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The effect of uncertainty in gas composition on the accuracy of gas-phase thermometry using Laser-Induced Thermal Grating Spectroscopy, LITGS, is studied. Temperatures are obtained from measurements of the sound speed derived from the frequency of oscillations ƒ_OSC imposed upon the LITGS signal arising from the transit of acoustic waves across the density modulation feature. The dependence of the sound speed, c_s on √γ/m, where γ is the ratio of specific heats and m is the mean molecular mass leads to a dependence upon gas composition. LITGS signals were generated in acetone vapour in a variety of gas mixtures in a temperature controlled cell at 4 bar total pressure using pump pulses from a frequency quadrupled Q-switched Nd:YAG laser at 266 nm and a cw diode pumped solid state probe laser at 671 nm. Studies were undertaken of the variation in ƒ_OSC with gas composition using gas mixtures of O₂ and N₂ with component concentrations in the range 0-100 %, and was found to agree with theoretical predictions. Measurement precision of the data (one standard deviation in 50 measurements) was found to be typically ± 1.7 % for measurements at 4 bar total pressure. The effect of varying concentrations in exhaust gas residuals (EGR) typical of pre-ignition gases in a spark ignition internal combustion engine were studied using synthetic air (N₂/ O₂ mixtures) containing variable amounts of simulated EGR components, CO₂ and H₂O. The effect of variation in CO₂ concentration in dry synthetic air was measured at 4 bar and 30°C and found to agree with theoretical predictions. Experiments conducted at 30°C, with the addition of a saturated vapour pressure of water indicate that the effect of a saturated vapour pressure of water on the oscillation frequency in synthetic EGR is on the borderline of resolution. The effect of variable amounts of typical hydrocarbon fuel vapour on ƒ_OSC was studied using 2,2,4-trimethyl-pentane in gas mixtures composed of synthetic air and variable amounts of EGR and water vapour at 80°C. Kinetic theory was used in order to model the dependence of the oscillation frequency ƒ_OSC on various gas compositions containing fuel and EGR, in order to construct an error surface for comparison with experimental measurements. Experimental data were found to agree with the model predictions to within experimental error for a representative data set within the range of calculated values. The results indicate that uncertainties in temperature values derived from LITGS thermometry can be estimated with confidence within reasonable estimates of composition variations in an internal combustion engine, and should lead to absolute temperature accuracy of within 2-3 %.

Doctor of Philosophy(PhD) / Abstract This thesis employs a statistical regression method to estimate models for testing the hypothesis of the thesis of vehicle emissions interdependencies. The thesis at the beginnings, reviews critically the formation of emissions in gasoline-fuelled engines, and also reviews existing and emerging models of automotive emissions. The thesis then, presents the relationships between the urban transport system and vehicle emissions. Particularly, it summarises different types of emissions and the contributory factors of the urban transport system to such emissions. Subsequently, the thesis presents the theory of vehicle emissions interdependencies and the empirical framework for testing the hypothesis of the thesis. The scope of testing the hypothesis of the thesis is only limited to gasoline-fuelled conventional vehicles in the urban transport environment. We use already available laboratory-based testing dataset of 542 passenger vehicles, to investigate the hypothesis of the thesis of vehicle emissions interdependencies. HC, CO, and NOX emissions were collected under six test drive-cycles, for each vehicle before and after vehicles were tuned. Prior to using any application, we transform the raw dataset into actionable information. We use three steps, namely conversion, cleaning, and screening, to process the data. We use classification and regression trees (CART) to narrow down the input number of variables in the models formulated for investigating the hypothesis of the thesis. We then, utilise initial results of the analysis to fix any remaining problems in the data. We employ three stage least squares (3SLS) regression to test the hypothesis of the thesis, and to estimate the maximum likelihood of vehicle variables and other emissions to influence HC, CO, and NOX emissions simultaneously. We estimate twelve models, each of which consists of a system of three simulations equations that accounts for the endogenous relations between HC, CO and NOX emissions when estimating vehicle emissions simultaneously under each test drive-cycle. The major contribution of the thesis is to investigate the inter-correlations between vehicle emissions within a well controlled data set, and to test the hypothesis of vehicle emissions interdependencies. We find that HC, CO, and NOX are endogenously or jointly dependent in a system of simultaneous-equations. The results of the analysis demonstrate that there is strong evidence against the null hypothesis (H0) in favour of the alternative hypothesis (H1) that HC, CO, and NOX are statistically significantly interdependent. We find, for the thesis sample, that NOX and CO are negatively related, whereas HC and CO emissions are positively related, and HC and NOX are positively related. The results of the thesis yield new insights. They bridge a very important gap in the current knowledge on vehicle emissions. They advance not only our current knowledge that HC, CO, and NOX should be predicted jointly since they are produced jointly, but also acknowledge the appropriateness of using 3SLS regression for estimating vehicle emissions simultaneously. The thesis measures the responses of emissions to changes with respect to changes in the other emissions. We investigate emission responses to a one percent increase in an emission with respect to the other emissions. We find the relationship between CO and NOX is of special interest. After vehicles were tuned, we find those vehicles that exhibit a one percent increase in NOX exhibit simultaneously a 0.35 percent average decrease in CO. Similarly, we find that vehicles which exhibit a one percent increase in CO exhibit simultaneously a 0.22 percent average decrease in NOX. We find that the responses of emission to changes with respect to other emissions vary with various test drive-cycles. Nonetheless, a band of upper and lower limits contains these variations. After vehicle tuning, a one percent increase in HC is associated with an increase in NOX between 0.5 percent and 0.8 percent, and an increase in CO between 0.5 percent and one percent Also, for post-tuning vehicles, a one percent increase in CO is associated with an increase in HC between 0.4 percent and 0.9 percent, and a decrease in NOX between 0.07 percent and 0.32 percent. Moreover, a one percent increase in NOX is associated with increase in HC between 0.8 percent and 1.3 percent, and a decrease in CO between 0.02 percent and 0.7 percent. These measures of the responses are very important derivatives of the hypothesis investigated in the thesis. They estimate the impacts of traffic management schemes and vehicle operations that target reducing one emission, on the other non-targeted emissions. However, we must be cautious in extending the results of the thesis to the modern vehicles fleet. The modern fleet differs