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<p>Numerical simulations have been done with the CHEMKINsoftware to study different aspects of wall effects in thecombustion of lean, laminar and premixed flames in anaxisymmetric boundary-layer flow.</p><p>The importance of the chemical wall effects compared to thethermal wall effects caused by the development of the thermaland velocity boundary layer has been investigated in thereaction zone by using different wall boundary conditions, walltemperatures and fuel/air ratios. Surface mechanisms include acatalytic surface (Platinum), a surface that promotesrecombination of active intermediates and a completely inertwall with no species and reactions as the simplest possibleboundary condition.</p><p>When hydrogen is the model fuel, the analysis of the resultsshow that for atmospheric pressure and a wall temperature of600 K, the surface chemistry gives significant wall effects atthe richer combustion case (f=0.5), while the thermal andvelocity boundary layer gives rather small effects. For theleaner combustion case (f=0.1) the thermal and velocityboundary layer gives more significant wall effects, whilesurface chemistry gives less significant wall effects comparedto the other case.</p><p>For methane as model fuel, the thermal and velocity boundarylayer gives significant wall effects at the lower walltemperature (600 K), while surface chemistry gives rather smalleffects. The wall can then be modelled as chemically inert forthe lean mixtures used (f=0.2 and 0.4). For the higher walltemperature (1200 K) the surface chemistry gives significantwall effects.</p><p>For both model fuels, the catalytic wall unexpectedlyretards homogeneous combustion of the fuel more than the wallthat acts like a sink for active intermediates. This is due toproduct inhibition by catalytic combustion. For hydrogen thisoccurs at atmospheric pressure, but for methane only at thehigher wall temperature (1200 K) and the higher pressure (10atm).</p><p>As expected, the overall wall effects (i.e. a lowerconversion) were more pronounced for the leaner fuel-air ratiosand at the lower wall temperatures.</p><p>To estimate a possible discrepancy in flame position as aresult of neglecting the axial diffusion in the boundary layerassumption, calculations have been performed with PREMIX, alsoa part of the CHEMKIN software. With PREMIX, where axialdiffusion is considered, steady, laminar, one-dimensionalpremixed flames can be modelled. Results obtained with the sameinitial conditions as in the boundary layer calculations showthat for the richer mixtures at atmospheric pressure the axialdiffusion generally has a strong impact on the flame position,but in the other cases the axial diffusion may beneglected.</p><p><strong>Keywords:</strong>wall effects, laminar premixed flames,platinum surfaces, boundary layer flow</p> / QC 20100504

Lors de la rentrée atmosphérique, la protection thermique d'un véhicule spatial est soumise à un plasma d'air. Le transfert de l'énergie cinétique de l'oxygène atomique à la surface conduit à une importante élévation de température. L'énergie libérée lors de la recombinaison de l'oxygène atomique provoque une élévation de température supplémentaire. Cet excès de température endommage la protection thermique. Cette étude a pour objectif de comprendre les transferts de matière (coefficient de recombinaison γ) et de chaleur (coefficient d'accommodation β) mis en jeu lors de cette rentrée atmosphérique. Ces phénomènes sont quantifiables par la mesure de la catalycité. Ces mesures seront réalisées sur des oxydes semi-conducteurs de type n jusqu'à 1100 K, avec pour objectif de réduire la recombinaison de l'oxygène atomique. Cette dernière conduit à l'oxydation de la surface et à un vieillissement accéléré de la surface. En effet grâce à l'utilisation de l'oxygène isotopique couplée à la spectrométrie de masse, les phénomènes d'oxydation (active et passive), d'ablation et de diffusion de l'oxygène à cœur ont été identifiés. A l'aide d'analyses SIMS, une cinétique de diffusion de l'oxygène sera établie dans ce genre de couche afin de tester leur effet barrière. Un nouveau genre de dépôt (UHTCs) possédant des propriétés de résistance à l'oxydation et de barrière de diffusion, sera également soumis à différents traitements thermiques (jusqu'à 1500 K). Enfin cette étude sera complétée par des simulations de recombinaison de l'oxygène atomique à l'aide du code de calcul Chemkin®.

