Moderate or Intense Low-oxygen Dilution (MILD) combustion is a combustion technology that can simultaneously improve the energy efficiency and reduce the pollutant emissions of combustion devices. It is characterised by highly preheated reactants and a small temperature rise during combustion due to the large dilution of the reactant mixture with products of combustion. These conditions are generally achieved using exhaust gas recirculation. However, the physical understanding of MILD combustion remains limited which prevents its more widely spread use. In this thesis, Direct Numerical Simulation (DNS) is used to study turbulence, premixed flames and MILD combustion to obtain these additional physical insights. In a first stage, the scale-locality of the energy cascade is analysed by applying a multiscale analysis methodology, called the bandpass filter method, on DNS of homogeneous isotropic turbulence. Evidence supporting this scale-locality were obtained and the results were found to be similar for Reynolds numbers ranging from 37 to 1131. Using the same method in turbulent premixed flames, the scale-locality of the energy cascade was still observed despite the presence of intense reactions. In addition, it was found that eddies of scales larger than the laminar flame thickness were imparting the most strain on the flame. In a second part, a methodology was developed to conduct the DNS of MILD combustion with mixture fraction variations. This methodology included the effect of mixing of exhaust gases with fuel and oxidiser in unburnt, burnt and reacting states. In addition, a specific chemical mechanism that includes the chemistry of ${\rm OH^*}$ was developed. From these DNS, the role of radicals on the inception of MILD combustion was studied. In particular, due to the reactions initiated by these radicals, the initial temperature rise in MILD combustion was occurring concurrently with an increase in the scalar dissipation rate of mixture fraction which is contrasting to conventional combustion. The reaction zones in MILD combustion were also analysed and extremely convoluted reaction zones were observed with frequent interactions among them. These interactions yielded the appearance of volumetrically distributed reactions. Furthermore, the adequacy of some species to identify these reaction zones was assessed and ${\rm OH}$ showed a poor correlation with regions of heat release. On the other hand, ${\rm OH^*}$, ${\rm HCO}$ or ${\rm OH} \times {\rm CH_2O}$ were found to be well correlated. Through the study of the flame index, the existence of non-premixed and premixed modes of combustion were also highlighted. The premixed mode was observed to be dominant but the contribution of the non-premixed mode to the total heat release was non negligible. Because of the presence of radicals and high reactant temperatures, auto-igniting regions and propagating reaction zones are both observed locally. The balance between these phenomena was investigated and it was found that this was strongly influenced by the typical lengthscale of the mixture fraction field, with a smaller lengthscale favouring sequential autoignition. Finally, using the bandpass filtering method, the effect of heat release rate in MILD combustion on the energy cascade was studied and this showed that the energy cascade was not unduly affected.

La combustion sans flamme est un mode de combustion innovant dans le domaine des économies d'énergie et de la réduction des émissions polluantes, qui reste toutefois encore peu étudié. L'objectif de cette étude est de caractériser ces mécanismes par une étude expérimentale. La première étape de cette étude a été réalisée sur une installation d'essais semi-industrielle. Différents moyens de mesure dans la flamme ont été utilisés pour caractériser le mode d'accrochage de la flamme, la structure des zones de réactives, les recirculations des produits de combustion et leur impact sur la combustion et les émissions polluantes. Parallèlement, une installation d'essais de laboratoire à été mise en place. La caractéristique essentielle de cette installation est la possibilité de changer une des dimensions de la chambre tout en gardant identiques les conditions opératoires. Différents paramètres (puissance, température de l'air, température des parois, taux d'aération, confinement de la flamme) sont modifiés afin de caractériser leur impact sur le régime de combustion sans flamme. <br />Les résultats montrent que la flamme peut être divisée en deux zones. Alors que la réaction principale se situe où le jet d'air et de gaz naturel se rejoignent, une première zone réactive est aussi observée dans la zone de recirculation interne, assurant la stabilisation de la flamme. La recirculation des produits de combustion est mise en évidence et quantifiée à partir des résultats des mesures dans la flamme. Entraînées par l'écoulement principal, ces recirculations induisent une importante dilution dans les zones réactives, où la concentration en CO et la température restent à de faibles niveaux. Les oxydes d'azote sont principalement produits par voie thermique, mais restent très faibles. Par ailleurs, un mécanisme de réduction dans la flamme des oxyde d'azote présents dans les recirculations peut être observé. L'étude paramétrique montre que si la puissance n'a pas d'influence sur le régime de combustion, la combustion sans flamme est possible avec de l'air non préchauffé. Par ailleurs, une augmentation du taux d'aération réduit la dilution de la zone réactive par les inertes. Pour de fortes augmentations du taux d'aération, on peut observer l'apparition d'une troisième zone de réaction à l'extérieur des jets de gaz naturel ; on quitte alors le régime de combustion sans flamme