significantly in technology from the dataset that we use in this thesis. The dataset consists of measurements of HC, CO, and NOX emissions for 542 gasoline-fuelled passenger vehicles, under six test drive-cycles, before and after the vehicles were tuned. Nevertheless, the dataset has a number of limitations such as limited model year range, limited representations of modal operations, and limitations of the measurements of emissions based only on averages of test drive-cycles, in addition to the exclusion of high-emitter emission measurements from the dataset. The dataset has a limited model year range, i.e., between 1980 and 1991. We highlight the age of the dataset, and acknowledge that the present vehicle fleet varies technologically from the vehicles in the dataset used in this thesis. Furthermore, the dataset has a limited number of makes - Holden, Ford, Toyota, Nissan, and Mitsubishi. There are also a limited number of modal operations. The model operations presented in the dataset are cold start, warming-up, and hot stabilised driving conditions. However, enrichment episodes are not adequately presented in the test-drive cycles of the dataset. Moreover, the dataset does not take into account driving behaviour influences, and all measurements are cycle-based averages. The emission measurements of laboratory-based testings are aggregated over a test drive cycle, and the test drive-cycle represents an average trip over an average speed. The exclusion of the measurements of high emitting vehicles from the dataset introduces further limitations. Remote sensing studies show that 20 percent of the on-road vehicle fleet is responsible for 80 percent of HC and CO emissions. The findings of the thesis assist in the identification of the best strategies to mitigate the most adverse effects of air-pollution, such as the most severe pollution that have the most undesirable pollution effects. Also, they provide decision-makers with valuable information on how changes in the operation of the transport system influence the urban air-quality. Moreover, the thesis provides information on how vehicle emissions affect the chemistry of the atmosphere and degrade the urban air-quality.

Doctor of Philosophy(PhD) / Abstract This thesis employs a statistical regression method to estimate models for testing the hypothesis of the thesis of vehicle emissions interdependencies. The thesis at the beginnings, reviews critically the formation of emissions in gasoline-fuelled engines, and also reviews existing and emerging models of automotive emissions. The thesis then, presents the relationships between the urban transport system and vehicle emissions. Particularly, it summarises different types of emissions and the contributory factors of the urban transport system to such emissions. Subsequently, the thesis presents the theory of vehicle emissions interdependencies and the empirical framework for testing the hypothesis of the thesis. The scope of testing the hypothesis of the thesis is only limited to gasoline-fuelled conventional vehicles in the urban transport environment. We use already available laboratory-based testing dataset of 542 passenger vehicles, to investigate the hypothesis of the thesis of vehicle emissions interdependencies. HC, CO, and NOX emissions were collected under six test drive-cycles, for each vehicle before and after vehicles were tuned. Prior to using any application, we transform the raw dataset into actionable information. We use three steps, namely conversion, cleaning, and screening, to process the data. We use classification and regression trees (CART) to narrow down the input number of variables in the models formulated for investigating the hypothesis of the thesis. We then, utilise initial results of the analysis to fix any remaining problems in the data. We employ three stage least squares (3SLS) regression to test the hypothesis of the thesis, and to estimate the maximum likelihood of vehicle variables and other emissions to influence HC, CO, and NOX emissions simultaneously. We estimate twelve models, each of which consists of a system of three simulations equations that accounts for the endogenous relations between HC, CO and NOX emissions when estimating vehicle emissions simultaneously under each test drive-cycle. The major contribution of the thesis is to investigate the inter-correlations between vehicle emissions within a well controlled data set, and to test the hypothesis of vehicle emissions interdependencies. We find that HC, CO, and NOX are endogenously or jointly dependent in a system of simultaneous-equations. The results of the analysis demonstrate that there is strong evidence against the null hypothesis (H0) in favour of the alternative hypothesis (H1) that HC, CO, and NOX are statistically significantly interdependent. We find, for the thesis sample, that NOX and CO are negatively related, whereas HC and CO emissions are positively related, and HC and NOX are positively related. The results of the thesis yield new insights. They bridge a very important gap in the current knowledge on vehicle emissions. They advance not only our current knowledge that HC, CO, and NOX should be predicted jointly since they are produced jointly, but also acknowledge the appropriateness of using 3SLS regression for estimating vehicle emissions simultaneously. The thesis measures the responses of emissions to changes with respect to changes in the other emissions. We investigate emission responses to a one percent increase in an emission with respect to the other emissions. We find the relationship between CO and NOX is of special interest. After vehicles were tuned, we find those vehicles that exhibit a one percent increase in NOX exhibit simultaneously a 0.35 percent average decrease in CO. Similarly, we find that vehicles which exhibit a one percent increase in CO exhibit simultaneously a 0.22 percent average decrease in NOX. We find that the responses of emission to changes with respect to other emissions vary with various test drive-cycles. Nonetheless, a band of upper and lower limits contains these variations. After vehicle tuning, a one percent increase in HC is associated with an increase in NOX between 0.5 percent and 0.8 percent, and an increase in CO between 0.5 percent and one percent Also, for post-tuning vehicles, a one percent increase in CO is associated with an increase in HC between 0.4 percent and 0.9 percent, and a decrease in NOX between 0.07 percent and 0.32 percent. Moreover, a one percent increase in NOX is associated with increase in HC between 0.8 percent and 1.3 percent, and a decrease in CO between 0.02 percent and 0.7 percent. These measures of the responses are very important derivatives of the hypothesis investigated in the thesis. They estimate the impacts of traffic management schemes and vehicle operations that target reducing one emission, on the other non-targeted emissions. However, we must be cautious in extending the results of the thesis to the modern vehicles fleet. The modern fleet differs significantly in technology from the dataset that we use in this thesis. The dataset consists of measurements of HC, CO, and NOX emissions for 542 gasoline-fuelled passenger vehicles, under six test drive-cycles, before and after the vehicles were tuned. Nevertheless, the dataset has a number of limitations such as limited model year range, limited representations of modal operations, and limitations of the measurements of emissions based only on averages of test drive-cycles, in addition to the exclusion of high-emitter emission measurements from the dataset. The