Progress on Combined Optic-Acoustic Monitoring of Combustion in a Gas Turbine

The need for better combustion monitoring in gas turbines has become more acute with the latest technical requirements, standards, and policies in terms of efficiency, environment, safety, operation flexibility and operation costs. Combustion Bay One and FH JOANNEUM initiated the project “emootion” (Engine health MOnitOring and refined combusTION control based on optical diagnostic techniques embedded in the combustor) in 2015, which is an experimental research program about the feasibility and first assessments of placing optical systems in the gas turbine's hot core, the combustion chamber.

The motivation of the project is twofold. On the one side, one wants to exploit the radiative feature of the flame and transform it into a piece of reliable information about the combustion status. On the other side, this information can be useful in terms of data interpretation or data reconciliation with other signals coming from conventional monitoring techniques such as temperature probes, fast pressure probes or accelerometers.

The focus is put on the detection of the flame, on the monitoring of the ignition process, on the quality assessment of combustion based on its spectral contents (including soot formation) and on the detection of possible combustion instabilities.

Paper GT2017-63626 showed promising results obtained using photodiodes, that offer an adequate trade-off between sensitivity and signal quality including narrow-band and time response. The present contribution reports on the progress done in terms of measurement chain, current high-temperature combined sensor concept and signal analysis. The concept Rayleigh Criterion Probe (RCP) has been extended with a probe redundancy and separate spectral bands of interest, such as the split in red, green and blue light components. Their further analysis allows mapping the different types of operation.

Regarding the probe packaging aspect, best practice shows that the optical sensor should be placed as near as possible to the optical interface and operate reliably despite the combustion heat. The minimum light collection intensity needed to keep an acceptable signal-to-noise ratio has so far been a limiting factor regarding the use of optical fibres.

The technology was tested on the new pressurised combustor developed by the authors in the frame of this project. Its specifications are presented in the paper. A probe assembly is proposed and tested, as well as a combination of redundant probes. The papers finishes with the proposal on a threefold RCPs monitoring strategy with a portable measurement chain.