Study On The Effect Of Natural Smoke Exhaust With Multiple Shafts

More and more tunnels have been constructed around the world in recent years to ease the pressure of urban traffic.However,the construction of tunnels also brings a serious fire risk.Because tunnel is a semi-confined space with long and narrow structure,in case of a catastrophic fire,if the smoke has not been timely exhausted,the hot and toxic smoke will accumulate beneath the ceiling and have high potential to cause disastrous casualties.Besides,for fire with large heat release rate(HRR),the flame will continuously impinge on the ceiling and destruct the tunnel structure,which may lead to the collapses of tunnel.Previous scholars have done a lot of works on the smoke exhaust effect by vertical shaft,and given some empirical correlations to predict the influence of atmospheric pressure,tunnel slope,longitudinal wind,maximum ceiling temperature,etc.,on the burning intensity in tunnel.In addition,the influencing factors,such as shaft height,width,and the shaft arrangements are studied,and a Ri number for judging the occurrence of plug-holing during the natural smoke exhaust by shaft is obtained.However,most of the previous works did not consider the influence of the smoke exhaust process on the fire behavior,and the HRR was always set as constant values.In fact,the natural smoke exhaust through the vertical shaft is a dynamic process,and the combustion intensity of the fire source will be affected by the induced supplement air flow owing to the smoke exhaust process.In practice,shaft groups are often used instead of one single shaft,the collaborative smoke exhaust process by shaft groups has rarely been considered.Therefore,in this paper,the effects of shaft height,shaft spacing,shaft position and burner size are studied,focusing on the combustion behavior of fire source and the smoke exhaust effect by shaft groups.In this paper,the pool fire combustion behavior was researched under the natural smoke exhaust conditions of single shaft and multiple shafts(shaft groups),by adopting a 1:10th scale model tunnel.The shaft positions,shaft heights and shaft spacing were changed.Results show that compared to the single shaft case,the scenario of multiple shafts can significantly enhance the combustion rate of the oil pool fire.The combustion rate increases monotonically with the shaft height,and it increases first and then becomes decreasing with the shaft spacing.To be specific,for multiple shafts,the combustion rate of the three shafts is much larger the case with double shafts.Moreover,with the increase of the pan size,the influence of the natural smoke exhaust on combustion rate becomes weakened.With regards to the ceiling temperature,it shows an exponential decay in the longitudinal direction of tunnel.Compared to the cases of single and double shafts,the three-shaft scenario has a more significant effect on the control of high-temperature smoke near the shaft region.By analyzing the smoke exhaust effect for different shaft arrangements,we can find that the phenomenon of plug-holing is easier to occur for higher and larger shaft,which will weaken the smoke exhaust effect.By contrast,the cases with three shafts can exhaust more smoke with higher temperature and larger CO volume fraction.In addition,the mass flow rate exhausted by shaft decreases with the increase of the fireshaft distance for multiple shafts.An ideal smoke extraction model is adopted to analyze the exhausted mass flow rate through the vertical shaft.It can be found that the smoke exhaust effect become worse for the cases with the occurance of plug-holing and boundary layer separation.The layout of two or three shafts can inhibit the negative influence of these phenomena,and thus impove the natural smoke exhaust effect.