Geometrical Improvement of Fuel Nozzle Locations in the SGT-750 Gas Turbine : An investigation into fuel-air mixing in main channels 1 & 2

Abstract: This study was conducted on SGT-750, which utilizes can type fourth generation DLE burner, developed at Siemens Energy AB, in Finspång, Sweden. The aim of this study is to obtain a flat fuel profile at the outlet of main channel 1 and main channel 2, demonstrating a proper mixing of fuel and air in the main channels leading to reducing local high-temperature points, and thus, reducing thermal NOx emissions. Furthermore, it is of prime importance to minimize fuel concentrations near the walls of the main channels to decrease laminar flame speed, such that if highly reactive fuels are used, the risk of flashbacks into the burner is eliminated. STAR-CCM+ is the software used for CFD simulations, and Design Manager is used for sweeping the location of fuel nozzles tangentially. Firstly, three grids are generated for the grid independence study with four, nine, and sixteen million cells. The parameters of interest for this evaluation are the mass flow rate and standard deviation of equivalence ratio, as well as velocity and fuel profiles at the outlet of main channels 1 and 2. It is found that the grid with nine million cells has an acceptable balance between the computational costs and the accuracy required. Moreover, the grid independence study revealed that as the grid becomes finer, tangential average of equivalence ratio is lower at the low radius and higher at the high radius. Thereafter, with the proper choice of the grid, three turbulence models namely, k-ε Realizable, k-ε Lag EB, and k-ω SST have been used to find the most stable RANS turbulence model to minimize the risk of non-convergence issues during design iterations in the improvement phase. It is depicted that the k-ε Realizable is the most stable RANS model among the models tested for this case, and this model is used for further improvements on the location of fuel nozzles. Ultimately three design improvements have been presented for both main channels based on dividing the outlet plane into four parts and minimizing the difference between the tangential average of equivalence ratio for the two middle portions and minimizing the tangential average of equivalence ratio for the near wall portions. It is demonstrated that design 3 is the best choice for main channel 1 due to lower fuel concentrations near the wall and flat fuel profile in the middle of the channel. However, for main channel 2, it was noticed that the Baseline Design is the best choice when non-highly reactive fuels are used, while Design 3 is most suitable for highly reactive fuels.