INVESTIGATION OF PLASMA ASSISTED COMBUSTION

Abstract

The combustion performance of methane/air triple flames has favorably been affected upon using modulated frequency, pulsed electric discharge via spark plugs. The computations predicted that the density/velocity gradients introduced by the spark pulsating temperature field upstream of the flame base increased the peak local acceleration to 251 m/s2. At 40 Hz, a velocity difference as high as 3.5 times the mixture local velocity was provided ahead of the flame lift-off position. At 120 Hz, the normal triple flame stability limit was extended to 14.9 m/s, where 0.011% of the input energy was required for the dielectric breakdown of gases across a 2 mm gap between the two electrodes to involve plasma assisted combustion. Although the peak temperature around the spark increased at least by 1200 K, the correspondingly reduced residence time across the flame zones by 93.3% reduced the NOx specific emissions by 76%. While the flame tip position was shifted downstream along the first 50% of the combustor length, the HC and CO emissions respectively reached magnitudes as low as 0.09% and 730 ppm. By displacing the high voltage electrode along the combustor centerline, the favorable turbulence effects of the spark discharge pulsating temperature on over-ventilated inverse triple flames were maximized at 120 Hz. As vi the mixture fraction gradient was thus reduced to 0.01 cm-1, a flame speed increase of 16.4 m/s provided a maximum firing intensity of 28.6 MW/m3; while the heat flux across the combustor increased with the flame stabilization height. Having the peak rate of radiation heat transfer earlier than the convection one, the efficiency was maintained above 29%. Increasing the mixtures' velocity difference to 3.0 m/s increased the turbulent kinetic energy to a maximum of 9.8 m2/s2 such that enlarging the gap distance to 4 mm increased the stability limit by 21.7%.