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.     

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.     

Effect of cross section on critical velocity in longitudinally ventilated tunnel fires

Numerical and theoretical work was conducted to investigate the effect of tunnel cross section on critical velocity for smoke control in longitudinally ventilated tunnel fires. The results show that for small fires, the critical velocity decreases with both the increasing tunnel height and tunnel width. For large fires, the critical velocity significantly increases with the increasing tunnel height but is independent of tunnel width. Different calculation models are compared with a focus on effect of tunnel cross section. A new correlation is proposed to account for the effect of tunnel width based on the previous model.     

An engineering tool to calculate heat release rates of multiple objects in underground structures

Simple theoretical calculations of the overall heat release rate (HRR) of multiple objects have been carried out. The results were compared to fire experiments in a model tunnel using wood cribs placed at equal distances from each other. Three different methods are presented which are based on physical relations for fire spread between the wood cribs. The first method uses a critical heat flux as ignition criteria while the other two methods use an ignition temperature. The method using the critical heat flux as ignition criteria shows very good agreement with the corresponding experimental results used. The two methods using the ignition temperature as ignition criteria did not agree well with the corresponding experimental results. The prerequisites, that the methods should be kept relatively simple to be of practical use and that the burning objects should not necessarily have to be of uniform composition, were fulfilled.     

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.     

Critical ventilation velocity for tunnel fires occurring near tunnel exits

Ventilation is an effective method for controlling smoke during a tunnel fire. The “critical ventilation velocity” u_cr is generally defined as the minimum velocity at which smoke is prevented from spreading against the longitudinal ventilation flow in tunnel fire situations. This study conducted small-scale experiments to investigate u_cr for situations when tunnel fire occurs near tunnel exits. The model tunnel was 4 m long, 0.6 m wide and 0.6 m tall, and the fires were located at 0.5 m, 1.0 m and 1.5 m from the tunnel exit. 6.3×6.3 cm² and 9.0×9.0 cm² square asoline fuel pans were used as fire source. Results show that u_cr decreases as the fire approaches the tunnel exit.     

Numerical analysis of tunnel thermal plume control using longitudinal ventilation

A CFD model of the 4th Beijing subway line was used to study the effect of longitudinal ventilation on heat and smoke plume movement in the tunnel. The critical ventilation velocity is correlated with the heat release rate for both a simplified heat fire source model and a complete combustion fire source model with methane gas as fuel. The influences of the heat source length and the fuel gas inlet geometry on the critical velocity are investigated for both fire source models. The results show that the influences of the combustion process and fire source area variation are not included in models based on Froude number preservation theory. Thus, Ri is no longer suitable as a dimensionless number for the critical ventilation velocity when the fire geometry or combustion conditions influence the results. The back-layering air temperature above the front of the fire source can be used to explain the different critical velocity variation regimes for all the simulation conditions.     

An experimental study on the effect of ventilation velocity on burning rate in tunnel fires—heptane pool fire case

The 1/20 reduced-scale experiments using Froude scaling are conducted to investigate the effect of longitudinal ventilation velocity on the burning rate in tunnel fires. The n-heptane pool fires with heat release rate ranging from 3.71 to 15.6 kW are used in this study. A load cell is used to measure the mass loss rate of burning fuel and the temperature distributions are measured by K-type thermocouples in order to investigate smoke movement. The ventilation velocity in the reduced-scale tunnel is controlled by the wind tunnel through an inverter. The increases in ventilation velocity lead to enhance burning rate of n-heptane fuel. The reason is that the oxygen supply effect prevails rather than the cooling effect as the ventilation velocity increases. As a result, the heat release rates in experiment are larger than constant heat release rates by 4.45–11.3 times in the n-heptane pool fires. Also, it is found that non-dimensional critical ventilation velocity is proportional to one-third power of non-dimensional heat release rate.     

