Surface temperature and emissivity assessment in an enclosed cavity at high temperature using IR thermography

• A novel method for numerical evaluation of cavity emissivity. • Relationship between apparent and intrinsic emissivity regardless of the complexity of the cavity. • Multispectral imaging to link simulation and experimental results. • Enhanced thermal imaging and remote sensing applications in harsh, high temperature and oxidizing environments. Assessing emissivity and temperature using IR thermography is challenging, particularly at high temperatures. Moreover, in an enclosed cavity, the multiple reflections of the signal before reaching the camera can lead to a geometry-dependent apparent increase in emissivity. In this work, we describe a novel approach for solving this problem in the context of measurements within a model aircraft engine combustion chamber. By using thermocouples and a multi-spectral camera, we experimentally validate our radiometric model for the cavity. We first show how to evaluate the amplification factor of a cavity using numerical tools, and we then use these results to apply corrections on the camera signals for in-band radiance (IBR) measurements. As a non-invasive and non-destructive technique, this approach can be used to monitor in real time the evolution of the temperature and emissivity over a large temperature range. As a specific example, we present and compare values measured by the camera and thermocouples inside the combustion chamber. Following the calibration step, we determine the emissivity and temperature distribution of the entire scene. The calculations are compared across 3 different wavebands to ensure their validity, with a difference lower than 2 %. Finally, we showcase the importance of assessing the in situ emissivity of a surface, which can change drastically with a large temperature variation and in a harsh environment. Using a calibration point given by a carefully placed thermocouple, the 2D temperature mapping of the whole scene is evaluated and compared in two different wavebands, leading to temperatures within Δ T = 10 0 C across the wavebands when the combustion chamber is at 700℃.