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Energie- und Umwelttechnik

Transverse Acoustic Modes and High-Frequency Instabilities in Gas Turbine Combustion Chambers

Supervisor: Subject:
    thermoacoustics, gas turbine, high-frequency instabilities, transverse modes
Editor: Cooperation/Funding:

Motivation

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.

Goals

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.

Approach

Two different test rigs are used to investigate

  • Auto-ignition kinetics with varying ignition time delay due to pressure fluctuations,
  • Secondary flow and fluctuating recirculation of exhaust gas and
  • Fluctuating air-fuel ratio normal to mean flow
as cause for high-frequency instabilities. Both rigs are equipped with multiple dynamic pressure sensors to identify mode shapes and to provide phase information for optical measures. Optical methods are applied to clarify the interaction of acoustic field, flame and flow.

Experimental Setup

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.

Figure 1: Atmospheric test rig.

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.

Figure 2: Second stage combustor in high pressure test rig.

Results

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.

Figure 3: Amplitude and phase of pressure signal.

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.

Figure 4: Velocity vectors measured by PIV.

Measurements of OH* chemiluminescence indicate a positive interaction of heat release and pressure fluctuations.

Acknowledgment

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.