Microwave air plasma supersonic hydrocarbon combustion enhancement experiments

Short combustion chamber residence times for supersonic fuel-air mixtures has made using hydrocarbon fuels in SCRAMjet engines challenging. To use hydrocarbon fuels, hydrocarbon-air lean flammability limits and flame speeds must be extended. Injecting air plasmas into the combustion zone can do this. For this investigation, 1 kW of 2.45 Ghz electromagnetic energy was input into a resonant cavity to create free-floating plasmas. Spectroscopic studies were conducted to determine the constituents of nitrogen, oxygen, and air plasmas. Nitrogen plasmas were dominated by ionized molecular nitrogen. Oxygen and air plasmas in the chamber were dominated by excited neutral molecular oxygen and ionized molecular oxygen. The air plasma plumes emitted black body radiation at approximately 10, 000 K, and consisted of approximately 75% atomic nitrogen, 20% atomic oxygen, 2.05% free electrons, 1.8% ionized atomic nitrogen, 0.4% molecular nitrogen, 0.23% ionized atomic oxygen, 0.01% nitric oxide, and.009% ionized nitric oxide. The ionic molecules in air plasmas react with fuel and air molecules between five and eight orders of magnitude faster than neutral molecules typically found in combustion processes. These results indicate injecting microwave air plasmas into hydrocarbon fuel-air combustion systems can increase the flame speeds and extend the lean flammability limits significantly.