Plasma-Assisted Combustion in a Coaxial Re-Entrant Microwave Cavity

Summary form only given. Potential advantages of combining electrical energy with combustion include a faster process, higher intensities, leaner combustion, pollutant reduction, improved fuel efficiency, more reliable ignition and combustion across a wider range of pressures, temperatures and stoichiometries. Recently we developed an experimental system that enabled the repeatable, controlled coupling of microwave energy into a premixed flame. In this experimental system microwave energy was coupled into a flame that was located inside tunable microwave cavity applicator. We describe an improved experimental microwave plasma-assisted combustion employing a new, more efficient, and compact microwave applicator. The applicator consists of a tunable system, re-entrant coaxial cavity that has been modified to allow a combustion flame to be located inside the applicator. This newly modified system has the potential to greatly reduce the system size and because of the improved microwave focus; to improve the microwave flame coupling efficiency. The re-entrant applicator employs a 2.54 cm diameter cylindrical coaxial cavity with a hollow 1.11 cm diameter center conductor. The flame is placed in the vicinity of the cavity applicator "gap region" at the end of a 5 mm diameter gas feed tube that has been placed axially within the center conductor. Microwave power at 2.45 GHz is coupled into the re-entrant cavity via a coaxial coupling loop antenna at power levels of 3-30 Watts. The flame is ignited at atmospheric pressure and the feed gas flows through a nozzle hole of 200-400 mum diameter located at the end of the center conductor. This flame applicator system is experimentally evaluated with a variety of feed gas mixtures including argon with nitrogen, methane and oxygen, and other hydrocarbon gases. Diagnostic measurements performed include (1) fuel lean extinction/flammability limits (2) discharge luminosity and power densities (3) discharge volume and size (4) spatial g- as temperature distribution measured by optical emission spectroscopy and (5) atomic radical species produced by the plasma-assisted combustion mode. These measurements are made versus absorbed microwave power, gas flow rate, and gas mixture composition. Experimental results indicate that the applicator system can be reduced by a factor of ten with an associated order of magnitude improvement in microwave/flame coupling efficiency.