Dust explosions in vented silos: Simulations and comparisons with current standards

Dust explosions represent a serious hazard to personnel and equipment in industries and silo facilities that handle combustible materials. Venting devices are the most common protective systems employed in silos, although their use may pose problems in large and low-strength silos. The main aim of the present work was to simulate dust explosions in silos using a commercial CFD program, the DESC code, to determine the pressures developed in vented explosions with vent areas of different sizes. Dust cloud characteristics were taken from studies carried out by the FSA (Research Centre for Applied System Safety and Industrial Medicine, Germany) in a 12 m³ silo with a mechanical feeding system. The pressures and associated vent areas in these simulations were compared to those contemplated in two venting standards. The simulated explosion pressures showed the expected trends for the associated vent areas and agreed reasonably well with the values contemplated in NFPA 68 (2007) [5]. However, when the reduced explosion overpressure was low, the vent area contemplated in EN 14491 (2006) [4] was much larger than in the present simulations.     

Validation and experimental calibration of 3D discrete element models for the simulation of the discharge flow in silos

The aim of the present work was to develop 3D discrete element models capable of simulating the observed flow of glass beads (simple glass spheres) and maize grains (represented as a combination of spheres) during their discharge from a small model silo. A preliminary model for each material was constructed based on values for variables measured in the laboratory or taken from the literature. The ability of the models to predict the flow of these materials was then tested by comparing their results with observed discharge flows. Three variables were recorded for this: the mean bulk density at the end of the filling phase, the discharge rate and the flow pattern. The comparison of the results for the last of these variables required the discharge process be filmed using a high speed camera in order to more easily recognise the details of the flow. The preliminary model for the glass beads made very reasonable predictions, but that for the maize grains required calibration. This involved modifying the values of the friction properties of the material until a model capable of making acceptable predictions was obtained. The results obtained highlighted the influence of friction properties on the characteristics of the discharge flow. Finally, some of the numerical results provided by the models were analysed in order to describe the flow characteristics and the behaviour of the discharge rate in more detail.     

Granular segregation in discharging cylindrical hoppers: A discrete element and experimental study

The segregation of granular materials due to differences in particle properties occurs during a variety of handling and transport processes, such as flow from a hopper. In the present work, the discrete element method (DEM) is used to investigate segregation of granular materials during discharge from a hopper. The effects of various particle properties and hopper geometries on the segregation of a spherical, bidisperse granular material during hopper discharge are studied. Particle contacts are modeled using a soft-particle model consisting of a hysteretic spring system and sliding friction. The effects of the particle diameter ratio, density ratio, fines mass fraction, hopper wall angle, hopper cross-sectional shape, and the initial fill conditions are investigated.These computational results are compared to those from a small experimental system with the same hopper dimensions and particle properties. The use of this small-scale system permits a novel, one-to-one comparison with the DEM model predictions for the purpose of model validation. The experiments utilize bidisperse glass spheres in a small, Plexiglas cylindrical hopper that is used in the ASTM International standard test for sifting segregation. Particles are discharged from either a ‘mass-flow’ or ‘funnel-flow’ hopper design and collected transiently in equal volumes until the hopper is empty. Analysis of the weight fractions of fine and coarse particles is conducted by sieving. A comparison of the computational and experimental results provides an indication of the model's success at predicting segregation during hopper discharge and the applicability of the DEM model to other granular flow systems.     

Numerical investigation of coarse powder and air flow in an electrostatic powder coating process

The work presented here reports on the numerical simulation of an electrostatic powder coating process that uses a commercial computational fluid dynamic code, FLUENT v6.1. The purpose of this study was to understand the gas and particle flow fields inside a coating booth under given operating conditions and the effect of particle sizes on its trajectories and the final coating quality. The air and powder particle flows in a coating booth were modeled as a three-dimensional turbulent continuous gas flow with solid particles as a discrete phase. The continuous gas flow was calculated by solving Navier-Stokes equations including the standard k − ε turbulence model with non-equilibrium wall function and the discrete phase was modeled based on the Langrangian approach. Since the solid phase volumetric fraction was less than 0.1%, the effect of particle-particle interaction on particle trajectories was not taken into account. In addition to drag force and gravity, the electrostatic force including the effect of space charge due to the free ions was considered in the equation of motion and implemented using user defined scalars and functions. The governing equations were solved using the second order upwind scheme. Information was provided on the particle trajectories with respect to the particle diameters that could be used to develop suitable operating conditions for the use of fine powders in a powder coating process.     

