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 safety, environment, efficiency, operation flexibility, and operation costs. Combustion Bay One e.U. and FH JOANNEUM GmbH initiated in 2015 an experimental research program about the feasibility and first assessments of placing optical systems near the combustor. The project's acronym "emootion"stands for "Engine health MOnitOring and refined combusTION control based on optical diagnostic techniques embedded in the combustor". The motivation of the project is twofold. On one side, one wants to exploit the radiative feature of the flame and to 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 information coming from other sensors such as temperature probes, fast pressure probes, or accelerometers. The focus is put on several aspects of combustor operations: On detection of the flame, on monitoring of the ignition process, on a quality assessment of combustion based on its spectral contents (including soot formation), and on the detection of possible combustion instabilities. Promising results were obtained using photodiodes that offer an adequate trade-off between narrow-band sensitivity and signal time response. It is shown that it is convenient to combine a fast-pressure sensor with an optical sensor in a compact form; this combination has led to the so-called Rayleigh Criterion Probe (RCP). The split in red, green, and blue (RGB) light components and their further analysis allows for mapping the different types of operation. Regarding the probe packaging aspect, it is discussed that the level of light collection needed to keep an acceptable signal-to-noise ratio has been so far a restraint for the use of optical fibres. Solutions are proposed to bring the optical sensor as close as possible to the optical interface and to make it operational and reliable in prevailing heat. This contribution closes with a description of the pressure tests in a new combustion facility built for this purpose. A compact and portable combustion monitoring system including at least 3 RCPs can become an instrumentation standard within the next decade.