Physique
Study of the spray to globular transition in gas metal arc welding: a spectroscopic investigation
The gas metal arc welding (GMAW) process is strongly influenced by the composition of the shielding gas. In particular, addition of CO2 increases the threshold current for the transition from unstable globular to more stable spray transfer mode. We report on the diagnostics—using optical emission spectroscopy—of a GMAW plasma in pure argon and in mixtures of argon, CO2 and N2 while operated in spray and globular transfer modes. The spatially resolved plasma parameters are obtained by applying the Abel transformation to laterally integrated emission data. The Stark widths of some iron lines are used to determine both electron density and temperature, and line intensities yield relative contents of neutral and ionized iron to argon.Our experimental results indicate a temperature drop on the arc axis in the case of spray arc transfer. This drop reduces with addition of N2 and disappears in globular transfer mode when CO2 is added. Despite the temperature increase, the electron density decreases with CO2 concentration. The highest concentration of iron is observed in the plasma column upper part (close to the anode) and for GMAW with CO2.Our results are compared with recently published works where the effect of non-homogeneous metal vapour concentration has been taken into account. Voir les détails
Mots clés : GMAW, Optical Emission Spectroscopy, Boltzmann Plot
Guided Wave Sensitivity for De- Bond Defects in Aluminum Skin Honeycomb Core
Sandwich plates are finding an increasing range ofapplication in the aircraft industry. The inspection of honeycomb composite structure by conventional ultrasonic technique is complexand very time consuming. The present study demonstrates a technique using guided Lamb waves at low frequencies to predict de-bond defects in aluminum skin_honeycomb core sandwich structure used in aeronautics. The numerical method was investigated for drawing the dispersion and displacement curves of ultrasonic Lamb wave propagated in Aluminum plate. An experimental study was carried out to check the theoretical prediction. The detection of unsticking between the skin and the core was tested by the two first modes for a low frequency. It was found that A0 mode is more sensitive to delamination defect compared to S0 mode. Voir les détails
Mots clés : Damage detection, delamination, guided waves, Sandwich structure
Notch detection in copper tubes immersed in water by a compressional guided waves
Tube- or pipe-like structures are used to convey fluids such as gas, oil or water. These fluids can be transported on land, in sea or in industrial buildings. Hence, tubes can suffer from mechanical, thermal and chemical stresses. The latter, can severely damage these structures due to the oxidation phenomena. This damage weakens the structure and reveals cracks and inhomogeneities. The objective of this work is to detect and characterise damage in tubes immersed in water by a non-destructive method. This research work is devoted to the development of an experimental device. This device can detect and locate axy- and nonaxy-symmetric circumferential small notches located on the outer surface of a filled tube and immersed in water using guided waves. Different sizes of notches are milled into copper tubes with a thin wall are considered. A single transducer is used for emission and reception of leaky guided waves by using the pulse-echo technique. The compressional L(0,2)mode is launched at relatively high frequency using oblique incidence of the transducer into the fluid. By using the wave leakages, notches of small sizes are detected and located. In addition, the sensitivity of the L(0,2) mode to the radial depth and circumferential extent of notches is evaluated. A roughly linear behaviour of the amplitude reflection coefficients is revealed which is in accordance with research works found in the literature. Voir les détails
Mots clés : guided waves, Immersed tubes, Circumferential notches, Experimental device
Theoretical prediction of the structural, electronic,and thermal properties of Al1-xBxAs ternary alloys
First-principles calculations are performed to study the structural, electronic, and thermalproperties of the AlAs and BAs bulk materials and Al1-xBxAs ternary alloys using the fullpotential-linearized augmented plane wave method within the density functional theory.The structural properties are investigated using the Wu–Cohen generalized gradientapproximation that is based on the optimization of total energy. For band structurecalculations, both Wu–Cohen generalized gradient approximation and modified Becke-Johnson of the exchange-correlation energy and potential, respectively, are used. Thedependence of the lattice constant, bulk modulus, and band gap on the composition x wasanalyzed. The lattice constant for Al1xBxAs alloys exhibits a marginal deviation from theVegard's law. A small deviation of the bulk modulus from linear concentration depen-dence was observed for these alloys. The composition dependence of the energy band gapwas found to be highly nonlinear. Using the approach of Zunger and coworkers, themicroscopic origins of the gap bowing were detailed and explained. The quasi-harmonicDebye model was used to determine the thermal properties of alloys up to 500 K. Voir les détails
Mots clés : Band structures, First principle calculations, Ternary alloys, Thermal Properties
Prediction of the conversion efficiency of a GaSb thermophotovoltaic converter heated by radioisotope source
