Methodology for High-Accuracy Infrared Calibration in Environments with Through-Wall Heat Flux

Abstract

This paper describes a new method for accurate in situ infrared (IR) calibration in environments with significant through-wall heat flux and surface temperature non-uniformity. In the context of turbine research environments, conventional approaches for in situ IR calibrations rely on thermocouples embedded in the surface or bonded to the surface using an adhesive layer. A review of the literature points to lack of emphasis on the uncertainty in the calibration arising from the effect of the adhesive substrate and paint on the temperature measured by the thermocouple, namely that under diabatic conditions (i.e. with through-wall heat flux) the measured temperature deviates from the true surface temperature. We present a systematic study of the sensitivity of the thermocouple temperature to installation conditions seen in typical laboratory IR calibration arrangements, and under realistic conditions of through-wall heat flux. A new technique is proposed that improves the calibration accuracy by reducing the difference between the thermocouple measurement and the external wall temperature seen by the infrared camera. The new technique has the additional advantage of reducing the uncertainty associated with selecting an appropriate pixel in the IR image, by providing a region with greater temperature uniformity especially in environments with significant underlying lateral surface temperature variation. The new approach is experimentally demonstrated and compared to more conventional measurement techniques on a heavily film-cooled nozzle guide vane assembly operated at highly engine-representative conditions. The proposed technique is demonstrated to significantly improve the measurement accuracy for IR in situ calibrations in environment with through-wall heat flux and surface temperature non-uniformity.

1D analytic and numerical analysis of thin film heat transfer gauges and infra-red cameras in non-planar applications

“…Thermography methods offer higher resolution thermal data compared to film gauges but are limited by line of sight access, often obstructed if analysing shrouded components. Methods for calibration and roughness compensation are described by Falsetti et al (2021), Michaud et al (2020), andCondren et al (2023). Many users choose to analyse thermal image data in the native 2D image plane, thus neglecting curvature and other geometric effects.…”

Effect of Porosity and Injection Ratio on the Performance of Transpiration Cooling through Gyroids

“…Whilst the window demonstrated no signs of heating during testing, fitting with a dependence of temperature allowed pre-calibration data to be used day-to-day by accounting for changes in room temperature through an in situ correction. The in situ correction was conducted using a reference temperature provided by a thermocouple mounted in a copper disk within the measurement region [17]. Finally, it should be noted that gains and offsets were applied to each pixel in the camera focal planar array before data collection, a process typically referred to as a Non-Uniformity Correction (NUC).…”

Method for Determining Adiabatic Film Effectiveness in Presence of Thermal Boundary Layer

“…Further details of the technique, and the corresponding uncertainty analysis, are reported in Ref. [14].…”