Flame size measurements of premixed propane-air mixtures ignited by microwave-enhanced plasma

The effects of microwave-enhanced plasma ignition on unsteady, expanding, premixed propane-air flames were studied using Schlieren technique in a constant volume chamber at various equivalence ratios and different microwave exposure durations. The plasma was generated by a spark discharge in engine-like conditions. Under these conditions, the dominant factors affecting the flame were stretching, heat transfer, and wall confinement effects. The spark was enhanced using microwaves, which accelerated the electrons in the spark, generating a non-thermal plasma. The flame size, flame speeds were computed from the visualized flame fronts. The experimental results indicated that an unsteady ignition kernel precedes the flame propagation and that there are multiple regimes prior to the final steady regime, in which the flame propagation is sustained. There is a minimum flame size which spark kernel must achieve, below which the flame could not sustain for successful combustion. The measured minimum flame size is a function of mixture equivalence ratio, and it decreases with increasing equivalence ratio. Increasing the microwave energy resulted in higher initial flame velocity and a reduction in the time taken to reach the critical flame size. Measurement of microwave-enhanced plasma size at non-reactive conditions indicated that plasma size is larger than spark-only case. At lean mixture conditions, enhancing the spark by microwave addition proven to enlarge the size of the ignition kernel, and achieve the required minimum flame size, that successfully helps in transitioning the initial spark kernel to a propagating flame, whereas spark-only operation failed to sustain the flame.