Standard thermography vs free emissivity dual color novel CIRA physics technique in the near-mid IR ranges: Studies for different emissivity class materials from low to high temperatures typical of aerospace re-entry

Abstract

The dual color infrared thermography is a non-intrusive temperature measurement technique, based on the ratio principle between the signals collected at two near wavelengths in local grey body hypotheses and applicable when a thermal camera is equipped with a suitable pair of narrow band filters. In this work, the applicability at high temperatures has been investigated by using an analytical model based on Planck's Law integration, convoluted with the real response curves of sensors, optics, filters and attenuators. The presented model has been validated at high temperature and used to optimize the hardware set up for dual color measurements both in terms of operative spectral band and also for choosing the best pair of filters. Furthermore, spectral emissivity trend curves for different materials, available in literature, and at different temperatures have been used in the theoretical model in order to simulate and replicate experimental results. The experimental simulations obtained with dual color technique have been compared with those obtained through classical techniques based on the a priori knowledge and setting of the emissivity average values.The aim is to establish in which conditions and for which class of Thermal Protection System (TPS) materials used to protect the inner cold structure of hypersonic space vehicles made of aluminium or metallic alloys, the dual color technique can be used for a more accurate and precise temperature measurement compared to the classical techniques. The applicability study was carried out up to high temperature (2000 °C) which are reached by the TPS surfaces during the re-entry phase from Earth orbit or from interplanetary trajectories due to the high heat flux loads produced by the strong shock wave during the deceleration phase.