Tunnel Fire Research Articles

Investigation on the natural smoke exhaust performance by vertical shaft in tunnel fires under different ambient pressures

In high-altitude tunnels, due to the ambient pressure and density of air being different from those under normal pressure, the convection and heat radiation in tunnels are also different when fires occur. As a result, the smoke exhaust capacity varies under different pressures. This work aimed to numerically explore the impact of ambient pressure on the smoke exhaust efficiency at high-altitude shallow tunnels with natural ventilation by shaft. The numerical results demonstrated that the smoke exhaust capacity is enhanced with increasing ambient pressure. This is because of the increase in the air entrainment coefficient and air density, and it causes plug-holing more easily to occur under higher ambient pressures. Once the plug-holing has taken place, the fresh air can be directly exhausted through the shaft, which results in a poor smoke exhaust capacity. By accounting for the factors of the smoke layer thickness, ambient pressure, the exhausted smoke temperature and heat release rate, the Richardson number was introduced as the criterion for determining whether plug-holing occurs. As ambient pressure was increased, the critical Richardson number for plug-holing was observed to decrease, a finding supported by existing research.

CFD Simulations to study the Effect of Ambient Temperature on the Ventilation in a Metro Tunnel under Different Heat Release Rate

Urban and inter-urban rail transport provide the most favoured commuting option for millions of individuals globally. In spite of massive investments of public money to develop rail technologies and related infrastructure such as tunnel systems, there remain challenges to ensuring safety in this area. Railway tunnels present specific and serious fire safety issues, based on their length and restricted width. Smoke and other emissions from fires are a central factor to be considered in relation to both structural safety and survival where tunnel fires occur. Patterns of smoke flow can impact upon how efficient exhaust systems are, and for naturally ventilated systems, establishing shaft smoke flow patterns in tunnel fires is a significant requirement. While earlier work has considered this issue, it has been based on standard atmospheric conditions, while external temperatures can influence smoke movement properties, heat distribution, and the pattern of smoke flows within vertically constructed shafts. This study takes the case study of an underground metro tunnel, applying computational fluid dynamics through ANSYS Fluent to simulate and examine the effects of more extreme external temperatures for fires of 5 and 15 mw occurring in this environment.

Experimental and Theoretical Investigation of Longitudinal Temperature Attenuation and Smoke Movement in Urban Utility Tunnel Fires

The urban utility tunnel is an indispensable part of modern engineering construction. However, the fire risk cannot be ignored due to the narrow space and limited ventilation of the utility tunnel. A study of smoke filling is performed in a 1/8-scaled utility tunnel (25 m × 0.5 m × 0.45 m). Five heat release rates (5, 10, 15, 20 and 25 kW) and four positions of fire sources are used for tests. The initial position of the one-dimensional smoke movement of strong plume is determined. Based on the traditional model, the longitudinal temperature attenuation model of tunnel smoke is established with consideration of radiation and convection heat losses. The theoretical value of the longitudinal temperature rise of smoke is in good agreement with the experimental value. A one-dimensional spreading velocity model is established that coincides well with the experimental value, and the relative error is less than 20%. The spreading velocity of smoke is increased by the heat release rate. The velocity of the smoke spreading at the near end is smaller than that at the center, due to the long spreading route. The current conclusions disclosed in this study provide important guidance for the ventilation design of utility tunnels for fire smoke scenarios.

Multi-dimensional investigation of ceiling jet temperature characteristics beneath curved ceiling produced by fire in the utility tunnel

To study the temperature profile beneath the curved ceiling from a multi-dimensional perspective, an experimental investigation was conducted in utility tunnels with two different sizes, and a series of models were established to predict a two-dimensional longitudinal temperature profile and transverse temperature attenuation in this study. The prediction models for the two-dimensional longitudinal temperature profile were developed by modeling the vertical temperature distribution, thermal boundary layer thickness, Gaussian thermal layer thickness and maximum longitudinal temperature attenuation. Through the experimental validation on two sizes, it is found that the proposed prediction model can express the longitudinal temperature profile from a two-dimensional view within the relative error range of 20%. Besides, the prediction model for transverse temperature attenuation was established by the definition of characteristic tilt angle, and the three detailed expressions of this model were proposed according to different types of ceiling jets after impingement. The proposed prediction model for transverse temperature attenuation was also validated, and the verification results showed that the relative error of the transverse temperature attenuation prediction model can be controlled within a 25%. The developed prediction models in this study can be applied in the fire protection engineering of utility tunnels.

