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.     

Temperature and velocity properties of a ceiling jet impinging on an unconfined inclined ceiling

Understanding the characteristics of ceiling jet flow is important because most fire detectors and suppression devices are designed to operate within the ceiling jet; the increases in temperature and smoke concentration within the ceiling jet become trigger occupants to begin fire-fighting action or to evacuation. A series of pool fire tests was conducted using a flat, unconfined model ceiling with dimensions of 2.5 m (D)×3.0 m (L) and changing the ceiling inclination angle of up to 40°. A single ceiling height is used. Two fire heat release rates were used to evaluate the effects: one with and the other without the flame tip touching the inclined ceiling under a steady-state condition. Maximum temperature and its position were determined based on the measurement using a rake consisting of 0.2-mm-diameter chromel–alumel thermocouples. The maximum velocity and its position were obtained by the particle image velocimetry method. These data were compared with the velocities obtained using a bi-directional flow probe and the relationship between them was clarified. Empirical formulae for the temperature rise and velocity versus the radial distance from the plume impingement point along the steepest run in the upward direction were developed considering the effect of the inclination angle. Variations in the Froude number and the Richardson number with radial distance were clarified with and without the flame tip touching the inclined ceiling.     

Temperature and velocity distributions from numerical simulations of ceiling jets under unconfined,inclined ceilings

Numerical simulations of ceiling jets under unconfined, inclined ceilings were conducted with the open-source code FireFOAM. A range of ceiling inclinations, 0–30° was considered with a 14 kW convective heat release rate (HRR) heptane fire used as the plume source, and the ceiling mid-point clearance from the top of the 0.228 m diameter burner kept fixed at 0.89 m. The predicted temperature and velocity in the developing ceiling jets were compared against the experimental data and empirical correlations. Temperature and velocity predictions on the elevated side of the ceiling are in general agreement with experimental data. Flow reversal in the lower side of the ceiling was predicted with good confidence, and comparison with experimental data was found to be reasonable. Following existing convention in the literature, the predicted results were non-dimensionalized using the convective HRR, ceiling height and radial distance from the ceiling mid-point. Comparison of the non-dimensional data on the elevated ceiling side showed better agreement for temperature against the correlation, whereas predicted velocity data showed a wider spread around the correlation values.     

Ceiling-jet thickness and vertical distribution along flat-ceilinged horizontal tunnel with natural ventilation

A series of fire tests was conducted in a 10.0 m (L) × 0.75 m (W) × 0.45 m (H) model tunnel with a rectangular cross section, and detailed measurements were taken of the temperature and velocity within a quasi-steady state fire-driven ceiling-jet running along the centre of a ceiling.The ceiling-jet thickness was defined as the distance from the tunnel ceiling to the point where the temperature and/or velocity dropped to half of their maximums. Correlations to represent the variation in the ceiling-jet thickness along the tunnel axis were developed with the aid of a theoretical approach. The coefficients included in these correlations were determined based on the experimental results obtained. It was found that the ceiling-jet thickness derived from the temperature was 1.17 times greater than that from the velocity in the tranquil flow region.In the tranquil region, both the velocity and temperature showed top-hat distributions, with a bulging shape from the apex of the distribution towards the tunnel floor. A cubic function and coordinate transformation were applied to develop empirical formulae for the temperature and velocity distributions, which were represented by the dimensionless distance from the tunnel ceiling and dimensionless temperature rise and/or velocity at a given distance from the fire source. The correlation developed for the temperature distribution was compared with the results of large- and full-scale tunnel experiments, which verified its applicability.     

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.     

