Jet entrainment effect on fire-induced flow in isothermal model applied to reduced-scale tunnel

Based on the entrainment rate of isothermal jet, an application method of the isothermal model in tunnel fire was presented through the more reasonable estimation of total smoke flow rate and its density at jet exit. Acetone LIF measurement was performed to identify the entrainment of ambient air qualitatively, and the entrainment constants were provided with the results of a previous study of which jet velocity corresponded to that of this study. The entrainment effect was more remarkable for the smaller fire, because the normalized axial distance to ceiling was increased with decreasing the fire size. These results suggested that the present model considering the jet entrainment might enhance the previous isothermal model in tunnel fire.     

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.     

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.     

A model experimental study on backdraught in tunnel fires

Backdraught is a special fire phenomenon in a limited-ventilation space. Research on the occurrence of backdraught in compartment fires has been extensive, but backdraught in subway tunnel fires, which occur in underground spaces with a large slenderness ratio, has received insufficient attention. In the present study, 31 cases, divided into two groups under conditions of natural ventilation and mechanical ventilation, were examined using a 1/8 reduced-scale model tunnel to investigate the critical conditions and characteristics of backdraught occurrence, as well as the differences between tunnel backdraught and compartment backdraught. The existing critical values of the mass fraction for different ventilation conditions are also discussed. The results indicated that the key parameter determining the occurrence of backdraught in subway tunnel fires is the mass fraction of the volatilized unburned fuel in the tunnel. The critical values of the mass fraction in natural ventilation and mechanical ventilation were 8.78% and 11.71%, respectively, with a humidity of 15% in fresh air. With natural ventilation, backdraught occurrence in a single tube tunnel configuration was similar to that in a compartment, and the ignition delay depended on the velocity of the gravity current; with mechanical ventilation, backdraught occurrence in a twin-tube tunnel configuration was different from that in a compartment, and its ignition delay was determined mainly by the ignition source delay. In addition, as the velocity of air flowing into the tunnel increased, so did the intensity of the backdraught. The experimental data were qualitatively validated and analyzed based on the flammability diagram of the fuel, and these results were similar to those obtained in previous experiments. Also, the humidity of the fresh air flowing into the tunnel affected the occurrence of backdraught; it would not occur under conditions of higher humidity even if the mass fraction mentioned above was higher. Furthermore, it may be economical and feasible to install a humidification device in the tunnel ventilation system to humidify the fresh air flowing into the tunnel to inhibit backdraught.     

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.     

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.     

Determination of smoke layer interface height of medium scale tunnel fire scenarios

Smoke layer interface height is an important parameter in fire safety science. In this paper, a series of experiments were conducted in a 1/6th scale model tunnel for determining the smoke layer interface height in medium scale tunnel fire scenarios. The commonly used approaches, including visual observation, N-percentage rule and integral method are reviewed firstly. Then, considering the subjectivity and empiricism of previous approaches, a buoyancy frequency method is put forward based on the vertical temperature distribution in tunnel, which has definite physical meaning and eliminates the subjectivity of previous methods. The smoke layer thicknesses determined by buoyancy frequency method are compared with the results of visual observation, N-percentage rule (N = 10, 20, 30) and integral ratio method, respectively. The comparison results reveal that the smoke layer thicknesses determined by buoyancy frequency method fit best with the visual values for all the experimental conditions. While the calculated values by integral ratio method are lower than the visual values. In addition, the selection of optimum N values for the N-percentage rule in different cases is also discussed.     

The influence of travelling fires on a concrete frame

When building fires occur in large, open compartments they rarely burn uniformly across an entire floor plate of a structure. Instead, they tend to travel, igniting fuel in their path and burning it out as they move to the next fuel package. Current structural fire design methods do not account for these types of fires. This paper applies a novel methodology for defining a family of possible heating regimes to a framed concrete structure using the concept of travelling fires. A finite-element model of a generic concrete structure is used to study the impact of the family of fires; both relative to one another and in comparison to the conventional codified temperature-time curves. It is found that travelling fires have a significant impact on the performance of the structure and that the current design approaches cannot be assumed to be conservative. Further, it is found that a travelling fire of approximately 25% of the floor plate in size is the most severe in terms of structural response. It is concluded that the new approach is simple to implement, provides more realistic fire scenarios, and is more conservative than current design methods.     

