Smoke Movement in a Sloping Subway Tunnel Under Longitudinal Ventilation with Blockage

Critical velocity and smoke back-layering length are two of the determining parameters to the fire risk assessment of subway tunnel. These two parameters of a sloping subway tunnel with train blockage were investigated both experimentally and numerically in this paper. To address the influences of slope, the slopes of 0, 3, 6, 9, 12, 15% in downhill subway tunnel were studied and the height (H) of the tunnel was replaced by the inclined tunnel height (\( H/\cos \theta \)). The train model with a dimension of 2 m (length) × 0.3 m (width) × 0.38 m (height) was also chosen in simulations and experiments for the tunnel blockage. Thenceforward, 30 reduced-scale experimental and 150 numerical scenarios were analyzed to predict the critical velocity and smoke back-layering length in various sloping subway tunnels. Six different heat release rates including 5.58, 11.17, 16.67, 22.35, 27.94, and 33.52 kW were considered in the experiments and five different heat release rates including 2.79, 5.58, 8.38, 11.17 and 16.67 kW were considered in the simulations. Based on the comparison in the horizontal tunnel, numerical results were quite consistent with the experiments. The results showed that train blockage influenced the smoke back-layering length, and the critical velocity increases with the tunnel slope. Finally, empirical models were developed to predict the critical velocity and smoke back-layering length in a sloping subway tunnel with train blockage.     

Fire simulation in road tunnels

The catastrophic tunnel fires since the year 1999 and a series of accidents in some tunnels in the summer of 2001 triggered extensive discussions and proposals relating to tunnel safety. When a fire occurs in a tunnel, and in absence of sufficient air supply, large quantities of smoke are generated, filling the vehicles and any space available around them. Unless a strong flow is created and maintained, hot gases and smoke migrate in all directions. With a weak airflow, smoke forms a layer along the tunnel ceiling and can flow against the direction of forced ventilation, interfering with personal evacuation. This paper shows the results of a computer fire simulation in a tunnel and the results of this simulation: air velocity, air temperature and wall temperature in the case of fire. The simulation started before the emergency ventilation system is activated and continued with the fans activated to control the smoke.     

海拔高度对公路隧道火灾烟气分布特性影响研究

高海拔地区存在大气压力低、空气密度小、含氧量低等特点,对公路隧道火灾燃烧和烟气扩散的影响与平原地区隧道有显著差异。为了得到不同海拔高度条件下公路隧道内火灾烟气扩散特性和温度场分布规律,依托米拉山高海拔特长隧道,采用FDS计算软件,对海拔0~ 6 km范围内不同海拔高度的隧道火灾进行三维数值模拟计算。分析不同海拔高度条件下火灾烟气逆流长度、隧道拱顶温度、距地面1.8 m高度处温度分布、隧道竖向温度分布等主要参数。研究结果表明:随着海拔高度的增加,火源上游区域烟气逆流长度不断减小,烟气逆流长度与海拔高度大致呈线性关系,海拔6 km情况下烟气逆流长度为平原地区的57%;与平原地区隧道相比,高海拔地区隧道火源上游区域温度较低,火源下游区域温度较高,高海拔隧道火源下游区域烟气温度降低幅度较大;火源下游区域,高海拔地区隧道竖向温度增长较快。     

Longitudinal ventilation for smoke control in a tilted tunnel by scale modeling

Longitudinal ventilation systems are commonly installed in new tunnels in large cities of the Far East including Mainland China, Hong Kong and Taiwan. Many tunnels are found in big cities and some of them are inclined at an angle to the horizontal. However, smoke movement in tilted tunnels is not fully understood. In some of the tunnels, the ventilation system was designed based on presumed smoke movement pattern without experimental demonstration. Smoke movement pattern in a tilted tunnel model was studied by using a scaled model. A 1/50 tunnel model of length 2 m with adjustable angle to the horizontal was constructed by transparent acrylic plastics. A small 0.097 kW propanol pool fire was used as the heat source combined with burning pellets generating smoke. A fan placed at the upstream end was used to create longitudinal ventilation. Different ventilation rates were set using a transformer to control or adjust the fan speed. Experiments were performed with the tunnel angle varying up to 30° to the horizontal. Effect of smoke screens was also studied. The observed smoke movement patterns indicated that the shape of the buoyant plume inside the tunnel depends on the tilted angle. Smoke would flow along the tunnel floor due to gravity. The bending angle of the plume depends on the tunnel angle. Tunnel inclined at greater angles to the horizontal would give larger amount of smoke flow. Smoke movement pattern for a tilted tunnel with smoke screens was observed to be very different from some design projects. All results will be reported in this paper.     

