Soot prediction in realistic systems is one of the most challenging problems in theoretical and applied combustion. Soot formation as a chemical process is very complicated and not fully understood up to the moment. The major difficulty stems from the chemical complexity of the soot formation processes as well as its strong coupling with the other thermochemical and fluid processes that occur simultaneously. Soot is a major byproduct of incomplete combustion, having a strong impact on the environment, as well as the combustion efficiency. Therefore, it needs to be predicted in realistic configurations in an accurate and yet computationally efficient way. In the current study, a new soot formation subgrid model is developed and reported here. The new model is designed to be used within the context of the Large Eddy Simulation (LES) framework, combined with Linear Eddy Mixing (LEM) as a subgrid combustion model. The final model can be applied equally to premixed and non-premixed flames over any required geometry and flow conditions in the free, the transition, and the continuum regimes. The soot dynamics is predicted using a Method of Moments approach with Lagrangian Interpolative Closure (MOMIC) for the fractional moments. Since, no prior knowledge of the particles distribution is required, the model is generally applicable. The effect of radiation is introduced as an optically thin model. As a validation the model is first applied to a non-premixed non-sooting flame, then a set of canonically premixed flames. Finally, the model is validated against a non-premixed jet sooting flame. Good results are predicted with reasonable accuracy.

Two new models to calculate the species instantaneous and filtered reaction rates for multi-step, multi-species chemical kinetics mechanisms are developed based on the artificial neural networks (ANN) approach. The proposed methodologies depend on training the ANNs off-line on a thermo-chemical database representative of the actual composition and turbulence level of interest. The thermo-chemical database is constructed by stand-alone linear eddy mixing (LEM) model simulations under both premixed and non-premixed conditions, where the unsteady interaction of turbulence with chemical kinetics is included as a part of the training database. In this approach, the information regarding the actual geometry of interest is not needed within the LEM computations. The developed models are validated extensively on the large eddy simulations (LES) of (i) premixed laminar-flame-vortex-turbulence interaction, (ii) temporally mixing non-premixed flame with extinction-reignition characteristics, and (iii) stagnation point reverse flow combustor, which utilizes exhaust gas re-circulation technique. Results in general are satisfactory, and it is shown that the ANN provides considerable amount of memory saving and speed-up with reasonable and reliable accuracy. The speed-up is strongly affected by the stiffness of the reduced mechanism used for the computations, whereas the memory saving is considerable regardless.

A new combustor, referred to as Stagnation Point Reverse Flow (SPRF) combustor has been developed at Georgia Tech to meet increasingly stringent emission regulations. The combustor incorporates a novel design to meet the conflicting requirements of low pollution and high efficiency in both premixed and non-premixed modes. The objective of this thesis is to perform Large Eddy Simulations (LES) on this lab-scale combustor and explain the underlying physics. To achieve this, numerical simulations are performed in both the premixed and non-premixed combustion modes. The velocity field, species field, entrainment characteristics, flame structure, emissions and mixing characteristics are then analyzed. Simulations have been carried out first for a non-reactive case and the flow features in the combustor are analyzed. Next, the simulations have been extended for the premixed reactive case by employing different sub-grid scale combustion chemistry closures - Eddy Break Up (EBU), Artificially Thickened Flame (TF) and Linear Eddy Mixing (LEM) models. Only LEMLES which is an advanced scalar approach is able to accurately predict both the velocity and species field in the combustor. The results from LEM with LES (LEMLES) using a reduced chemical mechanism have been analyzed in the premixed mode. The results showed that mass entrainment occurs along the shear layer in the combustor. The entrained mass carried products into the reactant stream and provided preheating. The product entrainment enhances the reaction rates and stabilizes the flame even at very lean conditions. These products are shown to enter into the flame through local extinction zones present on the flame surface. The flame structure is further analyzed and the combustion mode is found to be primarily in thin reaction zones. The emissions in the combustor are studied using simple global mechanisms for NOx. Computations show extremely low NOx values comparable to the measured emissions. These low emissions are shown to be primarily due to the low temperatures in the combustor. LEMLES computations are also performed with detailed chemistry to capture more accurately the flame structure. The flame in the detailed chemistry case is more sensitive to strain effects and show more extinction zones very near to the injector. LEMLES approach is also used to resolve the combustion mode in the non-premixed case. The studies indicate that mixing of fuel and air close to the injector controls the combustion process. The predictions in the near field are shown to be very sensitive to the inflow conditions. Analysis shows that fuel and air mixing occurs to lean proportions in the combustor before any burning takes place. The flame structure in the non-premixed mode is very similar to the premixed mode. Along with fuel-air mixing, the products also mix with the reactants and provide the preheating effects to stabilize the flame in the downstream region of the combustor.