Effect of cross section and ventilation on heat release rates in tunnel fires

Model scale fire tests were performed in tunnels with varying tunnel widths and heights in order to study the effect of tunnel cross-section and ventilation velocity on the heat release rate (HRR) for both liquid pool fires and solid fuel fires. The results showed that for well ventilated heptane pool fires, the tunnel width nearly has no influence on the HRR whilst a lower tunnel height clearly increases the HRR. For well ventilated solid fuel fires, the HRR increases by approximately 25% relative to a free burn test but the HRR is not sensitive to either tunnel width, tunnel height or ventilation velocity. For solid fuel fires that were not well ventilated, the HRRs could be less than those in free burn laboratory tests. In the case of ventilation controlled fires the HRRs approximately lie at the same level as for cases with natural ventilation.     

The maximum temperature of buoyancy-driven smoke flow beneath the ceiling in tunnel fires

In order to detect a fire and provide adequate fire protection to a tunnel structure, the maximum gas temperature beneath the ceiling to which the structure is exposed needs to be estimated. Theoretical analysis of maximum gas temperature beneath a tunnel ceiling based on a plume theory is given. The heat release rate, longitudinal ventilation velocity and tunnel geometry are taken into account. Two series of model-scale experimental tests were also carried out. The results of both analysis and experiments show that the maximum excess gas temperature beneath the ceiling can be divided into two regions. When the dimensionless ventilation velocity is greater than 0.19, the maximum excess gas temperature beneath the tunnel ceiling increases linearly with the heat release rate and decreases linearly with the longitudinal ventilation velocity. When the dimensionless ventilation velocity is less than 0.19, the maximum excess gas temperature beneath the ceiling varies as the two-thirds power of the dimensionless heat release rate, independent of the longitudinal ventilation velocity. In both regions, the maximum excess gas temperature varies as the −5/3 power of the vertical distance between the fire source bottom and tunnel ceiling. The investigation presented here considers only the cases when the continuous flame region is lower than the ceiling height.     

Effects of buoyancy induced roof ventilation systems for smoke removal in tunnel fires

The present article highlights the performance of natural roof ventilation systems and its effects on tunnel fire flow characteristics. Numerical analysis is performed using Large Eddy Simulations (LES) to predict fire growth rate and smoke movement in tunnel with single and multiple roof openings. The smoke venting performance of ceiling vents are investigated by varying the vent size and fire source locations. The critical parameters such as mass flow rate through ceiling openings, smoke traveling time and fire growth patterns are presented. The ceiling openings are effective in transferring hot gases and reduces the longitudinal smoke velocity. The heat source and ceiling vent locations significantly affects the vent performance and smoke behavior in tunnel. The present results are in good agreement with the experimental results available in literature.     

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.     

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.     

Investigation of the effect of tunnel ventilation on crib fires through small-scale experiments

This paper adopts a series of 1:20 scale tunnel experiments based on a series of large-scale tunnel experiments to study the influence of forced ventilation on fires. The small-scale tunnel has dimensions of 0.365 m (W)×0.26 m (H)×11.9 m (L). Cribs using a wood-based material provide the fuel source and forced ventilation velocities from 0.23 to 1.90 m/s are used. From the study of the measured heat release rate (HRR) and mass loss rate data it is found that the forced air velocity affects the fire spread rate and burning efficiency and further affects peak HRR values at different air velocities. A simple model to describe these influences is proposed. This model is used to reproduce the enhancement of peak HRR for cribs with different porosity factors noted by Ingason [1] and to assess the effects of using different length of cribs on peak HRR. The results from these analyses suggest that different porosity fuels result different involvement of burning surface area and result different changes in peak HRR. However, no significant difference to the enhancement on fire size is found when the burning surface area is similar. It is also found that the trend in the enhancement on fire size by using sufficiently long crib and available ventilation conditions matches the predictions of Carvel and Beard [2] for two-lane tunnel heavy goods vehicle fires.     

