Multispectral and multiscale infrared imaging for the study of heat and mass transfer in heterogeneous media

Abstract: In the aerospace industry, the implementation of composite materials is constantly increasing. During the elaboration process of these materials or during the life of the materials, it is important to monitor the thermophysical properties and their strength. Many techniques exist to verify the quality of fiber organization using destructive methods. Nevertheless, the Non-Destructive Technique (NDT) is a powerful tool for online monitoring and for quantitative and fast characterization. The first part of this presentation will be devoted to the development of InfraRed multiscale thermal characterization of such heterogeneous media. To realize such characterization the experimental set-up based on flying spot technique will be presented as well as processing, an analysis of the transient field of the temperature response based on analytical modelization. The use of new scanning systems based on a galvanometer mirror allows the easy control of the spatial and temporal displacements of a laser hot spot over a plane surface. Such systems are then suitable for use in developing new flying spot methods as alternatives to the initial flying spot technique, which is based on a constant velocity of the spot. Due to the great number of pulsed spots deposited with this technique it becomes possible to estimate in-plane fields of anisotropic thermal diffusivities by using the spatial logarithmic processing of the temperature response. Here, the method is presented as well as the first results on the heterogeneous anisotropic composite materials. In a second part, the study of multiscale heterogeneous solid materials has been extended to the study of multispectral heat and mass transfer in microfluidic systems. In fact, the recent development of the Fast Infrared Imaging Spectroscopic Technique (FIIST) allows simultaneous acquisition of temperature and concentration fields, using a non-intrusive method and also Thermal Imaging Velocimetry (TIV). The study in the middle-wave infrared range, from 2 to 6 µm, enables the study of compounds which can be transparent in visible, without the additions of tracers and its influence on mass diffusion. In this work, the exothermic acid-base reaction between sodium hydroxide and hydrochloric acid is performed in a co-flow microfluidic chip of high aspect ratio. This configuration makes it possible to obtain laminar flows where the mixture of the species is then only due to a radial diffusion process and advection. Mass diffusion cone makes out since water transmittance is increased in presence of the ion pair of Na+ and Cl-. Both heat and mass diffusivities are estimated from the logarithmic parabolas method, originally developed for thermal transfer but applied here also in the mass transfer case since the diffusion transport equation which is similar in both cases. Finally, in a last part, the beginning of thermospectroscopic tomography will be presented with imaging system that is able to measure transient temperature phenomena taking place inside a bulk by 3D tomography. This novel technique combines the power of multispectral waves and the high sensitivity of infrared imaging. The tomography reconstruction is achieved by the 3D motion of the sample at several angular positions followed by inverse Radon transform processing to retrieve the 3D transient temperatures. The aim of this novel volumetric imaging technique is to locate defects within the whole target body as well as to measure the temperature in the whole volume of the target. This new-fashioned thermal tomography will open research perspectives in the non-invasive monitoring techniques for volume inspection and in-situ properties estimations.