolidation stress, σ₁.     

Formation and Growth of Crystal Bridges in Bulk Solids

Solidification and caking of bulk solids often occurs during storage or while being transported to the customer. To investigate the formation and growth of solid bridges between two discrete particles, the modification of the contact region between these two particles stored in a climatic chamber is examined. The effect of load, temperature, relative humidity, and storage time on the formation of a bridge is analyzed. The objective is to describe the behavior of crystalline or salt‐like granules under real storage conditions.     

Measurements on Cohesive Powder with two Biaxial Shear Testers

At the Technical University of Braunschweig (Germany) and the Delft University of Technology (The Netherlands) two True Biaxial Shear Testers were used to perform comparative measurements. The Braunschweig Biaxial Tester (BBT) and the Flexible Wall Biaxial Tester (FWBT) are part of a new generation of advanced powder testers whose goal is to fundamentally investigate powder flow based on the same principles. The general aim of these investigations is to compare the results obtained with both testers and gain new experience in handling powders.     

Applying the Discrete Element Method in Process Engineering

This paper provides insight into the application of discrete element modeling in the field of bulk solids handling. This is shown with examples starting with the calibration of a virtual numerical material used in the simulation to mimic the actual material’s behavior. Based on the calibration of the material, solid handling problems such as the flow patterns and the stress distribution in silos are discussed.     

Chute Performance and Design for Rapid Flow Conditions

Many industrial chute applications are characterised by rapid flow conditions in which the bulk solid stream thickness or depth is less than the chute width. Under these conditions, it is possible to describe the stream flow by means of a lumped parameter model which takes into account the frictional drag around the chute boundaries as well as making allowance for inter‐particle friction. Equations of motion to describe the chute flow are presented and their application to the determination of chute profiles to achieve optimum flow is illustrated. By means of design examples, the problems associated with the feeding of bulk solids onto belt conveyors and conveyor transfers are discussed. Criteria for the selection of the most appropriate chute geometry to minimise chute wear and belt wear at the feed point are presented. The determination of optimum chute profiles to achieve specified performance criteria is outlined.     

Product Design of Cohesive Powders – Mechanical Properties,Compression and Flow Behavior

The fundamentals of cohesive powder consolidation and flow behavior are explained using a reasonable combination of particle and continuum mechanics. By the model “stiff particles with soft contacts”, universal models are presented which include the elastic‐plastic and viscoplastic particle contact behavior with adhesion, load‐unload hysteresis and thus energy dissipation, a history‐dependent and a nonlinear adhesion force function. With this as the physical basis, incipient powder consolidation, yield and cohesive steady‐state flow, consolidation and compression functions, compression and preshear works are explained. As an example, the flow properties of an ultrafine limestone powder are shown. These constitutive models are used to evaluate shear cell test results for apparatus design to ensure reliable powder flow. Finally, conclusions are drawn concerning particle stressing, powder handling behavior and product quality assessment in processing industries.     

Numerical Simulations of the Direct Shear Test

Numerical simulations of the direct shear test have been performed under constant normal stress conditions, using the Discrete Element Method. Results are presented to show how the average shear to normal stress ratio acting on the shear band compares with the force data at the boundaries. From the calculations of the stress tensor inside the shear band the corresponding Mohr’s circles are obtained and the principal stress directions identified. The principal directions of stress and strain‐rate are shown to be coaxial under steady‐state conditions. Using the state of stress and strain in the shear band, the interpretation of direct shear test data is discussed in the context of traditional experimental measurements.     

Modeling of Caked Contacts in DEMs

When modeling the caking properties of bulk solids, it is not only necessary to incorporate the yield properties of individual particle contacts, but also to extend them to a many‐particle system. To accomplish this by means of Distinct Element Method (DEM) simulations a contact model for (spherical) particles, including a yield criterion for combined load is proposed. An application to the simulation of a caking test is presented and compared to experiments.     