In recent years, Gallium Antimonide (GaSb), which has smallest bandgap among III-V semiconductors family, became the subject of extensive investigations in the field of thermophotovoltaic (TPV) converters, because of the recent improvements in optoelectronic technology. In this paper, we investigated the heat to electricity conversion efficiency of a GaSb radioisotope thermophotovoltaic (RTPV) converter, taking account of the photons with energy below the cells bandgap using a comprehensive analytical process. The results show that a conversion efficiency greater than 28% can be obtained for radiator’s temperature of 1600k, at ambiant temperature. This efficiency will decrease as the cell temperature increase. Voir les détails
Mots clés : Radioisotope, Emissivity, Thermophotovoltaic, Efficiency, Temperature
Analysis and optimization of In1-xGaxAsySb1-y thermophotovoltaic cells under low radiator temperatures
In this paper, we investigated the heat to electricity conversion efficiency of In1-xGaxAsySb1-yradioisotope thermophotovoltaic (RTPV) converter with x=0.8 and y=0.18, taking account of the photons with energy below the cells bandgap using a comprehensive analytical process. This was done with a computer program designed for this reason, which allowed the computation of the cell performance under a variety of specified incident radiation spectra as well as a variety of material parameters. The results show that for an emissivity value of 0.78, a cell thickness of about 7µm with low front recombination velocity(700cm/s),a conversion efficiency greater than 29% can be obtained for radiator’s temperature of1300°k at ambient temperature. This efficiency will decrease as the cell temperature increase. Voir les détails
Mots clés : Radioisotope, Emissivity, Recombination Velocity, Efficiency, Temperature
Correlation between structural and optical properties of SiO2/TiO2multibilayers processed by sol-gel technique and applied toBragg reflectors
SiO2 and TiO2 thin layers processed by sol-gel technique have been deposited,alternatively, on glass substrates and Si (111) wafer. Dip-coatedmultibilayerswere characterized by different experimental techniques:XRD,SEM, FTIRandUV-VIS-NIR. The obtainedX-ray diffraction patterns analysishave shown that our films crystallize in anatase and rutile phases whateveris the number of bilayers and the corresponding grain sizes increasefrom 5.48 nmto 16.11 nm. The SEMmicrograph shows that our layers arehomogeneous without any visual cracks. The FTIR spectra have shownthat the vibration of Si-O-Ti bonds becomes intense by the increase in thenumber of bilayers. This increase, on the on hand, decreases the transmissioncoefficient from 4.58% to 0.55% and increases the width of the stopband shown in UV-VIS-NIR spectra. On the other hand, the band-gap decreasesfrom3.73 eV to 3.59 eV. In addition, a pseudo band-gap is locatedbetween 300 nmand 400 increasing from1.76 eVto 2.29 eV.? 2013 Trade Science Inc. - INDIA Voir les détails
Mots clés : Sol-gel;, Anatase;, Rutile;, Stop band;, Si-O-Ti;, Band-gap.
Contribution à la caractérisation de la précipitation dans les alliages Cu-Ni-Si et Cu-Ni-Si-Cr
Notre travail porte sur des alliages des systèmes ternaire Cu-Ni-Si et quaternaire Cu-Ni-Si-Cr. Nous avons réalisé une étude expérimentale sur la cinétique de précipitation des phases présentes dans les systèmes étudiés via différentes méthodes d’analyse microstructurale telles que la Métallographie optique et électronique à balayage, les mesures de résistivité lors de traitements isothermes et la diffraction des rayons X in situ et ex situ. Nous avons étudié par calculs ab initio en appliquant la méthode FP-LAPW, la stabilité des phases précipitées dans l’alliage CuNiSi, cette méthode implémentée sur le code WIEN2k, ont permis de déterminer les propriétés structurales et énergétiques L12, DO22 et orthorhombique des phases intermétalliques N3Si, Cu2NiSi et Ni2Si. Des résultats originaux ont mis en évidence les similitudes qui existent lors des séquences de transformation de phases à l’état solide des deux alliages. Voir les détails
Mots clés : AA6061, polarisation, corrosion par piqûre, inhibiteur, pH
Evolution de l’écoulement stratifie en présence d’une Singularité
Ce travail de thèse a pour but l’étude des écoulements diphasiques dans une conduite horizontale avec et sans singularité. Une cartographie des régimes d’écoulements pour la configuration choisie a été établie dans un premier temps. Des mesures de la fréquence des bouchons, ainsi que les longueurs associées sans singularité à une distance égale à 107.5d ont été effectuées et comparées aux résultats obtenus à l’amont d’un élargissement brusque de rapport de section s=0.444. Les effets de la présence de la singularité sur les bouchons sont discutés. Voir les détails
Mots clés : Ecoulement diphasique, stratifié, bouchons, singularité
Étude de la convection naturelle dans une cavité inclinée
L’objectif de cette thèse est de faire une étude numérique tridimensionnelle des écoulements de convection naturelle dans une cavité d’air inclinée différentiellement chauffée. Pour cela, un code de calcul 3D a été utilisé pour résoudre les équations de Navier-Stokes tridimensionnelles pour des fluides incompressibles. Ce code est basé sur la méthode spectrale de « projection- diffusion » en collocation Chebyshev qui fait partie des méthodes dites de précision infinie. La mise sous forme adimensionnelle des équations gouvernantes fait apparaître des paramètres de contrôle, à savoir : le nombre de Rayleigh, le nombre de Prandtl Pr=0.71, le rapport de forme et enfin l’angle d’inclinaison de la cavité, dont nous avons examiné leurs influences sur la structure dynamique et thermique de l’écoulement, ainsi que sur le transfert thermique. Les résultats obtenus montrent qu’il existe une composante de vitesse non nulle suivant la troisième direction, sa distribution est notable près des parois terminales adiabatiques particulièrement aux coins de la cavité, sa valeur croit lorsque le nombre de Rayleigh augmente et diminue avec l’augmentation de l’angle d’inclinaison. Voir les détails
Mots clés : convection naturelle, milieu confiné, gradient de température