The dissertation presents experimental and kinetic modeling studies of ignition of hydrocarbon-air flows by a high voltage, repetitively pulsed, nanosecond pulse duration plasma. A high reduced electric field during the pulse results in efficient electronic excitation and molecular dissociation, and extremely low duty cycle of the repetitively pulsed nanosecond discharge improves the plasma stability and helps sustain a diffuse and uniform nonequilibrium plasma.
Gaseous fuel ignition experiments using a pulser (16-18 kV peak voltage, 20-30 nsec pulse duration, up to 50 kHz pulse repetition rate) generating a plasma in premixed ethylene-air and methane-air flows demonstrated flow ignition occurring at low air plasma temperatures, 200-300 0C. The experiments showed that adding fuel to the air flow increased the flow temperature in the plasma, up to 500-600 0C. At these conditions, the reacted fuel fraction was up to 80%, and significant amounts of combustion products were detected. Replacing air with nitrogen at the same flow and plasma conditions resulted in much less plasma temperature rise. This suggests that low-temperature plasma chemical reactions can oxidize significant amounts of hydrocarbons and increase the temperature of the air-fuel mixture, prior to ignition. Ignition occurs when the flow temperature becomes close to autoignition temperature. The present results also showed that plasma assisted ignition occurred at a low discharge power, ~1% of heat of combustion. Ignition was achieved for liquid methanol- and ethanol-air mixtures, and significant plasma temperature rise and fuel oxidation were detected.
A kinetic model was developed to simulate plasma assisted ignition of hydrocarbon-air mixtures by the repetitively pulsed nanosecond plasma. The model was validated by comparing with O atom concentration measurements in single-pulse air and air-fuel discharges. Kinetic modeling at the present experimental conditions did not predict significant fuel oxidation or ignition. The model predicts that ignition would occur if the discharge power is 2.5 times higher than measured in the experiments. The difference between two hydrocarbon oxidation mechanisms predictions suggests that neither of them might be applicable at the low-temperature conditions. This demonstrates the need for development and validation of a low-temperature hydrocarbon oxidation in non-equilibrium plasmas.