Interaction Behaviors Of Two Fire Sources Burning Under Cross Wind

Multiple fires burning is common in typical realistic accident scenes involving environmental wind,such as forests,urban junctions,industrial fuel storage tank areas,and ancient buildings.Under the influence of cross wind,the burning intensity of multiple fire sources increases significantly,and it is easy to spread and develop into regional fires.The deep investigation of the combustion characteristics and dynamics of multiple fires under the action of environmental wind can provide theoretical basis and scientific guidance for regional fires in the area of prevention,monitoring and early fire fighting.This work systematically carried out theoretical analysis,experimental study,and numerical simulation,focusing on the coupling mechanism of heat feedback and blocking and air entrainment restriction,and developed the interactive restricted behavior representation model of multiple fire plumes in cross wind.The main conclusions are shown as follows:(1)Experiments on two burning gas fires under the effect of cross wind were carried out.The Matlab video image processing method based on mathematical theory was used to quantify the flame merging probability in windless and windy environments.It was revealed that the blocking effect induces the asymmetry of upstream and downstream flame tilt angles.Based on the momentum conservation and force analysis,the model of flame tilt angles was developed.The temperature method and video processing method were used to compare and analyze the evolution characteristics of flame length.It was revealed that the non-monotonic trend of the flame length was caused by the cross wind effect.And,the flame length segmentation prediction model for different merging stages was then established based on the dimensional analysis.A triangular prism flame model was proposed to predict the downstream horizontal radiant heat flux received from the two interacting flames in terms of the behaviors of flame merging and flame drag.The accuracy of the aforementioned models was validated by comparing with the experimental results or conventional correlations.(2)The effects of cross wind on the burning rate and heat feedback mechanism of double heptane pool fires were investigated.At first,the evolution characteristics of the flame interaction,flame height and flame tilting were revealed under the action of superimposed heat feedback.Then,combined with the flame morphology,the multiple effects of pool size,pool spacing and wind velocity on burning rate were analyzed and explained.In general,with the increase of wind velocity,the burning rates of upstream fire sources can be divided into three areas:rapid growth zone,decay zone and slow growth and tend to be stable zone.However,due to the blocking and air entrainment restriction and heat feedback between the interacting flames,the burning rates of downstream fire sources were more complex,and it was also affected by the pool scale.It should be noted that when two flames merged together,part of the flame body of the upstream fire source would cover the downstream fire source under the influence of cross wind,which increases the heat feedback received by the downstream fire source,and ultimately leads to the phenomenon that the burning rate of the downstream fire is greater than that of the upstream fire.This phenomenon becomes more obvious as the extent of flame merging increases.With the increase of the distance,the interaction of the two fire sources under the action of cross wind presented a change process from interactive blocking and entrainment restricted dominance to thermal feedback enhancement dominance to single blocking and entrainment restricted dominance.The effects of convective heat feedback,radiant heat feedback,and conductive heat feedback received by the fuel were calculated and analyzed,respectively.The evolutions of the radiative and conductive heat feedback power with wind velocity were analyzed.According to the total heat feedback received by the fuel,the ratios of three kinds of heat feedbacks were calculated and analyzed.The process of the main control mechanism from radiation dominance to convection dominance with the increase of wind speed was revealed.A model for the evolution of the burning rate of upstream and downstream fire sources under the non-merging conditions was developed based on stagnant layer theory.(3)A numerical study on the flow characteristics around two propane fire sources in the tandem arrangement under cross wind was conducted.The heat release rate,cross wind velocity and fuel source spacing were changed.In still air,as the fuel source spacing increased,the flame interaction gradually weakened.The corresponding shapes of flow velocity vector field experienced three stages:a whole circular shape,two interacting circular shapes,and two separate circular shapes.As for the wind conditions,due to the impact of Rayleigh-Taylor instability,the counter-rotating vortex pair can be generated at the downstream of the fire sources.Under the influences of the counter-rotating vortex pair and blockage effect,a wake regime through flow entrainment was formed,which can reduce the longitudinal air flow velocity along the downstream centerline of fire sources.Under the impact of fire-wind enhancement,the local flow velocity there can exceed free stream velocity.When the velocity is relatively small,there will be a reverse flow region around the downstream fire source.In addition,the blockage effect showed a non-monotonous trend with the increase of wind velocity.