Investigations into the flame stability limits in a backward step micro scale combustor with premixed methane-air mixtures

In this paper, experimental and numerical investigations into the characterization of flame stabilization behavior in a 2.0 mm base diameter inlet with two backward steps and premixed methane-air mixture are reported. Parametric investigations are carried out to understand the effect of step length, mixture equivalence ratio (phi), flow rate and wall thermal conductivity on flame stability limits, flame position, wall temperature profile and pollutant emissions. It was observed that the recirculation zone created due to sudden flow expansion at the backward step modifies the flow velocity profile, helps in stabilizing the flame within the combustor and enhances the flame stability limits significantly. The increase in the length of the first step helps in enhancing the flame stability limits at both lower and higher flow rates. The increase in the length of the second step affects the flame stability limit at higher flow rates only. The effect of wall thermal conductivity on flame stability was investigated by employing various materials, such as copper, brass, steel and quartz for combustor fabrication. From these studies, it was inferred that fabrication with a high thermal conductivity material decreases the flame stability limits substantially and no stable flame was observed for a copper-based microcombustor. However, with a quartz-based microcombustor, stable flames were observed at minimum flow rates, which resulted in a minimum thermal input of similar to 5 W for a lean mixture (phi = 0.7). Measurements of pollutant emissions at the exit showed that the CO emission factor increases with equivalence ratio and no traces of NOx emission were found.