C. C. Pilgrim

Off-Line Temperature Profiling Utilizing Phosphorescent Thermal History Paints and Coatings

ABSTRACT Temperature profiling of components in gas turbines is of increasing importance as engineers drive to increase firing temperatures and optimise component’s cooling requirements in order to increase efficiency and lower CO 2 emissions. However, on-line temperature measurements and, particularly, temperature profiling are difficult, sometimes impossible, to perform due to inaccessibility of the components. A desirable alternative would be to record the exposure temperature in such a way that it can be determined later, off-line. The commercially available Thermal Paints are toxic in nature and come with a range of technical disadvantages such as subjective readout and limited durability. This paper proposes a novel alternative measurement technique which the authors call Thermal History Paints and Thermal History Coatings. These can be particularly useful in the design process, but further could provide benefits in the maintenance area where hotspots which occurred during operation can be detected during maintenance intervals when the engine is at ambient temperature. This novel temperature profiling technique uses optical active ions in a ceramic host material. When these ions are excited by light they start to phosphoresce. The host material undergoes irreversible changes when exposed to elevated temperatures and since these changes are on the atomic level they influence the phosphorescent properties such as the life time decay of the phosphorescence. The changes in phosphorescence can be related to temperature through calibration such that in-situ analysis will return the temperature experienced by the coating. A major benefit of this technique is in the automated interpretation of the coatings. An electronic instrument is used to measure the phosphorescence signal eliminating the need for a specialist interpreter and thus increasing readout speed. This paper reviews results from temperature measurements made with a water based paint for the temperature range 100˚C to 800˚C in controlled conditions. Repeatability of the tests and errors will be discussed. Further, some measurements are carried out using an electronic hand-held interrogation device which can scan a component surface and provide a spatial resolution of below 3mm. The instrument enables mobile measurements outside of laboratory conditions. Further a robust Thermal History Coating is introduced demonstrating the capability of the coating to withstand long term exposures. The coating is based on Thermal Barrier Coating architecture with a high temperature bondcoat and deposited using an air plasma spray process to manufacture a reliable long lasting coating. Such a coating could be employed over the life of the component to provide critical temperature information at regular maintenance intervals for example indicating hot spots on engine parts.

On-Line Temperature Measurement Inside a Thermal Barrier Sensor Coating During Engine Operation

Existing thermal barrier coatings (TBCs) can be adapted enhancing their functionalities such that they not only protect critical components from hot gases but also can sense their own material temperature or other physical properties. The self-sensing capability is introduced by embedding optically active rare earth ions into the thermal barrier ceramic. When illuminated by light, the material starts to phosphoresce and the phosphorescence can provide in situ information on temperature, phase changes, corrosion, or erosion of the coating subject to the coating design. The integration of an on-line temperature detection system enables the full potential of TBCs to be realized due to improved accuracy in temperature measurement and early warning of degradation. This in turn will increase fuel efficiency and will reduce CO2 emissions. This paper reviews the previous implementation of such a measurement system into a Rolls-Royce jet engine using dysprosium doped yttrium-stabilized-zirconia (YSZ) as a single layer and a dual layer sensor coating material. The temperature measurements were carried out on cooled and uncooled components on a combustion chamber liner and on nozzle guide vanes (NGVs), respectively. The paper investigates the interpretation of those results looking at coating thickness effects and temperature gradients across the TBC. For the study, a specialized cyclic thermal gradient burner test rig was operated and instrumented using equivalent instrumentation to that used for the engine test. This unique rig enables the controlled heating of the coatings at different temperature regimes. A long-wavelength pyrometer was employed detecting the surface temperature of the coating in combination with the phosphorescence detector. A correction was applied to compensate for changes in emissivity using two methods. A thermocouple was used continuously measuring the substrate temperature of the sample. Typical gradients across the coating are less than 1 K/μm. As the excitation laser penetrates the coating, it generates phosphorescence from several locations throughout the coating and hence provides an integrated signal. The study successfully proved that the temperature indication from the phosphorescence coating remains between the surface and substrate temperature for all operating conditions. This demonstrates the possibility to measure inside the coating closer to the bond coat. The knowledge of the bond coat temperature is relevant to the growth of the thermally grown oxide (TGO) which is linked to the delamination of the coating and hence determines its life. Further, the data are related to a one-dimensional phosphorescence model determining the penetration depth of the laser and the emission.

