Sustained propagation of ultra-lean methane/air flames with pulsed microwave energy deposition

An investigation of pulsed microwave energy addition to laminar methane/air flame fronts is undertaken. Microwave coupling efficiencies of up to 60% are measured in a microwave resonator with very low average power requirements (<30 W). Pulsed microwave energy addition to laminar flame fronts is quantified using power absorption measurements and compared with filtered Rayleigh scattering temperature measurements of a laminar stagnation flame. Relative increases in nitric oxide concentrations are measured using a selective radar REMPI laser technique. For outwardly propagating methane/air flames, short microwave pulse bursts increase the effective flame speed up to 25%. With microwave pulse bursts, results show the ability to sustain deflagration fronts at ultra-lean equivalence ratios of 0.3 (compared to the 0.6 normal lean limit) with microwave energy deposition of 10% of the available thermal energy content of the fuel.