Reliable Temperature Measurement With Thermal History Paints: An Uncertainty Estimation Model

Abstract

The operating temperature of turbomachinery components are increasing the drive towards higher efficiency, lower fuel consumption and reduced emissions. Accurate thermal models are required to simulate the operating temperature of gas turbine components and hence predict service life or other qualities. These models require validation through measurement. Therefore, the quality of the models and prediction are dependent on the uncertainty of the measurements used to validate them. Currently available temperature measurement techniques have limitations in the harsh operating conditions inside gas turbines. Thermocouples are widely used, however, are practically very challenging to apply on rotating components and only provide point measurements. Furthermore, over 80% of the surface must be measured to validate complex thermal models. A new technique under development called thermal history paints (THP) and coatings (THC) overcomes some of these limitations. While the uncertainty estimation model described in this work is directly related to THP, the principles can be applied in general to thermographic phosphors. The paint comprises a proprietary phosphor powder and a water-based silicate binder. The paint is applied to the surface of the test component. When the component is operated the paint records the maximum temperature of exposure across the complete surface of the component. After operation, the paint is read-out using automated instrumentation. The measurements are related to temperature through calibration to deliver a high-resolution temperature profile. An uncertainty model has been developed and described for the first time. The model assesses the uncertainty sources related to the generation of the calibration data and the measurement of the component. It has been applied to determine the uncertainty of the THP in the temperature range 400–750 °C. The estimated uncertainty in this case was, for most samples, ±3–6 °C (67% confidence level). The maximum estimated uncertainty was ±6.3 °C or ±13 °C for 67% or 95% confidence levels respectively. This is believed to be well within the uncertainty of thermal models and the requirements for temperature measurements in harsh environments on gas turbines. These results combined with the fact that the THP can record the temperature at many locations demonstrates that it is a very useful tool for the validation of thermal models and lifing predictions. The uncertainty model was validated by measuring separate test samples and comparing the temperature measured from the THP with the thermocouple data from the heat treatment. The difference was within ±7 °C and the uncertainty bounds determined by the model.

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