dataset has a limited model year range, i.e., between 1980 and 1991. We highlight the age of the dataset, and acknowledge that the present vehicle fleet varies technologically from the vehicles in the dataset used in this thesis. Furthermore, the dataset has a limited number of makes - Holden, Ford, Toyota, Nissan, and Mitsubishi. There are also a limited number of modal operations. The model operations presented in the dataset are cold start, warming-up, and hot stabilised driving conditions. However, enrichment episodes are not adequately presented in the test-drive cycles of the dataset. Moreover, the dataset does not take into account driving behaviour influences, and all measurements are cycle-based averages. The emission measurements of laboratory-based testings are aggregated over a test drive cycle, and the test drive-cycle represents an average trip over an average speed. The exclusion of the measurements of high emitting vehicles from the dataset introduces further limitations. Remote sensing studies show that 20 percent of the on-road vehicle fleet is responsible for 80 percent of HC and CO emissions. The findings of the thesis assist in the identification of the best strategies to mitigate the most adverse effects of air-pollution, such as the most severe pollution that have the most undesirable pollution effects. Also, they provide decision-makers with valuable information on how changes in the operation of the transport system influence the urban air-quality. Moreover, the thesis provides information on how vehicle emissions affect the chemistry of the atmosphere and degrade the urban air-quality.

The principle of controlled auto-ignition (CAI) is to mix fuel and air homogeneously before compressing the mixture to the point of auto-ignition. As ignition occurs simultaneously, CAI engines operate with lean mixtures preventing high cylinder pressures. CAI engines produce small amounts of nitrogen oxides (NOx) due to low combustion temperatures while maintaining high compression ratios and engine efficiencies. Due to simultaneous combustion and lean mixtures, CAI engines are restricted between low and mid load operations. Various strategies have been studied to improve the load limit of CAI engines. The scope of the project is to investigate the consequences of varying valve timing, as a method to control the mixture temperature within the combustion chamber and therefore, controlling the mixture auto-ignition point. This study presents computational fluid dynamics (CFD) modelling results of transient flow, inside a 0.45 litre Lotus single cylinder engine. After a validation process, a chemical kinetics model is combined with the CFD code, in order to study in-cylinder temperatures, the mixture distribution during compression and to predict the auto-ignition timing. The first part of the study focuses on validating the calculated in-cylinder velocities. A mesh sensitivity study is performed as well as a comparison of different turbulence models. A method to reduce computational time of the calculations is presented. The effects of engine speed on charge delay and charge amount inside the cylinder, the development of the in-cylinder flow field and the variation of turbulence parameters during the intake and compression stroke, are studied. The second part of the study focuses on the gasoline mixture and the variation of the valve timing, to retain different ratios of residual gases within the cylinder. After validation of the model, a final set of CFD calculations is performed, to investigate the effects of valve timing on flow and the engine parameters. The results are then compared to a fully homogeneous mixture model to study the benefits of varying valve duration. New key findings and contributions to CAI knowledge were found in this investigation. Reducing the intake and exhaust valve durations created a mixture temperature stratification and a fuel concentration distribution, prior to auto-ignition. It resulted in extending the heat release rate duration, improving combustion. However, shorter valve timing durations also showed an increase in heat transfer, pumping work and friction power, with a decrease of cylinder indicated efficiency. Valve timing, as a method to control auto-ignition, should only be used when the load limit of CAI engines, is to be improved.

This work is concerned with the fluid dynamic processes and the associated loss of acoustic energy produced by circular apertures within noise absorbing perforated walls. Although applicable to a wide range of engineering applications particular emphasis in this work is placed on the use of such features within a gas turbine combustion system. The primary aim for noise absorbers in gas turbine combustion systems is the elimination of thermo-acoustic instabilities, which are characterised by rapidly rising pressure amplitudes which are potentially damaging to the combustion system components. By increasing the amount of acoustic energy being absorbed the occurrence of thermo-acoustic instabilities can be avoided. The fundamental acoustic characteristics relating to linear acoustic absorption are presented. It is shown that changes in orifice geometry, in terms of gas turbine combustion system representative length-to-diameter ratios, result in changes in the measured Rayleigh Conductivity. Furthermore in the linear regime the maximum possible acoustic energy absorption for a given cooling mass flow budget of a conventional combustor wall will be identified. An investigation into current Rayleigh Conductivity and aperture impedance (1D) modelling techniques are assessed and the ranges of validity for these modelling techniques will be identified. Moreover possible improvements to the modelling techniques are discussed. Within a gas turbine system absorption can also occur in the non-linear operating regime. Hence the influence of the orifice geometry upon the optimum non-linear acoustic absorption is also investigated. Furthermore the performance of non-linear acoustic absorption modelling techniques is evaluated against the conducted measurements. As the amplitudes within the combustion system increase the acoustic absorption will transition from the linear to the non-linear regime. This is important for the design of absorbers or cooling geometries for gas turbine combustion systems as the propensity for hot gas ingestion increases. Hence the relevant parameters and phenomena are investigated during the transition process from linear to non-linear acoustic absorption. The unsteady velocity field during linear and non-linear acoustic absorption is captured using particle image velocimetry. A novel analysis technique is developed which enables the identification of the unsteady flow field associated with the acoustic absorption. In this way an investigation into the relevant mechanisms within the unsteady flow fields to describe the acoustic absorption behaviour of the investigated orifice plates is conducted. This methodology will also help in the development and optimisation of future damping systems and provide validation for more sophisticated 3D numerical modelling methods. Finally a set of design tools developed during this work will be discussed which enable a comprehensive preliminary design of non-resonant and resonant acoustic absorbers with multiple perforated liners within a gas turbine combustion system. The tool set is applied to assess the impact of the gas turbine combustion system space envelope, complex swirling flow fields and the propensity to hot gas ingestion in the preliminary design stages.