Scale similarity on ceiling jet flow

In this study, empirical formulae previously derived for describing the decrease in temperature rise, the decrease in velocity, the thermal boundary layer thickness, the momentum boundary layer thickness, the Gaussian thermal thickness, and the Gaussian momentum thickness of a ceiling jet flowing upward along the steepest run of an inclined ceiling were applied to a full-scale scenario. The coefficients in these formulae were determined through a series of pool fire tests conducted using a flat, unconfined model ceiling with dimensions of 2.5 m×3.0 m, and fixed ceiling clearance of 1.0 m. To verify the applicability of the developed formulae to actual fires, another series of pool fire tests were conducted using a flat, unconfined full-scale ceiling with dimensions of 7.0 m×14.0 m and a maximum ceiling clearance of 3.0 m. The proposed formulae were confirmed to be applicable to a full-scale scenario and to describe the ceiling jet flow accurately.     

Heat of combustion in spreading wood crib fires with application to ceiling jets

Responding to a challenge raised with respect to a 1989 revision of a 1979 paper on the ceiling jet of t-squared fires, we have measured the heat of combustion in the growth phase of wood cribs made of sugar pine, the test fuel in the original work, needed to generalize the ceiling jet measurements to any combustible. The present determination of the chemical heat of combustion in the growth phase, 14.1 kJ/g, is a little higher than adopted in 1989 (12.5 kJ/g, from wood sample burning with diffusion flame) but still considerably lower than employed in 1979 (20.9 kJ/g, from oxygen bomb calorimetry). More importantly, the convective heat of combustion was measured as 11.5 kJ/g, which has been employed to update the ceiling jet equations for temperature and velocity in t-squared fires. An explanation is offered for the varying, and often higher than expected ceiling-level temperatures measured with thermocouples directly over the fire in the original experiments, suggesting that both plume lean and thermocouple insertion depth may have affected the indicated temperature.     

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

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

An experimental study on the intermittent extension of flames in wind-driven fires

Experiments were conducted to study the intermittent extension of flames from wind-driven line fires using stationary burners. These fires are thought to share similar features with propagating wildland fires, where forward pulsations of flame have been observed to quickly ignite material far ahead of the mean flame front. However, stationary burners offer the ability to study the movement of the flame and its heating processes in greater detail than a spreading fire. In these stationary experiments, propane gas was used as a fuel with different burner sizes, 25–30 cm wide and 5–25 cm long in the direction of the flow. A specially-built wind tunnel was used to provide a well-characterized laminar flow for the experimental area. The free-stream flow velocity, measured by a hot-wire anemometer, ranged in the experiments from 0.2 to 2.7 m/s. The shape of the flame was measured using a high-speed video camera mounted perpendicular to the apparatus. A method was developed to track the extension of the flame close to the surface, simulating flame contact with unburnt fuel downstream of the fire. This extension length was then measured frame by frame and frequencies of flame presence/absence determined as a function of downstream distance. The location of maximum pulsation frequency, x_max, for each burner/wind configuration, was obtained using a level-crossing approach (essentially the variable-interval time-average (VITA) method). Further study indicates that x_max can be well estimated using mean flame properties. Probability distributions describing the location of the flame over time also showed that, the probability the flame extends far beyond the mean flame front is sensitive to increasing ambient winds and fire size.     

Numerical study of the behaviour of a surface fire propagating through a firebreak built in a Mediterranean shrub layer

The efficiency of a firebreak, built in a shrubland has been studied numerically using a multiphase physical model. The physical mechanisms governing the propagation of the surface fire and the consequences upon the temperature signal and the radiative heat flux received by a target located at 1 m above the ground level, have been firstly studied before positioning the firebreak. The role played by the flame and the recirculation of hot gases to the ignition of unburned fuel (especially the dry grass) ahead of the fire front have been clearly identified. Four values of the firebreak width L_C (ranged between 5 and 20 m) and 3 values of wind velocities (ranged between 1 and 8 m/s) have been tested. The simulations show that above a threshold value of this parameter, even if a small amount of the fuel located on the opposite side of the firebreak was ignited, the released energy was not sufficient to sustain the propagation of the surface fire after crossing the firebreak.     

