Critical ventilation velocity for multi-source tunnel fires

Ventilation is an effective method for controlling smoke during a fire. The “critical ventilation velocity” u_cr is defined as the minimum velocity at which smoke is prevented from spreading under longitudinal ventilation flow in tunnel fire situations. All previous studies on this topic have simulated fire scenarios in which only one fire source exists. This study conducted small-scale experiments and numerical simulations to investigate u_cr for cases in which two tunnel fires occur simultaneously. The tunnel was 4 m long, 0.6 m wide and 0.6 m tall. Three cases of two variously separated fires were experimentally explored and six cases were examined numerically. Both the experimental and simulation results indicated that for two identical fires, u_cr declines with separation. When the two fire sources are separate completely, u_cr can be determined by considering only a single fire. When the larger fire is upstream of the smaller downstream fire, u_cr also decreases with the separation. When two such fires sources are completely separate, u_cr can be evaluated by considering only the larger fire. The concurrent ventilation flow and flow of downstream smoke from the larger fire are strong enough to suppress the smoke flow from the smaller fire. However, when the smaller fire is upstream of the larger fire, the decrease in u_cr becomes insignificant as distance increases and the flow at u_cr must overcome the flow from both fires.     

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.     

纵向通风与竖井自然排烟下隧道火灾烟气特性实验研究

中国逐渐发展成为世界上隧道和地下工程最多的国 家,其长隧道数量和长度跻身世界前列。据统计,火灾中85%的 人员死亡是由热烟气造成的,目前隧道中采用较为广泛的排烟系 统有纵向排烟系统、集中排烟系统和横向排烟系统,而针对长隧道 来说,我国广泛采用的是竖井式纵向通风,因此,研究纵向通风与 竖井排烟综合效应下隧道火灾烟气流动特性及温度分布规律具有 重要意义。本文建立了1：10 缩尺寸竖井隧道模型,主隧道长度 16.5 m,宽度1.3 m,高度0.65 m;竖井通过排烟横通道与主隧道 连接,排烟横通道设置在主隧道侧面中部,尺寸为1.2 m 长、0.6 m 宽、0.4 m 高;竖井横截面为半径0.6 m 的1/4 圆,高4.6 m。在 竖井隧道模型中开展了一系列油池火实验,选取2 种方形燃烧池 （20 cm×20 cm、23 cm×23 cm）作为火源,设置2 个纵向火源位置 （位置A：火源中心线与排烟横通道中心线距离0.375 m;位置B： 火源中心线与排烟横通道中心线距离1.375 m）,7 种纵向通风风 速（0,0.18,0.27,0.35,0.44,0.52,0.69 m/s）,定量分析不同工 况下温度分布及烟气逆流长度。研究结果表明：当无纵向通风时, 火焰与隧道地板垂直,且呈轴对称形态;当有纵向通风时,火焰向 下游偏移,且纵向通风风速越大,火焰向下游偏移越明显;当纵向 通风风速为0 m/s 时,由于竖井的存在,火源上、下游两侧烟气温 度分布并非对称,火源下游（竖井侧）烟气温度下降速度较快,与单 洞隧道烟气温度分布明显不同;随纵向通风风速增加,烟气逆流长 度和烟气温度减小,而最大温度偏移距离整体呈增加趋势;当无量 纲纵向通风风速v′＜0.19 时,主隧道最大温升△Tmax 与Q2/3/ Hef 5/3 呈正比,而当无量纲纵向通风风速v′＞0.19 时,主隧道最大 温升△Tmax 与Q? /(vb1/3Hef 5/3)呈正比,但常数系数均小于Li 等预 测模型中的常数系数;竖井隧道内无量纲纵向烟气温度分布符合 Fan 和Ji 等建立的纵向温度衰减模型,衰减系数k′在1.36~1.63 范围内变化,但其值明显大于单洞隧道纵向温度衰减系数k′;另 外,当火源位于位置A 时,最大烟气温度低于火源位于位置B 时 的最大烟气温度,无量纲纵向烟气温度衰减速度慢于火源位于位 置B 时衰减速度。     

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.     

