Fire Behaviors And Smoke Transportation Law Of Tunnel Fires Under Confined Portal Boundaries

An increasing number of tunnels have been built around the world.They play an important role to relieve traffic congestions and facilitate goods transportation.However,in the event of a fire in tunnels,the consequences can be serious due to its narrow-long structure.The previous studies about tunnel fire dynamics and mitigation technologies mostly are based on good ventilation conditions in tunnels,such as longitudinal ventilation and natural ventilation with the premise that a tunnel has two open portals.However,the studies about the characteristics of tunnel fires under confined portal boundaries with complete or incomplete sealing at both portals are rare.Typical fire scenarios can appear in a subway train,a building corridor,an underground utility tunnel,a mining tunnel,a tunnel during construction and the application of sealing tunnel portals for fighting large tunnel fires and so on.The knowledge of tunnel fire dynamics for tunnels under good ventilation conditions is probably not applicable to the scenarios of tunnel fires under confined portal boundaries.Conducting the studies of tunnel fires under confined portal boundaries is of great significance for better understanding the characteristics of this type of tunnel fires and developing tunnel fire mitigation technologies.Therefore,by combining model-scale tunnel experiments and theoretical analysis,this thesis studies the fire behaviors and smoke transportation law of tunnel fires under confined portal boundaries.The main research contents include:1.Scaling effects of mass loss rates per unit area(mn)for fuel-controlled pool fires are studied by summarizing and analyzing large amounts of experimental data from the literature.As a further extension of tunnel fire similarity theory,it provides the basis and reference for later scale tunnel experiments.Results show that when a small-scale pool fire(D<1 m)occurs in the open,increasing wind speed tends to increase mn,especially for pools with D<0.2 m.This is because the ventilation increases significantly the conduction and convection heat feedback(leading role).But when the small-scale pool fire occurs in the tunnel with a short distance between the pool surface and ceiling height(Hef/D<3),the radiative heat feedback from tunnel ceiling is probably dominating,leading to a much higher m than that in the free burn.At this point,when subjected to longitudinal flows,the mn decreases due to the reduced radiation effect from tunnel ceiling.However,with a further increase of wind speed(at a higher wind speed level),the conductive and convective heat feedback probably become leading and therefore increase m".With the increase of pool diameter,the influence of wind on mn decreases gradually,no matter whether the pool is in the open or inside the tunnels.Finally,when the pool diameter exceeds 1 m,the radiation from flame itself is probably predominant.The mn most likely fluctuates within 30%across a wide range of wind speeds both in the open and inside the tunnels.2.The flame behaviors and the maximum gas temperature rise beneath the ceiling in an enclosed tunnel are studied by using a model-scale tunnel.Results show that when the fire(small fire)is not located at the tunnel center,the flame inclines towards the closer tunnel end due to the asymmetric flow field at both sides of the fire.The flame inclination angle keeps increasing when the fire is moving away from the tunnel center.When the fire is in Region I(0?d'?0.64),the maximum gas temperature rise decreases with the increasing dimensionless fire distance due to the increasing flame inclination angle.When the fire is in Region ?(0.64<d'<1),the maximum gas temperature rise increases with the increasing dimensionless fire distance due to the heat accumulation of returned hot smoke bounced from the end wall.By introducing a concept of equivalent ventilation velocity based on the flame inclination mechanism,a prediction model of maximum gas temperature rise beneath the ceiling in Region I is developed.Beyond that,an extra correction factor is proposed to the improved model in Region ? with a consideration of the heat accumulation of returned hot smoke bounced from the end wall.Besides,further dimensional analysis indicates that the normalized maximum gas temperature rise follows an exponential attenuation law with the dimensionless fire distance.3.The coupling control effects of sealing ratio and initial sealing time on the fire development(large fire)are studied by using a model-scale tunnel.Results show that sealing tunnel portals can decrease the mass loss rate of fuel and ceiling temperature,no matter whether the sealing is complete or incomplete.Moreover,the earlier the initial sealing time is,the better the fire can be suppressed.For the incomplete sealing,when the sealing is implemented during the violent burning stage,the sealing not only does not limit the fire growth but also exacerbates the tunnel fire,producing an extremely high CO concentration at tunnel portals and a longer ceiling flame jet.This will result in a huge threat to the rescue service at tunnel portals.Besides,if the tunnel portals are sealed incompletely,it will leave a small area for the exchange of smoke and air.The smoke will not continue to spread horizontally after leaving tunnel portals under the action of inertial forces.In order to maintain the combustion of fuel inside the tunnel,the fresh air from external environment flows into the tunnel vigorously and quickly from the gap and then uplifts the smoke out of the tunnel portals,which is also an important phenomenon for firefighters and needs to draw their attentions.4.The critical conditions for the occurrence of ventilated-controlled tunnel fires and the combustion mechanism are studied by using both model-scale and medium-scale tunnels.Results show that the critical equivalence ratio for the occurrence of ventilated-controlled tunnel fires is within 0.53-0.6,which is less than the theoretical value of 1.This is related to the occurrence of vitiation,consequently reducing the level of oxygen around the flame by diluting the O2 concentration.The low ventilation rate and vitiation result in low O2 volume fraction around the flame,and then mn starts to decrease and at the same time the air mass flow into tunnel becomes almost constant.Also,an oscillating mn and lifted flame is observed in the model-scale tests.Consequently the ventilation rate approaches and even reaches amount required for complete combustion of vaporized fuel.This means that the insufficient combustion in early ventilation-controlled tunnel fires has converted to sufficient combustion(from the perspective of the change of equivalence ratio,the combustion has converted from ventilation-controlled to fuel-controlled).As a result,no significant increase in CO production in ventilated-controlled tunnel fires is observed in both test series.5.The critical conditions for the occurrence of self-extinguishment and the corresponding influencing factors in ventilated-controlled tunnel fires are studied by using a model-scale tunnel during construction.The tunnel consists of an inclined access tunnel and a horizontal main tunnel.Results show that when a fire is in the horizontal main tunnel,the critical equivalence ratio for self-extinguishment is within 0.28-1.38 for the propane gas burner and 1.11-3.6 for the fibre board soaked with heptane.The difference is related to the burning behavior of the different fuels used.Moreover,the critical O2 volume fraction is about within 12-15%when the fire self-extinguishes.When a fire is at the closed end of the horizontal main tunnel,the stratification of smoke is destroyed after hitting the closed end,and then the smoke seems to spread over the entire cross section of the tunnel.The smoke spread velocity is proportional to the ventilation rate.However,when a fire occurs at the closed end of the inclined access tunnel,the fire does not self-extinguish,even when the ventilation rate is 0 m3/s.The corresponding smoke spread velocity is higher than that in the horizontal main tunnel.This is probably related to the increasing component of buoyancy in the longitudinal direction in the inclined access tunnel.Besides,no insignificant vitiation behind the fire is found.These two characteristics in the inclined access tunnel increase the temperature of smoke flowing out of the tunnel portal and in turn promote the natural ventilation and increase the O2 volume fraction.