横風中の火炎風下に発生する旋風に関する実験研究

摘要

大規模都市火災や森林火災では，旋風と呼ばれる竜 巻状の空気の渦が発生して大きな被害を引き起こすこ とがある．旋風は，その猛烈な風によって人や 物などを飛ばして被害をもたらすだけでなく，旋風 が火災域に近づけば，火炎を含んだ竜巻状の渦である 火災旋風に発展することもある．%Whirlwinds that occur downwind of a fire area were experimentally studied using flow visualization techniques and laser doppler velocimetry. Two types of experiments were performed. First, the effect of the complicated flow near the ground on the formation of whirlwinds downwind of a flame was investigated using an ethanol flame on a circular thin ceramic plate held in the middle of the test section of a wind tunnel using thin wires. It was demonstrated that the formation of whirlwinds downwind of the flame requires a floor beneath the flame. Secondly, the effects of cross-flow velocity and flame flickering on the characteristics of whirlwinds were investigated using the diffusion flame which was produced by injecting methane gas from a 30-mm diameter circular porous plate surface placed flush with the floor at a flow rate of 4.0 L/min into a cross-flow. The measured shedding frequency of the whirlwinds increased with increasing cross-flow velocity ranging from 0.13 to 0.37 m/s, corresponding to a Reynolds number from 262 to 727. The Strouhal number ranged from 0.05 to 0.42, which is much wider than the range of a Karman vortex wake in flow past a circular cylinder or of a jet wake in a cross-flow. The measured flame flickering frequency in the cross-flow ranged from 6.6 to 9.6 Hz and did not coincide with the whirlwind shedding frequency, which ranged from 0.3 to 4.2 Hz. The flickering frequency increased suddenly in a discontinuous manner at a cross-flow velocity of 0.28 m/s; this also did not coincide with the dependence of the whirlwind shedding frequency on the cross-flow velocity. The measured maximum tangential wind velocity of whirlwinds was almost constant at a cross-flow velocity ranging from 0.13 to 0.22 m/s.