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 gas 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.