Peter Weigand

Periodic combustion instabilities in a swirl burner studied by phase-locked planar laser-induced fluorescence

Quasi-simultaneous phase-correlated measurements of different species in a turbulent swirl flame with a self-excited instability are presented for the first time. Phase-resolved OH* chemiluminescence and planar laser-induced fluorescence (PLIF) spectroscopy of OH, CH, and H 2 CO were used to follow the temporal evolution of flame structures in a pulsating swirl-stabilized model injector for gas-turbine applications. H 2 CO is a suitable indicator for chemical heat release in combination with OH; CH LIF and OH* emission were shown to be suitable indicators for the average shape and location of flame fronts, while OH LIF marked regions with high temperature, both in the flame front and the burned gas regions. The combustor was operated on methane fuel at atmospheric pressure. Lasers and detectors were locked to the phase angle of the self-excited pressure oscillation using a trigger signal derived from a microphone. Measurements at different phase angles were performed by variable delays with respect to the trigger pulse. Due to a high degree of turbulence, a large number of single-pulse measurements at each phase angle had to be performed in order to retrieve phase-sensitive effects from the dominating turbulent fluctuations. Noticeable changes of the flame structure with phase angle, particularly near the injector exit, are indicative of a strong coupling between the flame and a periodically fluctuating flow field.

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 CH 4 -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 ≈300 Hz. 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.

Laser-Based Investigations of Periodic Combustion Instabilities in a Gas Turbine Model Combustor

The driving mechanism of pulsations in gas turbine combustors depends on a complex interaction between flow field, chemistry, heat release, and acoustics. Experimental data on all these factors are therefore required to obtain insight into the coupling mechanisms during a pulsation period. In order to develop a comprehensive experimental database to support a phenomenological understanding and to provide validation data for numerical simulation, a standard burner for optical investigations was established that exhibits strong self-excited oscillations. The burner was a swirl-stabilized nonpremixed model combustor designed for gas turbine applications and operated using methane as fuel at atmospheric pressure. It was mounted in a combustion chamber, which provides almost unobstructed optical access. The periodic combustion instabilities were studied by a variety of phase-resolved laser-based diagnostic techniques, locked to the frequency of the dominant pressure oscillation. Measurement techniques used were LDV for velocity measurements, planar laser-induced fluorescence for imaging of CH and OH radicals, and laser Roman scattering for the determination of the major species concentrations, temperature, and mixture fraction. The phase-resolved measurements revealed significant variations of all measured quantities in the vicinity of the nozzle exit, which trailed off quickly with increasing distance. A strong correlation of the heat release rate and axial velocity at the nozzle was observed, while the mean mixture fraction as well as the temperature in the periphery of the flame is phase shifted with respect to axial velocity oscillations. A qualitative interpretation of the experimental observations is given, which will help to form a better understanding of the interaction between flow field, mixing, heat release, and temperature in pulsating reacting flows, particularly when accompanied by corresponding CFD simulations that are currently underway.

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

Non-intrusive 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 about 300 Hz. The applied measurement techniques were 3-D LDV for velocity measurements, OH* chemiluminescence yielding information about the heat release, and point-wise laser Raman scattering for the determination of joint 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 which in turn induced the pressure fluctuations.Copyright

Investigation of Combustion Oscillations in a Lean Gas Turbine Model Combustor

This contribution is a continuation of ASME-GT2006-90300. While still working at atmospheric pressure, the range of operating conditions was extended to more realistic reduced mass flows to reproduce the engine pressure loss and air preheat up to 700K. The thermoacoustic behaviour of the burner was mapped over that operating range. Two different types of oscillations were observed for flames anchored at the nozzle or lifted from it. Both exhibited a frequency dependence on the Strouhal number for constant reduced mass flows. For a selected operating point with the lifted flame at a preheat temperature of 600K and a reduced mass flow of 0.3kg K0.5/(s bar), the thermoacoustic behaviour of the burner was characterised by phase locked Particle Image Velocimetry as well as phase locked OH- and OH-T- LIF measurements and correlated to the acoustic pressure signal obtained by microphones. The combined data showed pulsating combustion being supported through periodic reignition of the main flame zone by a recirculating volume of hot, OH-rich gas, the cycle time being connected to the observed frequency. The characterization of the preheated operating point was completed with a heat balance investigation quantifying the non-adiabatic combustion conditions of the uncooled combustor.