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Measuring gas turbine engine component temperatures using thermographic phosphors

The performance of turbine engines—which produce continuous power or thrust using a rotor fitted with blades that revolves via the fast-moving flow of combustion gases—is directly related to the maximum temperature achievable at the turbine inlet. This temperature is typically limited by the material of the turbine blades or vanes. As the push for higher thrust and greater efficiency drives engine temperatures closer to the material limits, it is becoming increasingly critical to accurately measure these temperatures. Engine developers need to know how blade temperatures respond to design changes, and maintenance personnel need to know to what temperatures engine components are being exposed. Advanced engines incorporate ceramic thermal barrier coatings (TBCs) on engine components to enable operation at high temperatures. Existing temperature sensors include thermocouples, which measure the electric potential between dissimilar metals, and pyrometers, which measure thermal radiation. Thermocouples are difficult to attach to ceramic surfaces and are not well suited for measurements on rotating parts. Furthermore, pyrometers often suffer from large errors caused by stray reflections. To avoid these problems, we are developing a temperature-sensing technique based on laser-induced luminescence of thermographic phosphors. Our approach directs a pulse of laser light to the target and measures the lifetime of the resulting luminescence.1 We coat a a ceramic phosphor, which is a material that produces luminescence when exposed to light of a particular wavelength, onto the target. The laser promotes atoms in the phosphor to excited states, which then emit photons as they return to the ground state. There are competing pathways to the ground state that Figure 1. Experimental setup to measure temperature of a nozzle guide vane supported in the exhaust of an afterburning turbojet engine. PMT: Photomultiplier tube.