Fire Heat Release Rate Research Articles

Experimental study on the characteristics of fire smoke longitudinal crossflow in adjacent tunnels

Tunnel fire smoke is an important fatal factor. For the adjacent tunnels, fire accidents in the upstream tunnels often produce a large amount of toxic smoke, which may have a certain impact on the downstream tunnels under the external wind. Based on the influence of external wind on fire smoke crossflow in adjacent tunnels, a series of experimental studies are carried out in this paper. Through experiments, it is found that the factors affecting the critical crossflow velocity between adjacent tunnels mainly includes the tunnel section itself, the separation distance between adjacent tunnels, and fire heat release rates. By changing different influencing variables, the critical crossflow velocity value under different test conditions is experimentally obtained. Combining experimental phenomena and theoretical analysis, it is also found that the Froude number (Fr), which represents the competition between external wind flow and thermal buoyancy, maintains a small range fluctuation of about 0.75 under the conditions of critical crossflow velocities. Under certain conditions, the critical crossflow velocity is proportional to the third power of fire heat release rate. Finally, a prediction correlation of the critical crossflow velocity of fire smoke in adjacent tunnels under the action of external wind was proposed.

Effect of lateral smoke extraction on transverse temperature distribution and smoke maximum temperature under ceiling in tunnel fires

This paper studied the effect of lateral smoke extraction on the transverse temperature distribution and maximum smoke temperature beneath the ceiling. The experiments were conducted in a reduced model tunnel (1:8). In the experimental study of optimal aspect ratio of the smoke extraction vent, a series of experimental conditions were considered. Keeping the same area of vents, four kind aspect ratios were set (n = 1, 2, 3, and 4). In experiment investigation of ceiling maximum smoke temperature, under the best vent aspect ratio n = 3, four fire heat release (50 kW, 60 kW, 70 kW, and 80 kW) rates were considered. The experimental results showed that the lateral smoke extraction system will affect the transverse temperature distribution, the transverse temperature distribution curve shows good stratification phenomenon, and the maximum temperature point will offset to the direction of the lateral extraction vent. The data of the maximum temperature under different test conditions were analyzed, and the influence factors of lateral smoke extraction were considered. Finally, the correlation of the maximum smoke temperature under the influence of lateral smoke extraction was proposed.

LES analysis of fire source aspect ratio effects on fire-wind enhancement

Enhancement of wind by bushfire, referred to as bushfire-wind enhancement phenomenon, causes damages to buildings located in bushfire-prone areas by increasing pressure load around the structures. This study focuses on the effects of point source aspect ratio (AR) on the wind enhanced by fire. FireFOAM solver of OpenFOAM platform is used to perform Large Eddy Simulation analysis for different fire source aspect ratios under two different fire source conditions: (i) identical fire intensity (fire heat release rate per unit area) and (ii) identical fire heat release rate conditions. Simulations were performed for three different fire source aspect ratios under these fire source boundary conditions. An appropriate normalization group based on fire source hydraulic diameter was introduced for fire-induced pressure gradient to explain the variation of wind enhancement with fire source aspect ratio. The results reveal that under a constant fire intensity condition, increasing the fire source aspect ratio causes a higher normalized fire-induced pressure gradient which leads to more intensified wind enhancement. In contrast, the increase of fire source aspect ratio while fire heat release rate is kept constant culminates in a reduction in the normalized fire-induced pressure gradient, reducing wind enhancement. Moreover, with the increase of the fire source aspect ratio, the area of counter-rotating vortices (CRV) where maximum wind enhancement occurs is expanded. The results also show that with the increase of fire source aspect ratio, the length of flame attachment to the ground immediately downstream of fire increases. In addition to the longitudinal wind enhancement, the effects of fire source aspect ratio on vertical velocity were also analyzed based on the Richardson number defined by hydraulic diameter and flow reference velocity. The effects of the aspect ratio on flame length were also studied. It was shown as a result of the increase of aspect ratio for one unit, flame length increases by approximately 14% and reduces by 7% under constant fire intensity and constant fire heat release rate condition, respectively.

