Nitric oxide emissions from the high‐temperature viscous boundary layers of hypersonic aircraft within the stratosphere

Abstract

Nitrogen oxides have been shown to catalyze the destruction of stratospheric ozone. Previous estimates of nitric oxide emissions from high‐altitude aircraft have taken into account engine emissions only. This study was undertaken to determine whether nitric oxide production in the hot viscous boundary layer surrounding the skin of a hypersonic aircraft can significantly increase estimates of total NO emissions. The described model approximates the viscous boundary layer as a series of finite stream tubes with time‐dependent and temperature‐dependent chemical kinetics and mass flow rates. Nitric oxide equilibrium mole fraction for air peaks at ≅ 3700 K. At hypersonic speeds the viscous effects near the aircraft skin will induce these very high temperatures. Along the skin surface of a 60 m craft, the NO mole fraction can be locally as high as 0.044. Nitric oxide output from this thin boundary layer becomes significant when integrated over the entire trailing edge of the aircraft. This model predicts that nitric oxide production in the boundary layer should be taken into account at speeds above Mach 8. Above Mach 8, boundary layer volume and temperature increase rapidly. At stratospheric speeds approaching Mach 16, the nitric oxide production in the boundary layer increases to the point where it roughly equals the nitric oxide output from the engines. Above Mach 16, the boundary layer produces the majority of the NO emissions. This additional source of nitric oxide will play an ever‐increasing role as aircraft fly at higher speeds and altitudes. Significant errors in total NO emission estimates will result if the viscous boundary layer is not taken into account at speeds above Mach 8.