X-shaped flames consisting of rotating slant flamelets

X-shaped flames were visually observed when a coaxial burner consisting of two coaxial streams of different premixed streams of methane–air was operated near blowout. High-speed photography showed that a rotating, slanted flamelet was formed; the shape of the flamelet was an asymmetric triangle with a curved surface. The rotational frequency was about 30 Hz and made no sound. Similar X-shaped flames have previously been observed in sudden-expansion tubes. Particle tracks were photographed by introducing seed particles into the flame and using a digital camera synchronized with the rotating flamelet to track the particle motion. The particle tracks show that the streamlines curve on the surface of the passing flamelet and consequently make a spiral flow. Because these flamelets rotate, the burning velocity has a circumferential component. This is different from the behavior reported for rotating polyhedral flames. The rotational frequency is strongly influenced by the mixture composition and gas velocity. The rotational frequency increases with increasing fuel–air ratio and decreases with either increasing inner-gas velocity or decreasing outer-gas velocity. From our tests, as the reactant preheat temperature was increased, the rotational frequency increased from 21–29 Hz at a temperature of 293 K to 27–35 Hz at a temperature of 346 K. The wave speed was higher than the laminar burning velocity. The ratio of the rotational frequencies of 27 Hz (at 346 K) to 21 Hz (at 293 K) is 1.29; this ratio is the same as the ratio of the laminar burning velocity at 346 K to that at 293 K.