| Fire whirl commonly appears in forest fires.Because of the strong suction and buoyancy,fire whirl enables the combustion faster,the temperature higher,and the flame longer.Fire whirl largely increase the difficulty in fighting fires,resulting huge casualties and economic losses.The atrium and inner channels of modern building provide favorable conditions in forming fire whirls,and bring huge potential hazards.In recent years,scholars have conducted a lot of researches on the formation mechanism,the movement patterns and the characteristics of the heat flow field of fire whirls.Vertical channel with side-slits has similar structure of inside channels in buildings.The research of fire whirl formed in vertical channel has significance in controlling building fires.In this paper,Fire Dynamic Simulator(FDS)is used for numerical simulation.Firstly,fire whirl simulations are conducted based on fire whirl experiments including a small-scale one and a middle-scale one.By comparing the characteristics of the flow field such as velocity and temperature,the accuracy of FDS is verified.Then,small-scale fire whirls formed in vertical channels of different side-wall heights,and middle-scale fire whirls formed in 15m high vertical channels with different side-slit lengths are studied.The buoyant flow velocities,the horizontal velocities,the angles of spiral flow lines and the partitioning feature of the flame are analyzed.The results show that,the higher side-wall results stronger suction,transverse shear force and entrainment force;smaller side slit lengths result stronger suction,transverse shear force and entrainment force;when the side slit length increases more than 10m,the continue increase no longer significantly affect the flow characteristics.Finally,dimensionless fittings of inflow jet velocity and plume fire temperature rise along the axis are conducted.The results verified that the dimensionless formula for critical lateral velocity of fire whirls in open space correlated by Kuwana is applicable for fire whirls formed in vertical channels;the dimensionless formula for the pool fire plume temperature rise along the axis that correlated by McCaffrey is amended for fire whirls,and the feasibility is verified by the fitting of simulation results. |