Laminar smoke points of coflowing flames in microgravity

Laminar smoke points were measured in nonbuoyant laminar jet diffusion flames in coflowing air. Microgravity was obtained on board the International Space Station. A total of 55 smoke points were found for ethylene, propane, propylene, and propylene/nitrogen mixtures. Burner diameters were 0.41, 0.76, and 1.6 mm, and coflow velocities varied from 5.4 to 65 cm/s. These flames allow extensive control over residence time via variations in dilution, burner diameter, and coflow velocity. The measured smoke-point lengths scaled with d^−0.91u_air^0.41, where d is burner diameter and u_air is coflow velocity. The measurements yielded estimates of sooting propensities of the present fuels in microgravity diffusion flames. Analytical models of residence times in gas jet flames are presented, and although residence time helps explain many of the observed trends it does not correlate the measured smoke points.     

Influence of the slope in the ventilation semi-transversal system of an urban tunnel

The covering of a section of the Inner Belt roadway (“Ronda del Mig”) in Barcelona gives rise to an urban tunnel of great length (1535 m). The tunnel is divided into two independent parallel galleries and its orientation is North–South, with a 2% upward slope towards the North. Although normal ventilation is achieved with jet fans, between the two galleries there is an interior passage for smoke extraction, in case of fire, through exhaust openings on both sides of this passage. Therefore, the tunnel has a semi-transversal ventilation system for fire incidents.The behavior of the smoke generated during those possible fire incidents in the traffic galleries was simulated with a commercial code, FLUENT^®, which allows a three-dimensional multispecies Navier–Stokes unsteady simulation. The mesh of each tunnel was made with about 250,000 triangular base prismatic cells. The simulated fire had a thermal power of 30 MW and the time step was set to one second, while the simulation covered 15 min.Special emphasis was put on the influence of the tunnel slope on the smoke’s behavior in each gallery. Simulation results showed that the fans’ capacity established in the project specifications was not enough to extract the smoke of a fire with the simulated power. A significant percentage of the smoke was aspirated through the exhaust openings but the rest continued rising to the tunnel portal due to the slope. This created a great risk mainly in the descending gallery with opposite traffic direction. For a more efficient extraction it was determined that the exhaust sections should be opened upward of the fire’s location. The standard opening, at both sides of the fire, reduced the capacity to extract smoke due to clean air aspiration from the lower portal.     

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。     

Wind tunnel tests on compartment fires with crossflow ventilation

When a fire occurs in a room at ground level or a compartment located in the higher floors of a very tall building , the strong ambient wind will play an important role in fire spreading and smoke movement behavior. However, wind effect on compartment fire in cross ventilation condition has not been fully studied so far. In the present study, an effort has been made to study the wind effect on compartment fire in cross ventilation condition through experimental investigations. The experimental fire was generated by 250 ml n-heptane on the floor center of a cube enclosure with two opposite vents on the walls. The inside and outside gas temperature profiles at different vertical and horizontal locations were recorded by two thermocouple matrixes. The ambient wind velocity was set to 0, 1.5 and 3 m s⁻¹. It is observed that the ambient wind would enhance the fire severity by increasing the compartment fire temperature and reducing the time to flashover. The spilled-out flame/plume would extend horizontally farther with the increase of wind speed. Simple theoretical analysis shows that there is a critical wind velocity, or a dimensional number, to differentiate whether the gas flow across the vents is bidirectional or unidirectional, which is believed to influence enclosure fire behavior greatly.     

