Jet engines have remained almost entirely mechanical machines for fail-safe reasons, despite the increasing sophistication of modern gas turbines. However, the trend goes toward more electronic devices for a better operation monitoring. This is the late approach called system of systems in aeronautics.

New regulations such as the ICAO/CAEP/10 nvPM Standard set limitations on soot emissions. CO reduction is also an issue. One possible strategy toward more efficient combustion and less pollutant emissions is an advanced management of the safety margins. This is combined with an obligation to reduce operation costs. Therefore new measurement techniques are required for precision combustion monitoring during operation. The specific data requested covers the success of ignition, the margin before the lean-blow-out limit, the effective burner load conditions and the stability of combustion. Many optical measurement techniques are available for advanced combustion diagnostics (Warnatz et al 2001). Their main features are precision and non-intrusivity. However, if these techniques are commonly used in a combustion laboratory or on a test-bench, no application had a breakthrough so far on a flying system. The implementation of optical devices in the aggressive environment of a combustor is challenging. Some critical details are for instance the need for a permanently transparent optical interface or the thermal protection of the sensitive parts.

In the scope of the project “emotion” subsidised by the FFG, a heat resistant probe combining optic and acoustic sensors was developed for this purpose. This probe will make advanced combustion monitoring possible. It will comply with the above mentioned rules or constraints. It could be mounted on the pressure casing with a view on the liner. It will monitor the presence or absence of a flame, it will report on the ignition success or failure, it will compare the observed flame power to the expected load, and detect the presence of a combustion instability.

In this paper, several sensors are considered. Three different circuits for optical light intensity measurement are assessed. A combined optical-acoustic sensor arrangement called the Rayleigh-Criterion probe is introduced. This most promising configuration is tested and validated on an atmospheric combustion test rig. The presented results support the further development of this probe, first for use on test benches where this technology can achieve maturity, then towards deployment first in power gas turbines and eventually in aeroengines.

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