Effects of radiation reabsorption on the laminar flame speed and NO emission during aviation kerosene combustion at elevated pressures

Abstract

Increasing attention has been paid on combustion stability and pollution emission of aviation kerosene due to the emerging interests on supersonic combustion scramjets. Whereas, the vitiation component H2O introduced by hydrogen-fueled heaters in high-enthalpy vitiated air during ground experiments has a considerable influence on kerosene combustion, especially through its radiation effect, which needs to be further investigated. In this paper, the radiation reabsorption effects on laminar flame speeds and NO emissions during RP-3/H2O/O2/N2 combustion was assessed numerically over a wide range of equivalence ratio and pressure (ϕ = 0.7–1.4 and P = 1–15 atm) using detailed chemical and radiation models. The surrogate model of RP-3 consisted of vol. 25% 1,3,5-trimethylbenzene (C9H12), 46.31% n-decane (C10H22) and 28.69% iso-dodecane (IC12H26), while the vitiated air had 12% H2O. It was revealed that the radiation reabsorption of H2O in the vitiated air had significant impact on the accurate simulation of laminar flame speeds. As equivalence ratios varied, the role of radiation reabsorption on laminar flame speeds was most pronounced at ϕ = 0.7. As the key radical, the generation of H through the reversed step of CH2OH + H = CH3 + OH was chemically inhibited due to radiation. The radiation reabsorption effect on flame speeds was strengthened with rising pressures, with the reaction H + O2 = O + OH dominant at the pressure range 1–10 atm. In contrast, a slight increase in the impact on laminar flame speeds between 10 and 15 atm was controlled by direct radiative effect. Finally, for NO emission, the reduction of downstream temperature caused by radiative heat loss and the increment of radical concentrations induced by preheating determined radiation reabsorption effects on NO generation.