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Turbine blades in gas turbine power generators are normally coated with a ceramic barrier coating to protect them from the intense temperatures they experience, up to 1500ºC. The only feasible way of monitoring the surface temperature of rotating blades is by radiation pyrometry. This is where the intensity of radiation emitted from the blades is measured and converted to temperature. Unfortunately, conventional pyrometry at visible and near-infrared wavelengths does not work due to the low emissivity of the translucent ceramic coatings at these wavelengths, thus the radiated signal is small, it is wavelength dependent, and also changes during the lifetime of the blades.

Work at ZAE Bayern, in Germany, found that the emissivity of these coating approaches unity around 10um wavelength and so a pyrometer was designed to operate at this wavelength. Careful analysis of the absorption spectrum of gaseous water in the intermediate space between the blade surface and the pyrometer led to the specific wavelength and spectral width to use.

The pyrometer probe consists of a optical collection and relay system using ZnSe lenses and an infrared transmitting polycrystalline fibre. The probe was purged with nitrogen gas to keep the optics clean and was also cooled with liquid water. Even so, the probe could only be held in the intensely hot gas flow for a few seconds before being retracted. A sketch of the probe is shown below:

Radiation is then passed through a few metres of fibre to the photodetector system. This consists of optics, a filter, a shutter, and an MCT detector (Mercury Cadmium Telluride). The reason for the shutter was to reduce the effects of background radiation. Even at room temperature, everything radiates in the 10um region and the effect of this is that the background signal from the optics and the mechanics is far larger than the signal from the turbine blade, so by closing the shutter and taking a background, dark, measurement immediately before and after measurement of the blade, the background may be subtracted. For example, the background signal was several Volts whereas the blade signal was around 100mV.

The photodetector system is shown below:

Using high-speed data acquisition, the thermal profile of each blade could be measured real-time. The plot below shows the passage of two consecutive blades measured on a gas turbine at the Siemens Berlin Test Facility. The thermal profile of the blades is clearly seen, enabling the effectiveness of cooling mechanisms, and degradation of the barrier coating, to be assessed.