Studies On Characteristics Of Confined Fire Behaviors And Method Of Smoke Exhaust Using Movable Fan In Arced Cross-sectional Road Tunnels

As the rapid development of the regional economy,the economic activities between regions have also been strengthened.This means that a large number of materials need to be transferred frequently,which have brought a heavy burden to transportation.Road tunnel is an effective form and it is widely adopted in modern traff-ic organizations.This is because it can greatly reduce the distance between the two places in space,thereby saving time and economic costs caused by inconvenient traffic.Although the construction of a large number of road tunnels brings a lot of convenience to transportation,it also increases the risk of fire.Once a tunnel fire occurs and effective measures are not taken to control it in the early stages of development,as a result,its consequence may be catastrophic.The tunnel fire not only damages to the tunnel structure,but also may result in serious casualties.Therefore,it is necessary to conduct in-depth research on the tunnel fire.In this paper,the full scale test,model scale experiment and numerical simulation method are used to study the characteristics of confined fire behavior and the method of smoke exhaust using movable fan in road tunnels with arced cross-section.The main contents are as follows:Firstly,the experimental and numerical methods are used to study the effect of sidewall constraint on the temperature distribution of ceiling jet driven by weak plume.Results showed that the constraint effect of sidewall has a significant influence on the maximum ceiling jet temperature rise at the impingement point.This temperature rise can be correlated with the fire horizontal distance to the tunnel wall by an exponential function.However,at a given location,the wall constraint effect on the normalized impingement ceiling jet temperature rise seemed independent of the heat release rate of the fire.The experimentally measured longitudinal distribution of the normalized ceiling jet temperature rise can be approximated by a power function,although there is a weak signature of exponential distribution according to the numerical simulation results.In addition,this study focused on the differences in temperature distribution between an arced ceiling and a flat ceiling.The results demonstrated that the longitudinal temperatures under the flat ceiling were lower than those under the arced ceiling for the same HRR when x/Hef>1.For the two different ceilings,although the correlation between the normalized temperature and dimensionless transverse distance can be fitted with a similar exponential function,the constraint effect under the arced ceiling was more significant in comparison to that of the flat ceiling owing to the existence of the transition from a weak plume to a strong plume as the transverse distance was reduced.Besides,a modified model was proposed to predict the maximum temperature rise under an arced ceiling in the range of HL<Hef.This model considers different transverse fire locations and ventilation velocities.Secondly,a series of model scale experiments were conducted to investigate the combustion characteristics of strong plumes in the presence of constraint effect.It was observed that the flame front beneath the arced ceiling occurs vortices.As the fire source closes to the sidewall,more air need to be entrained into between the pool and the sidewall for providing sufficient oxidizer to sustain a complete burning,consequently deviating the flame away from the vertical direction.On the whole,the control mechanisms of the sidewall constraint can be classified into two stages:the heat feedback controlled for the weak plume and the air entrainment controlled for the strong plume.As the transverse clearance between the fire and the sidewall is reduced,the fuel burning rate presents a non-monotone variation because the pool fire attached to the sidewall does not occur the maximum burning rate,but rather that near the sidewall.Except the wall fire,nonetheless,the sidewall constraint has a significant impact on the boiling occurrence of pool fires because a smaller clearance means a stronger radiation heat feedback and heat transfer to fuel,finally leading to a earlier boiling time.In addition,it was found that the air entrainment is a key factor to restrict the development of the wall fire beneath a confined ceiling by conducting different layouts of the pool fires against the sidewall.Finally,the method of smoke exhaust using movable fan was studied by conducting full scale tests.It was found that the distance of the movable fan away from fire shows a critical range when only a single fan was employed.The horizontal angle might be a better choice for exhaust smoke within a limited distance because it could enhance the decrease in the smoke temperature.This means that smoke flows out of the tunnel easier under such condition.However,the decay of the maximum smoke temperature under the ceiling did not have an obvious difference between the two inclined angles when the distance between the fan and the fire source exceeded the critical value.When two movable fans were employed,it was found that the longitudinal temperatures were notably reduced and the propagation velocity was enhanced if a movable fan was placed at downstream compared with the centralized layout.As a result,it is more effective to control backlayering that is conducive to extinguishment and rescue,but the descent speed of thermal flow is also accelerated,which is adverse to safe evacuation.Meanwhile,note that if the distance between the fan located downstream and the fire source was too far,the above conclusions might not be available.This suggests that there could exist a critical range for the arrangement of the movable fan at downstream.