Les agrégats de suie émis par les processus de combustion dans les turbines aéronautiques contribuent à modifier le bilan radiatif de l'atmosphère et la qualité de l'air. La connaissance de leurs caractéristiques physiques et chimiques est indispensable pour évaluer leur rôle dans les processus physico-chimiques atmosphériques et leur impact sur l'environnement et la santé publique. Dans ce contexte, notre étude vise à déterminer la taille et la morphologie d'agrégats de suie aéronautique à partir de mesures expérimentales menées en microscopie électronique en transmission (MET) et en tomographie électronique.Nous avons réalisé des clichés MET d'agrégats de suie émis par des turboréacteurs aéronautiques. Nous avons établi une méthode pour caractériser la morphologie des agrégats en déterminant leur allongement, leur compacité et la tortuosité de leur contour en analysant leur projection. Nous avons également développé un logiciel de traitement et d'analyse des images MET qui permet de reconstruire en 3D un agrégat à partir de ses projections et l'analyse de ses caractéristiques dimensionnelles et morphologiques à partir de sa reconstruction. Les résultats obtenus nous ont permis d'étudier la validité des relations liant les caractéristiques microphysiques 2D et 3D proposées dans la littérature et d'en proposer de nouvelles pour les agrégats étudiés.Ces résultats constituent la première caractérisation morphologique 3D d'agrégats de suie aéronautique à partir d'analyses par MET et tomographie électronique. Ils montrent que les propriétés morphologiques de ces agrégats ne permettent pas d'utiliser la méthode d'ensemble pour déterminer la dimension fractale massique. / Soot aggregates emitted by aircraft engines' combustion processes are involved in the modification of the global radiative budget and the air quality. The knowledge of their physical and chemical characteristics is a prerequisite to any evaluation of the way they may act in the atmospheric physical and chemical processes and their impact on the environment and public health. In this context, our study aims at determining the size and morphological characteristics of aircraft soot aggregates on the basis of experimental measurements by transmission electron microscopy (TEM) and electron tomography.We have acquired TEM pictures of soot aggregates emitted by aircraft engines. We have established a method to characterize the morphology of these aggregates by determining their elongation, their compacity and the tortuosity of their edge. This method is based on the analysis of their TEM projection. Besides, we have developed a software to process and analyse TEM pictures. It allows to reconstruct aggregates from their projections and to determine their size and morphological characteristics. Our results have lead us to study the validity of the relationships linking the 2D and 3D microphysical characteristics presented in the literature and to suggest new ones for the studied aggregates.These results constitute the first 3D morphological and size characterizations of aircraft soot aggregates using TEM and electron tomography. They highlight the fact that the morphological properties of these aggregates do not fulfil the hypotheses required for the use of the collective method to determine the mass fractal dimension.

