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Talence, France

Esnault V.,Lafarge | Michrafy A.,French National Center for Scientific Research | Heitzmann D.,Lafarge | Michrafy M.,BEM | Oulahna D.,French National Center for Scientific Research
Powder Technology | Year: 2014

The gas trapped into fine powders causes specific problems like feeding disturbances during roll compaction process. In this study, the gas flow and its effect on the rolling process are numerically investigated in the rolling direction, using Darcy's law and assuming the permeability as a function of both material density and particle size through Carman-Kozeny relationship. The solid properties evolution is based on the Johanson model, whereas the solid speed is determined from the conservation of the material mass. Computational results of solid properties and gas pressure distribution are presented by considering bentonite powder properties. According to material and process parameters, especially rolling speed and powder permeability, we discuss conditions for the escape of gas through the porous material during the process and stability conditions of the feeding at the rolls' entry. Beyond the simplicity of the model (1D), it allows for a better understanding of fine powders processing by roll press. It highlights the combined effect of the permeability of the powder and the rotating speed on the gas pressure, during roll compaction process. © 2014 Elsevier B.V. All rights reserved. Source


Michrafy A.,French National Center for Scientific Research | Diarra H.,French National Center for Scientific Research | Dodds J.A.,French National Center for Scientific Research | Michrafy M.,BEM
Powder Technology | Year: 2011

Homogeneity of properties over the width of strips produced by roll compaction of microcrystalline cellulose powder (MCC) has been examined by light transmission through the compact, by measurements of the porosity distributions and by three-dimensional finite element modeling. Light transmission through compacts revealed periodic heterogeneity in the form of alternate dark and light zones. The period seems to be connected to the geometry of the screw and independent on the feed screw velocity which was varied with the roll speed with a constant ratio. Measurements of the porosity of samples cut from the compacted strip show heterogeneity of the density over the width of strips with a higher density in the centre of the strip and a lower density on the sides. These two techniques clearly showed the heterogeneous behavior across the width of the compacted strip of MCC. However, the light zones (respectively dark zones) did not correspond to the lower porosity zones (respectively higher porosity zones). Three-dimensional finite element modeling (FEM) of roll compaction of powders was conducted with two inlet feed conditions: constant feed pressure and constant feed velocity. Results of the simulations using the constant feed pressure show a uniform maximum principal stress and density across the width of the strip. When a constant inlet feed velocity is assumed the maximum principal stress over the strip width was higher at the centre of the strip and decreases to the sides. This profile also corresponds to the density profile over the width of the strip. In this case, the predicted results present a similar tendency to that found by mercury intrusion porosimetery and are in agreement with the measured bulk density of strips produced with different roll speeds. © 2010 Elsevier B.V. Source


Michrafy A.,French National Center for Scientific Research | Diarra H.,Chemin des Cheuvreuils | Dodds J.A.,French National Center for Scientific Research | Michrafy M.,BEM | Penazzi L.,Ecole des Mines dAles
Powder Technology | Year: 2011

The process of drawing and densification of powdered microcrystalline cellulose by roller press in a steady state operation is analyzed using a 2D modeling with the finite element method and the modified Drucker-Prager Cap model as material behavior. Distributions of process variables in contact surface between powder and roller such as pressure, shear stress and relative speed were predicted and used to analyze the basic mechanisms of the transport and the densification of powder between rolls. The results show clearly the existence of three contiguous zones: a phase where the powder is drawn between rolls by a sliding mechanism, a sticking phase where the powder is transported with the same velocity as the roll and where the densification by deforming the powder bed is achieved under the increase of roll pressure that reaches its peak before the neutral angle. The formed compact is then expulsed out of the gap by a slip mechanism resulting from the change of the sign of the shear stress. The predicted density distribution between the rolls, shows a gradual increase. The density reaches its maximum before the neutral point and shows values in agreement with the density of strips prepared with an instrumented roll press. The effect of varying the material parameters on the maximum pressure and the nip angle is also investigated.Beyond the description of the basic mechanisms of roller compaction, this modeling shows a real potential of the optimization of the roller compaction process. © 2010 Elsevier B.V. Source


Esnault V.,ParisTech National School of Bridges and Roads | Esnault V.,Lafarge | Heitzmann D.,Lafarge | Michrafy M.,BEM | And 2 more authors.
Chemical Engineering Science | Year: 2013

