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Thiebaud F.,CNRS Mechanical Energy, Theories, and Applications Laboratory | Gelin J.C.,CNRS Femto ST Institute
Composites Science and Technology

This paper focuses on the numerical simulation of the polypropylene/multi-walled carbon nanotubes (PP/MWCNT) flow into a twin-screw mixer, during the mixing phase. The PP/MWCNT behavior obeys an innovating Carreau law enriched temperature built on the rheological properties carried out previously. The polypropylene was mixed with different MWCNTs contents (1, 2, 4 and 8 wt.% of MWCNT content) and the rheological tests were performed at shear rate ranges from 10-1 to 2 × 104 s-1 at four temperatures (180, 200, 220 and 240 °C). Thus the effects of the temperature and the MWCNTs content on the rheological properties of the PP/MWCNT composites were investigated. The finite element (FEM) analysis of the PP/MWCNT flow allows to compute the velocity, the shear rate and the temperature during the mixing phase period. A good agreement between the experimental measured torque on the screw and the calculated one is shown. Therefore, one can consider that the physical flow is generally well described, awaiting a numerical simulation of the PP/MWCNT mixing phase. © 2009 Elsevier Ltd. All rights reserved. Source

Feidt M.,CNRS Mechanical Energy, Theories, and Applications Laboratory

In a recent review an optimal thermodynamics and associated new upper bounds have been proposed, but it was only relative to power delivered by engines. In fact, it appears that for systems and processes with more than one utility (mainly mechanical or electrical power), energy conservation (First Law) is limited for representing their efficiency. Consequently, exergy analysis combining the First and Second Law seems essential for optimization of systems or processes situated in their environment. For thermomechanical systems recent papers report on comparisons between energy and exergy analysis and corresponding optimization, but the proposed models mainly use heat transfer conductance modelling, except for internal combustion engine. Here we propose to reconsider direct and inverse configurations of Carnot machines, with two examples. The first example is concerned with "thermofrigo-pump" where the two utilities are hot and cold thermal exergies due to the difference in the temperature level compared to the ambient one. The second one is relative to a "combined heat and power" (CHP) system. In the two cases, the model is developed based on the Carnot approach, and use of the efficiency-NTU method to characterize the heat exchangers. Obtained results are original thermodynamics optima, that represent exergy upper bounds for these two cases. Extension of the proposed method to other systems and processes is examined, with added technical constraints or not. © 2013 by the authors; licensee MDPI, Basel, Switzerland. Source

Panfilov M.,CNRS Mechanical Energy, Theories, and Applications Laboratory
Transport in Porous Media

In situ observations have shown that underground storage of hydrogen behaves like a natural chemical reactor and generates methane. The mechanism of this generation is the metabolic activity of methanogenic bacteria which consume hydrogen and carbon dioxide and transform them into methane and water. The coupled mathematical model of the reactive transport and population dynamics in a storage is suggested in this paper which also takes into account the fact that the population growth rate depends on the structure of the bacterium colony. The suggested system of equations is reduced to the Turing reaction-diffusion model which proves the appearance of non-attenuating self-oscillations in time which are uniform in space. These solutions are unstable and, once perturbed, generate regular spatial stationary waves which correspond to the alternations of zones which are rich in CH4 or CO2. This result predicts the effect of a natural in situ separation of gases, which was observed in practice. If the diffusivity of bacteria is neglected with respect to the effective diffusivity of the injected gas, then only large-scale spatial waves arise. A low but non-zero bacterium diffusivity causes the appearance of additional small-scale linear oscillations whose period is the intrinsic parameter of the process and is proportional to the bacteria-gas diffusivity ratio. The analysis is completed with numerical simulations of 2D problems and analytical solutions of 1D problems obtained using the technique of two-scale asymptotic expansion. The estimations for the parameters of the model were obtained. © 2010 Springer Science+Business Media B.V. Source

Abe Y.,CNRS Laboratory of Physical Chemistry and Microbiology for the Environment | Skali-Lami S.,CNRS Mechanical Energy, Theories, and Applications Laboratory | Block J.-C.,CNRS Laboratory of Physical Chemistry and Microbiology for the Environment | Francius G.,CNRS Laboratory of Physical Chemistry and Microbiology for the Environment
Water Research

Drinking water biofilms are complex microbial systems mainly composed of clusters of different size and age. Atomic force microscopy (AFM) measurements were performed on 4, 8 and 12 weeks old biofilms in order to quantify the mechanical detachment shear stress of the clusters, to estimate the biofilm entanglement rate ξ. This AFM approach showed that the removal of the clusters occurred generally for mechanical shear stress of about 100kPa only for clusters volumes greater than 200μm 3. This value appears 1000 times higher than hydrodynamic shear stress technically available meaning that the cleaning of pipe surfaces by water flushing remains always incomplete. To predict hydrodynamic detachment of biofilm clusters, a theoretical model has been developed regarding the averaging of elastic and viscous stresses in the cluster and by including the entanglement rate ξ. The results highlighted a slight increase of the detachment shear stress with age and also the dependence between the posting of clusters and their volume. Indeed, the experimental values of ξ allow predicting biofilm hydrodynamic detachment with same order of magnitude than was what reported in the literature. The apparent discrepancy between the mechanical and the hydrodynamic detachment is mainly due to the fact that AFM mechanical experiments are related to the clusters local properties whereas hydrodynamic measurements reflected the global properties of the whole biofilm. © 2011 Elsevier Ltd. Source

Dos Reis F.,CNRS Mechanical Energy, Theories, and Applications Laboratory | Ganghoffer J.F.,CNRS Mechanical Energy, Theories, and Applications Laboratory
Computational Materials Science

Auxetic materials having a network like structure are analyzed in terms of their deformation mechanisms and equivalent homogenized mechanical properties thanks to the discrete asymptotic homogenization method. This systematic and predictive methodology is exemplified for five different 2D periodical lattices: the re-entrant hexagonal, hexachiral, cross chiral, rafters and the re-entrant square. The equivalent moduli and Poisson's ratio are expressed in closed form versus the microbeam geometrical parameters and rigidities. As a novel result, the predicted homogenized properties depend on the slenderness of the beam, hence providing more accurate results in comparison to the literature. The studied lattices allow to explore the two main mechanisms responsible for negative Poisson's ratio, the re-entrant and the rolling-up mechanism. Non-standard overall behaviors, such as traction-shear coupling occurring for the cross chiral lattice, are evidenced. Negative values of the Poisson's ratio are obtained in a certain range of the configuration parameter of each lattice. Comparisons of the obtained homogenized moduli with finite element simulations show a very good accuracy of the predicted effective mechanical behavior. © 2011 Elsevier B.V. All rights reserved. Source

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