Numerical Investigation of Back-Layering Length and Critical Velocity in Curved Subway Tunnels with Different Turning Radius

Curved tunnels are inevitable subjected to the city underground geological conditions. Due to the catastrophic consequence of tunnel fires with high population density, the related researches on fire safety of curved tunnel are full of significance. Therefore, a series of curved subway tunnels with turning radius of 300–1000 m were investigated numerically by FDS 5.5.3 in terms of the smoke back-layering length and critical ventilation velocity under the heat release rate of 5–10 MW. Theoretical analysis shows that the curved tunnel with the local resistance has an advantage of preventing smoke spreading compared with straight tunnel. The simulation results also indicated that both the smoke back-layering length and the critical ventilation velocity increased with the rising turning radius, and the straight tunnel has the largest values. In fact, the local resistance impact factor for the smoke back-layering length in the curved tunnel, \( k_{f} \), was controlled by turning radius \( R \) and ventilation velocity \( V \). The dimensionless critical velocity increased slightly from \( 0.638Q^{*1/3} \) to \( 0.669Q^{*1/3} \) when the turning radius increased from 300 m to 1000 m. Without considering the influence of turning radius (local resistance), previous models cannot be applied to the curved tunnel. The improved prediction models about smoke back-layering length and critical velocity with the factor of turning radius could provide a technical guideline for the tunnel ventilation designs.     

大纵坡双向行车公路隧道火灾控制风速研究

城万二级公路白芷山隧道和八台山隧道为双向行车的大纵坡公路隧道,火灾后烟流控制难。采用火灾动力学计算软件FDS对其进行了火灾三维数值模拟,分析了仅火风压作用及0.5 m/s、1 m/s和-0.5 m/s控制风速作用下的烟流扩散和能见度的分布规律。结果表明:在大纵坡隧道中,火灾后仅靠火灾效应能引起火灾烟流的流动,其流动的规律是烟流从火源点流向高洞口方向,火灾烟流能完全从高洞口排出,而烟流向火源点下坡方向的蔓延距离仅在100 m左右;当风速为0.5 m/s时,烟流在火源两侧基本呈对称状蔓延;下坡方向的隧道中的烟流也因风速较小,产生的扰动比较小,这种烟流控制是最为合理,有利于灾害情况下逃生。因此,建议将白芷山隧道和八台山隧道的火灾控制风速取为0.5 m/s左右,且该速度应为自然风速、火风压引起的风速和风机提供风速的合速度。     

Analysis of FDS 6 Simulation Results for Planar Air Curtain Related Flows from Straight Rectangular Ducts

CFD results are discussed for jet flows through a straight square duct, which is an interesting configuration in the context of air curtain flows for smoke and heat control in buildings in case of fire. The CFD package Fire Dynamics Simulator, Version 6.0.1, is used. Special focus is given to the impact of the inlet boundary condition on the flow field in the near-field region. Investigation of different orifice configurations (W = 2 cm width, variable span-wise length), including calculations inside a straight square duct (2 cm × 2 cm, with variable length) ahead of the air orifice, reveals a small vena contracta effect when the orifice is flush with a solid boundary, leading to an acceleration of the flow in the symmetry plane in the near-field region. The vena contracta effect disappears if the co-flow at the nozzle exit is aligned with the jet. More important is the effect of the duct length (precursor domain length, serving as method to generate inflow turbulent conditions for the main computation): imposing a top hat velocity profile, a sufficiently long duct (i.e., L = 20W) is required for the flow to become fully developed at the orifice. The CFD results confirm an analytical correlation for the ratio of the entrance length to the hydraulic diameter of the duct as function of the Reynolds number, provided the duct width is used as characteristic length scale. Using a sufficiently fine mesh, i.e., 10 cells across the characteristic dimension of the nozzle, the evolution of the mean and RMS stream-wise velocity along the centerline, as well as their profiles across the nozzle width, are shown to be captured accurately in the CFD results.     

