半円柱後流の非対称渦列に形成される拡散火炎の構造と挙動

概要

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Effects of nonsymmetrical wake structure on turbulent diffusion flames, stabilized behind a semicircular cylinder are experimentally investigated. Interactions between two vortex sheets in cold flows are also examined. By varying the ratio of the lower airstream velocity U_2 to the higher U_1 from U_2/U_1-1.0 to 0.083, detailed optical observations and measurements of velocity and temperature are made on nine flames. Cold flows behind a semicircular cylinder are classified into three patterns depending on the velocity ratio : wake-type flow ( U_2/U_1 ≧ O.5), transition flow with strong interaction between two vortex sheets (0.5≦<U_2/U_1≦0.15), and free shear flow type (U_2/U_1≦0.15). Flames also exhibit a sudden transition from the bluff-body- stabilized wake flame to the plane diffusion flame, indicating good agreement with transition in the cold flow. A comparison of the vortex characteristics in the cold flow with those in the reacting flow shows that the flame-flow interaction originates mainly from a combined effect of the suppression of the K-H instability due to the positive temperature dependence of the kinematic viscosity, the expansion and acceleration due to the exothermic reaction and the strong pressure fluctuations associated with turbulent combustion.
1993-12-25