Carbon Monoxide Levels in Structure Fires: Effects of Wood in the Upper Layer of a Post-Flashover Compartment Fire

This experimental study was performed to determine the effects of wood pyrolyzing in a high-temperature, vitiated compartment upper layer on the environment inside the compartment and an adjacent hallway. This was done by comparing species concentrations and temperature measurements from tests with and without wood in the compartment upper layer. Experiments were performed with a window-type opening and a door-type opening between the compartment and the hallway. In these tests, the wood in the compartment upper layer caused CO concentrations inside the compartment to increase, on average, to 10.1% dry, which is approximately 3 times higher than levels measured without wood in the upper layer. Down the hallway 3.6 m from the compartment with wood in the upper layer, CO concentrations were measured to be as high as 2.5% dry. The use of the global equivalence ratio concept to predict species formation in a compartment was explored for situations where wood or other fuels pyrolyze in a vitiated upper layer at a high temperature.     

Simulating the effects of fuel type and geometry on post-flashover fire temperatures

This paper discusses the effect of fuel type and geometry on predicted compartment temperatures derived from computer modelling of post-flashover compartment fires. Many previous studies have investigated post-flashover fires with either wood crib or liquid pool fuels, but very few analytical or experimental studies have considered realistic wood-based fuels with different ratios of surface area to volume, combined with plastic-based fuels. A simple single zone fire model was used to calculate the temperatures in post-flashover compartment fires. The program includes a catalogue of furniture items, each with fuel mass loss rate evaluated on the basis of a constant regression rate on all exposed surfaces. The program also includes a pool-burning model and considers wood fuels and thermoplastic fuels burning together inside a compartment. Use of the model shows that the total fuels load alone is not sufficient to characterise a post-flashover fire. The fire temperature is highly dependent on the fuel type and geometry. For given ventilation and total fuel load, the resulting temperature depends greatly on the average thickness of the wood fuels and the presence of thermoplastic fuels. The ratio of the available fuel surface area to the ventilation opening is particularly important. Several fire scenarios involving different fuel types and characteristics are simulated and compared with Eurocode parametric fires.     

Assessment of the Use of Fire Dynamics Simulator in Performance-Based Design

This paper discusses a procedure for the use of fire modelling in the performance-based design environment to quantify design fires for commercial buildings. This procedure includes building surveys, medium-and full-scale experiments and computer modelling. In this study, a survey of commercial premises was conducted to determine fire loads and types of combustibles present in these buildings. Statistical data from the literature were analysed to determine the frequency of fires, ignition sources, and locations relevant to these premises. Based on the results of the survey and the statistical analyses a number of fuel packages were designed that represent fire loads and combustible materials in commercial buildings. The fuel packages were used to perform medium- and full-scale, post-flashover fire tests to collect data on heat release rates, compartment temperatures and production and concentration of toxic gases. Based on the experimental results, input data files for the computational model, Fire Dynamics Simulator (FDS), were developed to simulate the burning characteristics of the fuel packages observed in the experiments. Comparative analysis between FDS model predictions and experimental data of HRR, carbon monoxide (CO), and carbon dioxide (CO₂), indicated that FDS model was able to predict the HRR, temperature profile in the burn room, and the total production of CO and CO₂ for medium- and large-scale experiments as well as real size stores.     

Comparison of toxic product yields from bench-scale to ISO room

Toxic products are the main cause of fire injuries and deaths, but available methods for measuring or calculating toxic product yields have severe limitations. Full-scale or large-scale experimental re-creations of fire scenarios are sometimes used for the assessment of toxic hazard, but such tests are expensive, while small-scale or even larger-scale tests often provide poor simulations of full-scale conditions. From a testing and engineering calculation perspective there is a need for test methods to provide data-enabling calculations of toxic product yields in defined full-scale scenarios. Full-scale and large-scale tests have demonstrated that toxic product yields are highly dependent upon the combustion conditions. Fire stages and types can be characterised either in terms of CO₂/CO ratio, or preferably in terms of equivalence ratio, which provide reasonably good predictive metrics for product yields. The steady-state tube furnace (ISO TS 19700) allows individual fire stages to be replicated and shows a good general agreement with product yield data (measured for CO₂, CO, HCN, NO_x, total hydrocarbons and smoke particulates) obtained from large-scale ISO room tests for the five materials considered here and expressed as functions of equivalence ratio and CO₂/CO ratio. The closest direct agreement between the large- and small-scale data were obtained for pool fires involving PP and nylon 6.6 product yield. For materials burned as wall linings, with varying decomposition conditions at different room locations, and/or when a propane flame is also present, direct comparison with tube-furnace data is more problematic. Nevertheless MDF, MDF-FR and PS show reasonable agreement for CO, CO₂, HCN and hydrocarbon yields between the scales. Smoke yields tended to be more variable and may be influenced by the presence of different areas of flaming and non-flaming decomposition.     