The effects of air and particle density difference on segregation of powder mixtures during die filling

Segregation of mono-disperse binary mixtures with different particle densities during die filling in the presence of air was numerically analysed using a coupled discrete element method (DEM) and computational fluid dynamics (CFD) approach. Die filling with powders of different particle density ratios (i.e. the ratio of the heavy particles to the light particles) at various shoe speeds was simulated, in order to explore the effects of air and particle density difference on segregation. For die filling from a stationary shoe, the air can induce significant segregation by hindering the deposition of light particles (i.e., air-sensitive particles). As the particle density ratio increases, the light particles are deposited into the die at even lower speeds compared with the heavy ones due to the effect of air drag, resulting in an increase in the degree of segregation. For die filling with a moving shoe, segregation occurs due to different post-collisional velocities resulting from different particle inertia; and the degree of segregation increases as the particle density ratio increases due to the increasing difference in particle inertia. It is found that, as the shoe velocity increases, the powder flow pattern changes from nose flow dominated to bulk flow dominated and the degree of segregation generally decreases. The effect of air is limited for nose flow dominated die filling because the air can easily evacuate through the gap between the die walls and flowing powder stream. When bulk flow dominates in die filling, the air can be entrapped in the die, which has a significant impact on the powder flow and segregation behaviours. Finally, the effect of interparticle friction on segregation was investigated.     

The influence of ageing on consolidation and sinterability of a sub-micron alumina powder

A commercial sub-micron alumina powder was used for investigation of the influence of exposure to atmospheric humidity on powder characteristics, consolidation behaviour, densification and final microstructure of alumina ceramics. A significant uptake of atmospheric water by inadequate storage confirmed by thermal analysis, XPS, and FTIR, resulted in decreased sinterability of the powder, although no significant influence on the mean size of alumina grains was observed. In addition, the effect of low temperature heat treatment (drying at 120 °C and calcination at 700 °C) was also studied. The sinterability of pre-dried powder increased if a wet consolidation method (pressure filtration) was used, but a negative effect of pre-drying was observed in case of dry forming (axial pressing). The calcination decreased the ability of the powder to adsorb water. The presence of aggregates formed by calcination markedly decreased the green and sintered densities in compacts consolidated by axial pressing.     

Si粉和Al粉对N2保护热处理铝碳材料性能和显微结构的影响

为改善含碳材料埋炭保护热处理时的操作条件,并为提高含碳材料性能开辟新的途径,对添加Si粉或Al粉的铝碳材料在N2气氛中1200℃热处理5 h,测定试样热处理前后的质量变化率、抗折强度、体积密度、显气孔率、抗热震性、热膨胀系数和抗氧化指数,并用扫描电镜和XRD分析试样的显微结构和物相组成.结果发现,采用N2保护热处理工艺同埋炭热处理工艺一样可防止碳氧化,使铝碳材料形成碳结合并具有较好的性能指标.与添加Si粉的试样相比,添加Al粉的试样在热处理后具有较高的高温强度和较低的热膨胀系数;反应生成的Al2OC和AlN有益于材料抗热震性能和强度的提高,并减轻了其在埋炭保护热处理后由于生成Al4C3而产生的严重水化现象.部分Si粉与气氛反应生成纤维状的β-SiC、SixN和粒状的Si2N2O,它们对铝碳材料具有明显的增强作用.     

An experimental study on the effect of liquid content and viscosity on particle segregation in a rotating drum

The segregation phenomenon of wet granular materials was experimentally studied in a quasi-2D rotating drum. The mono-disperse systems and binary-mixture systems (with 4 mm and 2 mm glass beads and 40% filled volume fraction) were used. All the experiments were controlled so the Froude number of the rolling regime was 2.79 × 10^− 4. The effects of the volume and the viscosity of the liquid added to the granular system on the segregation index and angle of repose in the rotating drum were investigated and are discussed in this paper.The experimental results indicate that the volume and viscosity of the added liquid have significant effects on the wet granular flow. The results demonstrate that the segregation index decreases with an increase of the repose angle of the wet granular materials, regardless of the volume or viscosity of the added liquid.     

