Non-Boltzmann Effects in Oxygen and Nitrogen Recombination in Hypersonic Boundary Layers

A master equation model resolving individual vibrational states via a database of ab initio rate coefficients is coupled to the Lees–Dorodnitsyn boundary-layer equations for stagnation-line flow. Using this framework, nitrogen and oxygen boundary layers are analyzed at pressures ranging from [Formula: see text] to 10 atm with nose radii from 1 mm to 10 m, using a noncatalytic wall boundary condition. Significant differences between the translational–rotational and vibrational temperatures occur at the wall for small nose radii and low pressures, indicating incomplete vibrational thermalization. Overpopulation of high-lying vibrational levels due to recombination is observed in all calculations, even when the translational–rotational and vibrational temperatures are equilibrated. This non-Boltzmann overpopulation suppresses recombination rates by a factor of 1.4–6 relative to Boltzmann predictions, with stronger effects observed in oxygen than in nitrogen. These differences substantially alter wall heating: in oxygen, Boltzmann versus non-Boltzmann rates yield differences exceeding 50% when the nose radius is 1 m or larger and up to 20% for a 10 cm nose; in nitrogen, differences are typically 10% or less, except for the largest nose radius of 10 m. Sensitivity analysis further reveals that atomic mass fractions and convective heating are controlled by recombination pathways to intermediate vibrational levels.