Thermal History Sensors for Non-Destructive Temperature Measurements in Harsh Environments

The operating temperature is a critical physical parameter in many engineering applications, however, can be very challenging to measure in certain environments, particularly when access is limited or on rotating components. A new quantitative non-destructive temperature measurement technique has been proposed which relies on thermally induced permanent changes in ceramic phosphors. This technique has several distinct advantages over current methods for many different applications. The robust ceramic material stores the temperature information allowing long term thermal exposures in harsh environment to be measured at a convenient time. Additionally, rare earth dopants make the ceramic phosphorescent so that the temperature information can be interpreted by automated interrogation of the phosphorescent light. This technique has been demonstrated by application of YAG doped with dysprosium and europium as coatings through the air-plasma spray process. Either material can be used to measure temperature over a wide range, namely between 300°C and 900°C. Furthermore, results show that the material records the peak exposure temperature and prolonged exposure at lower temperatures would have no effect on the temperature measurement. This indicates that these materials could be used to measure peak operating temperatures in long-term testing.

Operation of a Burner Rig for Thermal Gradient Cycling of Thermal Barrier Coatings

Thermal barrier coatings (TBC), in combination with sophisticated cooling systems are crucial for the operation of highly efficient gas turbines. New generations of coatings will need to show increased cycling capability as a future energy mix will contain a high proportion of renewable energy which will be subject to rapid changes in supply. This will require gas turbines to be on stand-by to fill shortages in power supply with short notice. Furthermore, higher operating temperatures are sought to improve the efficiency of the engine. It is, therefore, an aim of the industry to find a coating composition or structure which will enable the operation at temperatures greater than 1250°C and with high cycling capability.Test methods are required to meet these new operating conditions to validate new coatings. The maximum temperature limit of commonly used isothermal or cyclic oxidation tests is usually the temperature at which the substrate will start to significantly oxidise. However, there is the technical need to test the ceramic top layer at elevated surface temperatures up to 1500°C while keeping the substrate ‘cool’. Such capability would allow the effects of ceramic sintering, and deposit induced damage to be assessed at the TBC surface. This only can be performed on a complete coating system, when a thermal gradient is established throughout the coating.This paper reviews a burner test facility, designed and built by Sensor Coating Systems Ltd. (SCS), which combines severe and frequent cycling with the exposure of the coating to high surface temperatures and active cooling of the substrate. Further, this test can include thermal shock by active cooling of the surface at the end of each cycle. The paper will consider different operating conditions and will review experiences in building and operating the rig, including results from thermal barrier coating tests on electron beam physical vapour deposition (EBPVD) and atmospheric plasma spray (APS) samples. Further, the rig is capable of testing optical techniques such as pyrometry and thermographic phosphor thermometry for measuring surface temperature in controlled laboratory conditions and example of this will be presented. The paper also will reflect on the ISO 13123:201 standard for this type of test.Copyright

Surface Temperature Measurements in an Industrial Gas Turbine Using Thermal History Paints

The measurement of surface temperatures of hot-gas path components of gas turbines under operating conditions provides a considerable challenge because the complexity of measurements under the prevailing conditions is substantial. The results from temperature measurements from an engine test using Thermal History Paint (THP) are presented here. The sensor material in the THP is an oxide ceramic which is doped with lanthanide ions to make the material luminescent. The properties of the luminescence depend on the temperature of exposure. The paper describes the first application of this technology in an extended, rather than dedicated, engine test in which components in both the hot gas path and the secondary air system were coated with THP. During the test campaign the engine components operated below maximum temperature for extended periods of time, which required a novel approach to the calibration of the paint. An overview over the correspondence between the temperatures measured with the THP, thermal paints and CFD calculations is provided for a sideplate and turbine blade. There is very good correlation between the results of the different methods. For the sideplate, the temperature measured with the THP was within 10K of the CFD calculation. Furthermore, the THP exhibited only minor erosion damage after over 50 hours of engine testing. The high durability and measurement accuracy demonstrate the feasibility of using the THP in extended engine tests.