Ignition of hydrocarbon fuels by a repetitively pulsed nanosecond pulse duration plasma.

摘要

The dissertation presents experimental and kinetic modeling studies of ignition of hydrocarbon-air flows by a high voltage, repetitively pulsed, nanosecond pulse duration plasma. This type of plasma offers two critical advantages. First, a high reduced electric field during the pulse results in efficient electronic excitation and molecular dissociation. Second, extremely low duty cycle of the repetitively pulsed nanosecond discharge greatly improves the plasma stability and helps sustain a diffuse and uniform nonequilibrium plasma.; Gaseous fuel ignition experiments using a Chemical Physics Technologies (CPT) 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°C. The experiments showed that adding fuel to the air flow increased the flow temperature in the plasma, up to 500-600°C. At these conditions, the reacted fuel fraction was up to 80%, and significant amounts of combustion products were detected. The experiments also showed significant fuel oxidation, with a resultant temperature rise, at conditions when there was no ignition detected. Replacing air with nitrogen at the same flow and plasma conditions resulted in much less plasma temperature rise. This demonstrates that the temperature increase is due to plasma chemical fuel oxidation reactions, rather than due to excited species quenching. 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, due to an additional energy release in plasma chemical reactions. The present results also showed that plasma assisted ignition occurred at a low discharge power, ∼1% of heat of combustion.; Experiments in hydrocarbon-air plasmas generated by an alternative, Fast Ionization Dynistor (FID) pulse generator (50 kV peak voltage, 5 nsec pulse duration, up to 100 kHz pulse repetition rate) did not result in ignition. It is concluded that a lower pulser energy coupled to the flow by the FID pulser resulted in less flow heating and presumably in lower radical concentrations generated in the plasma, thereby precluding ignition.; Ignition experiments in liquid methanol- and ethanol-air mixtures by the CPT pulser showed that preheating of the air flow up to 50-60°C is critical for producing ignition. Ignition was achieved, and significant plasma temperature rise and fuel oxidation were detected in preheated methanol-air and ethanol-air flows.; A kinetic model was developed to simulate plasma assisted ignition of hydrocarbon-air mixtures by a repetitively pulsed, nanosecond pulse duration, low-temperature plasma. The model was validated by comparing with O atom concentration measurements in single-pulse discharges in air, methane-air, and ethylene-air, showing good agreement. Kinetic modeling of a repetitively pulsed discharge at the present experimental conditions did not predict significant fuel oxidation or ignition at the measured discharge power. The model predicts that ignition would occur only 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 (starting at room temperature) of the present experiments. This demonstrates the need for development and validation of a low-temperature hydrocarbon oxidation in non-equilibrium plasmas.