Improved model for estimating sidewall effect on the fire heat release rate of horizontal cable tray

The occurrence rate of fire events involved with cables is high and a fire hazard analysis is needed. A cable tray may be commonly placed on a wall. The fire hazards of multiple cable trays are affected by the supporting wall to give a higher maximum heat release rate. Taking the wall constraint effect on cable burning into account, a new model for the fire heat release rate of a cable tray fire against a sidewall is developed based on small scale (cone calorimeter) measurements. The newly developed model introduces a constant m to reflect the acceleration in cable burning due to the sidewall. With a properly chosen value of m, the results obtained using the newly developed model agree well with the experimental data. The error in the peak fire heat release rate value is considerably smaller than those of other models. This work improves the understanding of the fire risk of a cable tray with a sidewall.

Fire Plume in a Sharply Stratified Ambient Fluid

The present work analyses theoretically and numerically fire plumes evolving in a two-layer stratified environment. The ambient fluid consists of a lower cold (heavy) layer and an upper warm (light) layer with a sharp interface. The fire plume is controlled by the fire heat release rate Q. Depending on the density interface location, the temperatures of the layers and on the fire source conditions, the fire plume can rise indefinitely or can be captured by the density stratification. To determine a criterion between these two situations, a theoretical model is proposed. With this model, it is found that the trapping/escaping criterion of the fire plume is determined in terms of a stratification parameter \(\varLambda > 0.88\). This stratification parameter \(\varLambda\) is a function of the fire heat release, the density interface location and the ambient stratification. The model also allows the plume height to be determined analytically. The comparison with the numerical data shows an excellent agreement. Finally, for practical purposes, we propose a simplified correlation to estimate the height of the trapped plume as a function of the interface height, the temperatures of the layers and the fire heat release rate.

Study on temperature decay characteristics of fire smoke backflow layer in tunnels with wide-shallow cross-section

The temperature distribution characteristics in tunnel fire scenarios are not only crucial for evaluating the hot damage of tunnel structure, but also pivotal fundation to study the smoke kinetic characteristics. In the present work, the scaled-model tests and the CFD simulations were adopted to investigate the effects of aspect ratio, longitudinal ventilation velocity and fire heat release rate on smoke temperature decay characteristics at both transverse and longitudinal directions in tunnel fires. Results indicate that: Tunnel aspect ratio has a significant influence on smoke temperature rate at both transverse and longitudinal directions. The increase of aspect ratio will increase the contact area between the smoke layer and the ceiling, enhancing the heat loss of the smoke backflow layer. As a result, smoke temperature decays faster in a wide-shallow cross-section tunnel. The longitudinal temperature decay rate in the wide-shallow tunnel (AR = 9.3) is almost eight times as large as that in the normal tunnel (AR = 1.1). The increase of fire heat release rate will reduce the smoke temperature decay rate at both transverse and longitudinal direction; As the ventilation velocity increases, the longitudinal temperature decay rate increases while the transverse temperature decay rate decreases. The definition of smoke backflow ratio was proposed to establish the fire smoke temperature decay model of the smoke backflow layer, which considers the effects of aspect ratio, longitudinal ventilation velocity and fire heat release rate. By comparing the test results in previous research work, the new model shows relatively good applicability in the full-scaled tunnel and is valid for a wide range of testing conditions.

Temperature evolution and external flame height through the opening of fire compartment: Scale effect on heat/mass transfer and revisited models

This paper investigates the temperature evolution inside a compartment (i.e., room) as well as the external flame height ejected through the opening (i.e., window) of fire compartment, which is associated with complex thermal heat/mass transfer and fluid dynamics. The scale effect on this problem is revealed and quantified employing the multi-scale compartments (inner scale ranged from 0.34 to 0.76 m). The fire heat release rate (Global Equivalence Ratio, GRE), opening sizes and compartment scales were varied with 468 experimental conditions. Models for predicting the upper-part temperature evolution with heat release rate (Global Equivalence Ratio, GRE), opening dimensions and compartment scales are established base on the heat balance analysis and the derived non-dimensional quantities. The external flame height shows a considerable difference for various compartment scales, due to the change of temperature inside the compartment hence buoyancy flux of the out-flow through the opening. A general model characterizing the external flame height of various compartment scales is proposed with the newly defined dimensionless excess heat release rate accounting for opening geometry length scale and the upper-part temperature inside the compartment, which shows a good agreement with the experimental results. This study provides important and new contributions over previous knowledge and has potential practical applications generally concerning the scale effect and for the validation of numerical models.

