A numerical study of the superadiabatic flame temperature phenomenon in HN3 flames

The phenomenon of superadiabatic flame temperature (SAFT) was discovered and investigated in a low-pressure HN₃/N₂ flame using numerical modelling. A previously developed mechanism of chemical reactions in the HN₃/N₂ flame at the pressure 50 Torr and the initial temperature T₀ = 296 K was revised. Rate constants of several important reactions involving HN₃ (HN₃ (+N₂) = N₂ + NH (+N₂), R1; HN₃ (+HN₃) = N₂ + NH (+HN₃), R2; HN₃ + H = N₂ + NH₂, R4; HN₃ + N = N₂ + NNH, R5; and HN₃ + NH₂ = NH₃ + N₃, R7) were calculated using quantum chemistry and reaction rate theories. Modified Arrhenius expressions for these reactions are provided for the 300–3500 K temperature range. Modelling of the flame structure and flame propagation velocity of the HN₃/N₂ flame at p = 50 Torr and T₀ = 296 K was performed using the revised mechanism. The results demonstrate the presence of the SAFT phenomenon in the HN₃/N₂ flame. Analysis of the flame structure and the kinetic mechanism indicates that the cause of SAFT is in the kinetic mechanism: exothermic reactions of radicals with hydrogen atoms occur in the post flame zone, which results in the formation of super equilibrium H₂ concentrations. The flame propagation velocity is largely determined by the second-order HN₃ decomposition reaction and not by the reaction of HN₃ with H, as was previously assumed. Calculation of the flame propagation velocity according to the Zeldovich-Frank-Kamenetsky theory with the decomposition reaction as a limiting stage yielded a value that agrees with that obtained in numerical modelling using the complete reaction mechanism.