Electrically assisted combustion has been studied in order to control or improve flame characteristics, and emphasizing efficiency and emission regulation. Many phenomenological observations have been reported on the positive impact of electric fields on flame, however there is a lack of detailed physical mechanisms for interpreting these. To clarify the effects of electric fields on flame, I have investigated flame structure, soot formation, and flow field with ionic wind electrical current responses in nonpremixed counterflow flames. The effects of direct current (DC) electric field on flame movement and flow field was also demonstrated in premixed Bunsen flames. When a DC electric field was applied to a lower nozzle, the flames moved toward the cathode side due to Lorentz force action on the positive ions, soot particles simultaneously disappeared completely and laser diagnostics was used to identify the results from the soot particles. To understand the effects of an electric field on flames, flow visualization was performed by Mie scattering to check the ionic wind effect, which is considered to play an important role in electric field assisted combustion. Results showed a bidirectional ionic wind, with a double-stagnant flow configuration, which blew from the flame (ionic source) toward both the cathode and the anode. This implies that the electric field affects strain rate and the axial location of stoichiometry, important factors in maintaining nonpremixed counterflow flames; thus, soot formation of the counterflow flame can also be affected by the electric field. In a test of premixed Bunsen flames having parallel electrodes, flame movement toward the cathode and bidirectional ionic wind were observed. Using PIV measurement it was found that a created radial velocity caused by positive ions (i.e. toward a cathode), was much faster than the velocity toward the anode. Even in a study of alternating current (AC) electric fields, bidirectional ionic wind could be observed, regardless of applied frequencies. Therefore, the effect of ionic wind cannot be considered negligible under both DC and AC electric fields. Detailed explanations for electrical current, flame behavior, and flow characteristics under various conditions are discussed herein.

Ce travail de Thèse est consacré à la caractérisation de la structure thermique des fronts de flammelaminaire de prémélange H2/CH4/Air et H2/CO/Air pauvres. L’étude a été réalisée sur un brûleur à jets opposés, permettant de stabiliser des flammes planes stationnaires, dans des conditions quasi-adiabatiques, pour différentes conditions d’étirement. Un diagnostic de Vélocimétrie par Imagerie de Particule (PIV) et un diagnostic bidimensionnel de diffusion Rayleigh induite par laser ont été utilisés successivement pour étudier l’influence de la richesse, de la concentration en hydrogène dans le combustible et de l’étirement sur le profil de température normal au front de flamme. Trois grandeurs fondamentales ont été étudiées : la température des gaz brûlés, le gradient maximum de température et l’épaisseur de flamme au sens de Spalding. Une attention particulière a été portée à l’interprétation du signal Rayleigh. Ce dernier dépendant notamment de la composition du gaz qui évolue à travers le front de flamme. Dans ce travaille de thèse, cette évolution a été évaluée numériquement (simulations 1D : CANTERA et OPPDIF) puis prise en compte pour améliorer le traitement des données expérimentales. Les résultats expérimentaux couvrent une gamme de richesses s’étalant pour H2/CH4/Air et H2/CO/Air, respectivement de 0.6 à 0.8 et de 0.4 à 0.6. Les concentrations en hydrogène dans le combustible s’étalent respectivement de 0 à 50% et de 10 à 50%. Une comparaison systématique a été faite avec les résultats de simulation numérique 1D (OPPDIF). / This Thesis is devoted to the characterization of the thermal structure of H2/CH4/Air and H2/CO/Air laminar flames. Counterflow flame setup has been used to study planar flames in steady and near-adiabatic conditions. Particle Image Velocimetry and laser induced Rayleigh scattering diagnostics has been successively applied to characterize the influence of equivalent ratio, hydrogen concentration in fuel and stretch on the temperature profile normal to the flame front. Three fundamental characteristics have been studied: the burned gas temperature, the maximum temperature gradient and the flame thickness defined by Spalding. Particular attention has been brought to the interpretation of the Rayleigh signal. Indeed, Rayleigh scattering depends on the gas composition which evolves across the flame front. This evolution has been numerical evaluated in this work (1D simulation: CANTERA and OPPDIF) and taken into account to improve Rayleigh data processing. Experimental results have been obtained for lean flames: equivalent ratio spreads from 0.6 to 0.8 and from 0.4 to 0.6 respectively for H2/CH4/Air and H2/CO/Air flames. A wide range of hydrogen concentration has been studied: from 0 to 50% of hydrogen in fuel for H2/CH4/Air flames and from 10 to 50% of hydrogen in fuel for H2/CO/Air flames. Experimental and numerical (OPPDIF) results have been systematically confronted.

