Ignition enhancement of ethylene/air by NOx addition

Recently, non-equilibrium plasma assisted combustion (PAC) has been found to be promising in reducing the ignition delay time in hypersonic propulsion system. NO_x produced by non-equilibrium plasma can react with intermediates during the fuel oxidation process and thereby has influence on the combustion process. In this study, the effects of NO_x addition on the ignition process of both the homogeneous ethylene/air mixtures and the non-premixed diffusion layer are examined numerically. The detailed chemistry for ethylene oxidization together with the NO_x sub-mechanism is included in the simulation. Reaction path analysis and sensitivity analysis are conducted to give a mechanistic interpretation for the ignition enhancement by NO_x addition. It is found that for both the homogenous and non-premixed ignition processes at normal and elevated pressures, NO₂ addition has little influence on the ignition delay time while NO addition can significantly promote the ignition process. The ignition enhancement is found to be caused by the promotion in hydroxyl radical production which quickly oxidizes ethylene. The promotion in hydroxyl radical production by NO addition is achieved in two ways: one is the direct production of OH through the reaction HO₂ + NO = NO₂ + OH, and the other is the indirect production of OH through the reactions NO + O₂ = NO₂ + O and C₂H₄ + O = C₂H₃ + OH. Moreover, it is found that similar to the homogeneous ignition process, the acceleration of the diffusion layer ignition is also controlled by the reaction HO₂ + NO = NO₂ + OH.