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Feidt M.,CNRS Mechanical Energy, Theories, and Applications Laboratory
Entropy | Year: 2013

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.

Jannot Y.,CNRS Mechanical Energy, Theories, and Applications Laboratory | Degiovanni A.,CNRS Mechanical Energy, Theories, and Applications Laboratory
Review of Scientific Instruments | Year: 2013

This paper presents a new method dedicated to thermal properties (conductivity and diffusivity) measurement of dry bulk materials including powders. The cylindrical three layers experimental device (brass/bulk material/stainless steel) and the principle of the measurement method based on a crenel thermal excitation are presented. The one-dimensional modeling of the system is used for a sensitivity analysis and to calculate the standard deviation of the estimation error. Experimental measurements are carried out on three bulk materials: glass beads, cork granules, and expanded polystyrene beads. The estimated thermal properties are compared with the values obtained by other measurement methods. Results are in good agreement with theoretical predictions: both thermal conductivity and diffusivity can be estimated with a good accuracy for low density material like cork granules or expanded polystyrene beads since only thermal diffusivity can be estimated for heavier materials like glass beads. It is finally shown that this method like all transient methods is not suited to the thermal characterization of wet bulk materials. © 2013 AIP Publishing LLC.

Thiebaud F.,CNRS Mechanical Energy, Theories, and Applications Laboratory | Gelin J.C.,CNRS Femto ST Institute
Composites Science and Technology | Year: 2010

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.

Asllanaj F.,CNRS Mechanical Energy, Theories, and Applications Laboratory | Fumeron S.,CNRS Mechanical Energy, Theories, and Applications Laboratory
Journal of Biomedical Optics | Year: 2012

Optical tomography is a medical imaging technique based on light propagation in the near infrared (NIR) part of the spectrum. We present a new way of predicting the short-pulsed NIR light propagation using a time-dependent two-dimensional-global radiative transfer equation in an absorbing and strongly anisotropically scattering medium. A cell-vertex finite-volume method is proposed for the discretization of the spatial domain. The closure relation based on the exponential scheme and linear interpolations was applied for the first time in the context of time-dependent radiative heat transfer problems. Details are given about the application of the original method on unstructured triangular meshes. The angular space (4πSr) is uniformly subdivided into discrete directions and a finite-differences discretization of the time domain is used. Numerical simulations for media with physical properties analogous to healthy and metastatic human liver subjected to a collimated short-pulsed NIR light are presented and discussed. As expected, discrepancies between the two kinds of tissues were found. In particular, the level of light flux was found to be weaker (inside the medium and at boundaries) in the healthy medium than in the metastatic one. © 2012 Society of Photo-Optical Instrumentation Engineers (SPIE).

Lemoine F.,CNRS Mechanical Energy, Theories, and Applications Laboratory | Castanet G.,CNRS Mechanical Energy, Theories, and Applications Laboratory
Experiments in Fluids | Year: 2013

The measurement of the sizes and the velocities of droplets relies upon widespread and well-established techniques, but characterizing their temperature and their composition remains challenging. The lack of standard methods is particularly detrimental, given the importance of these parameters for validating models and numerical simulations of many spray processes. Heat and mass transfers are dominant aspects in applications such as spray combustion in IC engines, spray cooling, spray drying, wet scrubbers in which liquid sprays capture gas pollutants and also the preparation of nanoparticles via spray route. This paper provides a review of the main techniques available to optically measure the temperature and chemical compositions of single droplets and sprays. Most of these techniques are based on phenomena related to light interaction with matter. Photoluminescence processes like fluorescence and phosphorescence have temperature and composition dependences which can be exploited, while other methods rely on light scattering by the droplets. In particular, the angular position of the rainbow is very sensitive to the refractive index and then to both the temperature and composition. Less widely used diagnostic methods include Raman scattering, thermochromic liquid crystals, thermographic phosphors, infrared thermography, morphology-dependent resonances and their subsequent effects on the stimulated emission of dye molecules. In this review, the emphasis is mainly placed on two groups of techniques - methods based on laser-induced fluorescence and those based on light scattering - but details about alternative methods will be also provided. The potential of combining fluorescence-based techniques or rainbow refractometry with a droplet sizing measurement technique to derive temperature and composition per size class will be also discussed. © 2013 Springer-Verlag Berlin Heidelberg.

Panfilov M.,CNRS Mechanical Energy, Theories, and Applications Laboratory
Transport in Porous Media | Year: 2010

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.

Dos Reis F.,CNRS Mechanical Energy, Theories, and Applications Laboratory | Ganghoffer J.F.,CNRS Mechanical Energy, Theories, and Applications Laboratory
Computational Materials Science | Year: 2012

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.

Haddag B.,CNRS Mechanical Energy, Theories, and Applications Laboratory | Nouari M.,CNRS Mechanical Energy, Theories, and Applications Laboratory
Wear | Year: 2013

In machining, tool-chip interface parameters such as pressure, temperature, sliding velocity and friction are extremely difficult to estimate only by experimental means. Theoretical methods can then give important solutions for predicting these quantities required for the assessment of tool wear. This work deals with a multi-steps modelling strategy based on several numerical calculations. The first step is a 3D thermomechanical analysis of the chip formation process. Cutting forces, chip morphology and chip flow direction as well as tool-chip interface parameters are obtained. The second step concerns the tool wear prediction using tool-chip interface parameters. The last step focuses on a 3D thermal analysis of the heat diffusion into the cutting tool using adequate thermal loading. An applied non-uniform heat flux is estimated using contact parameters obtained from the first step. Obtained results at each step of calculation are compared to experimental data. The predicted tool wear has been found in good agreement with experiments, and measured temperatures (using embedded thermocouples) very close to the temperature obtained by the last step of numerical calculation. © 2013 Elsevier B.V.

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 | Year: 2012

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.

Nouari M.,CNRS Mechanical Energy, Theories, and Applications Laboratory | Makich H.,CNRS Mechanical Energy, Theories, and Applications Laboratory
International Journal of Refractory Metals and Hard Materials | Year: 2013

An experimental investigation was conducted in this work to analyze the effect of the workpiece microstructure on tool wear behavior and stability of the cutting process during marching difficult to cut titanium alloys: Ti-6Al-4V and Ti-555. The analysis of tool-chip interface parameters such as friction, temperature rise, tool wear and workpiece microstructure evolution under different cutting conditions have been investigated. As the cutting speed increases, mean cutting forces and temperature show different progressions depending on the considered microstructure. Results show that wear modes of cutting tools used for machining the Ti-555 alloy exhibit contrast from those obtained for machining the Ti-6Al-4V alloy. Because of the fine-sized microstructure of the near-β titanium Ti-555, abrasion mode was often found to be the dominate wear mode for cemented cutting tools. However, adhesion and diffusion modes followed by coating delamination process were found as the main wear modes when machining the usual Ti-6Al-4V alloy by the same cutting tools. Moreover, a deformed layer was detected using SEM-EDS analysis from the sub-surface of the chip with β-grains orientation along the chip flow direction. The analysis of the microstructure confirms the intense deformation of the machined surface and shows a texture modification. © 2013 Elsevier Ltd. All rights reserved.

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