A numerical study on smoke movement in longitudinal ventilation tunnel fires for different aspect ratio

In this study, numerical simulation was carried out to analyze the effect of the aspect ratio on smoke movement in tunnel fires using FDS 3.0. Temperature distribution under the ceiling showed a relatively good agreement with experimental results within 10 °C. It confirmed the possibility of application of FDS code to tunnel fires. Results from varying of the aspect ratio showed good agreement with experimental data. Temperature near the fire source decreased with the increase of the aspect ratio. But, the rate of the temperature decrease was reduced by the decrease of the heat loss in the spanwise direction. Clear height of the simulation by the analysis of the velocity distribution was about 3% higher than that of the experimental result. Numerical results predicted the back-layering distance and the critical velocity reasonably.     

A comprehensive study of two fire sources in a road tunnel: Considering different arrangement of obstacles

This paper uses Fire Dynamics Simulator (FDS) to study various arrangements of different vehicles at upstream of two fire sources. In order to make a comprehensive study, the effects of two fire sources in both lateral and longitudinal directions are investigated. The results reveal that the behavior of two fire sources, in both perpendicular directions, is directly influenced by distance between them. For small vehicles, variations of the arrangement and distance between the vehicles and fire sources do not affect the calculated Critical Ventilation Velocity (CVV). However, the presence of medium vehicles strengthens the influence of inertia force rather than buoyant force of fire plume in the tunnel. Accordingly, when there is a short distance between fires and medium obstructions, less air ventilation is needed to prevent smoke back-layering. Eventually, far distance between the vehicles and the fires results in vanishing obstruction effects. Consequently, CVV is the same as the case in which there is no vehicle in the tunnel.     

Experimental study on transverse smoke temperature distribution in road tunnel fires

To assess the impact of smoke on the ceiling in tunnel fires, the smoke temperature under the ceiling was studied experimentally with small-scale experiments. This study focused on the transverse smoke temperature distribution in road tunnel fires as the longitudinal one has been widely researched. Comparison for the transverse and longitudinal smoke temperature distributions near the fire was conducted and the difference was researched. A correlation determining the transverse smoke temperature distribution under the ceiling was developed by taking the fire location into account.     

Prediction of fire and smoke propagation in an underwater tunnel

Different models and solvers are used to calculate the spread of fire and smoke in a tunnel. The methodology for obtaining the numerical solution of this fire dynamics problem involves commercial software and a research program. Both can handle geometries described in three dimensions. Particular emphasis was placed on road tunnels in which vehicles are present. The specific application of this work is a study of a fire scenario in the Louis-Hippolyte-Lafontaine Tunnel which runs under a river in the Montreal area. Besides standard representation, visualization is also used, with elements which consider the optical properties of the phenomenon for a realistic rendering of smoke and fire.     

A study on long tunnel smoke extraction strategies by numerical simulation

The performance of different smoke extraction strategies for a long vehicle tunnel was investigated for a 100 MW fire scenario. Computer modeling was used for the investigation with a numerical simulation method. The selection of single-point extraction (SPE) opening strategy versus multi-point extraction (MPE) opening strategy was analyzed. In the single-point extraction (SPE) opening strategy, the smoke spread was found to be contained between the fire site and the point of smoke extraction. This result was obtained when the tunnel air velocity reached the critical velocity for preventing back-layering of smoke. For multi-point extraction (MPE) strategies with more than one opening, the smoke spread to all the extraction openings. Moreover for MPE strategy, air velocities were found to be slower near the middle openings and could be less than the critical velocity. Distributions of smoke spread, CO, visibility and temperature were analyzed. It was found that visibility of smoke and temperature were the key factors for safe evacuation of the personnel in tunnel. The SPE system was found to be more effective than the MPE system for partial transverse ventilation systems.     