Numerical study on smoke temperature characteristics in T-shaped bifurcated tunnel fires

Numerical study on smoke temperature characteristics in T-shaped bifurcated tunnel fires

Performance of various water-based fire suppression systems in tunnels with longitudinal ventilation

Low pressure, medium pressure and high pressure water-based fire suppression systems were tested in a medium scale tunnel (scale 1:3). The primary objective was to investigate which of these systems are most effective in the suppression or control of different types of tunnel fires. The default low, medium and high pressure systems refer to full scale water flow rates of 10 mm/min, 6.8 mm/min and 3.7 mm/min, respectively. Some other water densities were also tested to investigate the effects, as well as different ventilation velocities and activation criteria. Several series of fire tests were conducted for different fire scenarios. The fire scenarios considered included idle wood pallet fires, loosely packed wood crib fires, loosely packed wood and plastic crib fires, and pool fires, with or without a top cover on the fuel load. Comparisons of the three default systems based on the three parameters: heat release rate, energy released and possibility of fire spread, show that the performance of the default low pressure system is usually the most effective based on the parameters studied. The default high pressure system usually yields results less effective in comparison to the default low pressure system. The performance of the default medium pressure system usually lies in between them. The high pressure system behaves very differently in comparison to the others, in terms of tunnel ventilation velocity, water density, operating pressure, and the presence of the top cover.

Characteristics of planar buoyant jets and plumes in a turbulent channel crossflow from direct numerical simulations

This paper is motivated by an interest in understanding the characteristics of buoyant fluids discharged from the bottom wall of channels, such as encountered during tunnel fires or in river effluent discharge. Direct numerical simulation is used to model the upward release of a planar buoyant jet or plume from the bottom wall of a channel into an incoming turbulent crossflow. The well-studied jet-in-crossflow with only a momentum source is simulated first, and subsequently, fixing the incoming Reynolds number, buoyancy source as heat flux is added alongside varying momentum source, with two cases where only a buoyancy source is present. Appropriate five non-dimensional parameters relevant for this flow are defined, of which three are fixed and two—source to channel momentum ratio and Richardson number—are varied. The changes in turbulence characteristics as the buoyant jet or plume evolves downstream are presented. In all cases with buoyancy, except for the pure jet case, the plume is initially confined to the lower half of the channel before it suddenly lifts to the top half, an effect that occurs at an increasingly smaller downstream distance with increasing buoyancy, and dividing the flow into a near and far field. The distributions of mean and Reynolds stresses in the near and far field of the source are reported, and it is found that the channel flow becomes more turbulent downstream of the source, and further, the turbulent vertical temperature flux switches sign from near to far field owing the a change in the mean temperature gradient sign. From the input parameters and using the integrated temperature equation a reasonable estimate of the far field mean channel temperature can be obtained by a reference temperature based on the heat conservation that includes the convective and diffusive source heat flux. A monotonic behaviour of the back-layering distance is also observed a function of this reference temperature, which was difficult to obtain with the two specified non-dimensional parameters.

Tunnel fire safety management and systems thinking: Adapting engineering practice through regulations and education

Society is changing ever faster, and tunnels are complex systems where performance is affected by many different stakeholders. These conditions suggest that safety management needs to be proactive and based on a systems perspective that acknowledges socio-technical theories. Although systems thinking principles are foundational in overarching European regulations and goals, system principles generally don't affect tunnel fire safety design principles or engineering practice. In the countries investigated in this study, tunnel fire safety management (TFSM) builds on experience-based and risk management-based principles that are optimized independently system by system. This is usually done with limited consideration of how these systems are interconnected and affect the overall tunnel system. The purpose of this paper is to investigate how systems thinking could support existing engineering practice. The work presented in this article is the outcome of a collaboration between fire safety researchers and practitioners from five countries and three continents. Through three workshops, current TFSM principles have been compiled and discussed. It is suggested that tunnel safety regulations be redesigned to strengthen the ability of engineers to work in design teams using systems thinking principles.

