1,721,195 research outputs found
Cohesive interparticle forces and bulk flow properties in fluidisation and powder flow: is a coordinated view possible and convenient?
No full textBinary cohesive interparticle forces are often used to describe the aggregative behaviour of particles in gas fluidization applications of fine powders and, sometime, to explain other phenomena in fluidization such as the delayed bubbling in aeratable powders. These forces, however, can be theoretically estimated but can be hardly measured. The direct measurement of bulk flow properties is a common procedure to predict the powder flow in silos and piles. Rumpf and Molerus suggest some approaches to relate interparticle cohesive forces with bulk properties. In theory, therefore, it would be possible to use the measurement of bulk flow properties to estimate interparticle forces in order to predict the aggregative behaviour of particles in fluidized systems.
In this presentation some recent experimental results will be reported trying to demonstrate the direct relationship between bulk flow properties and the aggregative behaviour of cohesive powders in vibrated fluidization and aerated discharge of solids. Other experiments will be also reported to validate the possibility of using the Rumpf approach to relate cohesive interparticle forces to the measurement of bulk flow properties
Polveri e solidi granulari, i problemi di movimentazione possono essere studiati e risolti
No full textPresso il laboratorio di Tecnologia delle Polveri del Dipartimento di Ingegneria Industriale dell’Università di Salerno sono disponibili diverse tra le più moderne apparecchiature per la completa caratterizzazione delle polveri e dei solidi granulari ai fini della corretta progettazione delle operazioni di stoccaggio e di movimentazione. In questo laboratorio, inoltre, si studiano metodi e tecnologie in grado di movimentare in sicurezza anche polveri molto coesive e si mettono a punto adeguate procedure di progettazione
THE ROLE OF AGGREGATION IN FLUIDIZATION AND FLOW OF COHESIVE POWDERS
No full textAggregation in cohesive powders is a well known behaviour and it is responsible of both poor fluidization of cohesive powders and of a reduced effect of aeration in the promotion of silo discharge. The size of the aggregates is determinant in the definition of the fluid dynamic behaviour of these powders. The aggregate size can be estimated starting from the measurement of flow properties, and it can be used to predict the solids discharge rate in the aerated discharge of cohesive powders and to calculate the minimum fluidization velocity in vibrated fluidization of similar powders. In the former case the aggregate size of the aggregate is the one for which the weight of the aggregate equals the cohesive forces keeping the aggregate with the solids bulk. In the latter case the approach is similar but the role of the weight is played by the aggregate inertia which depends on the vibrational acceleration rather than on the gravitational acceleration. It is, therefore, shown that aggregation can reasonably be estimated by the use of conventional techniques and that, if correctly accounted for, it allows to predict the fluid dynamics of cohesive powders
Review of the book: Introduction to granular flow. by K. KesavaRao and Prabhu R. Nott
No full textAn Introduction to Granular Flow
By K. Kesava Rao Prabhu R. Nott,
Cambridge University Press, Cambridge (UK), 2008
ISBN-13 978-0-521-57166-1
This book goes far beyond the promise of its title, and it is a wide compendium of the present state of the knowledge in the modeling of stress and velocity distributions for flowing and non flowing granular materials. This makes this book an important reference for scientists and engineers working on granular matter. The book is not only theory and most of the chapters start with an overview of the experimental observations that justify the specific modeling approach proposed in the corresponding chapter.
In the first chapter the fundamentals of the physics and the basic balance equation that are relevant to particulate systems are presented. Chapter 2 to 4 are devoted mainly to plain silo flow and, more in general, to those systems that can be approached by continuum mechanics in two dimensions. The simplified geometry allows to introduce and discuss in simpler cases most of the assumptions made in the stress and strain analyses involving granular materials. Chapters 5 and 6 extend the application of continuum mechanics to cases involving tree dimensional flow and, in particular, to axy-symmetric bins. The most important powder constitutive equations involving yield and non yield conditions are also discussed in these chapters. The last four chapters are devoted to the modeling of fast granular flow by the approach of the kinetic theory. In particular, chapters 7 and 8 address the kinetic theory that can be derived by assuming smooth inelastic particles and its applications. In chapter 9 the velocity distribution functions and the particulate solids constitutive equation are compared for the two cases of smooth and rough particles.
