Study On The Liftoff Behavior And Stabilization Mechanism Of Weak Fire Whirl

Fire whirl is an extreme fire behavior induced by the coupling effect of buoyant diffusion flame and rotating flow,often occurring in wildland and wildland-urban interface fires.Fire whirl can roll up firebrands to induce spotting fire,increase the fire spread rate,and is the primary reason for the generation of multiple fires.Thus,studying the flame structure and dynamic mechanism of fire whirl contributes to fire prevention and the development of fire safety technology.On the other hand,when the fire whirl occurs,the flame temperature,flame height,and burning rate increase significantly,which has potential application prospects.Under certain conditions,a fire whirl can lift off and blow out,which belongs to combustion instability behavior and may cause problems such as backflame,flameout,and extinction,leading to fires and explosion accidents.Therefore,the formation of a fire whirl contributes to safe combustion,improving combustion efficiency,and reducing pollution emissions in industrial production.The combustion behavior of fire whirl is related to two key parameters:the fire source intensity and the rotation strength.Previous research mainly focused on quasi-steady combustion characteristics of fire whirl,but the study on the liftoff behavior and stability mechanism of fire whirl is rare.The stability map of the weak whirl and the formation condition of the lifted flame is studied through experimental research and theoretical analysis.Then,the mechanism of lifted flame formation is analyzed,and the flame liftoff height,flame length,and the flow field structure of the lifted fire whirl with rotation strength and nitrogen volume fraction are investigated.Finally,the flame stability mechanism and criterion are analyzed.The main conclusions are as follows:The flame stability map of weak fire whirl and the critical condition for forming lifted flame is established,and the mechanism of lifted flame formation is revealed.The effect of fuel type and nitrogen dilution on the liftoff behavior of weak fire whirl is investigated.It is found that the lifted flame bottom of the weak fire whirl exhibited approximately a triple flame structure.By analyzing the formation of the lifted flame,it is found that the steady liftoff behavior of the weak fire whirl is related to the Schmidt number(Sc).When Sc>1,the stable lifted flame can not be formed,and there was a maximum liftoff height;When Sc<1,the flame can lift off steadily,and there was a minimum liftoff height;When Sc≈1,the flame can not lift off.It is proved that adding nitrogen affects the flame liftoff behavior by reducing the equivalent Schmidt number of the gas mixtures.The variations of flame liftoff height,flame length,and diameter with the imposed circulation and nitrogen volume fraction are revealed.The mean flame liftoff height increases first and then decreases rapidly with the increase of imposed circulation.And the flame liftoff height increases with the nitrogen volume fraction Based on dimensional analysis,a correlation of the liftoff height of a weak fire whirl is developed,showing that the liftoff height mainly depends on the imposed circulation,followed by nitrogen volume fraction and heat release rate.The length of the lifted flame is less than that of the free buoyancy flame under the same conditions.In addition,the relationship between flame curvature,fuel mass fraction gradient,and flame propagation velocity is revealed.It is found that the curvature of the flame stability point increases with the imposed circulation.And the flame propagation velocity decreases with the increase of flame curvature,which affects the flame liftoff height.The flow field structure of the axial plane in lifted weak fire whirl is revealed,and the flame stability mechanism is proposed.Using the three-dimensional particle image velocity measurement system(SPIV),the evolution of the center axial velocity of the lifted flame and the average velocity of the flame stability point are studied by varying the nitrogen volume fraction and the imposed circulation.It is found that the axial velocity of the lifted flame changes with the height approximately in the shape of "S".The mean axial velocity of the lifted flame base is within the range of 1-2 times the laminar flame propagation speed.It is revealed that the nitrogen dilution causes an umbalance between flame propagation velocity and gas axial velocity,which changes the flame stabilization position.Flame disturbance analysis proves that the lifted flame can only be stabilized in the declining region of axial velocity.