Analysis of large-scale fires in Makkah’s King Khaled Road tunnel using non-gray radiation models

The present study validated a fire dynamic simulation approach for reducing costs and losses in tunnel fire tests. It analyzed various radiation models, including gray, non-gray, and non-gray sooting models. The central processing unit (CPU) run time of the last model was very long, and the model is of little value in practice. The non-gray model was found to be the most accurate and efficient. This model was used to study fire behavior in the King Khaled Road tunnel in Makkah. This was done because the tunnel is small, there aren't many ways to get out, and it's hard to get firefighting teams to a place like Makkah during the Hajj season when there are a lot of people. Results showed that gas temperature, radiation, CO and CO2 concentrations, and air velocity increased with heat release rate, oxygen concentration, and soot yield. The air velocity was overpredicted during the simulations, having consequences for most of the other quantities, and this led to a cooler gas temperature, higher oxygen levels, and lower CO and CO2 concentrations. In addition, it is established that gas temperature and radiation increased when the ambient temperature increased, and oxygen concentration and air velocity were mostly unaffected by changing ambient temperature.

Fire-Retarding Asphalt Pavement for Urban Road Tunnels: A State-of-the-Art Review and Beyond

AbstractWith the rapid urbanization and development of metropolises, urban road tunnels have been constructed at an increasing rate, significantly alleviating urban traffic pressure, and improving urban resilience. Fire hazards have become a major threat to modern road tunnels due to the growing popularity of electric vehicles and high-density transportation of goods, particularly flammable materials. Asphalt pavements, as an essential component of road tunnels, may release harmful effluences and smoke under high temperatures, exacerbating the fire and adding risk to life safety. It is hence critical to investigate fire-retarding asphalt materials and their potential use in urban road tunnels pavements. This paper provides a comprehensive review of fire-retarding asphalt pavements for urban road tunnel pavements. The review covers tunnel fire generation mechanisms, evaluation methods, flame retardants for asphalt pavements, and recent developments in flame retardant technologies. By investigating these aspects, this paper aims to better understand the flammability of asphalt mixtures and asphalt pavements in urban road tunnels, promote the research of flame-retardant technology, and ultimately reduce the damage and loss caused by asphalt road tunnel fire accidents. Additionally, this study identifies the limitations of current research and provides an outlook for future research to contribute to the resilience of urban road tunnel structures and the longer service life of asphalt pavement in semi-closed road tunnels.

Heat transfer and mathematical model of flame geometry of rectangular-shaped pool fire in longitudinal ventilation tunnel

Fire is one of the most serious threats in operation of the tunnel. The thermal radiation disaster of fire is dependent on flame height and inclination angle which are affected by the longitudinal ventilation. The coupling effect of aspect ratio of rectangular container (n = 1, 2, 4, 8, 16) and longitudinal wind on flame geometry of heptane pool fire are revealed. The flame height decreases with an increase in the wind, while the inclined angle increases in longitudinal winds of 0–2.49 m/s. The variation of flame geometry is interpreted using the mechanical equilibrium of horizontal inertial force and gas uprising buoyancy induced by liquid fire. According to air entrainment theory and heat feedback theory, the flame height and inclined angle of rectangular-shaped fire change with characteristic scale ratio (n+1n) in mathematics. The measurements of flame height and inclined angle are analyzed and several quantitatively mathematical equations incorporating heat release rate, characteristic scale ratio, non-dimensional burning rate, non-dimensional air flow are proposed. The predicted results of flame geometry are fully validated by experimental measurements. The current finding is convenient for establishing radiation model of pool fire in air-blown condition.

Influence of Smoke Exhaust Volume and Smoke Vent Layout on the Ceiling Centralized Smoke Exhaust Effect in Tunnel Fires

This research focuses on the impact of smoke exhaust volume and smoke vent layout, which are two crucial factors affecting the smoke control efficiency in tunnels, on the smoke exhaust effect in tunnel fires. Numerical simulation methods are employed to investigate the impact of changing the smoke exhaust volume and the smoke vent number on the smoke exhaust performance in a curved tunnel with a ceiling centralized smoke exhaust system. This research primarily examines the length of the smoke distribution, the smoke temperature under the ceiling, the vertical visibility, and the exhausted smoke mass flow rate. The findings indicate that, in a tunnel with a single-side ceiling centralized smoke exhaust mode, an imbalance in smoke distribution occurs between the upstream and downstream of the fire source. The upstream area experiences a higher amount of smoke, while the downstream area has thinner smoke. Increasing the smoke exhaust volume yielded positive effects on smoke control, as evident in the reduced the smoke spread range, and improved the smoke exhaust efficiency. The influence of changing smoke vent number on the smoke exhaust effect was dependent on the smoke exhaust volume. When the smoke exhaust volume was excessive, altering the number of smoke vents had a minimal impact on smoke exhaust, while in cases with small smoke exhaust volumes, changes in smoke vent numbers obviously influenced the smoke control effect. Therefore, selecting an appropriate smoke exhaust volume and raising the smoke vent number can effectively optimize the performance of the ceiling centralized smoke exhaust system.