Heat Release Rate of Heavy Goods Vehicle Fire in Tunnels with Fixed Water Based Fire-Fighting System

A series of large-scale fire tests for road tunnel application was conducted in a test tunnel facility in Spain. The aim of this fire tests program was to investigate the magnitude of the heat release rate generated by a fire in heavy goods vehicles (HGV’s) with and without a fire suppression system in tunnels in Singapore; the possibility of interchanging a fire suppression system with other measures such as lowering the longitudinal flow velocity; and to acquire information on the appropriate design parameters (e.g., nozzle type, discharge density and activation time) to adopt based on the most probable fuel load used in these road tunnels. In order to ensure repeatability, simulated HGV’s consisting of 228 pallets with 48 plastic pallets (20%) and 180 wooden pallets (80%) were used in all fire tests. An air velocity of approximately 3 m/s was applied. As the scope of work covered in this fire test program is very large, only the setup of the fire test and the findings on the effects of heat release rate with (Test 4) and without (Test 7) a fixed water based fire-fighting system are covered. The test results indicate that a substantial reduction of fire heat release rate can be obtained using a low-pressure deluge fire suppression system, as long as timely activation of the water is provided. However, the influence of the suppression system on CO production is significant. Such experimental data address the current dearth of knowledge on the actual effect of low-pressure deluge systems on the heat release rate from HGVs in tunnel fires.     

具有横通道的双洞公路隧道火灾数值模拟研究

隧道内发生火灾时空气流动状况非常复杂,是一种带有剧烈放热化学反应的流动现象,它包含着流动、传热、传质和化学反应以及它们之间的相互作用,实际燃烧过程是三维非稳态湍流过程.文章采用三维数值模拟方法,用连续方程、动量方程、能量方程及气体组分方程描述其气流流动状态,并采用湍流粘性系数模型中应用最广泛和最成功的受浮力影响的湍流模型方程.通过对某实际隧道的不同火灾工况进行模拟计算,以及速度场、温度场的分析,从中可以较全面地了解隧道火灾特性和烟气发展规律,从而提出控制火灾烟气的优化通风方案,为确定人员逃生方式及时间提供理论依据.     

Effects of Ventilation and Water Spray in a Model-Scale Tunnel Fire

At the present time, eight new long tunnels are currently being constructed in Taiwan and plan to utilize sidewall sprinkler nozzles to conduct fire protection. To explore the effect of ventilation and water spray on tunnel fires, this research used a 1/5.5 model scale tunnel to perform experiments, while using a heptane oil pan of 0.45 m diameter as the fire source. Under five different ventilation velocities, the cooling effects of water spray on tunnel fires was discussed while controlling the water density at 2.3 mm/min. The critical velocity without water spray was 1.24 m/s, which fits empirical equations by past research. Under a ventilation velocity of 0.87 m/s, the smoke backlayering would be 2 m without water spray, while none was present with water spray. Overall quantitative analysis shows the significance of water spray in affecting backlayering, and it can be acknowledged that water spray can prevent backlayering from larger fires under the same velocity. This research used visual and temperature measurement methods to determine the location of backlayering, and it is recommended that maximum temperature gradient be used in future research as the basis for smoke backlayering location.     

Fire Detection Algorithm Combined with Image Processing and Flame Emission Spectroscopy

Fire poses a significant risk to the safety, health, and property of people around the world. However, traditional ‘‘point sensor’’ fire detection techniques for indoor buildings based on air particles, air temperatures, and smoke have a low sensitivity, long response time, and poor stability. Therefore, video-based fire detection has become a particularly efficient and important method for detecting the early signs of a fire. Due to image blur, low illumination, flame-like interference and other factors, there is a certain error rate of fire recognition using video flame recognition methods. According to our previous study of a multi-feature flame recognition algorithm, a novel flame recognition algorithm based on free radical emission spectroscopy during combustion is investigated in this paper. First, multiple features are extracted from the video images by employing our proposed processing scheme. Then, the features are post-processed by a temporal smoothing algorithm to eliminate the error recognition rate, which is caused by the similar characteristics of objects between flame-like and real flame areas. In the temporal smoothing experiments, the proposed method achieves the true positive rates of 0.965 and 0.937 for butane flames and forest fire, respectively. Additionally, the spectral signals of OH, CH, C₂ and other free radicals in the combustion objects were acquired by the spectrometer. The vibrational temperature and rotational temperature are calculated after identification of the A²Δ?→?X²Π transition of the CH (410–440 nm). The flames-like are completely rejected by the proposed method in the validation experiment. In the subsequent butane combustion experiment, the vibrational temperature of the butane was 4896 K, and the rotational temperature was 2290 K. The experimental results show that real fires can be precisely recognized and that the combustion temperature can be determined from the CH emission spectroscopy. This novel method provides a new viewpoint for fire detection and recognition.     