[ES] El proceso de atomización primaria es el mecanismo por el cual una vena líquida se disgrega en un ambiente gaseoso. Este proceso está presente en muchas aplicaciones de ingeniería realizando diferentes tareas. En ocasiones es un paso previo antes de ser quemado, como en la industria energética o de propulsión, donde el objetivo es extraer la energía específica del líquido. En otros sectores, como el revestimiento o la extinción de incendios, el objetivo es maximizar el área cubierta por el chorro. Sin embargo, aunque la atomización es una parte fundamental de varios procesos industriales, está lejos de comprenderse por completo. El proceso de atomización es una mezcla de fenómenos de interacción gas-líquido dentro de un campo turbulento que tiene lugar en el campo cercano, que es la región más densa del chorro. Cuando se trata de arrojar luz sobre el proceso de atomización primaria, el problema principal es la falta de teorías físicas definitivas capaces de vincular los complejos eventos de ruptura con la turbulencia. El principal obstáculo que impide investigar el proceso de atomización primaria es la incapacidad de las técnicas ópticas clásicas para proporcionar información de la región densa del chorro. Solo en los últimos años, las nuevas técnicas basadas en rayos X podrían proporcionar nueva información sobre las características de la atomización cerca de la salida de la tobera. Esto también afecta a los modelos computacionales de atomización primaria que, al no disponer de información experimental sobre la región densa, requieren una calibración precisa de sus constantes para proporcionar resultados fiables en el campo lejano. Esta tesis se centra en mejorar el conocimiento del proceso de atomización primaria, especialmente en cómo las condiciones de inyección afectan el desarrollo del chorro en el campo cercano desde dos puntos de vista diferentes. Por un lado, con un enfoque computacional usando Direct Numerical Simulations y, por otro lado, experimentalmente usando Near-Field Microscopy. El estudio computacional se centra en variar los números de Reynolds y Weber de inyección. Los resultados muestran que aumentar el número de Reynolds mejora la desintegración del líquido, mostrando un aumento de las gotas generadas y una nube de gotas más fina. Sin embargo, la falta de un perfil turbulento de flujo de entrada completamente desarrollado conduce a comportamientos inesperados en la longitud de ruptura de la vena líquida que también aumenta con el número de Reynolds. El número de gotas también aumenta cuando aumenta el número de Weber, pero los tamaños característicos de las gotas siguen siendo los mismos. La longitud de ruptura no varía, lo que sugiere que las variaciones de la tensión superficial afectan la ruptura de las gotas y los ligamentos, pero no la desintegración del núcleo líquido en sí. Con los resultados obtenidos de ambos estudios, se propone un modelo fenomenológico que predice la distribución del tamaño de gota en función de las condiciones de inyección. Además, también se ha estudiado el efecto de usar toberas elípticas. Se ha obtenido que el número de gotas detectadas aumenta en comparación con el chorro redondo manteniendo ángulos de apertura del chorro similares. Sin embargo, cuando se utilizan toberas extremadamente excéntricas, la disminución de la turbulencia del flujo de entrada contrarresta los beneficios de este tipo de inyectores. En cuanto al análisis experimental, usar Near-Field Microscopy permite magnificar la región densa y analizar las características macroscópicas del chorro. Por lo tanto, se varían las presiones de inyección y descarga, centrándose en el ángulo de apertura del chorro. Se observa el aumento esperado en el ángulo al aumentar tanto la presión de inyección como la de descarga. Sin embargo, adicionalmente, se realiza un análisis de las perturbaciones del contorno del chorro, concluyendo que, al aumentar la presión de inyección, y por lo tanto la turbulencia del flujo de / [CA] El procés d'atomització primària és el mecanisme pel qual una vena líquida es disgrega en un ambient gasós. Aquest procés és present en moltes aplicacions d'enginyeria fent diferents tasques. De vegades és un pas previ abans de ser cremat, com ara en la indústria energètica o de propulsió, on l'objectiu és extraure l'energia específica del líquid. En altres sectors, com ara el revestiment o l'extinció d'incendis, l'objectiu és maximitzar l'àrea coberta pel doll. No obstant això, tot i que l'atomització és una part fonamental de diversos processos industrials, està lluny de comprendre's per complet. El procés d'atomització és una barreja de fenòmens d'interacció gas-líquid dins d'un camp turbulent que té lloc en el camp pròxim, que és la regió més densa del doll. Quan es tracta de donar llum sobre el procés d'atomització primària, el problema principal és la falta de teories físiques definitives capaces de vincular els complexos esdeveniments de trencament amb la turbulència. El principal obstacle que impedeix investigar el procés d'atomització primària és la incapacitat de les tècniques òptiques clàssiques per a proporcionar informació de la regió densa del doll. Només en els últims anys, les noves tècniques basades en raigs X podrien proporcionar nova informació sobre les característiques de l'atomització prop de l'eixida de la tovera. Això també afecta els models computacionals d'atomització primària que, en no disposar d'informació experimental sobre la regió densa, requereixen un calibratge precís de les seues constants per a proporcionar resultats fiables en el camp llunyà. Aquesta tesi se centra a millorar el coneixement del procés d'atomització primària, especialment en com les condicions d'injecció afecten el desenvolupament del doll en el camp pròxim des de dos punts de vista diferents. D'una banda, amb un enfocament computacional usant Direct Numerical Simulations i, d'altra banda, experimentalment usant Near-Field Microscopy. L'estudi computacional se centra a variar els nombres de Reynolds i Weber d'injecció. Els resultats mostren que augmentar el nombre de Reynolds millora la desintegració del líquid, tot mostrant un augment de les gotes generades i un núvol de gotes més fi. No obstant això, la falta d'un perfil turbulent de flux d'entrada completament desenvolupat condueix a comportaments inesperats en la longitud de ruptura de la vena líquida que també augmenta amb el nombre de Reynolds. El nombre de gotes també augmenta quan creix el nombre de Weber, però les grandàries característiques de les gotes continuen sent les mateixes. La longitud de ruptura no varia, la qual cosa suggereix que les variacions de la tensió superficial afecten la ruptura de les gotes i els lligaments, però no la desintegració del nucli líquid en ell mateix. Amb els resultats obtinguts de tots dos estudis, es proposa un model fenomenològic que prediu la distribució de la grandària de gota en funció de les condicions