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A plasma injector element was designed to experimentally study the mechanism of methane-air diffusive flame stabilized by a discharge plasma. The air plasma was generated within the annulus gap of the injector by alternating current dielectric barrier discharge. The discharge voltage, current and photographs were recorded first. Three internal effects of the plasma on combustion were later investigated separately through several diagnostic methods, including optical emission spectrometry (OES), infrared thermography, thermocouple, infrared thermometer, schlieren imaging, photos and CH* chemiluminescence. Finally, the return on investment (ROI) was calculated. The results showed that a large number of filamentary micro discharge paths occur within the discharge gap. These discharge paths rotate anticlockwise at high speed and act as a virtual ‘fan’ to induce the flow jet. The velocity of the induced jet increases with increasing discharge voltage. The original jet expansion angle is enlarged by the radial velocity component of the induced jet, resulting in the mixing enhancement of the air and methane. The plasma rotational temperature (the first negative system ) obtained from OES is close to the discharge gas temperature measured by infrared thermography, indicating that the discharge gas temperature can be approximately represented by the rotational temperature. According to the measured temperature of the injector and the jet, the impact of the thermal effect of the plasma on flame stabilization is negligible. Due to the plasma, the height of the flame center and its representative length are generally reduced as the voltage rises, and the methane-air mixture becomes ignitable, and a stable flame can be reached under the conditions in which direct ignition fails. The combustion is enhanced with increasing heat release rate of the flame by the plasma. This finding revealed that the ROI of plasma-assisted flame stabilization is lower under a higher flowrate and a larger equivalence ratio for unstable flame situations. [ABSTRACT FROM AUTHOR]