Fire smoke diffusion and recirculation mechanism in an asymmetric street canyon under ambient wind conditions
Various street canyon structures are commonly observed in urban cities,and once a fire occurs,a large amount of harmful gas such as fire smoke is generated.Affected by the ambient wind,such fire smoke may tilt to downstream direction.The fire smoke may be obstructed by a leeward building so that it can not fully disperse to surrounding,resulting in the smoke recirculation inside the street canyon,which is very detri-mental to the safe evacuation and physical health of personnel there.This article applied the CFD fire simulation software FDS to construct a street canyon model,focusing on the effect of street canyon asymmetry on the fire smoke diffusion.By selecting typical street canyon conditions,changing the windward or leeward building height and ambient wind velocity,the evolution characteristics of fire smoke diffusion and flow field structure were indicated,and the critical recirculation wind velocity for each asymmetric street canyon condition were ob-tained.A factor of m,defined as the ratio of the windward building height to leeward building height,was intro-duced for the description of the street canyon asymmetry,and the prediction model between the critical recircula-tion wind velocity and the street canyon asymmetry factor was proposed.The results show that while maintaining the leeward building height but increasing the windward building height,the smoke diffusion exhibits three dif-ferent stages,i.e.,fully recirculation,semi-recirculation,and non-recirculation stages;on the contrary,while maintaining the windward building height but increasing the leeward building height,the smoke diffusion exhib-its two different stages,i.e.non-recirculation,and fully recirculation stages.It is found that with the increasing value of m,corresponding to a higher windward building height or leeward building height,the smoke recircula-tion is more unlikely to be formed,in that the critical wind velocity relative to the occurrence of smoke recircula-tion will be greater.When the factor of m is close,the smoke recirculation behavior of a street canyon fires is similar.A piecewise linear prediction model regarding to the critical recirculation wind velocity vc and the street canyon asymmetry factor m is well proposed.
street canyonasymmetry factorsmoke diffusionambient windcritical recirculation wind velocity
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