Kinetic analysis of H₂ addition effect on the laminar flame parameters of the C1-C4 n-alkane-air mixtures: From one step overall assumption to detailed reaction mechanism

In this work, numerical simulations on the laminar premixed C1-C4 n-alkanes with various hydrogen addition fractions (X_h) were conducted. Flame parameters, including laminar flame speed, adiabatic flame temperature (T_ad), flame thickness (δ_l), Lewis number (Le), Zeldovich number (Ze) and Markstein number (Ma) were calculated. Results show that hydrogen addition alters all flame parameters, especially at the sufficiently large X_h region. Based on one step overall reaction assumption, the hydrogen addition effect is demonstrated through three factors and it was found that the change in overall activation energy which represents the combustion kinetics, is the most prominent, compared to the change in adiabatic flame temperature and Lewis number. Detailed flame structure profiles show that the laminar flame speeds of all mixtures are linearly correlated with the maximum (H + OH) mole fraction and increase with X_h. To further understand the chemical kinetic effect of hydrogen addition on the increase of laminar flame speed, the reaction flux and rate of consumption of key radicals were examined. It is shown that at the small X_h region, the proportion of H radical consumption from R88 (CH₃+H(+M) = CH₄(+M)) increases, while the proportion of H radical consumption from R1 (H + O₂ = O + OH) decreases, thus relatively less OH radicals are produced. As a consequence, the increase in laminar flame speeds by hydrogen addition is slowed down. While at the large X_h region, the proportion of H radical consumption from R88 decreases dramatically, and the chain sequence R12 (H + O₂(+M) = HO₂(+M))/R16 (HO₂ + H = OH + OH) which is not sensitive at small X_h region is activated and significant amount of OH radicals are generated, leading to the remarkable increase of the laminar flame speed.