d'injecció. A més, també s'ha estudiat l'efecte d'usar toveres el·líptiques. S'ha obtingut que el nombre de gotes detectades augmenta en comparació amb el doll redó tot mantenint angles d'obertura del doll similars. No obstant això, quan s'utilitzen toveres extremadament excèntriques, la disminució de la turbulència del flux d'entrada contraresta els beneficis d'aquesta mena d'injectors. Quant a l'anàlisi experimental, usar Near-Field Microscopy permet magnificar la regió densa i analitzar les característiques macroscòpiques del doll. Per tant, es varien les pressions d'injecció i descàrrega, tot centrant-se en l'angle d'obertura del doll. S'observa l'augment esperat en l'angle en augmentar tant la pressió d'injecció com la de descàrrega. No obstant això, addicionalment, es realitza una anàlisi de les pertorbacions del contorn del doll i es conclou que en augmentar la pressió d'injecció, i per tant la turbulència del flux d'entrada, augmenten les pertorbacions en el contorn del ruixat, especialment a pressions de descàrrega mé / [EN] The primary atomisation process is the mechanism by which a liquid vein breaks into droplets in a gaseous ambient. This process is present in many engineering applications accomplishing different tasks. Sometimes it is a previous step before being burned, as in the energy or propulsion industry, where the objective is to extract the specific energy of the liquid. In other sectors, such as the coating or fire extinction, the objective is to maximise the area covered by the droplet cloud. However, although atomisation is a fundamental part of several industrial processes, it is far from fully understood. The atomisation process is a mixture of gas-liquid interaction phenomena within a turbulent field that takes place in the near-field, which is the denser region of the spray. When trying to shed light on the primary atomisation process, the main issue is the lack of definitive physical theories able to link the complex breakup events and the turbulence. The principal impediment that prevents the investigation from breaking through the atomisation process is the inability of the classic optical techniques to provide information from the dense region of the spray. Only in the last years, newer techniques based on X-Ray could provide new information on spray characteristics near the nozzle outlet. This also affects the computational primary atomisation models that, as there is no available experimental information on the dense region, require an accurate calibration of their constants to provide reliable results on the far-field. This thesis focuses on improving the knowledge of the primary atomisation process, especially on how the injection conditions affect the spray development in the near field from two different standpoints. On the one hand, with a computational approach using Direct Numerical Simulations and on the other hand, experimentally using Near-Field Microscopy. The computational study is focused on varying the inflow Reynolds and Weber numbers. Results show that increasing the Reynolds number improves the liquid disintegration, exhibiting an increase of generated droplets and a finer droplet cloud. However, the lack of a fully developed inflow turbulent profile leads to characteristic behaviours on the breakup length of the spray that also increases with the Reynolds number. The number of droplets increases when the Weber number increases, but the characteristic droplet sizes remain the same. The breakup length does not vary, suggesting that the surface tension variations affect the droplet and ligament breakup but not the core disintegration itself. With the results obtained from both studies, a phenomenological model is proposed to predict the droplet size distribution depending on the injection conditions. Additionally, using elliptical nozzles, the number of detected droplets increases compared with the round spray and maintain similar spray apertures. However, when using extremely eccentric nozzles, the inflow turbulence decrease counteracts the elliptical sprays' benefits. Regarding the experimental analysis, the Near-Field Microscopy magnifies the dense region and analyses the macroscopic features on the spray. So the injection and discharge pressure are varied, and the spotlight is put on the spray angle. The expected increase in the spray angle when increasing both the injection and discharge pressure is observed. Nevertheless, additionally, an analysis of the spray contour perturbations is performed, concluding that increasing the injection pressure, and thus the inflow turbulence, increases the perturbations on the spray contour, especially at lower discharge pressures. / González Montero, LA. (2022). Computational and Experimental Study of the Primary Atomisation Process under Different Injection Conditions [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/190635

[ES] El alto impacto del sector transporte respecto a las emisiones globales de CO2 y su efecto en el cambio climático ha llevado a que éste transite hacia tecnologías más eficientes y medioambientalmente sostenibles. Sin embargo, el ritmo de transformación es lento en relación a lo que se necesita para frenar el calentamiento global existente. En este sentido, en los últimos tiempos los caminos hacia la transformación se han diversificado; el concepto de "defossilization" ha surgido como alternativa a la descarbonización, ya que destaca la posibilidad de incluir una mayor cantidad de combustibles sintéticos y renovables, con los cuales se pueden obtener resultados igualmente efectivos. Dentro de estos, destacan los combustibles Polioximetileno dimetil éter (OMEn), su carácter oxigenado y no poseer enlaces carbono-carbono, los hace prometedores respecto a la formación de hollín. Además, presentan grandes similitudes y compatibilidades con el diésel convencional, lo cual posibilita el uso de la flota de vehículos con motores de combustión interna existente a nivel mundial, acelerando así la transición y siendo una alternativa con alcance global. La presente tesis tiene como objetivo llevar a cabo un estudio fundamental sobre el proceso de combustión y la estructura de la llama de chorros tipo Diésel cuando se utilizan combustibles tipo OMEn. Para la consecución de dicho objetivo, la metodología planteada es eminentemente computacional, encontrando aquí las mayores brechas en la literatura. Se lleva a cabo un estudio de la cinética química y el efecto de la difusión en los combustibles estudiados mediante configuraciones canónicas, como reactores homogéneos y flamelets de contraflujo. Posteriormente, se estudia detalladamente el proceso de combustión y la estructura de la llama mediante el uso extensivo de Dinámica de fluidos computacional (CFD, en inglés), con modelos de turbulencia RANS y LES, en conjunto con un modelo de combustión avanzado basado en el concepto de flamelets, denominado UFPV. Todos los casos estudiados