This thesis explores the effects of operating parameters on the location and shape of lifted flames in a Low Swirl Burner (LSB). In addition, it details the development and analysis of a CH PLIF imaging system for visualizing flames in lean combustion systems. The LSB is studied at atmospheric pressure using LDV and CH PLIF. CH* chemiluminescence is used for high pressure flame imaging. A four-level model of the fluorescing CH system is developed to predict the signal intensity in hydrocarbon flames. Results from imaging an atmospheric pressure laminar flame are used to validate the behavior of the signal intensity as predicted by the model. The results show that the fluorescence signal is greatly reduced at high pressure due to the decreased number of CH molecules and the increased collisional quenching rate. This restricts the use of this technique to increasingly narrow equivalence ratio ranges at high pressures. The limitation is somewhat alleviated by increasing the preheat temperature of the reactant mixture. The signal levels from high hydrogen-content syngas mixtures doped with methane are found to be high enough to make CH PLIF a feasible diagnostic to study such flames. Finally, the model predicts that signal levels are unlikely to be significantly affected by the presence of strain in the flow field, as long as the flames are not close to extinction. The results from the LSB flame investigation reveal that combustor provides reasonably robust flame stabilization at low and moderate values of combustor pressure and reference velocities. However, at very high velocities and pressures, the balance between the reactant velocity and the turbulent flame speed shifts in favor of the former resulting in the flame moving downstream. The extent of this movement is small, but indicates a tendency towards blow off at higher pressures and velocities that may be encountered in real world gas turbine applications. There is an increased tendency of relatively fuel-rich flames to behave like attached flames at high pressure. These results raise interesting questions about turbulent combustion at high pressure as well as provide usable data to gas turbine combustor designers by highlighting potential problems.

Numerical simulations have been done with the CHEMKINsoftware to study different aspects of wall effects in thecombustion of lean, laminar and premixed flames in anaxisymmetric boundary-layer flow. The importance of the chemical wall effects compared to thethermal wall effects caused by the development of the thermaland velocity boundary layer has been investigated in thereaction zone by using different wall boundary conditions, walltemperatures and fuel/air ratios. Surface mechanisms include acatalytic surface (Platinum), a surface that promotesrecombination of active intermediates and a completely inertwall with no species and reactions as the simplest possibleboundary condition. When hydrogen is the model fuel, the analysis of the resultsshow that for atmospheric pressure and a wall temperature of600 K, the surface chemistry gives significant wall effects atthe richer combustion case (f=0.5), while the thermal andvelocity boundary layer gives rather small effects. For theleaner combustion case (f=0.1) the thermal and velocityboundary layer gives more significant wall effects, whilesurface chemistry gives less significant wall effects comparedto the other case. For methane as model fuel, the thermal and velocity boundarylayer gives significant wall effects at the lower walltemperature (600 K), while surface chemistry gives rather smalleffects. The wall can then be modelled as chemically inert forthe lean mixtures used (f=0.2 and 0.4). For the higher walltemperature (1200 K) the surface chemistry gives significantwall effects. For both model fuels, the catalytic wall unexpectedlyretards homogeneous combustion of the fuel more than the wallthat acts like a sink for active intermediates. This is due toproduct inhibition by catalytic combustion. For hydrogen thisoccurs at atmospheric pressure, but for methane only at thehigher wall temperature (1200 K) and the higher pressure (10atm). As expected, the overall wall effects (i.e. a lowerconversion) were more pronounced for the leaner fuel-air ratiosand at the lower wall temperatures. To estimate a possible discrepancy in flame position as aresult of neglecting the axial diffusion in the boundary layerassumption, calculations have been performed with PREMIX, alsoa part of the CHEMKIN software. With PREMIX, where axialdiffusion is considered, steady, laminar, one-dimensionalpremixed flames can be modelled. Results obtained with the sameinitial conditions as in the boundary layer calculations showthat for the richer mixtures at atmospheric pressure the axialdiffusion generally has a strong impact on the flame position,but in the other cases the axial diffusion may beneglected. Keywords:wall effects, laminar premixed flames,platinum surfaces, boundary layer flow / QC 20100504

