Energy deposition and plasma propulsion during the time window of microwave-assisted CH4-O2 spark ignition with Ar/N2/CO2 dilution

Microwave-assisted spark ignition (MAI) is a promising technology to achieve ultra-lean combustion and thus satisfy the requirements in the low-carbon age. However, the coupling process between microwave and the early flame kernel is still a complex and critical question before its wide application and further optimization. Therefore, this work explores the energy deposition and plasma propulsion effect during the MAI through the ignition experiments fueled by the methane-air mixture under a range of equivalence ratios with argon (Ar), nitrogen (N2), and carbon dioxide (CO2) as the carrier gas. Both the flame and plasma characteristics are recorded and analyzed through the high-speed shadowgraph method. The assessment of the time window and the microwave energy depositing process is based on the voltage of the spark electrode and the detected voltage of reflected microwave power. Results show that in Ar and N2 cases, the microwave enhancing effect on early flame development decreases as the equivalence ratio approaches the stoichiometric ratio, but this tendency is less obvious in the CO2 case. During the MAI, there is a time window for the microwave energy deposition, and once it ends, the following microwave pulses will no longer be absorbed. In successfully ignited cases, the time window in MAI mode is 1.8 ms in the N2 case and 1.5 ms in the CO2 case, while it is extended to 2.4 ms when adopting Ar as the carrier gas causing one more microwave pulse absorbed. Generally, changes in equivalence ratio show no obvious influence on the energy-absorbing process. Microwave pulses during the time window periodically generate a plasma jet that originates from the electrodes gap region towards the flame front. This plasma jet acts as the medium for the microwave influencing the flame development, and thus the relative position between the plasma jet and the flame front determines the final microwave enhancing effect.