Plasma-Enhanced Ignition and Flame Stabilization in Microwave Plasma-Assisted Combustion of Premixed Methane/Oxygen/Argon Mixtures

The microwave plasma-assisted combustion (PAC) of premixed methane/oxygen/argon mixtures was investigated. A U-shaped dual-layer curve of fuel ignition/flame stabilization limit was observed, which shows the effects of the plasma power on the fuel ignition and flame stabilization. The PAC flame images revealed a quasi-stabilized flame region that is located in the gap between the upper (the fuel ignition limit) and lower layers (the flame stabilization limit). Optical emission spectroscopy showed different emission spectral features at the same plasma power but in different up-tunings (increasing the plasma power from low to high) and down-tunings (decreasing the plasma power from high to low) of the plasma power, which suggested that different chemical pathways were involved in these two processes. Cavity ringdown spectroscopy measurements of OH(X) number density outside the combustor showed that the OH(X) number density is on the order of 10¹⁶ molecule cm^-3 when the PAC flame was achieved yet zero when the PAC flame was not achieved. It is speculated that the OH(X) radical plays an important role in the flame stabilization in the PAC.