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.     

横截面形状和坡度对隧道火灾烟气分布影响研究

通过数值模拟建立了7种不同横截面形状的全尺寸跨座式单轨隧道模型,研究隧道横截面形状和坡度对隧道火灾烟气蔓延的影响.研究发现长方形隧道顶部温度普遍较高,其次为带有拱顶的隧道,正方形隧道顶部温度普遍较低,圆形隧道底部温度最低.在大火源功率条件下,坡度对临界风速的影响较小.离火源较远的位置,坡度对排烟效果的影响更加明显:随着...     

Effect of temperature control on a tunnel in permafrost

A key problem for tunneling in permafrost is how to coordinate the excavation and lining installation in a manner that allows good excavation productivity but installs the lining quickly enough to avoid stability problems caused by thawing of the rock around the tunnel. Excavation progress and tunnel stability benefits from a lower temperature of the rock, while construction of the lining benefits from a higher temperature for the development of concrete strength. Using finite element analyses of the temperatures inside a tunnel verified by physical measurements in a tunnel in permafrost, stability conditions related to the thawing depth and suitable for use during construction have been established. These have been found to be useful as a guide for the sequencing of excavation and lining of tunnels in permafrost.     

Velocity and temperature attenuation of a ceiling-jet along a horizontal tunnel with a flat ceiling and natural ventilation

A series of fire tests was conducted in a small-scale tunnel with dimensions of 10.0 m (L) × 0.75 m (W) × 0.45 m (H) and a rectangular cross-section. Detailed measurements of the velocity and temperature within a steady fire-driven ceiling-jet running along the centre of the ceiling were conducted.Referring to a theoretical derivation process described in the literature as a starting point, correlations representing the velocity and temperature attenuation along the tunnel axis were developed.The values of the coefficients included in the developed correlation for the velocity attenuation were measured using a particle image velocimetry system during the experiments conducted in the small-scale tunnel. The value of the Stanton number was determined by considering the ceiling-jet thickness, which was derived from the velocity distribution. The values of the coefficients included in the developed correlation for the temperature attenuation were also determined based on experimental results described in the literature, which were obtained in a large-scale tunnel constructed using good heat insulation properties.Through these correlations developed for the velocity and temperature attenuations along the tunnel axis, the variation in the Richardson number of the ceiling-jet based on the distance from the fire source position along the tunnel axis was examined, and the position where the ceiling-jet changed from a shooting flow to a tranquil flow was determined. The boundary positions between the shooting and tranquil flows were determined using correlations between the velocity and/or temperature attenuation, which were compared with the variation in the Richardson number along the tunnel axis to verify their appropriateness.     

非均匀火灾作用下方钢管混凝土柱受力机理研究

对单面、相对两面、三面及四面火灾作用下方钢管混凝土柱截面的典型温度场及其分布规律进行分析,研究方钢管混凝土柱轴向/侧向变形-时间关系曲线,得到不同受火条件对方钢管混凝土柱抗火性能的影响规律。结果表明,相对两面和四面受火的柱截面温度场双轴对称,因而截面材料强度场、温度应变、温度应力也呈双轴对称分布,柱受力机理与常温情况下类似;对于单面和三面受火情况,由于柱截面温度场单轴对称,使得柱受力模式与常温情况不同,受火过程中柱先弯向受火侧,然后弯向背火侧。与温度梯度产生的非均匀的温度变形和材料强度偏心相比,受火面的多少在更大程度上决定了试件的抗火性能,即在绝大多数情况下,四面受火、三面受火、相对两面受火及单面受火的方钢管混凝土柱的耐火极限依次增大。但在某些特殊情况（如长细比λ<30）下,三面受火的方钢管混凝土柱的耐火极限可能小于其四面受火情况,此时如按四面受火对方钢管混凝土柱进行抗火设计,将存在安全隐患。