影响块状模塑料均匀流动的主要因素

采用Brabender转矩流变仪,并结合同心圆取样测定块状模塑料（BMC）中玻纤质量分数的方法,讨论了预混料在模压中的流动性能;研究了温度、玻纤的质量分数及长度、填料粒度、增稠剂用量等因素对BMC预混料的流动粘度和玻纤在BMC材料中分布的影响。     

The Linear Thermal Expansion of Bulk Nanocrystalline Ingot Iron from Liquid Nitrogen to 300 K

The linear thermal expansions (LTE) of bulk nanocrystalline ingot iron (BNII) at six directions on rolling plane and conventional polycrystalline ingot iron (CPII) at one direction were measured from liquid nitrogen temperature to 300 K. Although the volume fraction of grain boundary and residual strain of BNII are larger than those of CPII, LTE of BNII at the six measurement directions were less than those of CPII. This phenomenon could be explained with Morse potential function and the crystalline structure of metals. Our LTE results ruled out that the grain boundary and residual strain of BNII did much contribution to its thermal expansion. The higher interaction potential energy of atoms, the less partial derivative of interaction potential energy with respect to temperature T and the porosity free at the grain boundary of BNII resulted in less LTE in comparison with CPII from liquid nitrogen temperature to 300 K. The higher LTE of many bulk nanocrystalline materials resulted from the porosity at their grain boundaries. However, many authors attributed the higher LTE of many nanocrystalline metal materials to their higher volume fraction of grain boundaries.     

The Influence of Shear Stress on Crystallization in an Ultrasound Levitator

Industrial precipitation processes often use chemical agents to influence crystal morphology and size distribution. This experimental study deals with the investigation of physical parameters including an alternative method to affect crystal growth, thus, avoiding the presence of additives as intrinsic impurities. The influence of shear stress acting on growing crystals within a droplet is investigated in an ultrasound levitator. An ultrasound levitator enables the suspension of a single droplet against gravity and the study of containerless precipitation with specific mechanical forces acting on crystals. The levitator is used as a three‐phase reactor with precipitation from the gas and liquid, and as a reactor for precipitation from two different solutions. Calcium carbonate is used as a model system. The variation of temperature and the amount of applied shear stress leads to different amounts of calcium carbonate morphologies. An increase in the shear stress results in more rounded or spherical crystals. The intensity of the shear stress also influences the particle size distributions of the precipitated crystals, i.e., with increasing shear stress, particle size distributions are shifted to smaller sizes.     

The role of contact stresses and wall friction on fluidization

Fluidization and defluidization experiments, where we increased the gas superficial velocity in small increments and then decreased it, were performed in tubes of different diameters to probe the role of wall friction on pressure drop and bed height. Such experiments, covering the regimes of packed bed, stable bed expansion and bubbling bed, were carried out for several different particles. The compressive yield strength of the particle assemblies at various volume fractions was determined by measuring the height of fully defluidized beds at various mass loading levels. The systematic effect of the tube diameter on pressure drop and bed height hysteresis could be rationalized in terms of a one-dimensional model that accounted for the effect of wall friction and path-dependent contact stresses in the particle phase. Bubbling seemed to set in when the yield stress in the particle assembly could be overcome by the inherent fluctuations. Our experiments, which focused primarily on gas velocities below the minimum bubbling conditions, did not reveal any dramatic change across the Geldart A-B boundary. This is consistent with the original observation by Geldart (Powder Technol. 7 (1973) 285). The distinct difference between beds of group A and B particles in the gently bubbling regime reported by Cody et al. (Powder Technol. 87 (1996) 211) is thus likely to be due to changes in the dynamics of the bubbles, as we observed no striking difference between these beds at gas velocities below minimum bubbling conditions.     

不同流场状态对球形氢氧化镍生长粒度的影响

在相同的搅拌桨线速度、晶化时间、化学条件下,采用化学沉淀法分别应用八斜叶涡轮圆盘桨、推进式螺旋桨、Intermig桨分别制备球形氢氧化镍晶体。采用SEM技术考察各个样品的微观形貌,并结合PIV物理模拟技术分析不同桨型状态下反应器内的流场特性,研究表明:采用八斜叶涡轮圆盘桨是样品的球形度最佳,采用Intermig桨时样品的粒度d0.5最大。讨论了不同桨型对球形氢氧化镍晶体松装密度的影响。     

Study on Bulk and Surface Properties of Poly(Lactic Acid)/Fluorinated Polymer Blends

The easy hydrolysis of poly(lactic acid) (PLA) leads to very poor mechanical stability. In order to improve water-resistance of PLA, fluorinated polymer has been chosen for its extremely low surface energy. Besides, n-butyl acrylate (BA) has been introduced into fluorinated polymer by copolymerizing BA with dodecafluorheptylmethylacrylate (FA) to enhance the compatibility of fluorinated polymer and PLA. The bulk and surface properties of PLA/fluorinated polymers blends have been studied. The results show that the compatibility of PLA and fluorinate polymers has been improved by BA and, more importantly, the hydrophobicity of PLA has increased obviously by blending with the fluorinate polymers.