纵向风对隧道火灾拱顶最高温度及其位置的影响

研究了燃烧风洞内不同纵向风速、不同火源功率条件下,隧道近火源区顶部温度沿纵向分布情况。结果表明,纵向风对不同尺寸火源条件下的顶部温度的影响呈不同特征。对较小尺寸火源,隧道顶部温升随风速增加而减小至稳定值;而对较大尺寸火源,顶部温升随风速增加先增加后减小。对于矩形火源,当纵向风较小(0.5~1.5m/s)时,长边平行于纵向风时顶部最高温升大于长边垂直于纵向风的情况;而当纵向风较大(≥2 m/s)时,两种油盘放置方式的顶部最高温升一致。纵向风作用下,顶部最高温升位置向下游呈现"两次移动"特征,即随着纵向风速增加该位置先向下游移动,当风速达到某一值时,隧道拱顶的加热机制由对流和辐射共同主控转变为辐射单独主控,最高温升位置突变回到上游后再次逐渐向下游移动。

Effects of longitudinal ventilation on maximum ceiling temperature and its position in tunnel fire

Experimental study of temperature distribution along tunnel ceiling with different longitudinal ventilation velocities and different typical fire sizes was conducted in a combustion wind tunnel with length of 20 m.Both square and rectangular pool fires were used as fire sources for longitudinal ventilation velocities ranged in 0-3 m/s.The smoke temperatures below the ceiling were recorded by K type thermocouple for near fire region and by thermal resistors for that relative far away from the fire source.The results indicate that longitudinal ventilation velocity and fire size have a great influence on temperature distribution.The variation of ceiling temperature distribution with the increase of longitudinal ventilation velocity was shown to be different for a small fire from that for a large fire.For small scale fires,temperature of tunnel ceiling reduced to a stable value with gradual increase of longitudinal ventilation velocity.However,for big scale fire,the temperature increased first,and then decreased.The position of maximum temperature rise point firstly moved downstream horizontally with the increase of the wind speed.When the wind speed reached a certain value,the maximum temperature rise point returned back to an upstream position,and then moved downstream again.This is due to the change of dominant heating mechanism with the increase of longitudinal ventilation velocity.When the longitudinal ventilation velocity is relative small,both convection from hot smoke and radiation from the flame provide considerable contribution.However,when the longitudinal ventilation velocity is relative large,radiation is dominant.The influence of the length of windward side of rectangular pool fire was also discussed.It was shown that when the longitudinal ventilation velocity is relative smaller(0.5-1.5 m/s),the maximum temperature is higher when the longer side is parallel to the wind direction than that under perpendicular condition.However,when the longitudinal ventilation velocity is relative higher(≥2 m/s),the maximum temperature is similar under the above two conditions.