Critical velocity and burning rate in pool fire during longitudinal ventilation

Since the prediction of ‘critical velocity’ is important to control the smoke in tunnel fires, many researches have been carried out to predict critical velocity with various fire sizes, tunnel shape, tunnel slope, and so forth. But few researches have been conducted to estimate critical ventilation velocity for varied burning rate by longitudinal ventilation, although burning rate of fuel is influenced by ventilation conditions. Therefore, there is a need to investigate the difference of upstream smoke layer (e.g., backlayering) between naturally ventilated heat release rate and varied heat release rate by longitudinal ventilation.In this study, the 1/20 reduced-scale experiments using Froude scaling are conducted to examine the difference of backlayering between naturally ventilated heat release rate and varied heat release rate by longitudinal ventilation. And the experimental results obtained are compared with numerical ones. Three-dimensional simulations of smoke flow in the tunnel fire with the measured burning rates have been carried out using Fire Dynamics Simulator; Ver. 406 code, which is developed by National Institute of Standards and Technology. They show a good degree of agreement, even if some deviation in temperature downstream of the fire is evident. Since ventilation velocity had a greater enhancing effect on the burning rate of fuel due to oxygen supply effect, the critical ventilation velocity should be calculated on the basis of varied HRR by ventilation velocity.     

Estimation of chemical heat release rate in rack storage fires based on flame volume

Quantification of heat release rate is crucial to many fire research works. Under certain conditions, such as very large fires and fire tests with sprinklers, measurements of fire heat release rate can be a challenging problem. This study attempted to develop a methodology of estimating chemical heat release rate using flame volume. This method is based on the theory that heat release rate per unit flame volume is relatively invariant, as long as the combustion is controlled by diffusion in buoyant fires under well-ventilated conditions. Test data were examined from a variety of fire experimental conditions to evaluate the proposed method. The results demonstrate that the flame-volume based method can provide reasonable estimation of heat release rate compared to oxygen-consumption based method.     

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.     

Correlation between temperatures and oxygen measurements in a tunnel flow

A correlation between measured oxygen concentrations and temperature measurements in a longitudinal tunnel flow has been validated. Such a correlation was first proposed by Newman [Experimental evaluation of fire-induced stratification. Combust Flame 1984;57:33–9] for use in mine and duct fires. Such correlations can be important when trying to estimate the heat release rate in tunnel fires where a lack of oxygen readings or technical problems with instruments forces one to find alternative solutions.     

Safe velocity of on-fire train running in the tunnel

The safety of a running train on fire in a tunnel is a key issue for rescue operations, and the train velocity is mainly related to its safety. In this study, the relationship between the wind velocity and heat release rate (HRR), temperature field around the train, and flame/smoke pervasion rule were investigated under the conditions of variable train velocity, fire location, and fire source location. Beijing Metro was considered as a typical example, in which the safe velocity was estimated to be ∼41.83 km h⁻¹. Assuming the occurrence of fire at the center of the train, the numerical simulations of the flow field using the sliding grid of CFD were performed for a full-scale tunnel under different HRRs. When the fire source reached to the target section, the velocities of all the monitoring points rapidly increased. The velocities increased as the train tail arrived at the target section. The velocities at the measuring points increased with the increase in height, excluding the value of the position with a distance of 0.025 m from the tunnel ceiling. The average temperature and concentration of smoke in the annular space between the train and tunnel ceiling had the minimum values when the running train on fire moved with a speed of 45 km h⁻¹. Thus, the safe velocity of a subway train on fire should be managed between 41.83 km h⁻¹ and 45 km h⁻¹.     

Measuring rate of heat release by oxygen consumption

This paper provides a comprehensive set of equations and guidelines to determine the rate of heat release in full-scale fire tests based on the O₂ consumption principle. The approach is different from other investigators as the enphasis is on full-scale fire test applications and the use of volumetric flow rates is avoided. Some general equations for flow rate (i.e., applicable irrespective of the configuration of the gas analysis system) are described first. In subsequent sections, distinctions are made between various gas analyzer combinations to derive the equations for rate of heat release. Procedures to calculate net rate of heat release from a specimen exposed to a gas burner or wood crib ignition source are also given. A summary at the end of the paper lists step by step procedures for all cases covered.     