Flame heat transfer in storage geometries

This paper presents measurements of the heat flux distribution to the surface of four square towers exposed to buoyant turbulent flames.The steel towers represent an idealisation of a rack storage configuration at reduced scale. Each tower was 1.8 m high and 0.3 m×0.3 m wide. The fuel was supplied from a circular gas burner at the floor. Three different gaseous fuels were used: carbon monoxide (CO), propane (C₃H₈), and propylene (C₃H₆). These fuels cover a wide range of flame sootiness resulting in distinctly different flame heat fluxes. At the same overall heat release rates the peak heat fluxes from C₃H₈ flames were twice those from CO flames, whereas the peak heat fluxes from C₃H₈ flames were 2.8 times those from CO flames. Heat fluxes were measured by thermocouples spot-welded to the back of the steel sheets. They were measured at 52 different locations. This measurement method turns out to be simple, accurate and robust in addition to being inexpensive. Formulas are provided for the flame heat flux distribution in terms of the overall fire heat release rate, fuel sootiness and separation distance between the towers. The formulas are suitable for direct use by engineering models of fire growth in storage geometries. The paper also provides additional data needed for the development of more general CFD models capable of predicting fire growth of other geometries.     

Inhibition of counterflow methane/air diffusion flame by water mist with varying mist diameter

In the present study, the effect of fine water mist on extinguishment of a methane–air counterflow diffusion flame was investigated to understand the underlying physics of fire extinguishment of highly stretched diffusion flame by water mist. Twin-fluid atomizers were used to generate polydisperse water mist of which Sauter mean diameters were 10, 20, 40, and 60 μm. When water mist is not added, the critical stretch rate at extinguishment is 439 s⁻¹ as compared to the theoretical value of 460 s⁻¹. For the case with water mist addition, when the stretch rate is small enough, almost all the water mist evaporates within the flame zone. On the other hand, for high stretch rate case, large mist droplets pass through the flame zone and can reach the stagnation plane. However, no oscillatory motion was found around the stagnation plane. Critical stretch rate at extinguishment decreases monotonously with the mass fraction of water mist independently of the mist diameter within the range of D₃₂ from 10 μm to 60 μm. On the other hand, with increase in the surface area parameter, the critical stretch rate at extinguishment decreases rapidly and becomes less sensitive at large surface area parameter, of which tendency is qualitatively in good agreement with theoretical predictions. For a constant surface area parameter, the critical stretch rate decreases with mist diameter because the mass fraction of water mist should increase in proportion to the mist diameter to keep the surface area parameter constant. When the water mist evaporates completely in the flame zone as in the present study, the mass fraction of the water mist is the dominant factor for fire extinguishment, rather than the surface area parameter. Therefore, an appropriate combination of stretch rate and water mist mass fraction should be provided to suppress effectively a given fire with a small amount of water mist.     

A study on ceiling jet characteristics in an inclined tunnel

The characteristics of a ceiling jet of an inclined tunnel in a fire will be studied and reported in this paper. Scale modeling experiments on a ceiling jet in a model tunnel of length 3.0 m, width 0.8 m and height 1.0 m inclined at different angles of 0°, 10°, 20° and 30° were carried out. Numerical studies by large eddy simulation were then performed. Both experimental observation and numerical simulation indicated that the characteristics of the temperature and velocity fields near the upper tunnel are different from those obtained using the empirical equations reported in the literature. Another set of empirical equations for gas temperature and flow velocity along the tunnel were fitted by experimental data. These derived empirical equations are useful for estimating the temperature and flow velocity patterns for the ceiling jet in an inclined tunnel with an angle within the range 0–30°.     

