Large Eddy Simulation of Turbulent Combustion with Chemical Kinetics
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The present doctoral thesis studies and develops methodologies for turbulent combustion with the Large Eddy Simulation (LES). Three main objectives for present doctoral thesis were. First, development of LES methodology in curvilinear coordinates. LES formulation in curvilinear coordinates can be achieved in two ways, (1) conventional approach, where filtering is performed prior to the transformation, and (2) alternate approach, where filtering is performed after the transformation. In present work the conventional approach was preferred than the alternate approach. Furthermore, filtering in physical space introduces a commutation error between filtering and differentiation due to non uniform meshes. The commutation filter, which can commute upto any order of accuracy were discussed in generalized coordinates. The LES module in generalized coordinate was implemented in the in-house RANS code. The methodology was validated by performing the LES of pipe, 3D lid driven cavity, Backward facing step, axisymmetric dump combustor with and without swirl. Furthermore, the influence of numerical scheme, discretization, subgrid model, grid resolution were explored for LES. The second objective of the present doctoral research was development of the Eddy Dissipation Concept for turbulent combustion (EDC) for LES. EDC assumes that combustion take place in the fine structure and they are located in the isolated regions. In RANS, the fine structure regions are estimated based on the full cascading in the each numerical cell, however this was not applicable for the LES, where either partial cascading or no cascading take place in the each numerical cell. In present work fine structure regions were formulated considering the partial cascading. Furthermore, in LES the turbulent kinetic energy and dissipation are not computed explicitly, therefore the fine structure length and velocity scales, based on the eddy viscosity were proposed. The LES-EDC was validated by performing the LES of many diffusion flames i.e. Sandia Flame D, Flame H3, Flame DLR and Flame ETH. The last objective was to extend the applicability of the EDC for predicting the pollutant such as NOx with low computational effort. Estimation of minor species such as NOx requires modelling of detailed chemical kinetics with eddy dissipation concept model, which is a computationally expensive procedure specially in large industrial burners with LES. A method was presented, where EDC with fast chemistry approach was used for major species mass fractions. A mixture fraction was formulated from the major species mass fractions and a transport equation for the NOx was solved, where source term was estimated as a function of mixture fraction from flamelet tables.