Specially designed optical combustion chambers (see for example BOSS-Rig of DLR-AT), use already established (laser-)optical diagnostics. Here, large-area window inserts allow for direct optical accessibility.

Probe-based measurement techniques (endoscopes, fiber optics) promise in principle the acquisition of temperature in test stands without direct optical accessibility, i.e. without the use of large-area window inserts. Thus, a major advantage is the reduced influence on reaction and flow processes as well as the reduced cooling requirements of the optical accesses. All in all, it is therefore possible to transfer such measurement methods to test rigs that operate close to real process conditions (full-ring test rigs) and have only very limited optical accessibility and installation space availability.

In the design of gas turbines, the temperature at the combustion chamber outlet is the decisive variable and has a significant influence on the service life of the units, the expenditure for cooling and, ultimately, the emission of pollutants via the combustion progress. Therefore, it is necessary to measure the temperature at any time and at the locations relevant for system stability. Classical measurement methods using thermocouples usually fail in this respect. Even though they have so far been seen as almost without replacement in the real process, they are subject to very rapid aging due to the high temperatures and pressures and have to be replaced at regular intervals. In addition to the high wear costs, this also means a significant increase in measurement uncertainties. Under the process boundary conditions in a gas turbine, attempts to cool the temperature probes have achieved only limited success and, moreover, do not solve the problem that such probes additionally disturb the system.

In addition to the influence on component properties, the effect of thermoacoustic instabilities is becoming increasingly prominent in gas turbines. The fluctuation of temperatures in the frequency range of acoustic oscillations is responsible for most of the noise emitted by gas turbines. In order to reduce noise, the operating range of the units is therefore restricted to low-vibration regions. However, in order to optimize gas turbines for low-consumption operation, it is essential to include the operating points that have not been used so far in the characteristic diagram.

For this purpose, a better understanding of the relationship between the thermoacoustic vibrations, the combustion behaviour and the design boundary conditions is crucial. Therefore, a coupling of high temporal resolution measurements of the temperature fluctuations with CFD simulation is necessary in order to better represent the thermoacoustic phenomenon in corresponding combustion chamber models.

Methodology

Tunable Diode Laser Absorption Spectroscopy: TDLAS will be used to resolve the fast changes of temperature (up to several kHz) and to relate them to the test rig parameters depending on the operating point.

Monostatic approach: Based on a probe technology developed in an earlier project, the fiber optics, the imaging optics and the cooling unit are to be adapted to the boundary conditions of the thermoacoustic test rig and characterized in terms of robustness to the stresses caused by the high temperatures, the pressures, the mechanical vibrations (sound pressure) and the soot loading, and qualified for permanent use on a full-ring test rig.

Key Scientific Takeaway

• Qualify a minimally invasive measurement method based on laser absorption spectroscopy on a thermoacoustic gas turbine test rig and to couple its temporally high-resolution measurement results with CFD simulation to determine the temperature transfer functions (TTF).
• Focus in particular on reducing measurement uncertainties while minimizing disturbances to the process.

Funding and cooperation

Robuste Turbomaschinen für den flexiblen Einsatz (Robust Turbomachinery for Flexible Use – RoBoFlex) is a joint research program in the frame of AG Turbo. It was financially supported by the Federal Ministry for Economic Affairs and Climate Action (BMWK) under grant number 03EE5013Q, and Rolls-Royce Deutschland.