Study On Formation Condition And Flame Characteristics Of Cylindrical Fire Whirl

Fire whirl is a typical extreme fire behavior in forest and urban fires.It is a swirling diffusion flame induced by the interaction between the buoyant fire plume and the ambient rotating flow field.Compared with the general free buoyant fire,the fire whirl involves a greater burning rate,flame height,flame temperature,and flow velocity.Moreover,fire whirl could induce long-distance spotting fire,resulting in the discontinuous spread of fire.Therefore,fire whirl is an important mechanism for the formation of large-scale fire and the acceleration of fire spread.Deeply revealing the combustion dynamics mechanism of fire whirl could provide the scientific basis for developing the prevention and control technology of forest and urban fires.The heat release rate(fire intensity)and the imposed circulation(the strength of the rotating flow field)are the two key parameters controlling the fire whirl behavior.The fire field,the wind,and the topographic conditions are complex and changeable in forest and urban open spaces,resulting in a wide range of changes in the intensity of the rotating flow field.Correspondingly,the combustion behaviors of real fire whirls are complicated,such as conical fire whirls under weak imposed rotation and cylindrical fire whirls under strong imposed rotation.Previous research mainly focused on the quasi-steady combustion characteristics of the conical fire whirls,while research on the cylindrical fire whirls is scarce.This thesis studied the formation and flame characteristics of cylindrical fire whirl through the combined experimental and theoretical research.The formation criterion and mechanism of the cylindrical fire whirl were revealed,and the variations of.the flow field,flame height,and flame diameter with the heat release rate and the imposed circulation were obtained.It mainly includes the following three parts:The formation mechanism is analyzed and the formation predicted model is derived for the cylindrical fire whirl.The heat release rate and the imposed circulation are decoupled,and the evolution of various flame patterns is revealed in the turbulent fire whirl.The vortex breakdown instability is the probable formation mechanism of the cylindrical fire whirl.Based on the governing equations of the buoyant diffusion flame under strong rotation conditions,the prediction model for the critical formation of cylindrical fire whirl is established by similarity analysis.It is found that the critical dimensionless imposed circulation depends on the dimensionless heat release rate by the 1/4 and 1/3 power laws for laminar and turbulent cylindrical fire whirl,respectively.The evolution of the velocity field of the cylindrical fire whirl is revealed.The stereo particle image velocimetry system(SPIV),applicable for the rotating-screen fire whirl facility,was developed,and the evolution of the axial plane velocity field with the imposed circulation is revealed.Two-celled vortex(TCV)and central recirculation zone(CRZ)structures were found in the plume zone of the cylindrical fire whirl,which confirmed the existence of vortex breakdown therein.It is found that the centerline axial velocity at the specific height and the mean flame height change synchronously:in the conical fire whirl,both of them increase with the increasing imposed circulation,while in the cylindrical fire whirl,both of them decrease with the increasing imposed circulation.The evolution of flame height and diameter of turbulent cylindrical fire whirl with the heat release rate and the imposed circulation was revealed.The mean flame height decreases with the increase of the imposed circulation,but it is always greater than that of the free buoyant flame with the same heat release rate.By radially integrating the governing equations of momentum and mixture fraction,a semi-physical model of the mean flame height of turbulent cylindrical fire whirls is established.It couples the two critical parameters of the heat release rate and the imposed circulation and agrees well with the experimental data of this work.The results show that the increase of turbulence intensity due to flow instability is a critical mechanism for the decrease of flame height of the cylindrical fire whirl.