están definidos siguiendo las directrices de la Engine Combustion Network (ECN), los cuales representan chorros inyectados en ambientes quiescentes con toberas monoorificio. Particularmente, se evalúan los Sprays A y D, y el impacto de variar la temperatura ambiente. Como conclusión general, se puede afirmar que estos modelos CFD predicen correctamente el desarrollo de la combustión bajo las condiciones analizadas, y que estos combustibles son capaces de desarrollar diferentes estructuras de llama altamente dependientes de las condiciones de contorno impuestas. / [CA] L'alt impacte del sector del transport respecte a les emissions globals de CO2 i el seu efecte en el canvi climàtic ha portat a que aquest transite cap a tecnologies més eficients i mediambientalment sostenibles. No obstant això, el ritme de transformació és lent en relació amb el que es necessita per frenar l'escalfament global existent. En aquest sentit, en els últims temps els camins cap a la transformació s'han diversificat; el concepte de "defossilització" ha sorgit com a alternativa a la descarbonització, ja que destaca la possibilitat d'incloure una major quantitat de combustibles sintètics i renovables, amb els quals es poden obtenir resultats igualment efectius. Dins d'aquests, destaquen els combustibles tipus polioximetilen dimetil èters (OMEn), el seu caràcter oxigenat i al no posseir enllaços carbó-carbó, els fa prometedors respecte a la formació de sutge. A més, presenten grans semblances i compatibilitats amb el dièsel convencional, la qual cosa possibilita l'ús de la flota de vehicles amb motors de combustió interna existent a nivell mundial, accelerant així la transició i essent una alternativa amb abast global. La present tesi té com a objectiu dur a terme un estudi basic sobre el procés de combustió i l'estructura de la flama de dolls tipus Dièsel quan s'utilitzen combustibles tipus OMEn. Per a la consecució d'aquest objectiu, la metodologia plantejada és eminentment computacional, trobant ací les majors mancances en la literatura. Es realitza un estudi de la cinètica química i l'efecte de la difusió en els combustibles estudiats mitjançant configuracions canòniques, com ara reactors homogenis i flamelets de contraflux. Posteriorment, s'estudia detalladament el procés de combustió i l'estructura de la flama mitjançant l'ús extensiu de dinàmica de fluids computacional (CFD, en anglés), amb models de turbulència RANS i LES, conjuntament amb un model de combustió avançat basat en el concepte de flamelets, anomenat UFPV. Tots els casos estudiats estan definits seguint les directrius de l'Engine Combustion Network (ECN, en anglés), els quals representen dolls injectats en ambients quiescents amb toveres mono-orifici. Particularment, s'avaluen els Sprays A i D, i l'impacte de variar la temperatura ambient. Com a conclusió general, es pot afirmar que aquests models CFD prediuen correctament el desenvolupament de la combustió sota les condicions analitzades, i que aquests combustibles són capaços de desenvolupar diferents estructures de flama altament dependents de les condicions de contorn imposades. / [EN] The significant impact of the transportation sector on global CO2 emissions and its effect on climate change has led to a shift towards more efficient and environmentally sustainable technologies. However, the pace of this transformation is slow relative to what is needed to mitigate existing global warming. In this regard, pathways toward transformation have diversified recently, with the concept of defossilization emerging as an alternative to decarbonization. Defossilization emphasizes the possibility of incorporating a greater variety of synthetic and renewable fuels, which can yield equally effective results. Among these alternatives, Polyoxymethylene dimethyl ether ($OMEn$) fuels stand out due to their oxygenated character and absence of carbon-carbon bonds, making them promising in reducing soot formation. Furthermore, their similarities and compatibilities with conventional diesel enable the utilization of the existing global fleet of internal combustion engine vehicles, thus potentially accelerating the transition on a global scale. This thesis aims to conduct a fundamental study on the combustion process and flame structure of Diesel-like sprays when OMEn-type fuels are utilized. To achieve this objective, the proposed methodology is eminently computational, addressing significant gaps in the existing literature. A study of chemical kinetics and diffusion effects in the fuels under investigation uses canonical configurations such as homogeneous reactors and counterflow flamelets. Subsequently, the combustion process and flame structure are examined in detail through extensive Computational fluid dynamics (CFD) simulations, employing RANS and LES turbulence models in conjunction with an advanced combustion model based on the flamelet concept, UFPV. All studied cases are defined according to the Engine Combustion Network (ECN) guidelines, representing sprays injected into quiescent environments with single-hole nozzles. Specifically, Spray A and D are evaluated, along with the impact of varying ambient temperatures. In conclusion, it can be affirmed that the CFD models accurately predict combustion development under the analysed conditions, and these fuels can develop different flame structures highly dependent on the imposed boundary conditions. / The respondent wishes to acknowledge the financial support received through a grant from Vicerrectorado de Investigación of Universitat Politècnica de València with reference FPI UPV SUBP2 (PAID-01-20) / De León Ceriani, D. (2024). Evaluation by Detailed CFD Modelling of the Effect of Renewable Fuels on the Flame Structure under Compression Ignition Engine Conditions [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/207008

[ES] Las emisiones de CO2 en el sector transporte se han incrementado considerablemente durante los últimos años debido al desarrollo económico mundial. El crecimiento de las flotas de transporte, junto con otros factores, ha contribuido al desequilibrio del ciclo de carbono del planeta. Es por ello que el CO2 se considera un gas de efecto invernadero de origen antropogénico que debe ser reducido para evitar el calentamiento global. Las estrategias para reducir el CO2 en el sector transporte están enfocadas a la electrificación y al uso de combustibles neutros o de bajo impacto al ambiente. Sin embargo, una efectiva implementación de esta última requiere un profundo entendimiento de la combustión con tales combustibles. En la presente tesis doctoral, se ha caracterizado experimentalmente la combustión de diferentes tipos de combustibles, entre ellos, algunos de bajo impacto en emisiones de CO2 como lo son el Aceite Vegetal Hidrotratado (HVO) y dos éteres de oximetileno (OME1 y OMEx).Además, por su potencial en la reducción de contaminantes se han evaluado mezclas de diésel y gasolina