重点排烟模式下坡度隧道烟气控制优化研究

针对不同坡度隧道,对重点排烟模式下的烟气控制效果进行研究。以某三车道隧道为例,采用FDS对不同坡度、单向与双向重点排烟以及不同的排烟口开启方式等多工况进行模拟研究,分析烟气蔓延长度、能见度和隧道洞口自然补风风速等关键性判据的变化规律,提出重点排烟模式下不同坡度隧道的烟气控制优化建议。     

公路隧道火灾CO浓度的数值模拟

国内针对公路隧道发生火灾时CO浓度变化规律的研究还很少。文章以厦门莲花隧道为对象，用FLUENT软件对否．固墨垫下哆火赛归气滇动鉴强尊彳亍了三维棼傅模拟，并分析了隧道不同高度不同横断面的CO浓度分布，得出了结论。     

Smoke spreading characteristics during a fire in a shallow urban road tunnel with roof openings under a longitudinal external wind blowing

A series of fire experiments was conducted using a 1:12 scale model of a shallow urban road tunnel with roof openings to clarify the flow structure of smoke and fresh air during a fire with a longitudinal external wind blowing above the roof openings. The model tunnel consisted of two road tubes separated by a pillar-type median structure. Five fire test cases were conducted by changing the heat release rate as the experimental parameter. When the smoke produced by a fire in the tunnel tube was exhausted by natural ventilation through the roof openings of the tunnel tube, fresh air was sucked in from the roof openings of the opposite tunnel tube. The flow of exhausted smoke and sucked-in fresh air created a complex three-dimensional flow structure inside the tunnel tubes. Stratified smoke that had formed under the ceiling of the tunnel tube was disturbed by the flow of sucked-in fresh air and was diffused on the upstream side of the fire. Compared to the condition without a longitudinal external wind, when a longitudinal external wind blew over the tunnel with the pillar median structure, the smoke spreading distance on the upstream side was longer than that without the external wind due to the diffusion of smoke. On the other hand, the smoke spreading distance on the downstream side of the fire was shorter than that without the external wind due to the improved smoke extraction performance by the Venturi effect of the longitudinal external wind. Furthermore, the smoke spreading distance on the downstream side was nearly constant and independent of the heat release rate of the fire, within the scope of our experimental conditions.     

高海拔隧道火灾烟气及温度场数值模拟研究

采用FDS建立典型双车道公路隧道,对海拔高度为500、4000 m的公路隧道发生火灾时的烟气蔓延特征及温度分布规律进行数值模拟分析,以得到低压、低温、低氧含量等高海拔环境对公路隧道火灾发展的影响规律.结果表明:相比较平原地区隧道,高海拔地区公路隧道火灾烟气最高温度更低,火焰高度更高,且近火源区的拱顶最高温度升温速度明显...     

设排烟道隧道的烟气逆流长度与临界风速研究

隧道火灾是运营公路隧道的主要灾害。为有效控制隧道火灾,采用理论分析和数值模拟相结合的方法研究了设排烟道隧道的火灾烟气逆流长度与临界风速。以国内常见的双车道隧道尺寸建立模型,分析了排烟速率和纵向通风速率对烟气逆流长度的影响,提出了临界风速的预测模型。并将其通风效果与常规未设排烟道的纵向通风做了比较。结果表明:未设排烟道时,纵向风速还未达到临界风速时,火灾下游烟气的层化状态就已破坏。设排烟道能及时排出火灾产生的烟气,有利于保持烟气的层化状态,有效改善火灾时的隧道环境,为火灾下游人员的疏散救援提供了有利条件。同时,设置排烟道有利于减小逆流长度和临界风速。随着排烟速率的增大,相应的临界风速呈指数函数递减的特性。     

Comparison of the Combustion of Pine Species with Two Sizes of Calorimeter: 10 g vs. 100 g

This study aims to analyze the scale effects on the combustion dynamics and smoke emitted during the burning of vegetation species under laboratory conditions. Experiments were carried out at two scales of combustion with two fuels (Pinus pinaster and Pinus laricio needles). Two oxygen consumption calorimetry devices were used: the cone calorimeter with samples in the scale of 10 g, and Large Scale Heat Release rate calorimeter (LSHR) with samples in the scale of 100 g. Fuel loads of 1.2 kg m^?2 and 1.04 kg m^?2 were considered respectively for Pinus pinaster and Pinus laricio needles. The parameters investigated to analyze the combustion dynamics are the heat release rate, combustion efficiency and mass loss rate. Measuring the extinction coefficient allowed us to obtain the soot emission factor. The emission factors of the combustion gases were obtained by using FTIR and NDIR detectors. A carbon mass balance showed that more than 95% of carbon was recovered. This study revealed an effect of scale on the combustion dynamics (peak HRR of 806 kW m^?2 and 651 kW m^?2 respectively at the bench scale and at full scale for Pinus pinaster), total smoke production, soot emission factor (1.32 g kg^?1 and 5.17 g kg^?1 respectively at the bench scale and at full scale for Pinus pinaster) and nitrogen compounds emission factors (2.47 g kg^?1 and 5.25 g kg^?1 for the NO respectively with the cone calorimeter and LSHR for Pinus laricio). Combustion efficiency appeared to be largely independent of the combustion scale (96.89% and 96.61% respectively at the bench scale and at full scale for Pinus pinaster). We also observed differences in the fire behavior for both types of needles.     