Experiments of evacuation speed in smoke-filled tunnel

Among tunnel fire safety strategies, evacuation speed in smoke, which is the basic evacuation performance characteristic, is one of the most important factors when assessing safety. An evacuation experiment in a full-scale tunnel filled with smoke has been done in order to clarify the relation between extinction coefficient up to Cs = 1.0 m⁻¹, which includes Cs = 0.4 m⁻¹ as a Japanese road tunnel fire prevention standard, and evacuation speed. The maximum, minimum and mean values of normal walking speeds are almost constant regardless of the extinction coefficient. As for the emergency evacuation speeds, the maximum speed is largely influenced by extinction coefficient, decreasing rapidly from 3.55 m/s at Cs = 0.30 m⁻¹ to 2.53 m/s at Cs = 0.75 m⁻¹ while the minimum and mean speeds are almost constant with a slight decrease as Cs increases. The maximum evacuation speed trends in the present experiments and those in Frantzich and Nillson (2003, 2004) and Fridolf et al. (2013), lie on the same decreasing logarithmic curve as a function of extinction coefficient.     

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.     

海底特长隧道利用横通道控烟技术研究

摘 要：为了解决特长海底隧道发生火灾时的排烟问题,提出利用服务通道和联络横通道辅助送风的通风方案。利用火灾动力学模拟软件（FDS）,建立隧道火灾通风模型,通过研究通风排烟时服务隧道内补风量与横通道开启数量对火灾烟气的控制效果,确定通风系统的技术参数。结果表明：火灾发生时,事故隧道内纵向通风风速2 m/s,同时开启火源上游3 个横通道,并在服务隧道两端各施加1.3 m/s 纵向通风风速,既可将烟气控制在火源一侧,同时不影响人员安全疏散,其控烟效果与通风网络解算结果一致。采用横通道辅助送风的通风方案,控制特长海底隧道内火灾烟气蔓延是具有理论可行性的。     

地铁区间隧道火灾通风模式的数值分析

介绍了地铁区间隧道火灾常见的几种通风排烟模式,对其中一种最复杂的模式进行了数值分析。模拟分析得出,对于地铁实际工程中的单线盾构圆形隧道,在10 MW火灾强度下,着火区间隧道内2.6~2.9 m/s左右的纵向风速可以有效阻止烟气发生逆流;在着火区间隧道2.9 m/s的纵向风速下,未着火区间隧道两端对送送风速度为1~1.5 m/s时,联络通道内有风速为6 m/s左右的气流流向着火区间隧道,可有效抑制烟气通过联络通道向未着火区间隧道蔓延,保证人员的安全疏散。     

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.     

胶州湾海底隧道防排烟设计探讨

分析海底隧道火灾烟气的特点及危害.结合青岛胶州湾海底隧道情况,运用FDS模拟分析海底隧道半横向通风在各种情况下的排烟效果,通过改变排烟口数量、排烟量以及隧道内纵向风速等参数研究烟气运动规律.结果表明,排烟量相同时,开启的排烟口越少,越有利于控制烟气向两端蔓延;存在纵向风时,火源上游排烟口越少,烟气抑制效果越好,烟气运动速率随纵向风速的增大而减小.     

Experimental investigation on transverse ceiling flame length and temperature distribution of sidewall confined tunnel fire

This paper presents an experimental investigation on the transverse ceiling flame length and the temperature distribution of a sidewall confined tunnel fire. The experiments were conducted in a 1/6th scale model tunnel with the fire source placed against the sidewall, 0 m, 0.17 m and 0.35 m above the floor, respectively. Experiments of fire against a wall without a ceiling, 0.35 m above the floor in a large space, were also conducted as a control group. Results shows that for small heat release rate (HRR), the flame is lower than the ceiling and extends along the sidewall. With the increase of HRR and elevation of burner height, the flame gradually impinges on the ceiling and spreads out radially along it. The flame impingement condition and the flame shapes of the wall fire with and without ceiling are presented. From the viewpoint of the physical meaning of flame impinging on the ceiling, the horizontal flame length should be a function of the unburned part of the fuel at the impinging point. Based on the proportional relation between the flame volume and HRR, the effective HRR (Q_ef) at the ceiling is determined and the effective dimensionless HRR, Q^*_ef is defined to correlate the horizontal ceiling flame length. Additionally, predictive correlations of transverse ceiling temperature distribution are proposed for the continuous flame region, the intermittent flame region and the buoyant plume region under the ceiling, respectively.     