Interaction of Heat Release Rate, Flame Tilt Angle and Flame Length of a Compartment Fire Under Stack Effect

This paper is a re-examination to quantify the flame tilt angle of fires induced by stack effect from a heat and mass transfer perspective. In previous studies, the critical transition and length of the deflection flame were formulated. In the current study, with different kinds of fuels, the flame tilt angles under different strength of stack effect are compared. Results show that under stack effect, the flame tilt angles are quite close at the quasi-steady state within a narrow range regardless of the heat release rate of fire and the position of the open window in a stairwell, which indicates that the inertial force of air flow induced by the stack effect equilibrates dynamically with the thermal buoyancy induced by fire plume. However, the velocity of air flow drawn into the fire compartment is directly related to the strength of stack effect, which is mainly influenced by the heat release rate of fire. A new correlation for the flame tilt angle under the influence of stack effect is developed by taking the dynamic equilibrium into account. The tangent value of the flame tilt angle is linearly proportional to the 4/15 power of the heat release rate of fire. Additionally, a correlation to predict the heat release rate with the titled flame length and flame tilt angle is formulated.

Study of propagation of harmful factors of fire in short road tunnels with different inclinations

The paper considers the spread of combustion products caused by fires with different heat release rates: 5, 10, 15, 20, 30, 50 MW in up to 400 m long tunnels. The slope of the tunnels on numerical models is - 0, 1, 3, 5, 7, 9%. The cross-sectional area of the tunnel is 42.5m2. The paper describes the dynamic variability of damaging factors caused by the “chimney effect” such as carbon monoxide and temperature. Modeling was done with FDS software by using the finite volume method. The time of process modeling is 180 s. The minimum cell size of the finite volume is 0.25x 0.25 x 0.25m. The hearth of fire is in the central part of the tunnel. The obtained results are given in the plane of the central longitudinal section of the tunnel. The boundary condition is given as an increment of the dynamic pressure caused by the height difference between the portals in normal conditions. The dependence of fire-induced backlayering distance on the fire heat release rate and the increment of the dynamic pressure caused by tunnel inclination are studied. The theoretical results and those obtained by modeling of the backlayering distance and velocity are analyzed and compared. The locations of high-risk factors for each damaging factor along the tunnel are identified. Their quasi-stationary nature is described and the intervals of transient processes typical to each factor are determined in the course of modeling. It is advisable to use the obtained results to respond to emergencies caused by fire in relevant tunnels.

LES analysis on the effects of baroclinic generation of vorticity on fire-wind enhancement

Substantial influence of fire flame on wind flow aerodynamic characteristics lead to enhancement of wind velocity that may cause considerable damages to buildings in bushfire-prone areas. Fire is associated with rotational vortical structures ranging from the small scales, sourced by baroclinic generation of vorticity (ωBGV), to large scale vortices arising from amalgamation mechanisms that can significantly affect wind flow characteristics during fire-wind interaction. This study aims to understand the extent to which baroclinic generation of vorticity affects wind enhanced by fire. Large eddy simulations (LES) of fire-wind interaction are conducted using fireFOAM solver of OpenFOAM platform for two different types of fuels with the aim to produce two different scenarios with similar heat release rate, but different flame temperatures. This will guarantee the production of different vortex structures in the two cases while flow expansion rate, which is proportional to the fire heat release rate, remains constant. FireFOAM solver was modified to extract fire-induced acceleration and vorticity components. The LES results show that under similar fire intensity and heat release rate conditions, wind enhancement is higher in the scenario with higher flame temperature along the centreline as well as the cross-sectional locations which are corresponding to the locations of counter-rotating vortices where the maximum wind enhancement appears. It was shown that in the case with higher flame temperature, baroclinic generation of vorticity is stronger, which causes stronger longitudinal fire-induced pressure gradient and consequently results in a higher wind enhancement in these cross-sectional locations. The distribution of maximum cross-sectional baroclinic generation of vorticity along longitudinal location are presented, confirming in both cases, baroclinic generation of vorticity reduces with an increase of distance from the fire source. However, in almost all distances, the scenario with a higher flame temperature generates stronger baroclinic generation of vorticity, so does the fire-induced pressure gradient and flow enhancement.