A Study of the Critical Velocity of Smoke Bifurcation Flow in Tunnel with Longitudinal Ventilation

Small longitudinal velocity cannot prevent backlayering in tunnel fire, while excessive longitudinal velocity will destroy stratification of smoke layer and lead to bifurcation flow. As smoke bifurcation flow proceeds, the longitudinal flow is divided into two streams and flow along both sidewalls of the tunnel ceiling. The critical velocity of bifurcation flow is the minimum value at which bifurcation flow starts to occur. To investigate the critical velocity of bifurcation flow, experiments and CFD simulations were conducted. Experiment was carried out in a reduced-scale tunnel, which is 8 m long, 1 m wide and 0.5 m high. The numerical research was performed using FDS. In simulation, the computational region of a tunnel is 200 m long, 10 m wide. The heat release rate (1 MW to 6 MW) and the height (4 m to 8 m) is changed in the 30 simulation scenarios. Theoretical analysis showed that the dimensionless critical velocity of bifurcation flow only depends on the dimensionless heat release rates, and a mathematical equation is proposed. The reduced-scale experiments indicated that the critical velocity of bifurcation flow is 1.48 times that of critical velocity for preventing backlayering, and the coefficient is in agreement with CFD simulation.     

Numerical 3D simulation of a longitudinal ventilation system: Memorial Tunnel case

In this work, a numerical 3D simulation of a longitudinal ventilation system (LVS) is developed to analyze the fire behaviour inside a road tunnel. The numerical modelling reproduces the Memorial Tunnel, a two-lane, 853 m long road tunnel, used for experimental purposes. On this tunnel, 98 full-scale fire ventilation tests with different ventilation systems were conducted, constituting the first significant experimental approach to analyze fire incidents inside road tunnels. A total number of 24 reversible jet fans were installed in groups of three, nearly equally spaced over the length of the tunnel, and cantilevered from the ceiling of the tunnel.

The validation of a numerical model is developed in the present paper. For that purpose, the behaviour of the smoke generated during a fire incident inside a road tunnel is predicted and compared with previous experimental data collected in the Memorial Tunnel Project. The smoke evolution and the performance of the LVS is simulated with a commercial code, FLUENT, which allows 3D unsteady simulations of the Navier–Stokes equations for multispecies mixtures of gases. A sufficient mesh density was introduced for the spatial discretization in order to obtain accurate results in a reasonable CPU time. Hence, typical ratios between total number of cells and the overall tunnel length were employed in the modelling. As a result, good agreement was achieved in all the tested cases, defining an accurate methodology to predict the performance of a LVS in case of fire inside a tunnel.     

Experimental studies on fire-induced buoyant smoke temperature distribution along tunnel ceiling

Twelve tests were conducted to study the distribution of smoke temperature along the tunnel ceiling in the one-dimensional spreading phase, two tests in a large-scale tunnel and the other ten in full scale vehicular tunnels. The fire size and the height above the floor, the tunnel section geometry and longitudinal ventilation velocity varied in these tests. Experimental results showed that when the fire size was larger, the smoke temperature below the ceiling was higher, but it decayed faster while traveling down the tunnel. The longitudinal ventilation velocity seemed to take much influence on the smoke temperature decay speed downstream. A “barrier effect” was shown for the smoke temperature distribution of the upstream back layering. The smoke temperatures measured were higher upstream than downstream before the “barrier”, and were much lower and decreased faster along the tunnel ceiling after the “barrier”. The temperature and the traveling velocity of the upstream smoke flow decreased largely when the longitudinal ventilation velocity increased a bit. The dimensionless excess smoke temperature distributions along the tunnel ceiling in all tests fell into good exponential decay. But the decay speed along the tunnel seemed to be much larger in the large-scale tunnel than that in full-scale tunnels. The measured data on ceiling jet temperature decay along the tunnel was compared with predictions of Delichatsios's model, a model built based on small-scale tests, with hydraulic diameter introduced. Results showed that Delichatsisos’ model over estimated the decay speed of ceiling jet temperature for the downstream flow. However, good agreement was achieved between the measured data and the model predictions for the upstream back layering. All the experimental data presented in this paper can be further applied for verification of numerical models, bench-scale results and building new models on ceiling jet temperature distribution.