Two features of this book make it suitable for adoption as a text book in a university course for graduate students on granular flow. The first is represented by more than sixty pages of 11 appendixes most of which summarize the basics of the tensor analysis. The second feature is represented by the large number of exercises promoting meditation on the main concepts presented in the text and their application
Pipe stability in aerated silos
Full text linkThe procedure to calculate the pipe stability criterion in aerated silos proposed by Johanson (2004) has been discussed and extended beyond the calculation limits present in the original paper. A table of the dimensionless function G(φ, A) = Dpgρb/fc has been calculated for angles of internal friction φ between 35 and 70° and dimensionless radial pressure gradients −A between 0 and 10. Values of G in this table have been fitted with an algebraic expressionwhich is able to correlate valueswith an approximation not larger than 10% and generally smaller than 3%. Theoretical findings were compared with previous experiments. The comparison between the theory and the experimental results indicates that aeration can produce significant changes in the effective body force acting on the powder to such an extent that the horizontal gas pressure gradient is larger than gravity and, therefore, beyond the limits provided by Johanson (2004). The theoretical approach is able to provide results which correlate reasonably well with the experiments if the appropriate bulk solid consolidation is used. In particular, if aeration is started before pipe formation, it can produce bulk solid compaction that increases pipe stability and has to be accounted for so as to evaluate critical aeration conditions for pipe stability
The activities of the EFCE Working Party Mechanics of Particulate Solids and some contributions from the University of Salerno
No full textWorking Parties and Sections are at the center of the scientific life of the European Federation of Chemical Engineers. Their role is to allow academic and industrial experts on scientific to meet and keep in contact in order to foster the development of knowledge in the field. They are made of delegates from the federated societies but also from elected members and guests.
The specific objective of the WP Mechanics of Particulate Solids is the coordination and the exchange of scientific information regarding the mechanics and the dynamics of granular systems, mainly considered as bulk solids. Typical applications taken in to account are the equipment for storage, handling and physical transformation of solid materials in granular form. Among its activities, the WP critically follows the evolution of characterization standards and can be a promoter of new ones. The WP follows the dissemination of scientific information in this field and creates opportunities for comparison. Presently the WP includes 36 among delegates, elected members, international guests and honorary members from 16 European countries, from the United States and from Australia. The WP also invites other scholars working in the field of its scientific interest to participate to its meetings.
The themes presently dealt with by the working party concern:
1) Wall friction project. This project is intended to find a more detailed procedure than the standard to obtain reproducible results with the measurements of the wall friction between bulk granular solids and silo walls. It was decided to carry out a ‘round robin’ test, following the model of the work carried out in the late 90’s by Haaker. In a thematic workshop in conjunction with the WP meeting in 2009 a modified measurement procedure was defined. WP participants from about 18 laboratories around the world take part to the work by carrying out experiments with their own equipment and with the material distributed by some industrial WP members.
2) Validation of DEM Simulation: A significant number of WP members take part to PARDEM project (www.pardem.eu), funded with ~3.3 million EUR by the EU 7th Research Framework Programme, Marie Curie Initial Training Network, for the period 2009-2013. The project is led by a Member of the WP, Prof. Jin Ooi, University of Edinburgh, UK. Thirteen PhD students and two Post Doc fellows are being trained.
3) K/lambda Testing: an experimental campaign is being carried out with the tester developed at the University of Salerno. The final objective of the experimental campaign is to find out a reproducible procedure which might be able to provide representative measures of the ratio of vertical to horizontal stresses. Experiments are carried out on both incompressible and compressible powders.
4) Flow properties of biomasses: Solid biomasses are renewable energy sources and can be made of different materials such as switch grass, corn stover, peanut hulls, bagasse. In energy production applications, these are often found as mixtures with traditional fossil fuel sources. These materials are typically fibrous, highly deformable.Their mechanical behavior is strongly affected by moisture and furthermore is not easily studied with conventional approach and instrumentation for bulk materials. Therefore, conventional silo and hopper design procedures are often inadequate. As a result, design errors are responsible of delayed start-ups and of reduced plant production capacities. A network has established within the working party trying to understand more deeply the flow behavior of these materials.
