Can vibrational pumping occur via O₂–N₂ collisions in nonequilibrium vibrationally excited air?

Abstract

The occurrence of vibrational pumping in air under nonequilibrium conditions is investigated as this phenomenon is not considered in the design of the current phenomenological models. It is shown that pumping can only happen during de-excitation and when the translational temperature is below around 1000 K. O2 is the molecule that would get pumped, and pumping will not occur when the initial equilibrium temperature is greater than around 1200–1600 K due to the formation of enough O to extinguish pumping via the O2–O vibration–translation reaction. The limiting initial temperature can be increased to around 2000 K if a nonequilibrium initial condition is considered. In cases where pumping does occur, constant–volume reactor simulations showed pumping of ≈5%. Nozzle simulations representative of that in hypersonic wind tunnels are conducted for an equilibrium temperature of 1100 K at the throat; pumping of up to around 10 K (≈1%) can be observed. It can be suggested that constant–volume reactors generally overestimate the manifestation of thermochemical nonequilibrium-associated phenomena and are a better zero-dimensional analogy for the relaxation process in flows with large length scales and no further expansion after an initial rapid expansion. After examination of the uncertainties of the most important rates used in the simulations, one may suggest that the current results correspond to the upper bound for the magnitude of pumping. It may be concluded that pumping is unimportant for practical intents and purposes in nonequilibrium hypersonic flows, and phenomenological models need not be able to recreate this phenomenon.