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Accueil du site > Thèmes de recherche > Propulsion spatiale & écoulements à grande vitesse > 3.2 Écoulements hypersoniques > 3.2.1 Rentrées atmosphériques

3.2.1 Rentrées atmosphériques

Activities of ICARE in this area are oriented towards the development of experimental research topics in connection with the ICARE facilities (Phedra, Marhy and Edith), whereas the numerical developments are conducted thanks to the collaborations established with the “Fluid Dynamics and Thermophysics” team of Marseille and the LEEM of Evry.

3.2.1.1 Supersonic plasma flows in thermal non equilibrium
Vehicles moving at high speed in a planetary atmosphere generate shock waves capable of emitting radiation. Due to high temperatures and low pressures, non-equilibrium effects occur behind the shock wave. They can lead to strong radiative fluxes that form a large part of the overall heat flux. This activity focuses on the study of radiative models of the created plasma and on the coupling between the involved physico-chemical processes. For high velocities, the 28 presence of electrons leads to additional physico-chemical processes : electronic collisions can be very efficient for electronic excitation and ionization of atoms, and electronic and vibrational excitation of molecules. The Phedra facility allows reproducing some of theses properties. Besides, it provides experimental data, which are useful to validate calculations of the emission spectra, non-equilibrium effects and modelling of the thermodynamic and transport properties. In the framework of the ANR project RAHYEN (2008-2011), our principal task is the development of an exhaustive and accurate spectroscopic database for terrestrial and Martian entries and the development of reliable physical models to simulate radiation and thermo-chemical non-equilibrium in atmospheric entry flows. To comply with the objectives, we have developed IR and VUV spectral diagnostics techniques and we have developed a theoretical spectral database necessary to analyze experimental results (ACL141, ACT I118).

3.2.1.2 Modelling of expanding plasmas
For polyatomic gases at high temperature, degrees of freedom other than the translational one must be taken into account and assigned a specific temperature : rotational, vibrational and electronic. Under atmospheric reentry conditions, the thermal nonequilibrium can reach 20% between translational temperature and rotational, vibrational or electronic ones. In wind tunnels, model flows are generated by mean of arc-jet nozzles, and from the inlet to the outlet of the throat, the flow can be heated from room temperature to more than 10 000K at trans-sonic speeds. Currently we are able to simulate a flow for realistic conditions of entry. The modeling of the Joule heating source gives quite consistent temperatures with the overall energy balance. More precise comparisons will be possible as soon as temperature measurements will be available. Different models of arc (more or less stable) were tested and compared. The dissipation of the energy, especially in chemical reactions has been demonstrated for a power ranging from 6 to 10 kW. The calculations also helped to highlight the influence of the heating source on the stability of the plasma jet. These computations have been applied to different atmospheres : Earth (N2, N2-O2), Titan (N2-CH4) and Mars (CO2-N2). The strong coupling between different species and their internal energies (electronic, vibrational and rotational) has been recently achieved (TH24). The evolution towards thermodynamic equilibrium between the translational and electronic temperatures in the collar has been demonstrated and validated for pure gas as Argon. We also considered the problem of the presence of species that once in their excited state, contribute significantly to the properties of the plasma. To examine the importance of these effects, a first study on Argon at low pressure in the arc-jet was carried out with four states : Ar, Ar (4s), Ar (4p) and Ar+ (ACL140, 150, 151, 163 ; ASCL8-10).