NITRIC-OXIDE EMISSIONS FROM THE HIGH-TEMPERATURE VISCOUS BOUNDARY-LAYERS OF HYPERSONIC AIRCRAFT WITHIN THE STRATOSPHERE

Nitrogen oxides have been shown to catalyze the destruction of stratos pheric ozone. Previous estimates of nitric oxide emissions from high-a ltitude aircraft have taken into account engine emissions only. This s tudy was undertaken to determine whether nitric oxide production in th e hot viscous boundary layer surrounding the skin of a hypersonic airc raft can significantly increase estimates of total NO emissions. The d escribed model approximates the viscous boundary layer as a series of finite stream tubes with time-dependent and temperature-dependent chem ical kinetics and mass flow rates. Nitric oxide equilibrium mole fract ion for air peaks at congruent-to 3700 K. At hypersonic speeds the vis cous effects near the aircraft skin will induce these very high temper atures. Along the skin surface of a 60 m craft, the NO mole fraction c an be locally as high as 0.044. Nitric oxide output froin this thin bo undary layer becomes significant when integrated over the entire trail ing edge of the aircraft. This model predicts that nitric oxide produc tion in the boundary layer should be taken into account at speeds abov e Mach 8. Above Mach 8, boundary layer volume and temperature increase rapidly. At stratospheric speeds approaching Mach 16, the nitric oxid e production in the boundary layer increases to t,he point where it ro ughly equals the nitric oxide output from the engines. Above Mach 16, the boundary layer produces the majority of the NO emissions. This add itional source of nitric oxide will play an ever-increasing role as ai rcraft fly at higher speeds and altitudes. Significant errors iii tota l NO emission estimates will result if the viscous boundary layer is n ot taken into account at speeds above Mach 8.