%0 Journal Article %K Buoyancy %K Burner rotation %K Lewis number %K Microgravity %K Premixed flame %A Hiroshi Gotoda %A Kazuyuki Maeda %A Toshihisa Ueda %A Robert K Cheng %B Combustion And Flame %D 2003 %F Combust %G eng %N 1-2 %P 67-79 %R 10.1016/S0010-2180(03)00082-8 %T Periodic motion of a Bunsen flame tip with burner rotation %V 134 %8 07/2003 %X

Effects of burner rotation on the shapes and dynamics of premixed Bunsen flames have been investigated experimentally in normal gravity and in microgravity. Mixtures of CH4-air and C3H8-air are issued from the burner tube with mean flow velocity U = 0.6 m/s. The burner tube is rotated up to 1400 rpm (swirl number S = 1.58). An oscillating flame with large amplitude is formed between a conical-shape flame and a plateau flame under the condition of Lewis number Le > 1 mixtures (rich CH4-air and lean C3H8-air mixtures). In contrast, for Le ≤ 1 mixtures (lean CH4-air and rich C3H8-air), asymmetric, eccentric flame or tilted flame is formed under the same swirl number range. Under microgravity condition, the oscillating flames are not formed, indicating that the oscillation is driven by buoyancy-induced instability associated with the unstable interface between the hot products and the ambient air. The flame tip flickering frequency ν is insensitive to burner rotation for S < 0.11. For S > 0.11, ν decreases linearly with increasing S. As S exceeds 0.11, a minimum value of axial mean velocity along the center line uj,m due to flow divergence is found and it has a linear relationship with ν. This result shows that uj,m has direct control of the oscillation frequency. When S approaches unity, the flame oscillation amplitude increases by a factor of 5, compared to the flickering amplitude of a conical-shape flame. This is accompanied by a hysteresis variation in the flame curvature from positive to negative and the thermo-diffusive zone thickness varying from small to large. With S > 1.3, the plateau flame has the same small flickering amplitudes as with S = 0. These results show that the competing centrifugal and buoyancy forces, and the non-unity Lewis number effect, play important roles in amplifying the flame-tip oscillation.