Study Of Ventilation Control For Fire-induced Smoke In Metro Tunnel Conjunction Area

Fire is one of the main threats to the safety of metro operation.Reasonable and effective smoke ventilation control method is of great significance to ensure passengers'life safety.With the development of metro in China gradually forming the trend of network operation,being the conjunction node of operation lines,the number of tunnel connected area is increasing,and the spread of smoke between the uplink and downlink tunnels in a same line and between the adjacent lines will greatly increase the range of fire risk,improve the difficulty of smoke control,emergency evacuation and rescue.However,the relevant standards and research have not yet formed a special smoke management technologies,and the current ventilation and exhaust system of single-line tunnels is hard to be applied on smoke control in tunnel connected areas.According to the typical structural characteristics,this paper divides the fire smoke control requirements in the connected area of metro tunnel into two research items:the longitudinal smoke ventilation control in the bifurcated contact area,and the ventilation and smoke exhaust mode in the large cross-section connected area.The following aspects of research on smoke diffusion,ventilation control and smoke exhaust modes were conducted by theory analysis,small-scale experiments,full-scale experiments and numerical simulation:Focusing on the smoke diffusion characteristics in metro tunnel connected area,the influence of bifurcation structure on ceiling temperature distribution was studied.In terms of fire source position,considering the fact that fire deviates from tunnel centerline in arched cross-section,the full-scale fire experiment was carried out in a bifurcation tunnel.It was found that as the fire source gradually approaching the sidewall,the maximum ceiling temperature is higher than in rectangular section,and the longitudinal ceiling temperature decayed lower.Accordingly,the decay coefficient was modified and the prediction model for ceiling temperature profile was established;In terms of tunnel slope,the model experiment for horizontal tunnel was carried out,and then the main and connected tunnel slopes were considered in numerical simulation.It was found that the maximum temperature of the ceiling decreased with the increase of the two tunnel slopes,and either uphill slope increase in a tunnel can lead to reduction of longitudinal temperature in another one.By modifying the prediction model in horizontal tunnel,the prediction model for the maximum temperature and longitudinal profile considering the slope of both the main and connected tunnel were established;In terms of bifurcation angle,the model experiments showed that the increase of the bifurcation angle can lead to the increase and decrease of ceiling temperature in main and connected tunnel,respectively.Based on the modification of the decay coefficient in ordinary single tunnel,the prediction model for the longitudinal ceiling temperature profile considering both bifurcation angle and connected tunnel slope was established.Focusing on the longitudinal ventilation control of the smoke in the bifurcation connected area,the smoke back-layering length and critical ventilation during smoke spread through connected tunnel to adjacent operation lines was studied.Firstly,the basic relationship between dimensionless back-layering length,fire power and longitudinal ventilation velocity was established through theoretical analysis.Then,on the basis of good agreement between 1:20 model experiment and numerical simulation,the formula between dimensionless back-layering length L*and Q*/V*3 was determined with various factors according to the measured data.The results showed that,when the fire occurred in node area,the critical velocity in connected tunnel increases and decreases with its increasing uphill and downhill slopes.And the thermal.pressure reduction effect of the longitudinal ventilation in main tunnel leads to the decrease of the critical velocity in connected tunnel.When the fire deviates from the node area,the critical velocity decayed exponentially with the increase of the deviation distance,and it decayed faster in case of fire occurring in downstream of node area;with the increase of the main tunnel slope,the critical velocity in connected tunnel increases while it's in the uphill from fire.Based on theoretical analysis,1:20 model experiment and numerical simulation results,the prediction model for critical ventilation velocity was established,which aims to effectively control the smoke diffuse to the adjacent operation lines through connected tunnel.Focusing on the difficulty of smoke control by longitudinal ventilation in large cross-sectional area of metro tunnel,the effect and optimization method of cooperated modes of current ventilation facilities were studied.Based on the full-scale fire experiment in the large cross-sectional metro tunnel connected area,the train fire scenarios with different co-operated modes of the tunnel ventilation fan,jet fan and ceiling exhaust duct in large cross-section connected area were constructed by numerical simulation.The results showed that under the longitudinal ventilation,the co-operated mode of activating all jet fans and downstream smoke exhausts brought about relatively high efficiency for fire occurred in front,middle and tail of the train,both the upstream longitudinal air velocity and downstream smoke exhaust efficiency were relatively high.In terms of optimization on operating parameters,the smoke control efficiency could be further improved by increasing the ceiling vent area and air volume.Based on the fire location and evacuation direction,the ventilation and smoke exhaust mode for train fire in large cross-sectional metro tunnel connected area was proposed.