Modeling of fire suppression by fuel cooling

Fire suppression with water spray was investigated, focusing on cases where fuel cooling is the dominant suppression mechanism, with the aim to add a specific suppression model addressing this mechanism in Fire Dynamics Simulator (FDS), which already involves a suppression model addressing effects related to flame cooling. A series of experiments was selected, involving round pools of either 25 or 35 cm diameter and using both diesel and fuel oil, in a well-ventilated room. The fire suppression system is designed with four nozzles delivering a total flow rate of 25 l/min and injecting droplets with mean Sauter diameter 112 μm. Among the 74 tests conducted in various conditions, 12 cases with early spray activation were especially considered, as suppression was observed to require a longer time to cool the fuel surface below the ignition temperature. This was quantified with fuel surface temperature measurements and flame video recordings in particular. A model was introduced simulating the reduction of the pyrolysis rate during the water spray application, in relation to the decrease of the fuel local temperature. The numerical implementation uses the free-burn step of the fire to identify the relationship between pyrolysis rate and fuel surface temperature, assuming that the same relationship is kept during the fire suppression step. As expected, numerical simulations reproduced a sharp HRR decrease following the spray activation in all tests and the suppression was predicted in all cases where it was observed experimentally. One specific case involving a water flow rate reduced such that it is too weak to allow complete suppression was successfully simulated. Indeed, the simulation showed a reduced HRR but a fire not yet suppressed. However, most of the tests showed an under-estimated duration before fire suppression (discrepancy up to 26 s for a spray activation lasting 73 s), which demonstrates the need for model improvement. In particular the simulation of the surface temperature should require a dedicated attention. Finally, when spray activation occurred in hotter environments, probably requiring a combination of fuel cooling and flame cooling effects, fire suppression was predicted but with an over-estimated duration. These results show the need for further modeling efforts to combine in a satisfactory manner the flame cooling model of FDS and the present suggested model for fuel cooling.     

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.     

Impacts of impregnation chemicals on combustion properties of the laminated wood materials produced combination of beech and poplar veneers

The aim of this study was to investigate the effects of impregnation with boron compounds Borax (B_X), Boric acid (B_A), B_X + B_A, Imersol-Aqua (I_AQUA) and Timbercare-Aqua (T_AQUA) on combustion properties of the laminated wood materials produced combination of Oriental beech and poplar veneers bonded with Desmodur-VTKA (D_V) and Poly(vinyl acetate) (P_VAc) adhesives. The test samples, prepared from beech (Fagus orientalis Lipsky) and poplar (Populus nigra Lipsky) woods, were impregnated boron compounds by vacuum, I_AQUA by dipping and T_AQUA by brushing methods according to ASTM D 1413-76-99 and directions of the manufacturer. The laminated wood materials were prepared in the form of five layers, 4 mm thickness from the impregnated beech and poplar veneers according to TS EN 386. Combustion properties of samples after laminated and impregnated process were determined according to ASTM E 160-50. Considering the interaction of combustion type and impregnation materials, combustion temperature was found the highest in flame source combustion (F_SC) + T_AQUA (528.150 °C) but the lowest in without flame source combustion (WF_SC) + B_A (391.333 °C). Consequently, boron compounds and I_AQUA showed a decreasing impact on combustion properties of the laminated wood materials, produced combination of beech and poplar veneers, bonded with D_V. In consequence, impregnation with boron compounds and Imersol-Aqua of the laminated wood materials, bonded with Desmodur-VTKA, provides security for the usage of having high risk of fire.     

Fire tests to evaluate CPVC pipe sprinkler systems without fire resistance barriers

Nine full-scale fire tests were conducted to assess the adequacy of exposed chlorinated poly vinyl chloride (CPVC) pipe and fitting sprinkler systems installed in light hazard occupancies. The tests were conducted in an enclosure using six different types of automatic sprinklers including QREC pendent and sidewall sprinklers, a pendent residential sprinkler, and a sidewall residential sprinkler. Two types of fires, fast growing and slow growing, were used as test fires.The sprinkler in each test was operated with the normal operation pressure until the test fire was almost completely suppressed. Next, to assess the integrity of piping systems after fire exposure, the water pressure was increased to 12.1 bar and maintained for 10 min and then were visually inspected for any leakage while hydrostatic pressure was maintained at 12.1 bar. No leakage was detected in any of the tests.The results of the tests indicate that sprinkler systems based on exposed CPVC pipe and fittings can be safely installed in light hazard occupancies and will provide adequate protection when installed to all the relevant standards.     

