21 Janvier – Thesis defense - Abderezak Aouali

09 h Amphi La Rochefoucauld-Liancourt (LRL), ENSAM, Talence

Thermo-spectroscopic tomography by 3D imaging for the study of plasma torches.

This thesis is part of a multi-partner ADEME IGAR (Injection of Reducing Gas) project which consists of studying, developing and validating this new process concept. In recent years, a major challenge for the steel industry has been to reduce greenhouse gas emissions. One of the solutions envisaged in blast furnaces is the injection of a reducing gas heated by plasma torches. In this context, one of the scientific objectives of this thesis is the measurement of temperature, concentration and flux within plasma torches, which are environments under extreme conditions due to the high temperatures and high powers involved.
In order to study this kind of heterogeneous and extreme environment, it is necessary to develop optical and non-contact instrumentation to obtain temperature, concentration and flux volume fields of flames on a laboratory scale using tomographic methods.
Therefore, the main challenge of this thesis work is the development of quantitative imaging methods. Initially, the work was focused on the development of an hyperspectral quantitative imaging fluxmeter under extreme conditions. This work is based on the understanding and modelling of the heat transfer within the sensor, the development of an inverse method based on the Wiener filter allowing the spatial and quantitative estimation of the excitation flux. These methods were accompanied by rigorous metrology to ensure complete control of the tool.
Once this sensor was developed, Flying spot methods for 3D hyperspectral imaging by Radon transform have been implemented. This new imaging concept made it possible to carry out hyperspectral 3D tomography (visible, infrared and terahertz) on spectrally heterogeneous objects with complex shapes. Finally, in a second phase, an experimental platform based on infrared spectroscopy coupled with cooled thermal cameras has been established to perform thermospectroscopic tomographic imaging. This measurement, based on a two-image method and a Radon-type tomography, made it possible to simultaneously obtain the temperature and concentration volume fields on laboratory micro-torch flames as well as various physical quantities (absorptivity, emissivity, transmissivity) related to the chemical composition of the flames.
In conclusion, the methods developed in this thesis, namely the development of fluxmeters and 3D thermospectroscopic measurement techniques on a laboratory scale, will allow a better understanding of the opto-thermo-chemical processes within plasma torches and more generally within heterogeneous media.

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