Numerical modeling of ignition enhancement of CH ₄ /O ₂ /He mixtures using a hybrid repetitive nanosecond and DC discharge

Ignition enhancement using a hybrid repetitive nanosecond and DC discharge is studied in CH ₄ /O ₂ /He mixtures at atmospheric pressure by using a hybrid ZDPlasKin-CHEMKIN method. Special attention is placed on the control of vibrational and electronic excitations of methane and oxygen using different electric field strengths. A plasma-ignition kinetic mechanism incorporating the reactions involving both vibrational and electronic excitations of CH ₄ and O ₂ as well as the low temperature methane oxidation pathways of O ₂ (a ¹ ? _g ) is developed and validated. The results show that the hybrid non-equilibrium plasma excitation is much more effective in ignition enhancement than thermal heating and the nanosecond discharge alone. O ₂ (a ¹ ? _g ) is generated more efficiently in the hybrid discharge and shortens the ignition delay time more effectively than O( ¹ D) and O produced in the nanosecond discharge. Vibrationally excited species CH ₄ (?) and O ₂ (?) are also produced but mainly contribute to ignition enhancement via energy relaxation and gas heating. The results also show that e+O ₂ ?e+O ₂ (a ¹ ? _g ) and e+O ₂ ?e+O+O( ¹ D) reactions compete with each other for oxygen consumption and play opposite roles in ignition enhancement in a hybrid NSD/DC discharge with a given plasma energy. For a given repetitive nanosecond discharge, there is an optimum DC electric field strength which has the minimum ignition delay time due to the selective production of excited species and the difference in electron density. This work provides fundamental understanding for the design of a hybrid discharge to optimize low temperature ignition enhancement and fuel reforming.