Simulation of fire scenarios due to different vehicle types with and without traffic in a bi-directional road tunnel

This paper presents findings obtained by CFD modelling for simulating the effects of fire due to different vehicle types in a bi-directional road tunnel. Four different burning vehicles placed in the centre of the driving lane at tunnel middle length were considered. Peaks of the heat release rate (HRR) of: 8, 30, 50, and 100 MW were simulated for the two cars, the bus, the heavy goods vehicle (HGV), and the petrol tanker, respectively. The fire effects on tunnel structure and on environmental conditions along people evacuation path were especially evaluated. The effects of the traffic jam, in contrast with the isolated vehicles, on temperatures, radiant heat flux, visibility distance, and toxic gases concentrations, were also investigated. The worst scenario was identified to be that pertaining to the petrol tanker and more critical conditions were also found when the tunnel was full of vehicles. The maximum gas temperatures reached in the presence of traffic at the side wall (and at the tunnel ceiling reported in brackets) were found to be: 360 °C (170 °C) for the two cars; 740 °C (465 °C) for the bus; 835 °C (735 °C) for the HGV and 1305 °C (1145 °C) for the petrol tanker, respectively. The presence of the traffic, in contrast with the isolated vehicle, involved an increase in the maximum temperatures equal to 16–17% for the two cars, and contained in the range 12–29% with percentages increasing starting from the tanker, to the HGV and to the bus. In other words when the maximum temperatures produced by the isolated vehicle are very high (e.g. for the tanker), the presence of the traffic had a minor effect. With reference to environmental conditions along the evacuation path, the results showed that in the case of petrol tanker fire the emergency ventilation ensures a tenable level of temperature, radiant heat flux, and toxic gases concentrations up to 5 min from the fire starting. This time increases up to 6.5 min for the HGV and 8 min for the bus. This means that the tunnel users in order to be safe in all scenarios should leave the tunnel within 5 min after the fire starting. Toxic gases concentrations, however, were found to be below the limit values in all cases and also in the presence of traffic. In the light of the aforementioned results, tunnel occupants should be promptly informed of the fire risk and guided to the exit portals. This might be done by equipping the tunnel with illuminated emergency signs located along the tunnel length and by installing traffic lights before the entrances so that the tunnel can be closed in case of emergency. By activating the traffic lights at the portals and the emergency signs (more especially those at the ceiling) at the same time as the emergency ventilation is activated, safer conditions for the people evacuation are expected.     

Critical ventilation velocity for multi-source tunnel fires

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

Full-scale experiment and CFD simulation on smoke movement and smoke control in a metro tunnel with one opening portal

Three full-scale model experiments were conducted in a unidirectional tube, which is a part of a metro tunnel with one end connected to an underground metro station and the other end opened to outside in Chongqing, PR China. Three fire HRRs, 1.35 MW, 3 MW and 3.8 MW were produced by pool fires with different oil pan sizes in the experiments. Temperature distributions under the tunnel ceiling along the longitudinal direction were measured. At the same time, CFD simulations were conducted under the same boundary conditions with the experiments by FDS 5.5. In addition, more FDS simulation cases were conducted after the FDS simulation results agreed with the experimental results. The simulation results show that the smoke temperature and the decay rate of the temperature distribution under the tunnel ceiling along the longitudinal direction increase as HRR increases. The smoke exhausts effectively from the tunnel under mechanical ventilation system, whether the emergency vent is activated as a smoke exhaust or an air supply vent. The operation mode of the mechanical ventilation system depends on the evacuation route.