CFD Modeling of Combustion in Flexi-Fuel Burners at Gas Turbine Conditions

Licentiate thesis, 2011

The main source of energy in the nearest future will come from fossil fuels. Increasing demands on environmentally friendly energy conversion implies that pollutants, such as CO and NOx, need to be reduced. This project aims to investigate recent developments in combustion modeling and turbulence modeling in the context of engineering type Computational Fluid Dynamics (CFD) analysis tools, applied to the main problem area of swirl-stabilized flexi-fuel flames. To perform a CFD simulation of reacting flows, kinetic information needs to be provided to the CFD code. Fully detailed kinetic mechanisms are expensive in terms of computer time when coupled with CFD and hence the use of global reaction mechanisms is preferable. Global mechanisms consist of a reduced number of reactions mostly one-way reactions which are governed by tuned Arrhenius rate expressions. The optimization at each equivalence ratio is here done by comparing results from Perfectly Stirred Reactor (PSR) calculations for both a detailed reference mechanism and the chosen multi-step global reaction mechanisms. Steady-state Reynolds Averaged Navier Stokes (RANS), hybrid Unsteady RANS/Large Eddy Simulation (URANS/LES) and LES turbulence models have been used in the CFD work. This thesis includes the development and derivation of new 3-step global reaction mechanism for methane-air flames, which has been implemented and evaluated in the CFD analysis for two different burner configurations. In papers I and II an atmospheric burner test rig at Lund University (LTH) has been modeled and the results have been compared to high quality experimental data (emission and Particle Image Velocimetry (PIV) data). There is good agreement between the CFD simulations and measurements of emissions, velocity field and flame visualization. The second configuration that has been modeled and compared with experimental data is the Sandia Flame D test case, which is reported in paper III. Keywords: CFD, Chemistry, Combustion, RANS, LES, SAS-SST, PSR, Global reaction mechanism