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22770号 / 工博第4769号 / 新制||工||1746(附属図書館) / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 黒瀬 良一, 教授 中部 主敬, 教授 岩井 裕 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Novel combustion technologies ensuring low emissions, high efficiency and fuel flexibility are essential to meet the future challenges associated to air pollution, climate change and energy source shortage, as well as to cope with the increasingly stricter environmental regulation. Among them, Moderate or Intense Low oxygen Dilution (MILD) combustion has recently drawn increasing attention. MILD combustion is achieved through the recirculation of flue gases within the reaction region, with the effect of diluting the reactant streams. As a result, the reactivity of the system is reduced, a more uniform reaction zone is obtained, thus leading to decreased NOx and soot emissions. As a consequence of the dilution and enhanced mixing, the ratio between the mixing and chemical time scale is strongly reduced in MILD combustion, indicating the existence of very strong interactions between chemistry and fluid dynamics. In such a context, the use of combustion models that can accurately account for turbulent mixing and detailed chemical kinetics becomes mandatory.Combustion models for conventional flames usually rely on the assumption of time-scale separation (i.e. flamelets and related models), which constrain the thermochemical space accessible in the numerical simulation. Whilst the use of transported PDF methods appears still computationally prohibitive, especially for practical combustion systems, there are a number of closures showing promise for the inclusion of detailed kinetic mechanisms with affordable computational cost. They include the Partially Stirred Reactor (PaSR) approach and the Eddy Dissipation Concept (EDC) model.In order to assess these models under non-conventional MILD combustion conditions, several prototype burners were selected. They include the Adelaide and Delft jet-in-hot coflow (JHC) burners, and the Cabra lifted flames in vitiated coflow. Both Reynolds Averaged Navier Stokes (RANS) and Large Eddy Simulations (LES) were carried out on these burners under various operating conditions and with different fuels. The results indicate the need to explicitly account for both the mixing and chemical time scales in the combustion model formulation. The generalised models developed currently show excellent predictive capabilities when compared with the available, high-fidelity experimental data, especially in their LES formulations. The advanced approaches for the evaluation of the mixing and chemical time scale were compared to several conventional estimation methods, showing their superior performances and wider range of applications. Moreover, the PaSR approach was compared with the steady Flamelet Progress Variable (FPV) model on predicting the lifted Cabra flame, proving that the unsteady behaviours associated to flame extinction and re-ignition should be appropriately considered for such kind of flame.Because of the distributed reaction area, the reacting structures in MILD combustion can be potentially resolved on a Large Eddy Simulation (LES) grid. To investigate that, a comparative study benchmarking the LES predictions for the JHC burner obtained with the PaSR closure and two implicit combustion models was carried out, with the implicit models having filtered source terms coming directly from the Arrhenius expression. Theresults showed that the implicit models are very similar with the conventional PaSR model on predicting the flame properties, for what concerns the mean and root-mean-square of the temperature and species mass fraction fields.To alleviate the cost associated to the use of large kinetic mechanisms, chemistry reduction and tabulation methods to dynamically reduce their size were tested and benchmarked, allowing to allocate the computational resources only where needed. Finally, advanced post-processing tools based on the theory of Computational Singular Perturbation (CSP) were employed to improve the current understanding of flame-turbulence interactions under MILD conditions, confirming the important role of both autoignition and self propagation in these flames. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

Autoignition characteristics of pre-vaporized iso-octane, primary reference fuels, gasolines, and dimethyl ether (DME) have been investigated experimentally in a coflow with elevated temperature of air. With the coflow air at relatively low initial temperatures below autoignition temperature Tauto, an external ignition source was required to stabilize the flame. Non-autoignited lifted flames had tribrachial edge structures and their liftoff heights correlated well with the jet velocity scaled by the stoichiometric laminar burning velocity, indicating the importance of the edge propagation speed on flame stabilization balanced with local flow velocity. At high initial temperatures over Tauto, the autoignited flames were stabilized without requiring an external ignition source. The autoignited lifted flames exhibited either tribrachial edge structures or Mild combustion behaviors depending on the level of fuel dilution. For the iso-octane and n-heptane fuels, two distinct transition behaviors were observed in the autoignition regime from a nozzle-attached flame to a lifted tribrachial-edge flame and then a sudden transition to lifted Mild combustion as the jet velocity increased at a certain fuel dilution level. The liftoff data of the autoignited flames with tribrachial edges were analyzed based on calculated ignition delay times for the pre-vaporized fuels. Analysis of the experimental data suggested that ignition delay time may be much less sensitive to initial temperature under atmospheric pressure conditions as compared with predictions. For the gasoline fuels for advanced combustion engines (FACEs), and primary reference fuels (PRFs), autoignited liftoff data were correlated with Research Octane Number and Cetane Number. For the DME fuel, planar laser-induced fluorescence (PLIF) of formaldehyde (CH2O) and CH* chemiluminescence were visualized qualitatively. In the autoignition regime for both tribrachial structure and mild combustion, formaldehyde were found mainly between the fuel nozzle and the lifted flame edge. On the other hand, they were formed just prior to the flame edge for the non-autoignited lifted flames. The effect of fuel pyrolysis and partial oxidation were found to be important in explaining autoignited liftoff heights, especially in the Mild combustion regime. Flame structures of autoignited flames were investigated numerically for syngas (CO/H2) and methane fuels. The simulations of syngas fuel accounting for the differential diffusion have been performed by adopting several kinetic mechanisms to test the models ability in predicting the flame behaviors observed previously. The results agreed well with the observed nozzle-attached flame characteristics in case of non-autoignited flames. For autoignited lifted flames in high temperature regime, a unique autoignition behavior can be predicted having HO2 and H2O2 radicals in a broad region between the nozzle and stabilized lifted flame edge. Autoignition characteristics of laminar nonpremixed methane jet flames in high- temperature coflow air were studied numerically. Several flame configurations were investigated by varying the initial temperature and fuel mole fraction. Characteristics of chemical kinetics structures for autoignited lifted flames were discussed based on the kinetic structures of homogeneous autoignition and flame propagation of premixed mixtures. Results showed that for autoignited lifted flame with tribrachial structure, a transition from autoignition to flame propagation modes occurs for reasonably stoichiometric mixtures. Characteristics of Mild combustion can be treated as an autoignited lean premixed lifted flame. Transition behavior from Mild combustion to a nozzle-attached flame was also investigated by increasing the fuel mole fraction.