Phase-resolved laser Raman scattering and laser Doppler velocimetry applied to periodic instabilities in a gas turbine model combustor

The understanding of periodic flame instabilities belongs to the major challenges in modem combustion research and technology and is of special importance for lean premixed gas turbine combustion. This paper presents experimental investigations in a gas turbine model combustor using laser diagnostic techniques. A partially premixed CH4/air flame operated at a thermal power of 10 kW at atmospheric pressure and an overall equivalence ratio of 0.75, which exhibited thermoacoustic oscillations at a frequency of 290 Hz, was investigated. Phase-locked laser Raman scattering was applied in order to determine the major species concentrations, temperature, and mixture fraction. In addition, laser Doppler velocimetry (LDV) was used separately for the measurement of the axial and radial velocity components. The measurements revealed pronounced phase-dependent variations of the velocity and the temperature, species, and mixture fraction distributions. The combined Raman and LDV results also enabled the determination of molecular species fluxes which showed that the fuel and air supply rates both varied during an oscillation cycle by +/- 33% but with a phase shift of 20-80 degrees between them. The correlations between temperature and mixture fraction revealed strong deviations from equilibrium composition and temperature, and their phase-dependent changes reflected the transport and mixing processes near the nozzle. The emphasis of the paper lies on the demonstration of the potential of phase-locked laser Raman scattering for the study of phenomena of periodic flame instabilities.