Simulating the mixing and segregation of solids in the transverse section of a rotating kiln

Rotating kilns are widely used in industry to process granular material. These processes include calcination of mineral ores, drying of foods and grains, combustion of wastes and manufacturing of pharmaceuticals. Since models developed for a particular process are often unique to that process there is a need to develop more generic models to predict the mixing and segregation in the transverse section of a rotary kiln.This paper presents a mathematical simulation - based on experimental observations - to estimate the mixing rate, final extent of mixing and the final distribution of material due to segregation. The models used in the simulation allow for scale-up of processes to produce a simulation that is applicable to a broad range of industrial processes. Independent experiments were used to verify the simulation and it was found that the mixing rate, the final extent of mixedness and the final segregated state could be predicted to within acceptable errors.     

Low-pressure chemical and photochemical reactions of oxides of nitrogen on alumina taken as a model substance for mineral dust in relation to air pollution

The interaction of γ-Al₂O₃, taken as a model substance of tropospheric mineral dust, with N₂O, NO and NO₂ has been studied using kinetic and temperature-programmed desorption (TPD) mass-spectrometry in presence and absence of UV irradiation. At low surface coverages (<0.001 ML), adsorption of N₂O and NO₂ is accompanied by dissociation and chemiluminescence, whereas adsorption of NO does not lead to appreciable dissociation. Upon UV irradiation of Al₂O₃ in a flow of N₂O, photoinduced decomposition and desorption of N₂O take place, whereas in a flow of NO, only photoinduced desorption is observed. Dark dissociative adsorption of N₂O and NO and photoinduced N₂O dissociation apparently occur by a mechanism involving electron capture from surface F- and F⁺-centers. Photoinduced desorption of N₂O and NO may be associated with decomposition of complexes of these molecules with Lewis acid sites, V-centers or OH-groups. TPD of N₂O and NO proceeds predominantly without decomposition, while NO₂ partially decomposes to NO and O₂.     

Power law and scaling for molecular weight dependence of crystal growth rate in polymeric materials

The molecular weight (M) dependence of the linear crystal growth rate (G) and the influence of the super-cooling on the relationship between M and G were studied. The molecular weight dependence of G has been expressed generally as G∝M^α at a given super-cooling. The temperature dependence of G shows a bell shape with the maximum growth rate (G_max). The value of α was −0.5 at the temperature (T_cmax) of G_max. However, the small super-cooling and the small molecular weight gave a large negative value of α. In other words, the value of α was dependent not only on the degree of super-cooling (ΔT) but also on the molecular weight. The effect on α by these two factors (ΔT and M) goes off to zero at T_cmax and α yields to −0.5. G_max can be defined as a characteristic intrinsic value to the crystal growth behavior. The molecular weight dependence of G_max was scaled and expressed as a −0.5 power to molecular weight for all crystalline polymers.     

Reynolds number scaling of flow in a Rushton turbine stirred tank. Part I—Mean flow, circular jet and tip vortex scaling

We consider scaling of flow within a stirred tank with increasing Reynolds number. Experimental results obtained from two different tanks of diameter 152.5 and 292.1 mm, with a Rushton turbine operating at a wide range of rotational speeds stirring the fluid, are considered. The Reynolds number ranges from 4000 to about 78,000. Phase-locked stereoscopic PIV measurements on three different vertical planes close to the impeller give phase-averaged mean flow on a cylindrical surface around the impeller. The scaling of θ- and plane-averaged radial, circumferential and axial mean velocity components is first explored. A theoretical model for the impeller-induced flow is used to extract the strength and size of the three dominant elements of the mean flow, namely the circumferential flow, the jet flow and the pairs of tip vortices. The scaling of these parameters with Reynolds number for the two different tanks is then obtained. The plane-averaged mean velocity scales with the blade tip velocity above a Reynolds number of about 15,000. However, parameters associated with the jet and tip vortices do not become Reynolds number independence until Re exceeds about 10⁵. The results for the two tanks exhibit similar Reynolds number dependence, however, a perfect collapse is not observed, suggesting a sensitive dependence of the mean flow to the finer details of the impeller.     

