Impact of Oxygen Enrichment on the Performance of Heat-Recirculating Micro-Scale Combustors

Abstract

Oxygen-enriched combustion (OEC) has shown the ability to improve the fuel efficiency and power density for burner-based power systems. By minimizing problems with flame quenching, OEC was also expected to improve the performance of microcombustors needed for micro-power generation. Oxygen-enriched microcombustors have the potential for compatibility with heavier fuels like JP-8 and elimination of the need for precious metal catalysts. This research effort sought to develop a fundamental understanding of OEC chemistry and apply that understanding to test the hypothesis that microcombustors could be made more robust through OEC. Using a flat-flame burner, this study experimentally investigated the flame structure and flame speed of oxygen-enriched methane and n-decane flames. The measured data was used to evaluate reaction mechanisms for the combustion of these fuels, as simple surrogates for real fuels, under elevated oxygen concentrations. Widely used reaction mechanisms had some limitations but were shown to correctly predict oxygenenriched methane flame-speed data at ambient pressure. Therefore, the reaction mechanisms were used in 2-D reacting flow simulations of heat-recirculating microcombustors at ambient pressure. Those simulations showed that oxygen enrichment modestly improved the lean limit for extinction. Subsequently, theoretical calculations were used to predict the low-flow and high-flow extinction limits for a range of design conditions. The theoretical model did not predict a low-flow heat-loss-induced extinction limit at the high adiabatic flame temperatures associated with OEC. The high-flow blow-off extinction limit was predicted to increase tenfold with OEC, although the expected pressure drop and device thermal stresses would be high at those operating conditions.