y de HVO y Gas Licuado de Petróleo (LPG), lo que requirió adecuar el sistema de inyección para evitar la evaporación a lo largo de la línea. Todos estos combustibles y mezclas han sido inyectados con una tobera mono-orificio y han sido evaluados mediante técnicas de visualización a alta velocidad bajo diferentes condiciones termodinámicas típicas de un motor de encendido por compresión operando en condiciones de combustión a baja temperatura, en instalaciones con accesos ópticos. Se ha analizado el efecto de las propiedades físico químicas de estos combustibles y mezclas sobre los parámetros característicos de un chorro como lo son la longitud líquida y la penetración de vapor. La combustión ha sido evaluada mediante la caracterización del tiempo de retraso, de la liberación de calor y la longitud del despegue de la llama, que viene condicionada por el proceso de mezcla. Igualmente, el estudio de la formación de hollín en función de las propiedades del combustible y de las características del proceso de mezcla, representa un aporte importante de esta tesis. En adición a los beneficios en reducción de CO2 que brindan los combustibles y mezclas utilizados en este estudio, estos también redujeron la formación de hollín en la cámara de combustión, destacándose entre ellos los combustibles oxigenados, especialmente el OMEx que además de no formar hollín, fue el de mayor reactividad en todas las condiciones de operación evaluadas. / [CA] Les emissions de CO2 en el sector transport s'han incrementat considerablement durant els últims anys a causa del desenvolupament econòmic mundial. El creixement de les flotes de transport, juntament amb altres factors, ha contribuït al desequilibri del cicle de carboni del planeta. És per això, que el CO2 es considera un gas d'efecte hivernacle d'origen antropogènic que ha de ser reduït per evitar l'escalfament global. Les estratègies per reduir el CO2 dins el sector transport, estan enfocades a l'electrificació i a l'ús de combustibles neutres o de baix impacte ambiental. No obstant això, una efectiva implementació d'aquesta última, requereix un profund coneixement del procés de combustió d'aquests combustibles. En la present tesi doctoral, s'ha caracteritzat experimentalment la combustió de diferents tipus de combustibles, entre ells, alguns de baix impacte en emissions de CO2 com són l'Oli Vegetal Hidrotratat (HVO) i dos èters de oximetileno (OME1 i OMEx) .A més , degut al seu alt potencial en la reducció de contaminants, s'han avaluat mescles de dièsel i gasolina, i de HVO i Gas Liquat de Petroli (LPG), el que va requerir adequar el sistema d'injecció per evitar l'evaporació al llarg de la línia. Tots aquests combustibles i mescles han estat injectats amb una tovera mono-orifici i han estat avaluats mitjançant tècniques de visualització a alta velocitat a través dels accessos òptics del que disposa la instal·lació. Les diferents condicions termodinàmiques utilitzades son típiques d'un motor d'encesa per compressió operant en condicions de combustió a baixa temperatura. S'ha analitzat l'efecte de les propietats fisicoquímiques d'aquests combustibles i de les mescles sobre els paràmetres característics d'un raig com són la longitud líquida i la penetració de vapor. La combustió ha estat avaluada mitjançant la caracterització del temps de retard, de l'alliberació de calor i de la longitud de l'enlairament de la flama que ve condicionada pel procés de mescla. A més, l'estudi de la formació de sutge en funció de les propietats del combustible i de les característiques del procés de mescla, representa una aportació important d'aquesta tesi evidenciant que a més dels beneficis en reducció de CO2 que brinden tots aquests combustibles i mescles, també varen reduir la formació de sutge a la cambra de combustió, destacant-se entre ells els combustibles oxigenats, especialment el OMEx, que a més de no formar sutge, va ser el de major reactivitat en totes les condicions d'operació avaluades. / [EN] CO2 emissions in the transport sector have increased considerably in recent years due to global economic development. The growth of transport fleets, along with other factors, has contributed to the imbalance of the planet's carbon cycle. For that, CO2 is considered a greenhouse gas from anthropogenic origin that must be reduced to avoid global warming. Strategies to reduce CO2 in the transport sector are focused on electrification and the use of neutral fuels or those with a low impact on the environment. However, an effective implementation of the latter requires a deep understanding of the combustion with those fuels. In this doctoral thesis, the combustion of different types of fuels has been experimentally characterized, including some with low impact on CO2 emissions such as Hydrotreated Vegetable Oil (HVO) and two oxymethylene ethers (OME1 and OMEx). Furthermore, due to their potential in reducing pollutants, blends of diesel and gasoline and HVO and Liquefied Petroleum Gas (LPG) have also been evaluated, which required adapting the injection system to avoid evaporation along the injection line. All these fuels and blends have been injected with a single-hole nozzle and they have been evaluated using high speed visualization techniques under different thermodynamic conditions typical of a compression ignition engine operating under low-temperature combustion conditions in installations with optical accesses. The effect of the physical-chemical properties of these fuels and blends on the characteristic parameters of a jet, such as the liquid length and the vapor penetration, has been analyzed. Combustion has been evaluated by characterizing the ignition delay, the heat release and the flame Lift-off length that is conditioned by the mixing process. Furthermore, the study of soot formation based on the fuel properties and the characteristics of the mixing process represents an important contribution of this thesis, showing that in addition to the benefits in CO2 reduction provided by the different fuels and blends used in this study, these fuels also reduced the soot formation in the combustion chamber, highlighting among them the oxygenated fuels, especially OMEx which, in addition to not forming soot, was the most reactive in all conditions of operation evaluated. / This research has been partly funded by the Government of Spain and FEDER under TRANCO project (TRA2017-87694-R), by the European Union’s Horizon 2020 Programme through the ENERXICO project, grant agreement n° 828947, and from the Mexican Department of Energy, CONACYT-SENER Hidrocarburos grant agreement n° B-S-69926 and by Universitat Politècnica de València through the Programa de Ayudas de Investigación y Desarrollo (PAID-01-18 and PAID-06-18). / García Carrero, AA. (2021). Experimental Study of the Fuel Effect on Diffusion Combustion and Soot Formation under Diesel Engine-Like Conditions [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/179997