Application of Laser-based Diagnostics to a Prototype Gas Turbine Burner at Selected Pressures

Abstract: The matured laser-diagnostic techniques of planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) were applied to a prototype gas turbine burner operating on various fuels. The work was performed to provide verification of computational fluid dynamic (CFD) models of the combustion of atypical fuels in a gas turbine combustor. The burner was operated using methane and three synthesized fuels of interest- one with hydrogen as the principle component and two with a low heating value (15 MJ/m3). Experiments were performed at pressures from 1 to 9 bar, with the fuel/air mixture at both ambient (~ 300 K) and elevated temperature. The burner, which was supplied by Siemens Industrial Turbomachinery, is a down-scaled prototype of that used in the SGT-750 gas turbine. It is composed of three individual sectors that are arranged concentrically, a centermost pilot sector, intermediate sector and main sector. Each sector contributes a premixed fuel/air flow, while swirl elements in each sector promote flame stabilization and recirculation in the combustion region. There are dedicated fuel feeds allowing for localized setting of fuel/air mixture at each of the sectors. The central pilot sector of the burner was separable from the full burner assembly and was examined in detail. Information was generated regarding the use of syngas to fuel the burner. This information is intended to be used for the validation of CFD models of the experiments, including optimization of reduced chemical kinetic mechanisms for the specific fuels. Laminar flame speed measurements were made for several syngas fuel candidates from which the high-hydrogen syngas fuel was selected. Burner performance at the lean stability limit was examined using the fuels of interest. It was found that increasing the fuel/air ratio in the central pilot sector improved the lean limit onset of flame extinction up to the point that the central pilot extinguished. Optimization of the burner nitrogen oxides (NOx) emission by fuel partitioning among the three sectors was performed. The response in emission level with fuel/air ratio was not universal among the fuels tested. The largest portion of work in this thesis is the visualization of the burner combustion field by laser diagnostic methods. The flame shape was imaged by the PLIF of the OH radical distribution. PLIF imaging of the central pilot sector was recorded for atmospheric and elevated pressure for iterations of inlet air temperature, fuel type and equivalence ratio. When comparing the OH-LIF distribution for various fuels and pressures it was found that equivalence ratio had the greatest effect on the distribution of OH signal from the exit of the central pilot sector. Lean equivalence ratios showed a diffuse signal typical of the post combustion region. Near stoichiometric equivalence ratios yielded a distribution having a clearly defined inner edge indicating combustion occurring outside of the pilot sector. At rich equivalence ratios the OH signal was lifted away from the pilot burner exit. Comparison of OH-PLIF and chemiluminescence signal for methane combustion supported the characterization that the pilot sector efflux varied from post combustion to attached and then lifted flame in conjunction with the increase in equivalence ratio from lean to rich. OH-PLIF imaging was collected for staging of fuel to all three sectors of the burner at atmospheric pressure. The flow field in the combustion region produced by the full burner was visualized using PIV for each of fuels of interest, illustrating the recirculation zone. Finally the OH-LIF distribution was imaged for the combustion region of the entire burner at elevated pressure during operation at a single equivalence ratio with various dilutions of natural gas. There was little discernible change in flame shape as the pressure was changed from 3, 4.5 and 6 bar and energy content was changed from 30, 40 and 45 MJ/m^3 Wobbe index.