Permeating air is known to have a negative impact on the roller compaction process, because the feed is destabilized by the flow of escaping gas, causing pressure to build-up and potentially damage compacts at release. Airflow during powder roller compaction and its effect on the rolling process are investigated in the rolling direction (1D), using an extension of the Johanson model for the solid. Fluid transport obeys Darcy's law, with permeability being a function of both material density and particle size, through the Kozeny-Carman relationship. In this modeling, the effect of the air pressure on the solid is neglected in the compaction zone. Assuming air at atmospheric pressure at the feeding angle and ignoring airflow through the gap, predictions of air pressure as a function of the rolling angle for bentonite material powder are presented and discussed. Results suggest the existence of two different stability zones within the operating conditions, where industrial systems could function without being affected by airflow effects. The model highlights the importance of the permeability/rotation speed ratio, which governs the proportion of air trapped in the compacts to the portion evacuated through the feed. We also investigate the effect of particle fragmentation during the rolling process. Finally, we provide guidelines for the scale-up of roller presses subjected to air flow issues, through a study of the effect of the system dimensions and rotation speed on the elimination of air.In spite of the lack of available experimental data, this model allows for a better understanding of how air escapes by diffusing through the material during the rolling process, and opens interesting perspectives for the mitigation of its effect on the process. © 2013 Elsevier Ltd. Source


News Article
Site: http://www.nrl.navy.mil/media/news-releases/

U.S. Naval Research Laboratory (NRL) research engineer, Dr. John Steuben, is awarded the prestigious American Society of Mechanical Engineers (ASME) 2015 Best Ph.D. Thesis of the Year Award for his dissertation entitled "Massively Parallel Engineering Simulations On Graphics Processors: Parallelization, Synchronization, and Approximation." Since 2013, the ASME Computers and Information in Engineering (CIE) Division has presented the award in recognition of promising young investigators who authored the best Ph.D. thesis of the year in the area of computers and information in engineering. Receiving a master's in mechanical engineering from Colorado School of Mines in 2011, Steuben continued his doctoral studies at Mines under the guidance of Dr. Cameron Turner, Associate Professor of Mechanical Engineering. In 2012 he began participating in the Naval Research Enterprise Internship Program (NREIP) under the supervision of Dr. John Michopoulos, head of the Computational Multiphysics Systems Laboratory (CMSL) at NRL. During this tenure, Steuben contributed in the development of a fast characterization approach that reduced the computing time required to characterize material specimens (tested in NRL's six degree-of-freedom automated test frame) from 24 hours to approximately 30 seconds. This research, hosted through a NREIP internship by NRL at CMSL, continues through present day and is devoted to the development of surrogate model approaches for the characterization of composite materials. Graduating from Mines in 2014 with a doctorate in engineering systems, Steuben's dissertation examined three computer-aided engineering (CAE) methods; boundary element method (BEM), discrete element method (DEM), and finite element method (FEM) for particular applications. These three methods are crucial elements of the core research upon which CIE was founded. Steuben's work led to a general-purpose computing on graphics processing unit (GPGPU) parallelized BEM solver, a GPGPU parallelized DEM model of icing effects of wind turbine blades incorporating frictional, thermal and phase change effects (requiring synchronization and parallelization), and a surrogate approximation enhanced FEM approach (parallelization, synchronization and approximation) to solving the inverse material characterization problem for non-isotropic composite materials including those being tested at NRL. "Dr. Steuben's research thesis was that parallelization must be considered in terms of both synchronization and approximation," said Turner. "As his work concluded on his dissertation, he proceeded to formulate extensions of his research into characterizing the as-built parameters of additively manufactured components using a combination of his DEM simulations and his FEM experience with composite materials." Since January 2015, Steuben has been conducting postdoctoral research at NRL with funding from the National Academies of Sciences, Engineering, and Medicine. This work has focused on the development of computationally efficient multiphysics simulations of additive manufacturing processes and components. This research will allow the design and fabrication of novel, useful, and efficient structures and components across a wide range of engineering disciplines. About the U.S. Naval Research Laboratory The U.S. Naval Research Laboratory provides the advanced scientific capabilities required to bolster our country's position of global naval leadership. The Laboratory, with a total complement of approximately 2,500 personnel, is located in southwest Washington, D.C., with other major sites at the Stennis Space Center, Miss., and Monterey, Calif. NRL has served the Navy and the nation for over 90 years and continues to advance research further than you can imagine. For more information, visit the NRL website or join the conversation on Twitter, Facebook, and YouTube.

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