Transport properties of high Mach number hypersonic air plasmas

Abstract The collisional and transport properties of hypersonic plasmas have been simulated by an air plasma chemistry model, which includes collisions with electrons, ions and neutral species, elastic scattering, vibrational relaxation and a transport model. Species and thermal fluxes at the surface of the vehicle have been investigated as a function of post-shock gas temperature and species sticking coefficient for a fixed altitude of 50 km. At temperatures below about 5000 K, the leading species are vibrationally excited nitrogen molecules, while at higher temperatures the atomic oxygen and nitrogen dominate. Large fluxes of oxygen atoms have been observed in the simulations, on the order of 10 20 cm − 2 s − 1 , which may lead to high rates of surface oxidation. Another subject of this study is temperature equilibration. For electrons, equilibration takes tens of microseconds, while the vibrational temperature equilibration time vary widely; from a few microseconds to hundreds of microseconds, depending on the gas temperature. Finally, the adiabatic parameter γ was calculated for post-shock gas temperatures 5000 K and 10 000 K and total gas density <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>1.2</mml:mn> <mml:mo>×</mml:mo> <mml:mrow> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mn>17</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> <mml:mstyle scriptlevel="0"/> <mml:mrow> <mml:mtext>c</mml:mtext> </mml:mrow> <mml:mrow> <mml:msup> <mml:mrow> <mml:mtext>m</mml:mtext> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> . For post-shock gas temperature 5000 K, only a slight reduction of γ from the ideal gas value of γ = 1.4 is observed, while for 10 000 K it drops to γ ≅ 1.26, which has a considerable impact on plasma properties. It was further established that the vibrational kinetics plays a leading role. While a single excited vibrational level of the N 2 molecule captures the salient properties of the plasma, a detailed vibration kinetics is required for high-fidelity simulations.