Mesure de température par pyrométrie 2D à bande spectrale et pyrométrie spectrale de métaux chauffés par laser dans un environnement fortement oxydant

La calibration et la validation de deux techniques de mesure de température complémentaires basées toutes les deux sur la pyrométrie optique (pyrométrie 2D monobande et pyrométrie spectrale), utilisables dans le cadre de l'étude des métaux chauffés dans des conditions hautement oxydantes et plus généralement au cours des procédés laser sur des métaux dans la gamme de température 2000-4000 K ont été réalisées. Une bonne correspondance des résultats des deux méthodes est obtenue lorsque l'émissivité de l'objet est connue et varie peu, mais seule la pyrométrie spectrale est performante lors de grandes variations d'émissivité, fournissant à la fois une mesure de température et d'émissivité au cours du procédé. Les incertitudes ont été calculées et représentent respectivement 6 et 3% dans une gamme de 1800 à 4500 K pour la pyrométrie 2D et la pyrométrie spectrale

Temperature measurement of laser heated metals in highly oxidizing environment using 2D single-band and spectral pyrometry

Calibration and validation of two temperature measurement techniques both using optical pyrometry, usable in the framework of the study of the heated metals in highly oxidizing environments and more generally during laser processing of materials in the range of 2000-4000 K have been done. The 2D single-band pyrometry technique using a fast camera provides 2D temperature measurement, whereas spectral pyrometry uses a spectrometer analyzing the spectra emitted by a spot on the observed surface, with uncertainties calculated to be, respectively, within ±3 and 6 of the temperature. Both techniques have been used simultaneously for temperature measurement of laser heated V, Nb, Ta, and W rods under argon and to measure the temperature of steel and iron rods during combustion under oxygen. Results obtained with both techniques are very similar and within the error bars of each other when emissivity remains constant. Moreover, spectral pyrometry has proved to be able to provide correct measurement of temperature, even with unexpected variations of the emissivity during the observed process, and to give a relevant value of this emissivity. A validation of a comsol numerical model of the heating cycle of W, Ta, Nb, V rods has been obtained by comparison with the measurement

On-Shell Description of Unsteady Flames

The problem of non-perturbative description of unsteady premixed flames with arbitrary gas expansion is solved in the two-dimensional case. Considering the flame as a surface of discontinuity with arbitrary local burning rate and gas velocity jumps given on it, we show that the front dynamics can be determined without having to solve the flow equations in the bulk. On the basis of the Thomson circulation theorem, an implicit integral representation of the gas velocity downstream is constructed. It is then simplified by a successive stripping of the potential contributions to obtain an explicit expression for the vortex component near the flame front. We prove that the unknown potential component is left bounded and divergence-free by this procedure, and hence can be eliminated using the dispersion relation for its on-shell value (i.e., the value along the flame front). The resulting system of integro-differential equations relates the on-shell fuel velocity and the front position. As limiting cases, these equations contain all theoretical results on flame dynamics established so far, including the linear equation describing the Darrieus-Landau instability of planar flames, and the nonlinear Sivashinsky-Clavin equation for flames with weak gas expansion.Comment: 21 pages, 3 figures; extended discussion of causality, new references adde

Temperature measurement of laser heated metals in highly oxidizing environment using 2D single-band and spectral pyrometry

Calibration and validation of two temperature measurement techniques both using optical pyrometry, usable in the framework of the study of the heated metals in highly oxidizing environments and more generally during laser processing of materials in the range of 2000-4000 K have been done. The 2D single-band pyrometry technique using a fast camera provides 2D temperature measurement, whereas spectral pyrometry uses a spectrometer analyzing the spectra emitted by a spot on the observed surface, with uncertainties calculated to be, respectively, within ±3 and 6 of the temperature. Both techniques have been used simultaneously for temperature measurement of laser heated V, Nb, Ta, and W rods under argon and to measure the temperature of steel and iron rods during combustion under oxygen. Results obtained with both techniques are very similar and within the error bars of each other when emissivity remains constant. Moreover, spectral pyrometry has proved to be able to provide correct measurement of temperature, even with unexpected variations of the emissivity during the observed process, and to give a relevant value of this emissivity. A validation of a comsol numerical model of the heating cycle of W, Ta, Nb, V rods has been obtained by comparison with the measurement

