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

抄録

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 U₂ to the higher U₁ from U₂/U₁-1.0 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 ( U₂/U₁ ≥ O.5), transition flow with strong interaction between two vortex sheets (0.5≤<U₂/U₁≤0.15), and free shear flow type (U₂/U₁≤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.