Enhanced activity of in and Ga-supported sol-gel alumina catalysts for NO reduction by hydrocarbons in lean conditions

Nitrogen monoxide was reduced efficiently by hydrocarbons in the presence of oxygen over sol-gel alumina supported indium, gallium, cobalt and tin catalysts. The support alumina prepared by a sol-gel method had high surface area and accordingly active alumina sites for the reaction. Particularly indium/alumina showed a high activity to reduce NO preferably by propene, propane and ethene but also by alcohols in the absence and the presence of water vapor. The activities of alumina supported cobalt, silver and tin catalysts were increased when calcinating the catalysts at 800°C instead of 600°C. In the case of gallium/alumina, NO₂ has higher reactivity than NO to nitrogen when propene was used as a reductant, proving the significance of the oxidation step of NO to NO₂. The step of NO oxidation was promoted by preparing a physical mixture of 5 wt% Mn₃O₄ with indium/alumina or gallium/alumina. The NO conversion to nitrogen was increased from 58 to 84% with the manganese oxide promotion over indium/alumina in the presence of water. The reaction mechanistic differences between the alumina supported catalysts and Cu/ZSM-5 were also discussed.     

On the importance of fibers' cross-sectional shape for air filters operating in the slip flow regime

In this paper, we investigate the effects of fibers' cross-sectional shape on the performance of a fibrous filter in the slip and no-slip flow regimes. The slip flow regime is expected to prevail when fiber diameter is comparable in size to the mean free path of the gas molecules (about 65 nm at normal temperatures and pressures), whereas the no-slip flow regime describes the aerodynamic condition of flow through media with large fibers. Our numerical simulations conducted for flow around single fibers with different geometries indicate that, while the collection efficiency is only weakly affected by the cross-sectional shape of nanofibers, the fiber drag (i.e., permeability of the media) can be considerably influenced by the fiber's shape. Simulating the flow field around nano- and microfibers with circular, square, trilobal, and elliptical cross-sections, it was found that the more streamlined the fiber geometry, the lower the fiber drag caused by a nanofiber relative to that generated by its micron-sized counterpart.     

Hydrodynamic and heat transfer characteristics of a centrally heated cylindrical enclosure: CFD simulations and experimental measurements

Natural convection in enclosures is of importance in many engineering applications. The stratification arising out of natural convection may be desirable/undesirable depending on applications. In order to control the degree of stratification, understanding of flow pattern and temperature profiles is required. In the present work, transient natural convection in a cylindrical enclosure has been investigated for water with CFD simulations and flow visualization [using particle image velocimetry (PIV) and hot film anemometry (HFA)] over a wide range of parameters namely Rayleigh number (1.08 × 10¹¹ ≤ Ra ≤ 3.76 × 10¹³) and aspect ratio (1 ≤ H/R ≤ 2). The effect of various parameters like pressure, tube diameter and aspect ratio on the extent of stratification has been studied. PIV measurements have been performed to understand the transient flow behavior. Multiple thermocouples were used to measure the temperature profiles. CFD simulations have been performed using SST k–ω model and the results have been compared with the PIV measurements. The CFD simulations have been carried out for 2D axi-symmetric cases and the effect of boundary conditions (free-slip and no-slip) has been investigated. An excellent agreement was found between the CFD predictions and the experimental measurements of flow and temperature patterns. The extent of stratification has been quantified using dimensionless parameters like stratification number and stratification time. The kinetic energy profiles and kinetic energy dissipation profiles show that almost 75% of the enclosure is stratified (after different times depending on Ra number and the aspect ratio). The turbulence parameters were found to weaken with time in the stratified region and these predictions are corroborated with HFA measurements.     

