In the recent past, LES methodology has emerged as a viable tool for modeling turbulent combustion. LES computes the large scale mixing process accurately, thereby providing a better starting point for small-scale models that describe the combustion process. Significant effort has been made over past decades to improve accuracy and applicability of the LES approach to a wide range of flows, though the current conventions often lack consistency to the problems at hand. To this end, the two main objectives of this dissertation are to develop a dynamic transport equation-based combustion model for large- eddy simulation (LES) of turbulent spray combustion and to investigate grid- independent LES modeling for scalar mixing. Long-standing combustion modeling approaches have shown to be suc- cessful for a wide range of gas-phase flames, however, the assumptions required to derive these formulations are invalidated in the presence of liquid fuels and non-negligible evaporation rates. In the first part of this work, a novel ap- proach is developed to account for these evaporation effects and the resulting multi-regime combustion process. First, the mathematical formulation is de- rived and the numerical implementation in a low-Mach number computational solver is verified against one-dimensional and lab scale, both non-reacting and reacting spray-laden flows. In order to clarify the modeling requirements in LES for spray combustion applications, results from a suite of fully-resolved direct numerical simulations (DNS) of a spray laden planar jet flame are fil- tered at a range of length scales. LES results are then validated against two sets of experimental jet flames, one having a pilot and allowing for reduced chemistry modeling and the second requiring the use of detail chemistry with in situ tabulation to reduce the computational cost of the direct integration of a chemical mechanism. The conventional LES governing equations are derived from a low-pass filtering of the Navier-Stokes equations. In practice, the filter used to derive the LES governing equations is not formally defined and instead, it is assumed that the discretization of LES equations will implicitly act as a low-pass filter. The second part of this study investigates an alternative derivation of the LES governing equations that requires the formal definition of the filtering operator, known as explicitly filtered LES. It has been shown that decoupling the filter- ing operation from the underlying grid allows for the isolation of subfilter-scale modeling errors from numerical discretization errors. Specific to combustion modeling are the aggregate errors associated with modeling sub-filter distribu- tions of scalars that are transported by numerical impacted turbulent fields. Quantities of interest to commonly-used combustion models, including sub- filter scalar variance and filtered scalar dissipation rate, are investigated for both homogeneous and shear-driven turbulent mixing. / text

La simulation de scenarios d’incendie nécessite de modéliser de nombreux processus complexe, particulièrement la combustion gazeuse d’hydrocarbure incluant la production de suie et les transferts radiatifs dans un écoulement turbulent. La nature turbulente de l’écoulement fait apparaitre des interactions qui doivent être prises en compte entre ces processus. L’objectif de cette thèse est d’implémenter une méthode de transport de la fonction de densité de probabilité afin de modéliser ces interactions de manière précise. En conjonction avec un modèle de flammelettes, le modèle de Lindstedt et un modèle à large-bande k-corrélé, l’équation de transport de la PDF jointe de composition est résolue avec la méthode des Champs Eulérien Stochastiques. Le modèle est validé en simulant 12 flammes turbulentes recouvrant une large gamme de nombre de Reynolds et de propension à former de la suie par les combustibles. Dans un second temps, les effets des interactions rayonnement-turbulence (TRI) sur l’émission de la suie sont étudiés en détails, montrant que la TRI tend à augmenter l’émission radiative de la suie à cause des fluctuations de température, mais que cette augmentation est plus faible pour des nombres de Reynolds élevés ou des quantités de suie plus élevées. Ceci est dû à la corrélation négative entre le coefficient d’absorption des suies et la fonction de Planck. Finalement, l’influence de la corrélation entre la fraction de mélange et le paramètre de non-adiabaticité est étudiée sur une flamme d’éthylène, montrant qu’elle a peu d’effet sur la structure moyenne de flamme mais tend à limiter les fluctuations de température et les pertes radiatives. / The simulation of fire scenarios requires the numerical modeling of various complex process, particularly the gaseous combustion of hydrocarbons including soot production and radiative transfers in a turbulent. The turbulent nature of the flow induces interactions between these processes that need to be taken accurately into account. The purpose of this thesis is to implement a transported Probability Density function method to model these interactions precisely. In conjunction with the flamelet model, the Lindstedt model, and a wide-band correlated-k model, the composition joint-PDF transport equation is solved using the Stochastic Eulerian Fields method. The model is validated by simulating 12 turbulent jet flames covering a large range of Reynolds numbers and fuel sooting propensity. Model prediction are found to be in reasonable agreement with experimental data. Second, the effects of turbulence-radiation interactions (TRI) on soot emission are studied in details, showing that TRI tends to increase soot radiative emission due to temperature fluctuations, but that this increase is smaller for higher Reynolds numbers and higher soot loads. This is due to the negative correlation between soot absorption coefficient and the Planck function. Finally, the effects of taking into account the correlation between mixture fraction and enthalpy defect on flame structure and radiative characteristics are also studied on an ethylene flame, showing that it has weak effect on the mean flame structure but tends to inhibit both temperature fluctuations and radiative loss.

