Bandeau CORIA
Boundary layer analysis in air plasma
by laser diagnostics
and optical spectroscopy

Français-French     English-Anglais Pascal BOUBERT .  Auteur 
Damien STUDER .  Auteur 
Pierre VERVISCH .  Auteur 
Le 17 07 2010

Bullet    Abstract
During the hypersonic atmospheric entry of a space probe in a planetary atmosphere, the thermal protection system encounters heat flux densities up to 1 MW/m2 due to the radiation, the convection and the chemical reactions occurring behind the shock front.
This medium is reproduced in laboratory using an inductively-coupled plasma (ICP) torch to study the interaction of high-enthalpy plasmas with materials. In the frame of this study, the boundary layer over a water-cooled stainless steel plate in an air plasma was analysed by determining temperatures and densities of the majority molecular species N2, O2 and NO.
Laser and spectroscopy techniques allowed to demonstrate the non-equilibrium behaviour of the plasma and the production of NO close to the wall.

Bullet    Working conditions
  • Inductively-coupled plasma (h = 9 MJ/kg)
  • Mixture: synthetic air (80% N2 – 20% O2)
  • Mass flow rate: 2.4 g/s
  • Chamber pressure: 3800 Pa
  • Jet velocity: 250 m/s
  • Temperature in the free jet : 3000 K
  • Leading edge position: x = 80 mm
 		Set-up


Bullet    Diagnostic techniques
Objectives: Temperatures (Tvib,Trot) and Densities.
  • LIF (laser-induced fluorescence):               NO (X)
  • OES (optical emission spectroscopy):         NO (A)
  • SRS (spontaneous Raman spectroscopy):   N2 (X), O2 (X)
    •


    Bullet    Non-equilibrium between ground and excited states
    Measurements of rotational temperature made by various techniques on N2, O2 and NO show coherent result for ground states in agreement with the translational temperature. For the excited levels, the equality between Trot and Ttrans is not valid because they are created by excitation exchange for example with the metastable state of N2 in the case of NO. Ttrans can not be measured by OES in this case.
    		Non-equilibrium between ground and excited states


    Bullet    Vibration – Rotation non-equilibrium
    Whatever the species (N2, O2, NO on their ground states), vibrational and rotational temperatures are not equal in the free jet as well as within the boundary layer. This result was expected for N2 because of its long vibrational relaxation time but unexpected for O2 and NO. The outcome is that each vibrational mode should me modelled separately in a calculation.
    		Vibration – Rotation non-equilibrium on NO


    Bullet    NO production at the wall and in the gas
    Equilibrium and frozen chemistries are not able to explain the behaviour of NO and O2 densities within the boundary layer. The density measurements confirm both way of NO production: a catalytic one at the wall
    Ogas + Nadsorbed → NOgas
    and a homogeneous one along the Zel'dovich reaction
    O2 + N → NO + O
    following oxygen recombination at the wall.
    NO profile in the boundary layer O2 profile in the boundary layer


    Bullet    References
    - D. Studer, P. Vervisch – "Raman scattering measurements within a flat plate boundary layer in an inductively coupled plasma wind tunnel." Journal of Applied Physics (2007), 102, 033303
    - D. Studer, P. Boubert, P. Vervisch – "Demonstration of NO production in air plasma–metallic surface interaction by broadband laser-induced fluorescence." Journal of Physics D: Applied Physics (2010) 43 315202
    - D. Studer, P. Boubert, P. Vervisch – "NO excitation and thermal non-equilibrium within a flat plate boundary layer in an air plasma." Lasers and Optics (2010)


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