Measurements of combustion efficiency in nonequilibrium RF plasma-ignited flows

The paper presents results of combustion efficiency measurements using Fourier transform infrared absorption spectroscopy in premixed hydrocarbon–air and CO–air flows excited by a low-temperature transverse radiofrequency (RF) discharge plasma. The results demonstrate that significant fractions of these fuels are burned in RF plasma-generated and -stabilized flames (up to 100% of ethylene, 70% of methane, and 40% of CO). The results also show that fuel oxidation in the RF plasma-excited flows is most efficient in lean mixtures. Measurements of reaction product concentrations in hydrocarbon–air and CO–air mixtures show that significant amounts of fuel react under conditions when there is no flame detected in the test section. Under these conditions, fuel species oxidation occurs in plasma chemical reactions, without producing ignition. Also, experiments in CO–air flows demonstrated ignition and combustion at the equivalence ratios well below the lean flammability limit for CO. Finally, experiments in higher power RF discharges (500 W) showed that nonequilibrium plasma ignition occurs at flow velocities up to at least u=60 m/su=60 m/s and at flow residence times exceeding ∼1 s∼1 s. The effect of significant fuel oxidation, which is observed for lean fuel–air mixtures outside the flammable range, before the ignition occurs, provides additional evidence for the nonthermal fuel oxidation triggered by plasma-generated radicals. The present results, combined with previous experiments in which RF plasma ignition was demonstrated to occur in a low-temperature plasma, suggest the following nonequilibrium plasma ignition mechanism: (i) active radical species generation by the nonequilibrium plasma, (ii) plasma chemical reactions of fuel species oxidation with participation of these radicals, (iii) flow heating due to net exothermal plasma chemical fuel oxidation process, and (iv) subsequent thermal ignition and combustion.