Sensitivity analysis was performed on a soot model using a partially stirred reactor (PaSR) in order to determine the effects of mixing model parameters on soot scalar values. The sensitivities of the mixture fraction zeta and progress variable C to the mixing model constant C_phi were calculated; these values were used to compute the sensitivity of water mass fraction Y_H2O to C_phi and several soot quantities to soot moments. Results were validated by evaluating the mean mixture fraction sensitivity and a long simulation time case. From the baseline case, it was noted that soot moment sensitivities tended to peak on the rich side of the stoichiometric mixture fraction zeta_st. Timestep, number of notional particles, mixing timescale tau_mix, and residence time tau_res were varied independently. Choices for timestep and notional particle count were shown to be sufficient to capture relevant scalar profiles, and did not greatly affect sensitivity calculations. Altering tau_mix or tau_res was shown to affect sensitivity to mixing, and it was concluded that the soot model is more heavily influenced by the chemistry than mixing. / text

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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

[pt] O presente trabalho tem como finalidade avaliar os diferentes modelos de mistura para o cálculo da combustão de reagentes pré- misturados utilizando a abordagem de Reator Parcialmente Misturado (PaSR). Os modelos de mistura considerados neste trabalho foram os modelos IEM estendido, Langevin e Langevin estendido. Investiga-se aqui o grau de mistura previsto por tais modelos e sua influência sobre as propriedades termoquímicas em um processo de combustão. A primeira parte deste trabalho consiste na apresentação e avaliação destes modelos de mistura, considerando-se um campo escalar inerte em presença de um campo turbulento homogêneo e isotrópico. Uma vez que estes modelos de mistura envolvem formulações do tipo estocástico, sua implementação foi realizada utilizando o método de Monte Carlo, mediante a utilização de esquemas numéricos adequados à resolução de equações diferenciais estocásticas. Assim, estuda-se a evolução da Função Densidade de Probabilidade (PDF) e das principais propriedades do campo escalar para cada modelo implementado. Os resultados obtidos também são comparados com simulação numérica direta e com resultados analáticos disponsáveis. Um ótimo acordo em termos qualitativos e quantitativos é obtido. A segunda parte deste trabalho utiliza estes modelos para o estudo numérico de um PaSR no qual são modelados os processos difusivos e reativos presentes durante a combustão. O PaSR é usado para avaliar a influência dos modelos de mistura nas propriedades termoquímicas da mistura em uma situação de combustão de tipo pré-misturada, que é modelada utilizando-se uma variável de progresso de uma reação. Os resultados obtidos com os diferentes modelos de mistura são comparados para diferentes regimes de funcionamento do PaSR, mostrando que, em situações de mistura rápida e reação intensa, os diferentes modelos apresentam resultados similares. Porém, nos casos de mistura lenta e reação moderada, discrepancias importantes são observadas entre os resultados dos modelos; as quais atingem até 65% para o valor médio da variável de progresso da reação. / [en] The present work evaluates several mixing models for the prediction of premixed combustion in a Partially Stirred Reactor (PaSR). The models considered in this work were the extended IEM, Langevin and extended Langevin models. The degree of mixing and its influence on the termochemical properties in a combustion process are investigated here. The first part of this work consists on the presentation and the assesment of these mixing models in which a single scalar field was considered in presence of a homogeneous and isotropic turbulent field. Since these mixing models involve stochastic terms, their implementation is performed by the Monte Carlo method using numerical schemes which solve the corresponding Stochastic Differential Equations (SDE). The evolution of the Probability Density Function (PDF) and the main properties for a single scalar field are studied for each mixing model. The numerical results are compared with Direct Numerical Simulation and available analytical results. Excellent qualitative and quantitative agreements are obtained. In the second part of this work, mixing models are used for numerical simulation of a PaSR where the diffusive and reactive processes occur. The PaSR is used to assess the mixing model influence on the termochemical properties of the mixture in a premixed combustion process, which is modeled using a reaction progress variable. The results obtained with the different mixing models are compared in several operating regimes of the PaSR, showing that when mixing is fast and reaction is intense, the different models lead to similar results. However, when mixing is slow and reaction is weak, important discrepancies are observed between the model results, which reach 65%, as far as the averaged reaction progress variable is concerned