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Theses

Etude expérimentale et modélisation des propriétés radiatives des mélanges gazeux de type CO2-N2 à très haute température en vue de l'application aux rentrées atmosphériques martiennes,

Abstract : Modeling of the radiation of CO2 above 2000 K remains a challenge for the prediction of radiative transfer at high temperatures, especially for the design of thermal protection of space vehicles entering the Martian atmosphere. The challenge for both the creation of databases that the development of models to perform calculations coupled between Aerothermodynamics and radiative transfer. Several spectroscopic databases have been developed in recent decades. Simulations of CO2-N2 mixtures at equilibrium showed that the IR emission of CO2 is important, if not predominant, up to 4000 K. But the experimental data to verify their accuracy at high temperature are few and limited to the low spectral resolution, at lower temperatures, or involve significant uncertainties. The main objective of this work is to provide experimental data at medium or high spectral resolution and high temperature in order to verify the accuracy of the theoretical foundations developed. The gas mixture at high temperature we measure the emission spectra is achieved through a microwave discharge through a stream of pure CO2. The temperatures reached (about 6000 K), the mixture is in local thermodynamic equilibrium (LTE) and mainly contains the following species: CO2, CO, O2 and O. The measurement of the spectra is carried out using a Fourier transform spectrometer in the infrared. The harmonic emission bands of CO in the region 3800-4400 cm-1 power allows accurate determination of the radial temperature field in the plasma. The theoretical CO2 emissions, both in emission coefficient or radiance integrated on an optical path, in the regions of 2.7 and 4.3 _m _m, is then calculated from the spectroscopic database and compared measures. The databases are based spectroscopic studied CDSD-4000 and recently published EM2C-1994 which is older. The results show that the latter is sufficient for combustion in conventional applications, typically requiring low spectral resolutions and temperatures below 2500 K, for which it was originally developed. However, based CDSD-4000 is generally in good agreement with experimental data, especially in the wings of bands indicating reliability for applications at very high temperatures. Although comparisons have revealed that the line positions for high energy transitions are unclear in this basis, the high degree of overlap of the lines in the regions 2.7 and 4.3 _m _m should greatly reduce the impact of this imprecision in practice. The other objective of this work concerns the difficulty of coupling between the calculations of radiative transfer and aerothermodynamics. Indeed, calculations of radiative transfer iv involving polyatomic molecules must take into account a large number of rovibronic transitions. Therefore, an approximate model of radiative properties (Statistics narrowband model) was developed and its accuracy has been studied for electronic systems optically thick carbon diatomic molecules encountered in problems of Martian atmospheric reentry. These systems are CO 4th positive, C2 Swan and CN violet. Various conditions to the non-LTE and LTE, and various mixtures have been studied for different regimes of line broadening. Comparisons with calculations "line by line" showed excellent agreement. The model parameters were tabulated in terms of two temperatures, Tg (vibrational temperature) and Tr (rotational temperature).
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Sebastien Depraz. Etude expérimentale et modélisation des propriétés radiatives des mélanges gazeux de type CO2-N2 à très haute température en vue de l'application aux rentrées atmosphériques martiennes,. Autre. Ecole Centrale Paris, 2011. Français. ⟨NNT : 2011ECAP0051⟩. ⟨tel-00697001⟩

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