Role of H₂ doping in freely-propagating C₂H₂–O₂ flames: Sensitivity and flame structure

The effects of hydrogen (H₂) addition on the freely-propagating acetylene-oxygen (C₂H₂–O₂) flames in a doubly-infinite domain are investigated, considering separate and coupled chemical and transport phenomena. Results show that increasing proportions of H₂ enhance the laminar flame speeds of C₂H₂–O₂ mixtures. Moreover, H₂ promotes the C₂H₂–O₂ flame to contain both light (H, H₂) and heavy (C₂H₂) species. Compared with the Soret diffusion and radiation effect, the unity Lewis number assumption can induce substantial inaccuracies. Sensitivity analysis indicates that the key elementary reaction H + O₂[dbnd]O + OH exerts stronger influences on the laminar flame speed. Additionally, due to the effective enthalpy of C₂H₂ being greater than H₂, adding H₂ decreases both adiabatic flame temperature and enthalpy of C₂H₂–O₂ flames. Notably, reactions C₂H₂ + O[dbnd]H + HCCO and C₂H + H₂[dbnd]H + C₂H₂ in the C₂H₂ sub-mechanism are identified as important exothermic- and endothermic-reactions respectively. Furthermore, it is demonstrated that dilution effects from CO₂ have a larger impact compared to those from H₂O and Ar. Finally, the effects of initial pressure and temperature on the laminar flame speeds are also discussed. The results of the present work provide guidance for the effective use of binary fuel and risk control as well as fire suppression.