Increased knowledge into the physics and chemistrycontrolling emissions from flame-surface interactions shouldhelp in the design of combustion engines featuring improvedfuel economy and reduced emissions. The overall aim of this work has been to obtain afundamental understanding of wall-related, premixed combustionusing numerical modeling with detailed chemical kinetics. Thiswork has utilized CHEMKIN®, one of the leading softwarepackages for modeling combustion kinetics. The simple fuels hydrogen and methane as well as the morecomplex fuels propane and gasified biomass have been used inthe model. The main emphasis has been on lean combustion, andthe principal flow field studied is a laminar boundary layerflow in two-dimensional channels. The assumption has been madethat the wall effects may at least in principle be the same forlaminar and turbulent flames. Different flame geometries have been investigated, includingfor example autoignition flames (Papers I and II) and premixedflame fronts propagating toward a wall (Papers III and IV).Analysis of the results has shown that the wall effects arisingdue to the surface chemistry are strongly affected by changesin flame geometry. When a wall material promoting catalyticcombustion (Pt) is used, the homogeneous reactions in theboundary layer are inhibited (Papers I, II and IV). This isexplained by a process whereby water produced by catalyticcombustion increases the rate of the third-body recombinationreaction: H+O2+M ⇔ HO2+M. In addition, the water produced at higherpressures increases the rate of the 2CH3(+M) ⇔ C2H6(+M) reaction, giving rise to increased unburnedhydrocarbon emissions (Paper IV). The thermal coupling between the flame and the wall (theheat transfer and development of the boundary layers) issignificant in lean combustion. This leads to a sloweroxidation rate of the fuel than of the intermediatehydrocarbons (Paper III). Finally in Paper V, a well-known problem in the combustionof gasified biomass has been addressed, being the formation offuel-NOx due to the presence of NH3 in the biogas. A hybridcatalytic gas-turbine combustor has been designed, which cansignificantly reduce fuel-NOx formation. Keywords:wall effects, premixed flames, flamequenching, numerical modeling, CHEMKIN, boundarylayerapproximation, gasified biomass, fuel-NOx, hybrid catalytic combustor. / QC 20100504

Non-thermal plasma generated in a dielectric barrier packed-bed reactor has been used for the remediation of chlorinated volatile organic compounds. Chlorinated VOCs are important air pollutant gases which affect both the environment and human health. This thesis uses non-thermal plasma generated in single and multiple packed-bed plasma reactors for the decomposition of dichloromethane (CH2Cl2, DCM) and methyl chloride (CH3Cl). The overall aim of this thesis is to optimize the removal efficiency of DCM and CH3Cl in air plasma by investigating the influence of key process parameters. This thesis starts by investigating the influence of process parameters such as oxygen concentration, initial VOC concentration, energy density, and plasma residence time and background gas on the removal efficiency of both DCM and CH3Cl. Results of these investigations showed maximum removal efficiency with the addition of 2 to 4 % oxygen to nitrogen plasma. Oxygen concentrations in excess of 4 % decreased the decomposition of chlorinated VOCs as a result of ozone and NOx formation. This was improved by adding an alkene, propylene (C3H6), to the gas stream. With propylene additives, the maximum remediation of DCM was achieved in air plasma. It is thought that adding propylene resulted in the generation of more active radicals that play an important role in the decomposition process of DCM as well as a further oxidation of NO to NO2. Results in the single bed also showed that increasing the residence time increased the removal efficiency of chlorinated VOCs in plasma. This was optimized by designing a multiple packed-bed reactor consisting of three packed-bed cells in series, giving a total residence time of 4.2 seconds in the plasma region of the reactor. This reactor was used for both the removal of DCM, and a mixture of DCM and C3H6 in a nitrogen-oxygen gas mixture. A maximum removal efficiency of about 85 % for DCM was achieved in air plasma with the use of three plasma cells and the addition of C3H6 to the gas stream. Nitrogen oxides are air pollutants which are formed as by-products during the decomposition of chlorinated VOCs in plasmas containing nitrogen and oxygen. Results illustrate that the addition of a mixture of DCM and C3H6 resulted in the formation of the lowest concentration of nitric oxide, whilst the total nitrogen oxides concentrations did not increase. A summary of the findings of this work is presented in chapter eight as well as further work. To conclude, the maximum removal efficiency of dichloromethane was achieved in air plasma with the addition of 1000 ppm of propylene and the use of three packed-bed plasma cells in series. The lowest concentration of nitric oxide was formed in this situation.