Full-scale experimental study on smoke flow in natural ventilation road tunnel fires with shafts

Three full-scale burning tests were conducted in a natural ventilation city road tunnel with shafts. Fire sources were placed to be at different locations but its peak release heats were all around 5 MW. Results showed that large amounts of smoke and heat were released through shafts. The maximum smoke temperatures under the ceiling were below than 100 °C, and being lower than 110 °C at the safe height farther 3 m away from fires. The maximum smoke spreading horizontal lengths were less than 240 m both in the upwind and downwind. During the late stages, many smoke particles descended from the ceiling and downdraught occurred at shafts due to low smoke temperatures, but the visibility was not very bad and people needn’t evacuate. All These results are valuable for fire protection and construction of natural ventilation road tunnel with shafts.     

Estimation of smoke arrival time in tunnel fires

The estimation of smoke arrival time in tunnel fires is helpful to comprehensive fire risk assessment and effective fire evacuation, while few studies focused on this topic. A model to estimate the arrival time of fire smoke in tunnels is derived based on the smoke temperature distribution along the tunnel ceiling. The predictions from the model are compared to experimental data from one past study, which shows good agreements. The influencing factors of the smoke arrival time are studied based on the model. Results show that the Stanton number is the main influencing factor. The smoke arrival time increases with the increase of the Stanton number.     

Simulation of temperature and smoke distribution of a tunnel fire based on modifications of multi-layer zone model

Some modifications on Suzuki’s multi-layer zone model (MLZ) have been done to predict temperature and smoke distribution of a tunnel fires, i.e., the radiation heat loss of fire source is taken into account and a four-surface radiation heat transfer model is introduced. Like Suzuki’s model, as a special long and narrow space, the tunnel space is also divided into a number of layers in vertical direction and regions in longitudinal direction. The physical properties like temperature and species (CO, CO₂, etc.) are assumed uniform in every zone like two-zone model. However, the different heat transfer model is introduced. The MLZ model prediction is compared with the experiments of USTC and CFD model (FDS). It shows good agreement between the model prediction, experiments and CFD models (FDS). And the MLZ model needs less time than CFD model.     

Simulation of smoke from a burning vehicle and pollution levels caused by traffic jam in a road tunnel

Detailed analyses of smoke movement from a burning vehicle in a road tunnel have been carried out for the westbound Melbourne City Link tunnel. The time-averaged equations for velocity, pressure, temperature, and mass fraction of emissions were solved for transient condition using the CFD software FLUENT 6.0. For the analysis, a burning bus was assumed to release an equivalent energy of burning 500 l of diesel in 6 min, with vehicles upstream of the fire at a standstill. On the other hand, the vehicles downstream of the fire had enough time to escape from the tunnel through the exit portal. Due to the action of jet fans, most of the smoke was pushed downstream of the fire. The smoke had also dispersed about 55 m upstream of the fire, putting the passengers in this region at great risk. The emissions released from the vehicles in the jam, with their engines running, also posed a threat to human health. Within 8 min after the fire had started, the mass concentrations of O₂, CO₂ and CO were in the ranges of 0.12–0.15, 0.08–0.11 and 0.0006–0.0014, respectively. Therefore, quick evacuation of the passengers is essential in the event of a fire in the tunnel.     

Modeling for predicting the temperature distribution of smoke during a fire in an underground road tunnel with vertical shafts

In this study, fire experiments using a 1:20 model-scale tunnel were conducted to investigate the performance of natural ventilation in an underground road tunnel with six vertical shafts. The experimental parameters were the heat release rate of a fire source and the height of the shafts, and nine experiments were conducted in total. Furthermore, simple models were developed for predicting the temperature distribution of the smoke flowing under the tunnel ceiling. The following results were obtained: (1) In the experiments, the form of the smoke exhausted from the shaft became plug-holing when the shaft height was 1.0H_t, and became boundary layer separation when the height was 0.24H_t. (2) The average efficiency of heat exhaust was 0.16 when the form was plug-holing, and was 0.12 when the form was boundary layer separation. (3) When the form was plug-holing, the ratio of entrainment of fresh air became almost constant regardless of Ri. On the other hand, when the form was boundary layer separation, the ratio of entrainment of fresh air was smaller than that under the condition of plug-holing. (4) The temperature distribution under the tunnel ceiling predicted by the models agreed with that measured by the fire experiments in all cases.     