The effect of fuel area size on behavior of fires in a reduced-scale single-track railway tunnel

A set of experiments was carried out in a 1/9 reduced-scale single-track railway tunnel to investigate the effect of fuel area size on the temperature distribution and behavior of fires in a tunnel with natural ventilation. Methanol pool fires with four different fuel areas 0.6 × 0.3 m² (1 pan), 1.2 × 0.3 m² (2 pans), 2.4 × 0.3 m² (4 pans) and 3.6 × 0.3 m² (6 pans), were used in these experiments. Data were collected on temperatures, radiative heat flux and mass loss rates. The temperature distribution and smoke layer in the tunnel, along with overflow dimensions and radiant heat at the tunnel entrance were analyzed. The results show that as the fuel area enlarges, the fire gradually becomes ventilation-controlled and the ceiling temperature over the center of fire source declines. Burning at the central region of fire source is depressed due to lack of oxygen. This makes the temperature distribution along the tunnel ceiling change from a typical inverted V-shape to an M-shape. As observed in the experiments, a jet flame appeared at tunnel entrances and both the size and temperature of the flame increased with the enlargement of fuel area leading to a great threat to firefighters and evacuees in actual tunnel fires.     

Hydraulic conductivity of fly ash-sewage sludge mixes for use in landfill cover liners

Secondary materials could help meeting the increasing demand of landfill cover liner materials. In this study, the effect of compaction energy, water content, ash ratio, freezing, drying and biological activity on the hydraulic conductivity of two fly ash-sewage sludge mixes was investigated using a 2^7-1 fractional factorial design. The aim was to identify the factors that influence hydraulic conductivity, to quantify their effects and to assess how a sufficiently low hydraulic conductivity can be achieved. The factors compaction energy and drying, as well as the factor interactions material × ash ratio and ash ratio × compaction energy affected hydraulic conductivity significantly (α = 0.05). Freezing on five freeze-thaw cycles did not affect hydraulic conductivity. Water content affected hydraulic conductivity only initially. The hydraulic conductivity data were modelled using multiple linear regression. The derived models were reliable as indicated by R_adjusted² values between 0.75 and 0.86. Independent on the ash ratio and the material, hydraulic conductivity was predicted to be between 1.7 × 10⁻¹¹ m s⁻¹ and 8.9 × 10⁻¹⁰ m s⁻¹ if the compaction energy was 2.4 J cm⁻³, the ash ratio between 20% and 75% and drying did not occur. Thus, the investigated materials met the limit value for non-hazardous waste landfills of 10⁻⁹ m s⁻¹.     

实体隧道火灾温度分布试验研究

建立实体隧道火灾现场试验系统及数据采集系统,通过在岩门界隧道开展的实体隧道火灾试验,测试了不同通风模式、不同火灾工况下隧道火源上、下游不同断面处的温度变化及隧道拱顶的温度变化。试验结果表明,与车行方向相同的1.8m/s机械通风能在一定程度上延缓烟气蔓延;有与车行方向相反的自然风时,火源下游温度较低,最高温度出现在火源上游;有机械通风时,温度随风向、风速而变化。     

无纵向通风曲线隧道和直线隧道火灾烟气特性数值模拟

基于数值模拟的方法,采用PyroSim 软件搭建半径分别为250、300、400、500、600 m 的曲线隧道模型及长度为130.8 m 的直线模型,模拟隧道火灾发生后无纵向通风时的烟气运动,对比分析两种模型中心线上不同高度的烟气温度。模拟分析得到：火灾前期,直线模型中烟气蔓延时基本关于隧道中心线对称,而曲线模型中烟气运动时在上游偏向凹壁下游偏向凸壁;达到稳定状态时,直线模型中火源正上方温度高于曲线模型,无论近火源区还是远火源区,直线模型温度纵向分布关于火源位置均具有很好的对称性,而曲线模型中表现为近火源区波动较大,远火源区温度衰减梯度大于直线模型,1.6 m 高度上游温度衰减梯度大于下游;提出曲线模型中顶棚温度纵向衰减指数模型。