Repeatability and reproducibility of the ISO/TS 19700 steady state tube furnace

The ISO/TS 19700 steady state tube furnace is designed to measure the yields of combustion products, and in particular toxic combustion products, under a range of different specified decomposition conditions occurring in full-scale compartment fires. In order to establish the repeatability and reproducibility of the method, a “Round Robin” inter-laboratory study and statistical analysis has been carried out involving three laboratories. Pre-existing data from a fourth laboratory have also been considered where appropriate. Samples of different polymers (PMMA, LDPE, PA6.6 and rigid PVC) were tested under two fire conditions: well-ventilated flaming at 650 °C and post-flashover under-ventilated flaming at 825 °C, with each laboratory performing three replicate runs on each polymer under each condition. The results showed good agreement between laboratories and against calculated stoichiometric maxima, demonstrating satisfactory levels of repeatability and reproducibility for key combustion gases. These included O₂ consumption, and the yields of CO₂, CO, HCN and HCl. A number of amendments to the standard have been recommended as a result of minor procedural issues arising during preliminary work.     

The role of temperature on carbon monoxide production in compartment fires

The objective of this study was to assess the effect of temperature on carbon monoxide production in compartment fires in order to resolve the difference between global equivalence ratio-yield correlations obtained in simplified upper layer environments and more realistic compartment fires. The chemical reactivity of upper layer gases was studied using a detailed chemical kinetics model. An analysis of the modeling and experimental data in the literature provided insights into the effect of temperature on carbon monoxide production.

The effect of changing temperature on compartment fire upper layer composition is twofold: (1) the generation of species in the fire plume is changed; and (2) oxidation of post-flame gases in the layer is affected. Elevated compartment temperatures correlate with increased fire plume temperatures and more complete oxidation of the fuel to CO₂ and H₂O within the plume. The layer temperature dictates post-flame oxidation in the layer. For most situations, upper layer temperatures below 800K indicate chemically unreactive layers. As such, combustion within the fire plume dictates final CO production in the compartment. Reactions in the upper layer dictate final CO levels when upper layer temperatures are about 900K and higher.     

Risk analysis of unburnt gas ignition in an exhaust system connected to a confined and mechanically ventilated enclosure fire

The main purpose is to focus on the assessment of an ignition risk due to a large amount of unburnt fuel gases accumulated in the extraction duct connected to a confined and mechanically ventilated enclosure fire combining numeric and experiment. The current numerical study includes the initial well ventilated fire, spreading of flame in the enclosure, subsequent decay during under-ventilated conditions and exhaust of unburnt gas ignition in an extraction duct. Globally, Large Eddy Simulation (LES) combined with an Eddy Dissipation Concept (EDC) combustion model shows the feasibility for simulations of the air vitiation effect on transient combustion events occurring in a closed environment. A particular effort is undertaken to properly predict the pressure level inside a confined facility, and consequently, air inflow supply rate by using a HVAC system. Overall, the numerical results are in fair agreement with the experimental data for the minor species production (CO, H₂), and good agreement for pressure pulse, temperature peak, the major species and heat release rate. In spite of results for minor species that could be improved, the current work confirms the feasibility of a numerical treatment of under-ventilated fire phenomena. The possibility of simulating an ignition risk in an extraction duct connected to a very under-ventilated enclosure fire, has been demonstrated with success in medium-scale facility.     