In this keynote an overview of the Working party Mechanics of Particulate solids activity will be given and some details will be provided of the research activities carried out by the University of Salerno for the Working Party and in particular on the K/lambda Testing and in the field of the biomass flow properties
Aggregative behaviour in gas fluidisation of mechanically vibrated cohesive powders
No full textPresentato alla 8th European Conference in Chemical Engineering, Berlin (D), 26-29 settembre 2011. Session Particle Technology 29/9/2011 h11:00
ABSTRACT:
Gas fluidization of mechanically vibrated cohesive silica powder (120 Hz) and a potato starch powder (70 Hz) was tested. The peak acceleration was the main parameter of the mechanical vibration which was changed. It was varied between 2g and 10g.
Cohesion of both powders was such that proper fluidization was impossible without the application of mechanical vibration. The starch powder showed gas channelling which tended to disappear for high values of acceleration. At increasing acceleration, increasing bubbling and bubble size were observed. At low acceleration silica powder exhibited cracks which tended to disappear with increasing acceleration.
Aggregate diameters were estimated by means of the Ergun equation and compared with those derived from a model in which the aggregate diameter was derived from powder flow properties measured with a shear tester. Results of the aggregate diameter obtained with both procedures show a decrease with increasing acceleration for both powders. In particular, the quantitative agreement between the two modelling approaches is good for silica powder in the entire range of the accelerations tested, and only for high acceleration for the starch powder.
The minimum fluidization velocity of the powder was estimated from the aggregate diameter derived from the powder flow properties and the application of the Ergun equation, assuming a voidage between the aggregates constant with the acceleration. Model results are in satisfactory agreement with the experiments
A study on the measurement error source of the vertical to horizontal stress ratio on coarse model materials
No full textThe correct structural design of storage silos for granular solids depends on the lateral
and vertical stresses that are exerted by the stored solids on the silo walls. Standard structural
design specifications are reported in international design codes as Eurocode EN 1991-4:2006
[1] and in related national standards. The estimate of the load that acts on a vertical silo wall
is usually performed by applying the Janssen [2] method of differential slices, which requires
the evaluation of the horizontal to vertical stress ratio or the lateral pressure ratio, K.
An experimental device for the measurement of the horizontal to vertical stress ratio had
been purposely built and tested for fine and cohesive solids [3]. With powder testing the
source of error in measurement procedure for powder flow properties may arise from different
sources and, in particular from a) the story of the powder sample or b) some forces not
properly accounted in the measurement, such as those due to unduly neglected frictions.
Proper measurement procedures should determine measurement conditions in which both
sources of errors are minimized or appropriately accounted for. For example, for the
developed device [3], it was found that that a twisting action on the lid of the cylindrical
powder cell was able to produce reliable repeatable results, by possibly minimizing both the
effect of the powder history and of the uncounted powder friction on the cell lid.
In order to asses on the possible sources of errors in the proposed experimental
procedures a large set of experiments was carried out by using model material made of Glass
spheres (6mm), plastic spheres (5 mm), steel spheres (2mm). Comparison on the scatter of the
value of the vertical to horizontal stress ratio obtained within a single experiment at different
load values and the scatter of data between different sets of experiments it can be concluded
that with these materials the largest source of error is due to unpredictable differences in the
internal structure of the particulate materials. Furthermore, the comparison between the
Coulomb representation of the flow properties of the particulate solids and the Mohr
representation of the state of stress in the device obtained by assuming the vertical and the
horizontal stress as principal stresses confirms the hypothesis that the condition of incipient
internal shear of the material is reached by showing Mohr circles tangent to the yield locus.
This conditions confirms that the proposed measurement procedure is in fact able to minimize
the effects of the lid friction on the sample.
Well known hysteresis effects in the unloading part of the experiments are also observed
in our experiments and reported. It is verified that, for the tested material, no memory of the
previous compression experiment is found only in the case in which the complete sample
unloading is reached between the two subsequent experiments.
References
[1] Eurocode 1. DD ENV 1991-4 (1996).
[2] Janssen, H.A. Zeitschr. d. Vereines deutscher Ingenieure, 39, 1045-1049 (1895). As
reported by Sperl, M., Granular Matter 8, 59-65 (2006).
[3] Barletta, D. and Poletto M. Granular Matter 15, 487–497 (2013)
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