Experimental Investigation of CH_4∕Air Inverse Diffusion Flame Stabilization by Nonequilibrium Plasma

摘要

A plasma injector is designed to improve the flame stabilization of a typical coaxial injector. Based on the plasma injector experimental system, the effects of an ac dielectric barrier discharge generated plasma on a CH4/air inverse diffusion flame are investigated. The electrical characteristics, discharge images, plasma species, thermal effect, gas dynamic effect, flame shape, and heat release are analyzed in detail. The results show that species including C, C-2, C-3, and CH are generated in the methane discharge plasma. The concentration of CH radically increases linearly with increasing voltage, while its spatial distribution tends to be uniform. However, the CH radical concentration decreases with an increasing flow rate. For application to plasma-assisted combustion, the thermal effect of the ac dielectric barrier discharge plasma on the CH4 jet can be neglected. The heating is limited to within the injector. The CH4 jet angle, which reflects the transverse movement degree of the methane flow, is expanded by the discharge. Because of the gas dynamic effect of the plasma, which changes the pattern of the original CH4 jet, the mixing between the fuel and the oxidizer is improved when the discharge is active. The original unstable flame with a single layer is transformed into a stable double-layer flame, and the heat release from the flame is enhanced. However, the stable flame might become unstable if the discharge voltage is too high. The plasma shows better performance with respect to flame stabilization under a high flow rate or a fuel-rich condition. The lowest cost-effectiveness ratio is only 0.7% in the experiment.