The Use of CFD Calculations to Evaluate Fire-Fighting Tactics in a Possible Backdraft Situation

This paper is an attempt to integrate theoretical Computational Fluid Dynamics (CFD) calculations with practical fire-fighting tactics commonly used when arriving at the scene of an underventilated fire. The paper shows that CFD has a great potential in improving understanding and creating better effectiveness in the estimation of fire-fighting tactics. If burning has occurred in a lack of oxygen for a long time, excessive pyrolysis products may have accumulated in the fire compartment. If air is suddenly introduced in the compartment a backdraft may occur. The CFD code used for the simulations is fire dynamics simulator (FDS). In this paper, we focus on the conditions that can lead to backdraft, and not the deflagration or rapid combustion in itself. Therefore, the simulations focus on the gravity current and the mixing process between cold fresh air and hot smoke gases by considering a uniform temperature inside the building as initial condition. The different tactics studied include natural ventilation, positive pressure ventilation (PPV) and dilution by water mist. Their effectiveness is observed comparing them with a reference scenario, where no action is taken. The main objective of natural ventilation is to find the fire source, and the venting is more effective with several openings. Tactics involving PPV are very effective in evacuating the unburnt gases, but increases the mixing, and consequently the probability of backdraft during the early stage of operation. On the other hand, the addition of water mist can reduce the danger of backdraft by reducing the concentration of unreacted combustible gases below the critical fuel volume fraction (CFVF), where ignition cannot occur. If the dilution level is insufficient the danger of backdraft is increased, mainly because the process of gases evacuation is longer due to cooling, which reduces the density difference between hot and cold gases. During a fire-fighting operation, the choice of tactic depends mainly on whether there are people left in the building or not, but also on the fire-fighters’ knowledge of the building’s geometry and the fire conditions. If the situation shows signs of strongly underventilated conditions, the danger of backdraft has to be considered and the most appropriate mitigation tactics must be applied.     

Correlations Among Signatures for Detection of Different Types of Fires

One way of overcoming the problem of false alarms encountered in a single parameter detection system due to non-fire stimulii is the simultaneous use of multiple signatures. Researchers have found significant benefits of multi-sensor detection in reducing false triggering. Appreciable interest has been expressed in using carbon monoxide (CO) or carbon dioxide (CO₂) gas sensors in combination with smoke sensors. The present study has been carried out to determine the correlation and inter-dependence between two different fire signatures like CO–OD (optical density), CO–CO₂, CO₂–OD. The signatures have been investigated using experimental measurements of a fire inside a closed compartment measuring 7 x 7 x 4.2 m. A range of fuels are used, and both smouldering and flaming combustion are examined. Attempts have been made to examine whether correlation coefficients between two signatures can form a basis of detection and be exploited as one of the components in multi- criteria fire detection algorithm. The CO/CO₂ ratio as a criterion for detector operation has also been examined and discussed in the light of existing literature and codal provisions.     

A novel artificial neural network fire model for prediction of thermal interface location in single compartment fire

Thermal interface is the boundary between the hot and cold gases layers in a compartment fire. The height of the interface depends predominantly on the mass of air entrained into the fire plume. However, the analytical determination of the air mass flow rate is complicated since it is highly nonlinear in nature. Currently, computer models including zone models and field models can be applied to predict fire phenomena effectively. In the zone model computation, the compartment on fire is commonly divided into two layers to which conservation equations are applied to evaluate the fire behaviour. However, the locations of the fire bed and the openings are ignored in the computation. Computational fluid dynamics techniques may be employed, but a major shortcoming is the requirement for extensive computational resources and lengthy computational time. A unique, new and novel artificial neural network (ANN) model, denoted as GRNNFA, is developed for predicting parameters in compartment fires and is an extremely fast alternative approach. The GRNNFA model is capable of capturing the nonlinear system behaviour by training the network using relevant historical data. Since noise is usually embedded in most of the collected fire data, traditional ANN models (e.g. feed-forward multi-layer-perceptron, general regression neural network, radial basis function, etc.) are unable to separate the embedded noise from the genuine characteristics of the system during the course of network training. The GRNNFA has been developed particularly for processing noisy fire data. The model was applied to predict the location of the thermal interface in a single compartment fire and compared with the experiments conducted by Steckler et al. (Flow induced by fire in a compartment, NBSIR 82-2520, National Bureau of Standards, Washington, DC, 1982). The results show that the GRNNFA fire model can predict the location of the thermal interface with up to 94.5% accuracy and minimum computational times and resources. The trained GRNNFA model was also applied to rapidly determine the height of the thermal interface with different locations of fire on the compartment floor and different widths of the opening against field model predictions. Among the five test cases, four of them were predicted well within the minimum error range of the experiment results. It also demonstrated that the prediction accuracy is related to the amount of knowledge provided for network training.     