隧道火灾烟气运动的数值模拟

对不同入口风速和隧道高宽比情况下隧道内的烟气运动进行模拟,分析隧道内的烟气分布情况.结果表明:随着入口风速的增大,隧道内整体的烟气浓度会逐渐降低.入口风速过大,加强了气流的湍流程度,使烟气层较早降至路面,隧道断面提前充满烟气,使火源附近近地面处的烟气浓度过高.当入口风速较小时,火源上部的部分烟气会逆着风向流动,产生回流现象,所研究工况下烟气不发生回流的临界入口风速为2.2 m/s左右.横截面积相同的情况下,隧道高宽比越小,烟气上升高度越高,隧道下部烟气量越少,利于救援和人员疏散.     

Experimental study on heat exhaust coefficient of transversal smoke extraction system in tunnel under fire

Heat exhaust coefficient of transversal smoke extraction system in tunnel under fire is studied by experimental means with a 1:10 model tunnel using Froude scaling. Heat exhaust coefficient is defined as the proportion of the heat exhausted by individual exhaust inlet, smoke duct and exhaust fans in total heat released by the fire in the tunnel, respectively. Results of a series of fire tests in a model tunnel are presented. Heat exhaust coefficient of single exhaust inlet and the smoke duct are strongly influenced by the configuration of the exhaust inlets. Heat exhaust coefficient of the exhaust fans varies in the range of 13–20% and is smaller than the heat exhaust coefficient of the smoke duct which varies from 17% to 83% and tends to be about 35% with the increase of the total area of the exhaust inlets. Activating small number of the exhaust inlets is beneficial for enhancing the heat exhaust coefficient of the smoke duct. The heat exhaust coefficient of the smoke duct and exhaust fans is high when the exhaust inlets are set close to the fire. Due to the cooling effect of the solid boundaries on the smoke while traveling in the tunnel and smoke duct, the heat exhaust coefficient of the exhaust fans in unilateral exhaust mode is slightly smaller than that in bilateral exhaust mode.     

Characteristics of smoke extraction by natural ventilation during a fire in a shallow urban road tunnel with roof openings

The characteristics of the spread of smoke were investigated for a fire occurring in a shallow urban road tunnel with roof openings in its ceiling. In this type of tunnel, the smoke produced by a fire is ventilated through the openings in the ceiling given the natural buoyancy of hot smoke. A fire experiment was conducted using a 1/12 scale model tunnel to ascertain whether natural ventilation via the roof openings was sufficient to maintain a safe evacuation environment for tunnel users. The distance from the fire to the tip position of the spreading smoke and the thickness of smoke layers along the ceiling were investigated by changing the heat release rate and using two types of median structure as experimental parameters. The two types of median structure dividing the tunnel into two road tubes were pillars and walls. It was clarified that the smoke spreading distance was constant and independent of the heat release rate of the fire under our experimental conditions. Moreover, it was confirmed that the thickness of the smoke layers in the tunnel thinned out quickly due to the natural ventilation.     

火源横向位置及面积对双车道公路隧道临界风速影响研究

为了探明火源横向位置对临界风速的影响规律,运用FDS研究马蹄形断面双车道公路隧道内火源位于隧道中心与侧壁两种场景下的临界风速,并改变火源面积,结合理论分析,与前人矩形断面隧道内的研究结果进行对比。结果表明：单位面积热释放速率一定时,临界风速随火源面积的增大而增大;壁面火的临界风速小于中心火的临界风速,与矩形断面隧道存在差异;且随着火源面积的扩大,壁面火与中心火的临界风速比值趋近于1;不能用“镜面效应”解释马蹄形隧道内壁面火与中心火临界风速差异的原因。     