Numerical and experimental studies on the flow multiplicity phenomenon for gas-solids two-phase flows in CFB risers

The flow multiplicity phenomenon in circulating fluidized bed (CFB) risers, i.e. under the same superficial gas velocity and solids circulation rate, the CFB risers may sometimes exhibit multiple flow structures, was numerically and experimentally investigated in this study. To investigate the flow multiplicity phenomenon, the experiments of gas-solids two-phase flows in a 2-D CFB riser with different flow profiles at the inlet of the CFB riser were conducted. Specially designed gas inlet distributors with add-ons are used to generate different flow profiles at the inlet of the CFB rise. The CFD model using Eulerian-Eulerian approach with k-ε turbulence model for each phase was employed to numerically analyze the flow multiplicity phenomenon. It is experimentally and numerically proved that for gas-solids two-phase flows, the flow profiles in the fully-developed region are dominated by the flow profiles at the inlet. The solids concentration profile is closely coupled with the velocity profile, and the inlet solids concentration and velocity profiles can largely influence the fully-developed solids concentration and velocity profiles.     

Studies on the performance of a hydrocyclone and modeling for flow characterization in presence and absence of air core

Hydrocyclones are getting more and more interest from various industries. They are widely used to separate particulates from liquid at high throughput because of their advantages like simple structure, low cost, large capacity and small volume, require little way of maintenance and support structure. Modeling of complex and multiphase flow behavior inside the hydrocyclone is done usually with the help of computational fluid dynamic study. Current study involves experimental investigation of separation performance characteristics of the hydrocyclone using new design parameters. For experimental purpose, a new hydrocyclone was designed with insertion of solid rod, at central portion of conical section of hydrocyclone, inside the hydrocyclone . By which air core could be eliminated effectively and hydrocyclone performance is improved. This effect may be observed due to reduction of radial and axial components of velocity and turbulence in the area near the entrance of the vortex finder. Therefore, the flow field characteristics inside the hydrocyclone with no air core become more suitable for separation. Also the effect of flow rate, vortex finder depths, air core and particle interaction were studied experimentally. A new arrangement was suggested to eliminate the air core formed inside the hydrocyclone. In this case, effect of diameter and height of solid rod inserted inside the hydrocyclone with changing total inlet flow rate was studied experimentally. Three-dimensional geometry and meshing of hydrocyclone is created in Gambit, preprocessor of commercial software—Fluent, for hydrodynamic study.     

Deactivation and regeneration of Pt/γ-alumina and Pt/ceria–alumina catalysts for methane combustion in the presence of H2S

Ceria has been widely explored as an additive in alumina-supported precious metal catalysts due to a number of unique properties. The success of ceria and ceria-based materials is mainly attributed to the unique combination of an elevated oxygen transport capacity coupled with the ability to shift easily between reduced and oxidised sates. In this study the influence of CeO₂ addition to a Pt/Al₂O₃ catalyst for low temperature (<540 °C) methane oxidation in an oxidising environment has been investigated. The resistance to H₂S-poisoning and influence on catalyst regeneration by oxidation or reductive treatments has been studied. The addition of CeO₂ to the support creates an increase in the level of activity based primarily on the oxygen storage capacity offered by the cerium oxide, causing an increase in oxygen activation. The ceria–alumina-supported catalyst showed a greater shift to poorer activity upon exposure to H₂S. It appears sulphur compounds react with the oxygen storage component causing a decrease in oxygen transfer, removing any benefit offered by the ceria. However, the level of Pt-agglomeration and support changes were reduced with the incorporation of ceria, emphasising the stabilising effect and promotion of metal particle dispersion associated with ceria. In order to obtain the maximum benefit of ceria addition to the support structure in terms of activity a reductive pretreatment is required. Upon exposure to a reducing atmosphere, it appears a Pt–CeO₂ interaction generates greater levels of activity.     

Optimization of a multiphase sensor for detection of phosphonates in air

The objective of this article is to report the modeling and optimization of a new MEMS‐based phosphonate sensor that utilizes a porous membrane between a gas and a liquid stream to allow operation at low‐liquid and high‐gas flow rates. Previous work from our laboratory demonstrated that phosphonate molecules can be detected with such a device, but the sensitivity was insufficient for certain applications (e.g., detection of pesticides in foodstuffs). In this article, COMSOL simulations and design of experiments were used to optimize the device. We find that both the simulation and the experiment show that (i) the size of the pores in the membranes and (ii) the liquid channel height make the most difference to the sensor response. Also, by optimizing the geometry, the sensitivity of the device could be enhanced. The optimized device can detect 10⁹ molecules with good signal to noise. © 2009 American Institute of Chemical Engineers AIChE J, 2010