Investigation of wind and smoke concentration effects on thermal instability of cylindrical tanks with fixed roof subjected to an adjacent fire

Fire loading on structures generally involves considerable degrees of uncertainties due to surrounding environmental conditions. Therefore, a reliable structural analysis depends on a proper evaluation of fire disturbing parameters such as wind and smoke concentrations. Wind intensity and direction could affect the heat flux distribution on the structures. Also, smoke (soot) concentration would decrease radiation intensity. Fair prediction of such uncertainties requires multi-physics structural and fluid dynamic approaches, which will be more complicated in the case of structural instability problems. In this research, for the first time, the effect of wind and soot concentration on thermal instability of cylindrical tanks is studied through fire simulation and nonlinear structural analysis. The Large Eddy Simulation (LES) approach is implemented for the fire dynamics analysis, and the Arc-length method is exploited in the case of structural stability calculations. In the Computational Fluid Dynamics (CFD) model, the wind intensity and direction and soot concentrations are considered as disturbing parameters. By conducting LES simulations, heat flux distribution on the fire exposing surfaces of the cylindrical tanks has been estimated. Afterward, the obtained data is transferred into Abaqus/heat transfer, and ultimately, the nonlinear Arc-length structural analyses are conducted. The results reveal that normal direction wind against the container could escalate the heat flux up to 25 times of the windless condition. Also, it is observed that there is a wind speed domain, in which the thermal stability threshold would be maximized. Moreover, the Heat Release Rate (HRR) of fire plays an important role in agitating the instability, while its effects on thermal instability threshold and corresponding mode shapes are negligible.

Study on flame merging behavior and air entrainment restriction of multiple fires

Fire is a typical thermal runaway process of uncontrolled energy release. The possible ignition of adjacent leaked fuel might induce multiple fires burning simultaneously. A series of numerical simulations have been conducted to study the flame merging behavior and air entrainment restriction effect from square propane fire arrays. The simulation results were first systematically compared to experimental data, showing that the flame characteristics including the flame height, flame centerline temperature, and velocity distributions can be successfully reproduced numerically. Then, the flame merging phenomenon was studied by varying fire spacing, the number of fires, and the heat release rate of an individual fire in the array. Results show that the critical flame merging spacing of the fire array equals to 0.4 times the flame height of a single fire, which is independent of the fire array size. More importantly, a parameter denoting the extent of air entrainment restriction α was introduced, which enables a characterization of the air entrainment restriction effect of a fire array. A maximum air entrainment restriction model was established, which is shown only related to the number of fires in the array.

On the Use of Solid Curtains Near Smoke Extraction Vents to Control Smoke Spread Resulting From Fire in Road Tunnels

AbstractThe effectiveness of the smoke control strategy plays an important role in increasing safety levels when fire accidents occur in road tunnels. This paper introduces clarifications about how the efficiency of smoke extraction control using solid curtains can be increased by placing smoke extraction vents close to the solid curtains. The effect of adding a solid curtain with different heights and at various positions relative to a smoke extraction vent was studied in this paper. A 14.3% increase in the vent flowrate occurs at the time corresponding to the fire peak heat release rate when the distance between the solid curtain and the vent is equivalent to 90% of the tunnel height and when the solid curtain height is equal to 16% of the tunnel height. High temperature and low visibility conditions occur near the solid curtain at the smoke-trapped area when the smoke curtain height exceeds 40% of the tunnel height. Using a solid curtain positioned far away from the vent with a distance equals to 90% of the tunnel height and with a height in the range from 16% to 30% of the tunnel height achieves the best results in terms of suppression of smoke spread and attaining acceptable visibility and temperature levels at the region where the smoke is trapped by the solid curtain.

