Vibrational relaxation and recombination of nitrogen and air in hypersonic nozzle flows

The effects of vibrational relaxation and recombination of high-temperature nitrogen and air were investigated in a hypersonic shock tunnel at high flow Mach numbers. The equilibrium temperature and pressure ranges after the reflected shock wave were 1400° to 7000 °K and 100 to 500 psia. Static pressure measurements were made along the axis of the nozzle to determine the freezing of the vibrational mode during the expansion process. Vibrational relaxation times for nitrogen obtained from shock-tube experiments were used to calculate the relaxation length. Once the nitrogen vibrational mode freezes in the expansion region, there is very little exchange of the vibrational energy into the translational and rotational energies, and the static pressures are significantly lower than those calculated for an equilibrium expansion. The nonequilibrium effects for nitrogen are much less than those for air, and the large difference is caused mainly by the presence of oxygen. With air, the reservoir pressure has a large influence upon the departure from equilibrium expansion.