Laser-Based Investigations of Thermoacoustic Instabilities in a Lean Premixed Gas Turbine Model Combustor

Nonintrusive laser-based and optical measurements were performed in a gas turbine model combustor with a lean premixed swirl-stabilized $CH4$-air flame at atmospheric pressure. The main objective was to gain spatially and temporally resolved experimental data to enable the validation of numerical CFD results of oscillating flames. The investigated flame was operated at 25 kW and ϕ=0.70, and exhibited self-excited oscillations of more than 135 dB at $≈300Hz$⁠. The applied measurement techniques were three-dimensional (3D) laser doppler velocimetry (LDV) for velocity measurements, $OH*$ chemiluminescence yielding information about the heat release and pointwise laser Raman scattering for the determination of joint probability density functions (PDFs) of the major species concentrations, temperature, and mixture fraction. Each of these techniques was applied with phase resolution with respect to the periodic fluctuation of the pressure in the combustion chamber that was measured with a microphone probe. The measurements finally revealed that the mixing of fuel and air in this technical premixing system was strongly affected by the pressure fluctuations leading to changes in equivalence ratio during an oscillation cycle that, in turn, induced the pressure fluctuations.