Numerical simulation of the fire emergency evacuation for a metro platform accident

A coupled analysis of agent behavior and Computational Fluid Dynamics (CFD) model is applied to investigate the fire evacuation effectiveness in a popular metro station in Guangzhou, China. Due to the high density and complexity of traffic, the concept of Required Safe Escape Time and Available Safe Escape Time (RSET/ASET), which is more flexible and adaptable than the “6 minutes” principle, is applied in the safety assessment of fire evacuation. To pursue a stable simulation of the coupled model, the standard Critical Radiant Flux is used to deter the tenability criteria for exposure to fire and heat. Various related factors, including the fire location, the Heat Release Rate (HRR) of fire, the crowd density, and the operation mode of escalators, are analyzed through a series of simulations. Results indicate that the interaction between fire and humans should not be neglected in the evacuation simulation. Both the fire location and the crowd density have a significant effect on the evacuation, while the HRR of fire has a minor impact. When the accident happens at the entrance of an escalator, RSET is 58.3% longer than that when the accident occurs in the middle of the platform. RSET grows with the increase of the crowd density linearly. Besides, the evacuation efficiency could be partly improved by changing escalators that usually operate in the descending mode into ascending mode.

An experimental study on the transition velocity and smoke back-layering length induced by carriage fire in a ventilated tunnel

Smoke is one of the important causes of tunnel fire casualties. The critical velocity and smoke back-layering length are important parameters of smoke control in the tunnel. This paper focused on experimentally studying the characteristics of the critical control parameter (such as, critical velocity, transition velocity) and smoke back-layering length induced by carriage fire in the longitudinal ventilated tunnel, which has not been quantified in previous studies. Overall, many repeatable test conditions were involved on different fire heat release rate (HRR), the longitudinal ventilation velocity and opening sizes. It is found that, when the fire heat release rate in the tunnel is small, the critical velocity increases with the increasing heat release rate. The critical velocities will no longer change when the fire heat release rates further increases to a certain value. The relationship between dimensionless smoke back-layering length and critical velocity is piecewise function. With the longitudinal ventilation velocity increasing to a certain value, the smoke back-layering length will become not sensitive to variations with the longitudinal ventilation velocity any more. So, for this part, the new parameter called “transition velocity” implies that the dimensionless smoke back-layering length changes slowly with the dimensionless critical velocity. Then, the dimensionless correlation of smoke back-layering length and transition velocity induced by carriage fire in ventilated tunnel was proposed.

Experimental study on maximum temperature beneath tunnel ceiling under the condition of double fire sources

In order to study the maximum temperature beneath tunnel ceiling of double fire sources, experiments were carried out in a model tunnel (1:10) with different heat release rates, longitudinal ventilation velocities and fire source pitches considered. Results showed that under the condition without wind effect, two flames merging phenomenon gradually disappeared as the increase of fire source pitch. Larger fire source pitch resulted in less impact on maximum temperature beneath tunnel ceiling. Under the condition with longitudinal ventilation, the maximum temperature beneath tunnel ceiling decreased significantly, however, the effect of fire source pitch on the maximum temperature beneath tunnel ceiling is only reflected in the tests with relatively higher fire heat release rates. Based on previous single fire source model, a prediction model for maximum temperature beneath tunnel ceiling of double fire sources was developed. It is showed that the proposed model has a good correlation with the experimental results.