<p dir="ltr">Addressing the pressing need for reduced carbon emissions, Turbulent Jet Ignition (TJI) emerges as a promising technology for ultra-lean combustion, offering enhanced thermal efficiencies and minimized cyclic variability in spark-ignited engines. To facilitate rapid testing and integration of this technology, a robust computational modeling framework is crucial. This study delves into the predictive capabilities of computational models for main-chamber ignition and premixed flame propagation using a single-cycle TJI rig measured by Biswas et al. (Applied Thermal Engineering, volume 106, 2016). Employing an open-source compressible flow simulation solver with Large Eddy Simulation (LES) for turbulence modeling, the investigation integrates the conventional Laminar Finite Rate Chemistry (LFRC) model alongside the transported Probability Density Method (PDF) for turbulence-chemistry interaction. A fully-consistent Eulerian Monte-Carlo Fields (EMCF) method is utilized to approximate the transported PDF, while Interaction by Exchange with Mean is employed to close micro-mixing terms in stochastic differential equations. A reduced chemical reaction mechanism with 21 species and 84 reactions (DRM-19) is used for solving chemical kinetics, and a double Gaussian energy deposition model is used to approximate the spark ignition in the pre-chamber. An unstructured O-grid mesh with 0.3 million cells in the pre-chamber and 1 million cells in the main chamber is employed. Results are divided into two phases: pre-chamber initialization and full TJI simulations. Validation of the predicted pre-chamber flame propagation and the lean ignition in the main-chamber is carried out by using available experimental data. Under quiescent conditions, both the LFRC and transported PDF methods largely underestimate the flame speed and subsequent pressure growth in the pre-chamber. A linear momentum forcing technique is applied to investigate the impact of initial turbulence in the pre-chamber, demonstrating a notable influence on flame propagation. Fine-tuning of the forcing coefficient reproduces the sudden pressure growth observed in the experiment. The experimentally validated pre-chamber simulation serves as the initial condition for the full TJI simulations. It is found that the LFRC model fails to predict lean-ignition in the main-chamber, resulting in a misfiring event. Incorporation of turbulence-chemistry interaction using the transported PDF method substantially improves the prediction of the ignition event in the main-chamber, achieving fair qualitative agreement and quantitative validation of combustion parameters within 10% of the reported experimental data. The rich simulation results consisting of a full set of statistical description of the thermo-chemical states enable us to gain deep insights into the ignition mechanisms in the main chamber, which is limited when done experimentally. A novel dual ignition phenomenon is revealed in the TJI rig for the first time. Initially, a primary ignition kernel is formed at a downstream location which eventually detaches from the main jet. As the jet momentum decreases, a secondary ignition event follows, this time at a more upstream location which eventually combines with the primary ignition kernel to form a single connected flame front. Investigation of these ignition sequences in chemical composition space reveal distinct differences between the two. The primary ignition event in the main-chamber is followed by a large concentration of active radicals from the pre-chamber jet, accelerating the chain-branching steps, characterizing what has been referred to as flame ignition. In contrast, the secondary ignition occurs in the absence of active radicals in the pre-chamber jet, hence characterized as jet ignition. Further analysis of the effect of pre-chamber jet characteristics on lean ignition in the main-chamber is conducted by setting up cases with different initial pressure ratios (p_r^o) between the two chambers, a non-dimensional parameter, ranging from 1.2 to 3.2. As the initial pressure ratio increases, jet momentum increases, with dual ignition observed in cases above p_r^o= 2.2. Case with p_r^o= 3.2 lead to misfiring. The effect of ignition sequence on global combustion characteristics of TJI is analyzed. Dual ignition events lead to non-monotonicity in combustion characteristics such as global reaction progress variable, flame penetration, and global heat release rate. In dual ignition events, although the rate of fuel consumption and global heat release rate is initially lower, the secondary ignition leads to a sudden increase in flame surface area, resulting in a sudden jump and promoting the overall performance of the TJI system.</p>