3-D RANS Simulation of Turbulent Reacting Flow and Emissions in a Lean Premixed Reverse Flow Type Gas Turbine Combustor

DoctorThis thesis describes simulation of a small stationary gas turbine combustor of a reverse flow, low NOx type in which methane and air are partially mixed inside swirl vanes. Different turbulent combustion models are evaluated to provide insights into mixing, temperature distribution and emission in the combustor. Validation is performed for the models in STAR-CCM+ against the measurement data for a simple swirl flame (SM1, http://public.ca.sandia.gov/TNF/swirlflames.html). The realizable k-ε model with enhanced wall treatment is employed to model turbulent swirl flow, whereas Eddy Break-Up (EBU), Presumed Probability Density Function (PPDF), Laminar Flamelet Model (LFM) and Partially premixed Coherent Flame Model (PCFM) are tried for reacting flow in the combustor. Independent simulations are carried out for the main and the pilot nozzle to avoid flashback and to provide realistic inflow boundary conditions for the combustor. Geometrical details such as air swirlers, vane passages and liner holes are all taken into account. Tested combustion models show similar downstream distributions of mean flow and temperature, while EBU and PCFM show a lifted flame with stronger effects of swirl due to limited increase of axial momentum by expansion. PCFM is applied in whole combustor system including solid/fluid interface with an imposed downstream Flame Area Density (FAD) to avoid flashback and incomplete combustion. Physical models are validated against the measurements of outlet temperature, product gas composition and NO emission at the low operating pressure. Parametric study is performed to investigate the effect of load and Pilot/Total (P/T) fuel ratio on mixing characteristics and the resulting temperature distribution and pollutant emissions. As the P/T fuel ratio increases, the high temperature region over 1800 K enhances reaction of the mixture from the main nozzle in the primary mixing zone. For low P/T ratios the pilot stream dilutes the mixture, on the contrary, to suppress reaction with an increasing height of the lifted flame. The NO is associated with the unmixedness as well as the mean temperature level and tends to increase with increasing load and P/T ratio. The high operating pressure does not affect overall velocity and temperature distribution, while it tends to increase NO and liner temperature under the given boundary conditions.Partially premixed combustion model was implemented combining modified Weller and steady laminar flamelet models in OpenFOAM to consider non-equilibrium and finite rate chemistry. Validation is done against measurement data for mean flow and scalar fields of swirling methane/air flames (SMA1, http://public.ca.sandia.gov/TNF/swirlflames.html). NOx is post-processed considering turbulence-chemistry interaction and these approaches are applied to a lean premixed stationary gas turbine combustor of a reverse flow, semi silo type for power generation. Physical models are also validated against the measurements of outlet temperature, product gas composition and NO emission at the low operating pressure. In this study NOx formation including Zeldovich, nitrous oxide, prompt and NNH were investigated for gas turbine conditions