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 are temperature-dependent, causing the lifetime of the excited state to be a function of temperature. Lifetime typically decreases monotonically with increasing temperature.²

To demonstrate the technique, we performed measurements on an engine component mounted in the exhaust flow of an afterburning turbojet engine.³ Figure 1 shows the experimental layout, in which a nozzle guide vane from an engine was mounted on a support in the exhaust stream of the engine. The support structure was cooled by water and served to protect the optics used to transmit the laser light and collect the luminescence signal. The support also directed an optional stream of air through small cooling passages in the vane surface, but the vane itself was not water-cooled. The vane was coated with a TBC layer of yttria-stabilized zirconia (YSZ), which is a zirconium oxide-based ceramic material, and a phosphor layer of dysprosium-doped yttrium aluminum garnet (Dy:YAG). A fiber optic probe—consisting of a central laser-delivery fiber (600μm in diameter) with 82 surrounding collection fibers (each 200μm in diameter)—was used to deliver light from a pulsed neodymium-doped yttrium aluminum garnet (Nd:YAG) laser with a wavelength of 355nm to excite a measurement spot on the vane. Luminescence signals from the phosphor coating were collected by the probe, transmitted through an optical fiber extension, and detected by a photomultiplier tube (PMT). Signals from the PMT were digitized with an oscilloscope and stored on a computer.

Figure 1. Experimental setup to measure temperature of a nozzle guide vane supported in the exhaust of an afterburning turbojet engine. PMT: Photomultiplier tube.

Raw signals from the experiments are shown in Figure 2, in which the luminescence signal is plotted as a function of time after the laser pulse. For each laser pulse, the decay lifetime was obtained by performing an exponential curve fit to the data after subtracting off the baseline signal. Data from four separate laser pulses are shown, for which the measured luminescence lifetime is seen to vary from 114 to 338μs. These lifetimes correspond to temperatures in the range 1187–1276°C and were obtained using a previous calibration.⁴ The lifetime is what ultimately limits the time resolution of the measurement, which can be in the hundreds of microseconds in this case.

Figure 2. Luminescence decays (with lifetimes given in yellow) from a phosphor-coated vane in an engine exhaust flow at various temperatures (T). Dy:YAG: Dysprosium-doped yttrium aluminum garnet.

We conducted a time series experiment of temperatures measured with this technique, from which a 13 second period of an engine run with the afterburner on is shown in Figure 3. We acquired data records at a rate of 5Hz, each consisting of 10,000 voltage measurements of a luminescence decay from which the lifetime was obtained. Significant levels of background emission from the exhaust plume were observed in the raw data. However, by subtracting this background, most of the data was usable. Fluctuations in temperature seen in Figure 3 are mainly attributable to actual temperature variations, as can be inferred by the fact that they vary in a continuous way rather than randomly. We estimated the random error from data taken with the engine at low power to avoid large fluctuations, and calculated a standard deviation of 7μs. This level of uncertainty corresponds to a noise level of 6°C at 1050°C, or about 3°C at 1250°C. By comparison, uncertainties in current methods are rarely better than 50°C. A post-test photograph of the vane is shown in Figure 4, which shows damage caused by the hot exhaust gases.

Figure 3. Time series of temperatures measured on a nozzle guide vane supported in the exhaust stream of a turbojet engine showing the capability to measure temperatures near 1200°C on a ceramic coating. BW: Bandwidth of the filter.

Figure 4. Post-test photograph of the nozzle guide vane doublet (two airfoils), showing damage to one of the airfoils.

We have demonstrated that temperatures of thermal barrier-coated engine components in the vicinity of 1250°C can be measured in the presence of hot combustion gases using a luminescence lifetime technique. Random variations in the measured lifetime at low engine power settings corresponded to a noise level of 3°C at 1250°C. These findings should lead to sensors that will aid in the development of advanced turbine engines. In the coming months, we will demonstrate this technique on active components within an operating engine.