Numerical analysis on the combustion characteristics of NH3/H2/air flames with elevated initial pressure and temperature

Abstract

In response to the global warming crisis, NH3/H2 blended fuel has aroused great interest as a promising energy carrier. Consequently, this study aims to construct a laminar burning velocity (LBV) correlation and perform a comprehensive analysis of combustion characteristics for NH3/H2/air flames with elevated initial pressure and temperature. For a = 0–0.4 (H2 mole fraction), the model (SL=exp[∑i=12(ai⋅ΔHi∑i=12(ai⋅ΔHi)⋅lnSL,i)]) based on the energy fraction mixing rule has sufficient accuracy in predicting the LBVs of NH3/H2/air flames at various work conditions. For a = 0.4–1, the model SL=exp[∑i=12(ai⋅lnSL,i)] has a better performance. For NH3/H2/air flames, branching reactions (R1: H + O2 <=> O + OH and R68: NH2 + NH <=> N2H2 + H) are the main reactions that promote LBVs, while the recombination reactions (R9: H + O2(+M) <=> HO2(+M) and R67: NH2 + NH = N2H3) are the main reactions that suppress LBVs. Raising the initial pressure and temperature can accelerate the reaction rates of NH3/H2/air flames. Furthermore, it is discovered that the decrease in LBVs of NH3/H2/air flames with elevated initial pressure is mainly due to the thermal diffusivity greatly decrease. The increase in LBVs of NH3/H2/air flames with elevated initial temperature is the result of the increase both in reaction rate and thermal diffusivity. The overall reaction orders of NH3/H2/air flames decrease with rising initial pressure and H2 mole fraction. Finally, it is discovered that the hydrodynamic instabilities of NH3/H2/air flames obviously increase with elevated initial pressure, and decrease with elevated initial temperature. The diffusional-thermal instability slightly increases with elevated initial temperature, and the initial pressure has little influence on it.