On the formation of spinning flames and combustion completeness for premixed fuel–air mixtures in stepped tube microcombustors

Various premixed fuel–air mixtures are introduced in stepped tube microcombustors. The experimental investigations show the formation of various steady and unsteady spinning flame propagation modes for a range of flow rates and mixture ratio conditions due to thermal-wall coupling. The spinning flames of very high frequency, ∼150 Hz have been observed for the first time with various fuel–air mixtures. The spinning frequency of these flames is almost twice that of flames observed in earlier work. The spinning flames were observed for both lean and rich mixtures of methane-, LPG- and propane–air mixtures. The formation of these spinning flames depends on the mixture flow rate, equivalence ratio and number of steps in the microcombustor. Exhaust gas analysis was carried out for different operating conditions. It was observed that contrary to general perception of low combustion efficiency and high CO emissions associated with such rotating and spinning flames, these high frequency spinning flames led to complete combustion with almost zero CO emissions.