Numerical study on smoke temperature and exhaust efficiency in electric cable tunnel

Cable tunnels are the lifeline of cities. Once a serious fire occurs on cables, the toxic smoke generated by cable combustion spreads and escapes, causing serious harm to the surrounding environment and personnel. The heat generated can also ignite nearby combustibles, causing casualties and property damage. Based on the above issues, using fluid dynamics methods, fire dynamic simulation was used to simulate cable tunnel fire scenarios under four variables: wind speed, fire source power, smoke outlet height, and smoke exhaust airflow. The temperature distribution law of the ceiling smoke layer inside the cable tunnel, smoke generation law, tunnel smoke exhaust efficiency, and other factors were clarified, revealing the impact mechanism of the above factors on tunnel fire smoke exhaust efficiency and smoke performance improvement methods. The research results indicate that the combination mode of exhaust air volume and ventilation air speed can effectively control tunnel fire smoke, providing a reference for the fire prevention design of cables in urban cable tunnels.

A numerical study on smoke propagation in metro tunnel fires with different door opening scenarios

Abstract Numerical study was carried out to analyze the effect of door opening scenario on the smoke propagation in metro tunnel fires. The smoke back-layering length, critical velocity, and temperature distribution were studied under a total of 8 working conditions. Results show that the opening of emergency exit doors could result in a shorter smoke back-layering length in the train compartment. In such conditions, a smaller driving force was needed to approach the critical velocity. On the other hand, the high temperature area of the tunnel evacuation platform would change under different opening scenarios. The research result could be used as a deeper understanding of the smoke propagation of metro fire and a reference for the formulation of smoke control.

AIoT-enabled digital twin system for smart tunnel fire safety management

High traffic flow in a confined tunnel makes fire safety a critical issue. This paper proposed a digital twin framework for tunnel fire safety management in real-time, driven by dynamic sensor data and AIoT technologies. A deep learning model trained by the Transformer network and simulation dataset is used to predict real-time fire location and size. Then, the AI model is integrated into a 3D digital twin platform developed by the game engine Unity 3D. The performance of the proposed digital twin framework is demonstrated using numerical experiments and large-scale tunnel fire tests. Results show that the established AI model achieved promising accuracy in predicting fire location and power for both numerical and experimental data. The digital twin platform can also visualize the 3D fire scene that supports evacuation, firefighting, and emergency rescue. This research demonstrates the feasibility of using a 3D environment and digital twin in real-time fire safety management.

Investigation of loading intensity effect (LIE) on the failure mechanism of shield tunnel subjected to fire

The overload damage seriously threatens the service life of the tunnel structure, and the elevated temperatures under the extreme load of fire pose a more serious threat to the structure. In this study, a thermal–mechanical methodology is proposed to analyze the combined effects of loading intensity and fire on tunnel structures. The methodology is also validated by experimental studies. Impacts of loading intensity effect (LIE) on temperature distribution, deformation development and inner force distribution are discussed respectively. Results indicate that LIE aggravates heat flow invasion, making uneven temperature distribution of the segmental ring. On the other hand, LIE has a direct influence on the development of mid-span deformation of lining segments, and induces the failure of lining segments due to excessive deformation at the initial stage of heating. higher loading intensity induces a higher initial axial stress level of the tunnel structure. It is also found that the failure of the segmental ring can be attributed to the development of plastic hinges originated from the coupled thermo-mechanical effect. The findings not only reveal the combined effects of overload damage and fire on tunnel structures, but also provide valuable insights on fire prevention in case of tunnel fire.

Experimental study and newly proposed correlations for maximum ceiling gas temperature rise in tunnel fires with natural ventilation

As a critical factor in tunnel fire safety, previous research on model predictions of the maximum ceiling gas temperature rise under natural ventilation has not demonstrated consistent results. These discrepancies are commonly attributed to variations in tunnel dimensions and materials. To address this issue, an experimental investigation utilizing heptane pool fires with different dimensions was conducted in a model-scale tunnel. The results show that the burning rate increases with an increasing aspect ratio of the fire source for the selected diameter range of 0.078–0.169 m. The flame length was characterized using a modified diameter of the fire source considering the air entrainment, and found that the non-dimensional flame length is 2.4 times the 2/5th power of the non-dimensional heat release rate. Finally, incorporating the concept of flame virtual origin, two different forms of correlations were developed to quantify the maximum ceiling gas temperature rise under different impingement conditions of the fire plume, and the coefficients were determined as 0.065 and 0.215 by numerous experimental results from different studies. The newly proposed models demonstrate good applicability to the fire sources with different dimensions, especially for the rectangular fire source with a large aspect ratio, thus solving the problem of insufficient accuracy of classical models under this condition.