Microgravity combustion of polyethylene droplet in drop tower

Microgravity experiments of polyethylene (PE) droplet combustion were conducted by a 3.6-s drop tower with the gravity level of 10−3~10−4 g to investigate the burning behaviors and fire hazards of molten thermoplastics in the spacecraft. Pre-ignited droplets with a diameter of about 3 mm were continually generated and detached from burning PE tubes. Once the drop capsule started free-fall, droplets entered the microgravity environment with an initial velocity of 10–35 cm/s (Stage I). A comet-shape flame with an intense bubbling and ejecting process of the moving droplet was observed, and the burning-rate constant (K) was found around 2.6 ± 0.3 mm2/s. After the droplet landed on the floor, it could rebound with a near-zero velocity, showing as a spherical flame (Stage II). The combustion of PE droplet followed the classical d-square law with K = 1.3 ± 0.1 mm2/s. The measured large burning-rate constant (or the volume shrinkage rate) of the moving droplet was caused by the robust bubbling process, which reduced the bulk density of molten PE and ejected unburnt fuel (about 25% of total mass loss). However, the actual mass burning rate of the PE droplet should be smaller than most hydrocarbon liquids because of a smaller mass-transfer number (B ≈ 2). The flame burning rate of PE droplet is 4 ± 1 g/m2-s per unit flame-sheet area that may be used to estimate the fuel mass-loss rate and fire heat release rate in microgravity. This novel microgravity combustion experiment on the thermoplastic droplet could expand the physical understanding of fire risk and hazard of plastic material in the spacecraft environment.

Characteristics of Heat Release Rate Predictions of Fire by a Fire Dynamics Simulator for Solid Combustible Materials

The heat release rate (HRR) of fire for solid combustibles, consisting of multi-materials, was measured using the ISO 9705 room corner test, and a computational analysis was conducted to simulate the fire using an HRR prediction model that was provided by a fire dynamics simulator (FDS). As the solid combustible consisted of multi-materials, a cinema chair composed primarily of PU foam, PP, and steel was employed. The method for predicting the HRR provided by the FDS can be categorized into a simple model and a pyrolysis model. Because each model was applied and computational analysis was conducted under the same conditions, the HRR and fire growth rate predicted by the pyrolysis model had good agreement with the results obtained using the ISO 9705 room corner test.

Heat release rate of high-speed train fire in railway tunnels

In this study, a cone calorimeter and an ignition temperature tester were used to conduct experiments on the combustible materials of the main structure of a train for determining its heat release rate and ignition temperature. According to the experimental data of the materials, a numerical model for high-speed train fires was established, and it was validated on the basis of full-scale train fire experiments reported in the literature. Numerical calculations were performed to study the effects of the area of the train carriage vents, power of the fire source, position of the fire source, and longitudinal ventilation velocity of the tunnel on the heat release rate. The results indicated that a higher power of the fire source corresponds to an earlier peak of the heat release rate of the train fire. When the fire source was set at the end wall corner of the carriage and the fire source power was 150–1000 kW, the peak heat release rate was 33.6–36.4 MW. The peak heat release rate of high-speed train fires has an exponential relationship with the area of the train carriage vents and an exponential decay relationship with the longitudinal ventilation velocity of the tunnel. When the fire source is below a seat and its power is low, the flame height is so small that the temperature of the smoke at the top of the train carriage is lower than the ignition temperature of the ceiling material. Hence, the ceiling material does not burn, and it is difficult for the fire to spread in the carriage. The flame spread characteristics of the fire are similar when the source is at other positions. The flame spreads along the ceiling to the two ends of the carriage, causing the seat and floor to burn owing to the heat radiation from the ceiling.

Critical longitudinal ventilation velocity for smoke control in a tunnel induced by two nearby fires of various distances: Experiments and a revisited model

Fire smoke in a tunnel fire is a key factor causing casualties, thus the critical longitudinal ventilation velocity is a key parameter in tunnel fire safety ventilation design. The correlation of critical longitudinal ventilation velocity for smoke control in a tunnel induced by two nearby fires has rarely been studied experimentally and quantified before, despite the fact that this scenario exists in practice. In this paper, the critical longitudinal ventilation velocities for smoke control in the tunnel, induced by two nearby fires were experimentally quantified, involving various separation distances between the two fires of both axi-symmetrical and linear sources of various heat release rates, with a total of 72 test conditions. Results show that the effect of separation distance between the two fires on the critical longitudinal ventilation velocity, decreases with the increase of fire heat release rates. The critical longitudinal ventilation velocities are found to firstly decrease with increasing separation distance between the two fires, and then remain unchanged when the separation distance between two fires beyond a critical value. A new modified model, involving two fires of various separation distances, is proposed. The model is shown to well represent the experimental results.