火灾时隧道内烟流流动状态试验研究

通过大比例火灾模型试验,研究火灾时隧道内烟流流动状态、烟流速度变化以及通风对烟流流动状态的影响。试验模型隧道长100m,内径1.8m。火源采用燃烧床盛放油料模拟,试验中设定了A、B、C三个火灾规模用以模拟实际隧道火灾场景。试验结果表明,点火后,隧道内火区、火区下游烟流速度在2～8min内增加很快,明显大于点火前风速,且其增幅随通风风速、火灾规模的不同而变化。同时,随着火势的逐渐减弱隧道内烟流速度也逐渐减小,并趋于初始风速。试验结果建议对于一般的限制或禁止油罐车通行的隧道,火灾时,隧道内应尽快建立起2～3m/s的纵向风流以抑止烟气的逆流。     

Impact of location of jet fan on airflow structure in tunnel fire

Airflow structure and its magnitude are important for smoke management in longitudinally ventilated tunnels. This paper presents a sensitivity study by Computational Fluid Dynamics technique, which considers different dimensions, orientations and the natures of fire source. Its impact on the airflow velocity and temperature distribution is investigated with various locations of fan group activated. A theoretical correlation between the distance of active fan group from the fire source and the upstream velocity is proposed individually for pool fire and solid fire. It reveals that the nature of the fire source imposes adverse effect on the upstream air velocity and the airstream pattern over the entire tunnel. The upstream velocity remains fairly constant when the active fan group is located 200 m or beyond from a vehicle fire that occupies a significant cross-sectional area of tunnel.     

A study of an optimal smoke control strategy for an Urban Traffic Link Tunnel fire

An Urban Traffic Link Tunnel (UTLT) is a novel type of underground transportation system consisting of a main tunnel in a loop shape and several linked tunnels. It has a higher level of fire risk compared to other common road tunnels. In this study, numerical study was conducted to investigate the smoke control strategies for a designed fire scenario in the Beijing Center Business District (CBD) UTLT. An optimal smoke control strategy was developed and evaluated using three criteria: critical velocity, minimal smoke spreading area and available safe evacuation time. In developing the optimal smoke control strategy, six different smoke control strategies for the UTLT were evaluated initially by the criterion of critical velocity, and one of those control strategies was chosen as the primary strategy by steady simulation. This strategy was then modified according to the dynamic simulated results of smoke movements to satisfy the other two criteria. Consequently, the optimal smoke control strategy for the UTLT based on the primary strategy was obtained. The transient distributions of the smoke spread, smoke temperature and CO₂ concentration, were analyzed.     

Experimental study of the effectiveness of a water system in blocking fire-induced smoke and heat in reduced-scale tunnel tests

A water system, consisting of several water mist nozzles, has been installed in a reduced-scale tunnel. Its effectiveness in blocking fire-induced smoke and heat is tested, with and without longitudinal ventilation. A total of 14 fire tests have been carried out, with 250 ml methanol in an iron tray (25 cm × 20 cm) as fuel. Temperatures have been measured by 30 thermocouples, located upstream and downstream of the fire location. The aim is to assess the effectiveness of the water system in preventing smoke spread and in reducing the temperature in the tunnel. Interaction of the water with the fire is avoided. The impact of water pressure, ventilation velocity and nozzle arrangement on the effectiveness in smoke blocking and temperature reduction is discussed. The result confirms that the water system effectively reduces the temperatures and prevents smoke spreading in the absence of longitudinal ventilation. However, strong longitudinal ventilation (0.8 m/s ventilation velocity in the reduced-scale tunnel, corresponding to critical velocity in full-scale (1:10) tunnel) reduces the effectiveness in blocking the smoke spreading by the water system, although the temperature reduction downstream the water system remains in place. Higher water pressure makes the cooling effect stronger, because more and smaller water droplets are injected into the tunnel. For a given level of water pressure level, the impact of the nozzle row configuration is small in the tests.