Ignition of Ethylene–Air and Methane–Air Flows by Low-Temperature Repetitively Pulsed Nanosecond Discharge Plasma

This paper presents results of low-temperature plasma-assisted combustion experiments in premixed ethylene-air and methane-air flows. The plasma was generated by high-voltage, nanosecond pulse duration, high repetition rate pulses. The high reduced electric field during the pulse allows efficient electronic excitation and molecular dissociation, thereby generating a pool of chemically active radical species. The low duty cycle of the repetitively pulsed discharge improves the discharge stability and helps sustain diffuse, uniform, and volume filling nonequilibrium plasma. Plasma temperature was inferred from nitrogen second positive band system emission spectra and calibrated using thermocouple measurements in preheated flows (without plasma). The experiments showed that adding fuel to the air flow considerably increases the flow temperature in the plasma, up to DeltaT = 250degC-350degC. On the other hand, adding fuel to nitrogen flow at the same flow and discharge conditions resulted in a much less pronounced plasma temperature rise, only by about DeltaT = 50degC. This shows that temperature rise in the air-fuel plasma is due to plasma chemical fuel oxidation reactions initiated by the radicals generated in the plasma. In a wide range of conditions, generating the plasma in air-fuel flows resulted in flow ignition, flameholding, and steady combustion downstream of the discharge. Plasma-assisted ignition occurred at low air plasma temperatures, 100degC-200 degC, and low discharge powers, ~100 W (~1% of heat of reaction). At these conditions, the reacted fuel fraction is up to 85%-95%. The present results suggest that the flow temperature rise caused by plasma chemical fuel oxidation results in flow ignition downstream of the plasma.