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.

Dynamics of Inelastic and Reactive Collisions of 16O(3P) with 15N18O

“…The formation of both NO and O 2 was considered in models of the chemistry of hypersonic boundary layers. ,, For example, Armenise and Esposito showed that, at the interface of the boundary layer and a surface, the inclusion of NO formation/destruction in their model strongly affects the mass fraction of nitrogen, oxygen, and nitric oxide. In particular, O 2 becomes 5 times smaller, while NO is more than 6 orders of magnitude lower compared to the case where these reactions are not included .…”

“…In addition, when a hypersonic vehicle travels at high velocities through the atmosphere, NO may be produced in the shock layer . In these aerothermodynamic conditions, dissociation of molecular oxygen and nitrogen occurs, and the resultant atomic species can then undergo collisions with the thermal protection system (TPS or “heat shield”) or collisions with other molecules present in the boundary layer.…”

“…7 In addition, when a hypersonic vehicle travels at high velocities through the atmosphere, NO may be produced in the shock layer. 8 In these aerothermodynamic conditions, dissociation of molecular oxygen and nitrogen occurs, and the resultant atomic species can then undergo collisions with the thermal protection system (TPS or "heat shield") or collisions with other molecules present in the boundary layer. In the first case, the gas−surface interactions between the O and N atoms and the high-temperature TPS 9 may lead to the degradation of the TPS and the release of additional gaseous products into the boundary layer.…”

The Impact on the Ozone Layer of a Potential Fleet of Civil Hypersonic Aircraft

“…That report also indicated that the amount of nitrogen oxides produced in the HST flow field by the shock wave occurring around the nose of the aircraft would be an order of magnitude lower than the emission by a ramjet engine, thus of the same order as the 10.1029/2020EF001626 emission by a classic turbojet. Brooks et al (1993) indicate that, for Mach numbers between 1 to 10, the effect of NO output produced by the high-temperature boundary layer of the HST can be neglected compared to the emission by the engines. At Mach 16, however, the output of NO from the boundary layer of the aircraft would become roughly equal to the emission from a scramjet engine, and thus considerably higher than the emission of a classic air-breathing turbojet.…”

Experimental and Theoretical Studies of Hyperthermal N + O2 Collisions

“…Dissociation of molecular nitrogen and oxygen produces N and O atoms, which can in turn collide with O 2 and N 2 and lead to nitric oxide formation. 3,4 These reactions are commonly referred to as Zeldovich reactions: 5…”