Study On The Characteristics Of Smoke Flow In Highway Tunnel Fire With Central Extraction

The central extraction mode is the development trend in the highway tunnel fire,and the effective utilization of the smoke extraction system is the key to the engineering design.The heat release rate and exhaust volume of smoke directly affect the effect of smoke extraction.Therefore,it is of great significance to study the influence of heat release rate and smoke volume on the spread of smoke.This paper takes a highway tunnel in Wuhan as the prototype,according to the fluid mechanics similarity principle,the length of the tunnel test model of 22 m for1:20 is established.By collecting the smoke temperature,smoke flow velocity,smoke back-layering length and other test data under different situations to analyze the law of the test data.The conclusions under different smoke extraction volume and the heat release rate are as follows:In central extraction mode,the more the smoke extraction volume and the heat release rate are,the stronger the air entrainment extent is.Through theoretical analysis,the calculation expression of the entrainment coefficient is derived.In this experiment,the entrainment coefficient is between 0.01 and 0.03,which is much larger than that of natural smoke extraction.Through the experiment,it is found that the smoke back-layering length decreases with the increase of the smoke extraction volume,and increases with increasing of heat release rate.The natural longitudinal air velocity increases with the smoke extraction volume,but keeps constant with the heat release rate varies.The relationship between the smoke back-layering length and the natural longitudinal air velocity,as well as the heat release rate,is obtained.The smoke extraction volume has little effect on the critical velocity which can be ignored.When the dimensionless heat release rate is less than 0.12,the dimensionless critical velocity increases with the increase of the dimensionless heat release rate,and the critical velocity does not very when the dimensionless heat release rate is greater than 0.12..In central extraction mode,the critical velocity is smaller than that in longitudinal ventilation mode.The calculation formula of the critical velocity is obtained.