Concerning gas turbine combustion chambers mainly low- and medium-frequency oscillations have been investigated in the past. The feedback mechanism -- the interaction of acoustics and flame -- is understood quite well. Low- and medium-frequency instabilities are related to compact flames, i.e. the size of the flame is small compared to the wavelength. Only little is known about non-compact flames and high-frequency oscillations in gas turbine combustion chambers.
The goal of this project is to understand the feedback mechanism of high-frequency instabilities in gas turbine combustion chambers and identify the main influence parameters. The knowledge on influence parameters and feedback mechanism shall be used to derive a predictive model on high-frequency instabilities.
Two different test rigs are used to investigate
In the atmospheric tests fuel and air are perfectly premixed and preheated to investigate secondary flow phenomena. However, self-excited high-frequency oscillations can be observed under certain operating conditions.
The second test rig is operated at elevated pressure and includes a two-stage combustor. Exhaust gas of the first stage is used as oxidizer in the second stage. Auto-ignition, secondary flow and fluctuating air-fuel ratio have to be considered here. The high pressure rig can be excited by a siren.
Pressure measurements at the atmospheric test rig peak at a frequency of 3.3 kHz, see Fig. 3. The phase shift at different measuring positions clearly indicate a first transverse (T1) mode. The T1 mode is rotating with the frequency of the oscillation.
High-speed Mie-scattering images recorded at 20 kHz and particle image velocimetry (PIV) with a repetition rate of 10 kHz reveal the interaction of acoustics and mean flow at the shear layer between unburnt mixture and combustion products in the outer recirculation zone. Fig. 4 shows vortices in the unburnt mixture containing engulfed combustion pro\-ducts.
Measurements of OH* chemiluminescence indicate a positive interaction of heat release and pressure fluctuations.
The project is funded by Alstom Power Systems, Bayerisches Staatsministerium für Wissenschaft, Forschung und Kunst und Bayerisches Staatsministerium für Wirtschaft, Infrastruktur, Verkehr und Technologie whose support is gratefully acknowledged.