The main objective of this Thesis is the study of the autoignition phenomenon of reactive mixtures from a theoretical and experimental point of view. A wide parametric study has been carried out in a Rapid Compression-Expansion Machine (RCEM) for different initial temperatures, compression ratios, equivalence ratios and molar fractions of oxygen (by using synthetic EGR) for different fuels. The ignition delay referred to cool flames (if it can be identified), as well as the ignition delay referred to the high-temperature stage of the ignition, have been experimentally obtained and their trends have been explained regarding the chemical kinetics of each fuel. The different effects of the species that compose the synthetic EGR on the ignition delay have been studied, decoupling the thermodynamic effects from the chemical ones. Different compositions have been taken into account to generate the synthetic EGR, and validation limits have been obtained for each mixture. The thermodynamic and the chemical effects have shown to be opposed, while the dominant one is different depending on the working temperature. Several chemical kinetic mechanisms have been validated by comparison to the experimental results. A detailed mechanism for iso-octane and n-heptane blends and a reduced mechanisms for n-dodecane have been analyzed. Moreover, a sub-model for the generation and decay of excited OH* has been validated by comparison to chemiluminescence and spectroscopy results. The different radiation sources have been studied for iso-octane and n-heptane by means of spectroscopy techniques. Besides, chemiluminescence measurements filtered at 310nm (OH* emission wavelength) have been performed in order to analyze the generalization and propagation velocity of the autoignition front. The ignition propagation has shown to depend on the thermodynamic conditions reached in the combustion chamber when the first ignition spot occurs and not on the global reactivity of the mixture. Furthermore, two different radiation sources have been found at 310nm in the spectroscopic analysis depending on the ignition intensity: the decay of the OH* radical from excited to ground state and the oxidation of CO to CO2 (CO continuum). However, these optical techniques have been applied only in the experiments carried out with iso-octane and n-heptane due to technical limitations. Finally, a new predictive model has been theoretically developed starting from the Glassman's model for autoignition. This method is based on modeling the accumulation rate of chain carriers up to reach their critical concentration (obtaining the ignition delay referred to cool flames) and, afterwards, modeling the disappearance rate of such chain carriers up to their consumption (when the maximum heat release rate is reached, obtaining the ignition delay referred to the high-temperature stage of the process). The predictive capability of the model has been compared to the ability of other methods that can be found in the literature, such as the Livengood & Wu integral method. The validity of each method has been tested, defining a working methodology to obtain reasonable predictions for the ignition delay. / El objetivo de esta Tesis Doctoral es el estudio del fenómeno de autoencendido de mezclas reactivas desde un punto de vista teórico y experimental. Se ha realizado un amplio estudio paramétrico en una Máquina de Compresión-Expansión Rápida (RCEM por sus siglas en inglés) barriendo diversas temperaturas iniciales, relaciones de compresión, dosados relativos y fracciones molares de oxígeno (mediante el uso de EGR sintético) para distintos combustibles. El tiempo de retraso del fenómeno de llamas frías (en el caso de existir), así como el tiempo de retraso de la etapa de alta temperatura, han sido obtenidos experimentalmente y sus tendencias explicadas mediante cinética química. Se han estudiado los diferentes efectos de las distintas especies involucradas en el EGR sintético sobre el tiempo de retraso, desligando aquellos de carácter termodinámico de los efectos puramente químicos. Se han tenido en cuenta distintas composiciones para definir dicho EGR, estableciendo límites de validez para cada una de las mezclas propuestas. Los efectos termodinámicos y químicos resultaron ser opuestos, siendo dominante uno u otro a distintos rangos de temperatura de trabajo. Varios mecanismos de cinética química han sido validados gracias a los resultados experimentales obtenidos. Además de un mecanismo detallado para mezclas PRF de iso-octano y n-heptano, se ha llevado a cabo la validación de otro mecanismo simplificado para el n-dodecano. Por otro lado, un submodelo de formación y decaimiento de OH* excitado ha sido validado contra resultados de quimioluminiscencia y espectroscopía. Se han estudiado las diferentes fuentes de radiación del proceso de autoencendido para el iso-octano y el n-heptano mediante técnicas de espectroscopía. Además, se han realizado medidas de quimioluminiscencia filtrada a 310nm (longitud de onda de emisión del radical OH*) para el análisis de la generalización y velocidad de propagación del frente de autoencendido. La propagación