Characteristics of the Flame Propagating through a Mixture Flow with Periodic Concentration Fluctuation

Abstract

An experimental study was made on the flame propagating through a lean methane/air mixture flow with periodic concentration fluctuation by using an original stagnation flow type burner. The burner was devised so as to fluctuate the mixture concentration (equivalence ratio) only in the direction of flow of constant velocity. For the fluctuation with the mean equivalence ratio of 0.85, the amplitude of 0.06 and the frequency ranging from 3 Hz to 50 Hz, the following results were obtained : (1) Phase difference exists between the fluctuations of burning velocity and flame position in the stream and the former fluctuation precedes the latter one, meaning that the fluctuating flame has the maximum burning velocity (the minimum burning velocity) before it gets to the uppermost stream (the lowermost stream). (2) For a certain range of the fluctuation frequency, the maximum and minimum burning velocities of the flame are higher and lower, respectively, than those estimated for the mixture equivalence ratio fluctuation at the burner exit. Similar tendencies are found for both the temperature and the CO concentration of the burnt gas. This can be caused by the periodic change of the direction of flame movement ; when the flame moves against (along) the fuel flow, the mass flux at the flame front increases (decreases) as compared with the stationary flame.