Super adiabatic flame temperature phenomenon for NH3/O2/N2 mixtures

Abstract

The super adiabatic flame temperature (SAFT) phenomenon is important for understanding and controlling the flame stability, NO emission, reaction rate and other aspects of combustion. Therefore, a numerical study was conducted to study the SAFT phenomenon for NH3/O2/N2 mixtures under different initial conditions. It was found that the main characteristics of SAFT for NH3/O2/N2 mixture are the negative heat release in the post-flame zone and the overshoot of H2O. The chemical nature of SAFT was also analyzed and found that the reverse reaction of R4 (H2 + OH = H2O + H) in the post-flame zone, resulting in the overshoot of H2O concentration, is one of the reasons of SAFT. The study also found that H radical plays an important role in SAFT phenomenon, and the relative lack of H radical in the main reaction zone is another one of the important factors of SAFT phenomenon. In addition, the study found that the effects of initial temperatures, pressures and oxygen content on SAFT have opposite trends at different equivalence ratios. When the equivalence ratio is less than 1.6, the increase of initial temperatures and oxygen contents leads to the increase of SAFT; the increase of initial pressure leads to the decrease of SAFT. The opposite tendency occurs when the equivalence ratio exceeds 1.6. This is because the flame temperature and structure changes with the equivalence ratio increasing, and the chemical nature of SAFT changes accordingly.