Effect of width and slope of large cross-section tunnel on ceiling smoke temperature under natural ventilation

ABSTRACT When a fire occurs in a large cross-section tunnel with slope, the distribution characteristic of ceiling smoke temperature illustrates special laws under the effect of chimney effect, the thermal radiation feedback, and the air entrance limit from walls. A series of numerical simulations are conducted through FDS simulation software based on the actual size of highway tunnel. The influence of large cross-section tunnel width and slope on ceiling smoke temperature when fire occurs are discussed. The research results show that (1) as the tunnel slope increases from 5° to 20°, the distance between the maximum ceiling smoke temperature point and the fire source is 0.5 m, 1 m, 1.7 m, 2.4 m, respectively, and the distance between the ceiling impact point and the fire source is 1 m, 1.8 m, 2.3 m, 3 m, respectively. As the tunnel width increases from 10 m to 20 m, the distance between maximum ceiling smoke temperature point and fire source increases 0.9 m, 1.1 m, 1.3 m, 1.8 m, 2.5 m, and 2.6 m. (2) The prediction formula of maximum ceiling smoke temperature is modified, and a dimensionless relationship among the maximum ceiling smoke temperature and tunnel slope and tunnel width is established. (3) The formula of longitudinal ceiling smoke temperature distribution is modified, and the relationship among tunnel slope, tunnel width, tunnel height, and longitudinal ceiling smoke temperature distribution is established. The research conclusions can provide a theoretical basis for fire prevention design, smoke control and evacuation of this kind of large cross-section tunnel fire.

A physics-informed artificial fish swarm algorithm for multiple tunnel fire source locations prediction

To address multiple tunnel fire source locations prediction for better firefighting, this work aims to establish a model-driven and data-driven fusion algorithm, which is named as the physics-informed artificial fish swarm algorithm. In the algorithm, the artificial fish swarm algorithm is utilized to avoid complex fire mechanisms modelling, which makes the algorithm owns the advantage of the data-driven methods. Meanwhile, a physical model of the longitudinal ceiling temperature distribution relative to fire source locations and some physical mechanisms are utilized to guide the data-driven artificial fish swarm algorithm for converging to the reasonable prediction, which makes the algorithm owns the advantage of the model-driven methods. Two full-scale tunnel fire experiments with two pool fire sources and three pool fire sources as well as one full-scale cable fire experiment in the tunnel are utilized to support and verify the algorithm, which can be utilized to achieve multiple tunnel fire sources prediction and ceiling temperature distribution prediction in tunnel fires.

Integrating FBN and FDS for quantitative risk assessment of cable fire in utility tunnel

The highly integrated underground utility tunnel (UUT) is a kind of critical infrastructure for process clusters and cities. However, this integration inevitably entails potential risks, including gas leakage, explosions, cable fires, and the accumulation of toxic gases within sewage pipes, and the concern regarding cable fires supersedes others due to their multifactorial correlations and severe consequences. Especially in utility tunnels within chemical industrial parks, cable fires can precipitate the shutdown of equipment, potentially culminating in grave incidents such as chemical leaks or uncontrolled reactions, owing to failures in control systems. This paper presents a novel dynamic risk assessment model for cable compartment fire in UUTs, integrating the Fuzzy Bayesian Network (FBN), Fire Dynamic Simulator (FDS). By employing sensitivity analysis and vulnerability path analysis techniques within the FBN framework, critical hazards associated with cable compartment fires are identified. Subsequently, a comprehensive FDS analysis is conducted for a standardized UUT cable compartment fire consequence, considering the chain response of the safety barriers. The obtained results reveal that even with normal safety barrier response, the risk in the early stage of a fire cannot be disregarded. Notably, among various safety barrier failure scenarios, the failure of a single fire door has the lowest integrated risk, while sensor failure may lead to a higher peak of integrated risk, as well as a higher average integrated risk due to sprinkler failure. The results indicate that the proposed model can offer practical supports in accident mitigation, risk management, and safety barrier optimization of cable compartments in UUTs.