Liquid phase combustion of iron in an oxygen atmosphere

In this article, we report an investigation of laser-initiated ignition of pure iron rods, using optical pyrometry, video observations, and analysis of metallographic cross section of quenched burning liquid on copper plates. When ignition occurs, caused by the melting of metal, the combustion takes place in the liquid. Two distinct superposed phases (L1 and L2) are identified in the liquid, according to the known phase diagram of the iron oxide system. Our observations show that the L1 and L2 phases can be either distinct and immiscible or mixing together. The temperature of the transition at which the mixing occurs is around 2350 K. Two mechanisms are proposed to explain the mixing occurring at high temperature: the spontaneous emulsification resulting from a strong decrease of the interfacial tension between L1 and L2 and the reduction of the miscibility gap between them at high temperature. Based on the experimental data of the evolution of the temperature and the video observation of the melt for different ignition conditions, we provide a complete description of the combustion process of iron induced by laser. Eventually, an extrapolation of the iron–oxygen phase diagram, to temperatures higher than 2000 K, is proposed

Laser Ignition of Bulk Iron, Mild Steel, and Stainless Steel in Oxygen Atmospheres

The ignition of pure iron, mild steel S355J, and stainless steel 316L has been investigated. The whole ignition and combustion processes have been monitored using a high-speed video camera and adapted pyrometry. Our results show that the absorptivity of the iron and mild steel to laser radiation increases rapidly at 850 K, from 0.45 to 0.7, and that of stainless steel increases more gradually during the heating process from 0.45 to 0.7. The ignition of iron, mild steel, and stainless steel is controlled by a transition temperature, at which the diffusivity of the metal increases sharply. The transition temperature of pure iron and mild steel is around 1750 K, when molten material appears, and that of stainless steel is around 1900 K, when the solid oxide layer loses its protective properties. These temperatures are independent of the oxygen pressure (from 2 to 20 bar) and of the laser intensity (from 1.6 to 34 kW•cm ). During ignition, the temperature increases very strongly at first, and after that a change in the heating rate of the surface is observed. A diffusive-reactive model, provided with equations describing the diffusion of oxygen in the metal and the transfer of heat released by the oxidation reactions has been solved. The model correctly reproduces the sharp rise of temperature as well as the decrease in the heating rate that follows. Comparison between calculated and experimental data shows that, without liquid convection flow in the melt, combustion would extinguish as soon as the metal surface is fully oxidized and that the combustion front moves into the metal

Experimental and theoretical study of iron and mild steel combustion in oxygen flows

The effects of oxygen flow speed and pressure on the iron and mild steel combustion are investigated experimentally and theoretically. The studied specimens are vertical cylindrical rods subjected to an axial oxygen flow and ignited at the upper end by laser irradiation. Three main stages of the combustion process have been identified experimentally: (1) induction period, during which the rod is heated until an intensive metal oxidation begins at its upper end; (2) static combustion, during which a laminar liquid “cap’’ slowly grows on the upper rod end, and, after the liquid cap detachment from the sample; (3) dynamic combustion, which is characterized by a rapid metal consumption and turbulent liquid motions. An analytical description of these stages is given. In particular, a model of the dynamic combustion is constructed based on the turbulent oxygen transport through the liquid metal-oxide flow. This model yields a simple expression for the fraction of metal burned in the process and allows one to calculate the normal propagation speed of the solid metal–liquid interface as a function of the oxygen flow speed and pressure. A comparison of the theory with the experimental results is made, and its potential application is mentioned