Cette étude est consacrée à l'élaboration d'une méthodologie de détermination des vitessesfondamentales des flammes laminaires, en utilisant un diagnostic de Vélocimétrie par Imagerie deParticules (PIV). Ce dernier est appliqué aux écoulements réactifs avec point de stagnation, permettant lastabilisation de flammes planes, stationnaires et en conditions quasi adiabatiques. Les effets d’étirementssubits par la flamme sont également quantifiables et parfaitement maîtrisés. L’approche ici développée atout d’abord été appliquée aux mélanges méthane/air pour validation. Une comparaison exhaustive desrésultats obtenus avec les données de la littérature est effectuée. Les codes de combustion 1D (PREMIX,OPPDIF) et 2D (Fluent©) ont été utilisés afin de confirmer la fiabilité et la précision de l’approche proposée.Une attention particulière a été accordée à la caractérisation du mouvement des particules ensemencéesdans les écoulements réactifs divergents, avec notamment la prise en considération de la force dethermophorèse. La méthode développée a ensuite été appliquée à la détermination des vitesses deflammes laminaires de divers mélanges de syngas (H2+CO). Une étude comparative sur ces mélanges aété conduite en utilisant des approches expérimentales multiples comprenant : les flammes à contrecourant,les flammes à propagation sphérique ainsi que les flammes stabilisées coniques. Les résultatsobtenus pour chaque approche ont été confrontés et la sensibilité à l’étirement des flammes de syngas aété caractérisée pour une large gamme de richesses (E.R.=0.4 to 5.0) et de compositions de mélanges(5/95 to 50/50 % H2/CO). / In the context of CO2 emission reduction, the present study is devoted to the development of alaminar flame speed measurement methodology, using the Digital Particle Image Velocimetry (DPIV)diagnostic. The latter is applied to stagnation flow flames, seen to have considerable assets for suchstudies. Indeed, flames stabilized in these diverging flows are planar, steady and in near-adiabaticconditions, while subtraction of strain effects on flame is intrinsically allowed. The methodology developedherein has been applied to the well-characterized methane/air mixtures for validation. An extensivecomparison with the literature datasets has been provided. Both 1D (PREMIX, OPPDIF) as well as 2D(Fluent©) numerical tools have been used to confirm the reliability and accuracy of the developed approach.A particular attention has been given to the characterization of the seeding particle motion within thediverging flow, with consideration of the often-neglected thermophoretic force. Fundamental flame velocitiesof various syngas (H2+CO) mixtures have been investigated using multiple experimental approachesincluding the aforementioned counterflow methodology as well as spherical and conical flameconfigurations. Performed measurements from the different approaches have been confronted and flamesensitivities to stretch have been characterized for a wide range of equivalence ratios (E.R.=0.4 to 5.0) andmixture compositions (5/95 to 50/50 % H2/CO).

Soot prediction from combustion systems is still a major challenge in high-fidelity simulations of reactive flows, especially in turbulent conditions. Among the critical aspects, due to its slow characteristic formation times, soot sensitivity to strain rate and turbulence-chemistry interaction models for combustion closure can be found. Starting from the laminar problem, Soot Formation (SF) and Soot Formation Oxidation (SFO) counterflow flames are studied, allowing assessment of the roles of the different underlying phenomena concurring at soot formation with varying strain rates, depending on their relevance in each configuration. Attention is devoted to the inception model, which always regulates the onset of soot formation, and entirely determines the soot sensitivity to strain rate in the SF configuration through nucleation and condensation. Besides, surface growth and oxidation are analyzed in the SFO configuration, where they are predominant. The corresponding models are fine-tuned and generalized, and improved predictions are obtained in both configurations. Afterwards, a 2-points flame-controlling continuation method with soot module inclusion is developed to build a tool capable of flamelets generation inclusive of soot effects on the gas phase. The implementation is first tested discussing general features of the S-curve and verifying the consistency with previous works. The tool is finally used to compute the S-curve of ethylene pressurized sooting flames. The models and tools developed are incorporated into an OpenFOAM-based solver to perform Computational Fluid Dynamic (CFD) simulations of sooting turbulent flames. These are studied in pressurized, highly turbulent environments, to validate the soot model at a fundamental level but with practically relevant operative conditions. The numerical results are found to satisfactorily depict the soot volume fraction (SVF) formation, even though a few quantitative and qualitative discrepancies are discussed. Furthermore, soot intermittency and pressure scaling are analyzed. Finally, an alternative turbulence-chemistry interaction model for combustion closure is explored. A generalized partially-stirred reactor model is developed which accounts for all chemical times in a consistent manner. While the applicability of available models is confined to specific turbulence-chemistry interaction regimes, the incorporation of detailed chemistry description in the proposed approach improves synergistic predictions of all species and makes it suitable for systems with characteristic times very different from each other, such as soot and NOx.