A simplified characterization of upholstered furniture heat release rates

The technology for measuring uphostered furniture heat release rates was established with the development of the furniture calorimeter. Analysis of a large number of tests in the furniture calorimeter has now demonstrated that for most specimens a good approximation to the rate of heat release as a function of time may take the form of a triangle. Methods of generating such curves, suitable for fire protection engineering hazard assessment purposes, have been developed.     

区间隧道列车火灾通风临界时间研究

计算地铁区间列车火灾人员所需安全疏散时间,与模拟所得可用安全疏散时间对比,确定区间人员疏散策略及通风临界时间。研究表明：地铁列车外部中间位置着火停靠在区间,火源功率分别为5、7.5、10 MW,需启动纵向通风排烟系统,组织人员向上风向疏散。火源功率为5 MW,纵向通风风速为2.0 m/s时,150~180 s 开始通风可保证人员安全疏散;火源功率为7.5、10 MW,纵向通风风速分别为2.4、2.6 m/s 时,120~180 s 开始通风可保证人员安全疏散。风机由静止转换为事故工况的通风临界时间为120 s,由运转转换为事故工况的通风临界时间为90 s。     

Experimental evaluation of the heat flux induced by tunnel fires

Traffic tunnel closures are highly undesirable and some of the lengthiest are attributed to structural failures. Historical data shows that these failures are closely linked to fire. Furthermore, the parameters necessary for proper thermo-mechanical analysis of structural members are poorly defined or absent from literature. The energy transferred to the structure (i.e. heat flux) is the fundamental parameter for determining structural performance in fire. However, current research focuses on identifying heat release rates and temperature histories which are difficult to use for structural analyses. In this study, full-scale experiments were undertaken on passenger vehicles resulting in heat fluxes between 20 and 70 kW/m². The analyses show fire duration is linked to a vehicle’s mass and small vehicle fires can be scaled as a function of the ratio between the tunnel diameter and the characteristic height of the desired vehicle. Appropriate design values are outlined for engineers to undertake informed thermo-mechanical analyses to minimise the risk of structural tunnel failure in fires.     

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.     

Predicting the heat release rates of liquid pool fires in mechanically ventilated compartments

In this paper we perform predictive simulations of liquid pool fires in mechanically ventilated compartments. We show that steady state burning rates are accurately predicted using a detailed model for the liquid phase heat transfer. The effect of lowered oxygen vitiation on the burning rate of pool fires is correctly captured. Simulations were done using the Fire Dynamics Simulator and the experiments considered were conducted in the OECD PRISME project. The main difference between the present study and previous simulation studies is the use of a detailed liquid evaporation model and the direct calculation of the vitiation and thermal environment interactions through the CFD solver.     

The influence of vehicular obstacles on longitudinal ventilation control in tunnel fires

The effect of the vehicular blockage in a tunnel under longitudinal ventilation smoke control was systematically studied using a small-scale tunnel (1:30 of a standard tunnel section) with a helium-air mixture as the buoyant plume. The experimental results showed excellent agreement with full-scale data and reference correlations from former studies. When there are vehicular obstacles in the tunnel, the critical velocity decreased as a function of the blockage ratio. Notwithstanding, it was found that the relative size of the vehicular obstacle and the relative location of the fire source can have a reversed effect, inasmuch as the presence vehicular obstacle exerted an influence on the critical and confinement velocities. Moreover, the backlayering distance was evidently affected by the vehicular blockage. A parallel analysis was carried out for the backlayering distance for lower and upper regimes of the dimensionless heat release rate, where the current data was compared against data from other studies. The method and experimental set-up proved their ability to reproduce several phenomena and thus also their capability to supply relevant and valuable information on the effect of the vehicular blockage on tunnel fire dynamics.