del encendido ha mostrado ser dependiente de las condiciones termodinámicas alcanzadas en la cámara de combustión en el instante de ignición más que de la reactividad de la mezcla. Se han encontrado dos fuentes de radiación distintas a 310nm mediante espectroscopía, dependiendo de la intensidad del encendido: el decaimiento del radical OH* de estado excitado a estado natural y la oxidación del CO a CO2 (continuo del CO). No obstante, estas técnicas han sido utilizadas solamente para los dos combustibles de referencia de la escala de octanaje debido a limitaciones técnicas. Finalmente, se ha desarrollado un nuevo modelo predictivo de manera teórica partiendo del modelo de Glassman para el autoencendido. Este método se basa en modelar primero la tasa de acumulación de portadores de cadena hasta su concentración crítica (obteniendo así el tiempo de retraso referido a la etapa de llamas frías) y, tras dicho instante, modelar la tasa de consumo de dichos portadores de cadena hasta su completa desaparición (instante en el cual se produce la máxima exotermia del proceso, prediciendo el tiempo de retraso referido a la etapa de alta temperatura del encendido). La capacidad predictiva del modelo ha sido comprobada para cada uno de los seis combustibles ensayados. Además, dicha capacidad predictiva ha sido comparada con la de otros métodos existentes en la literatura, como la integral de Livengood & Wu. La validez de cada uno de los métodos ha sido analizada, definiendo una metodología de uso para obtener predicciones razonables del tiempo de retraso. / L'objectiu d'aquesta Tesi Doctoral és l'estudi del fenomen d'autoencesa de mescles reactives des d'un punt de vista teòric i experimental. S'ha realitzat un ampli estudi paramètric en una Màquina de Compressió-Expansió Ràpida (RCEM per les seues sigles en anglès) cobrint diverses temperatures inicials, relacions de compressió, dosatges relatius i fraccions molars d'oxigen (mitjançant l'ús de EGR sintètic) per a diferents combustibles. El temps de retard del fenomen de flames fredes (en el cas d'existir), així com el temps de retard de l'etapa d'alta temperatura, han sigut obtinguts experimentalment i les seues tendències explicades mitjançant cinètica química. S'han estudiat els diferents efectes de les diferents espècies involucrades en l'EGR sintètic sobre el temps de retard, deslligant aquells de caràcter termodinàmic dels efectes purament químics. S'han tingut en compte diferents composicions per a definir aquest EGR, establint límits de validesa per a cadascuna de les mescles proposades. Els efectes termodinàmics i químics van resultar ser oposats, sent dominant un o un altre a diferents rangs de temperatura de treball. Diversos mecanismes de cinètica química han sigut validats gràcies als resultats experimentals obtinguts. A més d'un mecanisme detallat per a mescles PRF d'iso-octà i n-heptà, s'ha dut a terme la validació d'un altre mecanisme simplificat per al n-dodecà. D'altra banda, un submodel de formació i decaïment d'OH* excitat ha sigut validat contra resultats de quimioluminescència i espectroscopía. S'han estudiat les diferents fonts de radiació del procés d'autoencesa per a l'iso-octà i l'n-heptà mitjançant tècniques d'espectroscopía. A més, s'han realitzat mesures de quimioluminescència filtrada a 310nm (longitud d'ona d'emissió del radical OH*) per a l'anàlisi de la generalització i velocitat de propagació del front d'autoencesa. La propagació de l'encesa ha mostrat ser depenent de les condicions termodinàmiques aconseguides en la cambra de combustió en l'instant d'ignició més que de la reactivitat de la mescla. S'han trobat dues fonts de radiació diferents a 310nm mitjançant espectroscopía, depenent de la intensitat de l'encesa: el decaïment del radical OH* d'estat excitat a estat natural i l'oxidació del CO a CO2 (continu del CO). No obstant açò, aquestes tècniques han sigut utilitzades solament per als dos combustibles de referència de l'escala de octanaje a causa de limitacions tècniques. Finalment, s'ha desenvolupat un nou model predictiu de manera teòrica partint del model de Glassman per a l'autoencesa. Aquest mètode es basa a modelar primer la taxa d'acumulació de portadors de cadena fins a la seua concentració crítica (obtenint així el temps de retard referit a l'etapa de flames fredes) i, després d'aquest instant, modelar la taxa de consum d'aquests portadors de cadena fins a la seua completa desaparició (instant en el qual es produeix la màxima exotermia del procés, predient el temps de retard referit a l'etapa d'alta temperatura de l'encesa). La capacitat predictiva del model ha sigut comprovada per a cadascun dels sis combustibles assajats. A més, aquesta capacitat predictiva ha sigut comparada amb la d'altres mètodes existents en la literatura, com la integral de Livengood & Wu. La validesa de cadascun dels mètodes ha sigut analitzada, definint una metodologia d'ús per a obtenir prediccions raonables del temps de retard. / López Pintor, D. (2017). Theoretical and experimental study on the autoignition phenomena of homogeneous reactive mixtures [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/90642 / TESIS