钢筋混凝土楼板隔火能力的耐火当量时间

我国现行规范在规定钢筋混凝土楼板耐火极限时,并未考虑火灾荷载和通风条件.从隔热性方面研究了钢筋混凝土楼板的耐火极限,以室内火灾轰燃后的房间热平衡和热传导理论为基础,以钢筋混凝土楼板火灾中背火面的最高温度相等为原则,利用计算机数值分析方法计算出楼板的隔火当量时间,并用最小二乘法回归出当量时间关于火灾荷载密度和开口因子的计算公式,可使设计方法得到简化,为性能化防火设计提供参考.     

Air pumping action of a plume in a room fire

Air pumping effect of a fire plume to give higher intake rate through vertical openings in a post-flashover room fire will be discussed in this paper. The thermal balance equation was set up with known fire phenomena in a room. The hydrostatic model was applied to study the air intake rate through vertical openings. An equation relating heat release rate to room air temperature rise with empirical constants was then justified by reported experimental data on post-flashover room fire. The heat release rate was measured by the oxygen consumption method in that experiment. The predicted heat release rate from the empirical equation reported in the literature was observed to be proportional to the room air temperature rise as derived from hydrostatics. However, the proportionality constant is lower than the experimental value. A possible explanation is due to neglecting another fire phenomenon on air pumping action of the fire plume in a real room fire. Higher pressure differences across the door would give higher airflow rates across an opening. This would supply more air to give higher heat release rate as observed in the experiment. In this paper, the pressure due to air pumping of the fire plume is taken as a proportion of the hydrostatic pressure due to temperature differences between the upper hot layer and lower cool layer. Comparing the measured heat release rate with the estimated heat release rate due only to hydrostatics will give the air pumping action. The possible increase in heat release rate in a post-flashover fire can then be estimated accordingly.     

A model experimental study on backdraught in tunnel fires

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

Temperature Calculation of Insulated Steel Columns Exposed to Natural Fire

A method suitable for design purposes has been developed which allows the approximate post-flashover compartment fire temperature to be plotted versus time in one curve, the general natural fire curve; time is then modified or scaled to take into consideration ventilation conditions and wall properties. In the analysis the common assumptions of constant and ventilation controlled combustion, uniform temperature, and wall losses proportional to the thermal inertia are made.The general natural fire curve is given an analytical expression, which is then used to calculate temperature in fire exposed insulated columns by a simple integration procedure. The results are plotted in handy diagrams, and temperatures obtained in columns exposed to natural fires and standard fires according to ISO 834 are compared.     

Experimental review of the homogeneous temperature assumption in post-flashover compartment fires

Traditional methods for quantifying and modelling compartment fires for structural engineering analysis assume spatially homogeneous temperature conditions. The accuracy and range of validity of this assumption is examined here using the previously conducted fire tests of Cardington (1999) and Dalmarnock (2006). Statistical analyses of the test measurements provide insights into the temperature field in the compartments. The temperature distributions are statistically examined in terms of dispersion from the spatial compartment average. The results clearly show that uniform temperature conditions are not present and variation from the compartment average exists. Peak local temperatures range from 23% to 75% higher than the compartment average, with a mean peak increase of 38%. Local minimum temperatures range from 29% to 99% below the spatial average, with a mean local minimum temperature of 49%. The experimental data are then applied to typical structural elements as a case study to examine the potential impact of the gas temperature dispersion above the compartment average on the element heating. Compared to calculations using the compartment average, this analysis results in increased element temperature rises of up to 25% and reductions of the time to attain a pre-defined critical temperature of up to 31% for the 80th percentile temperature increase. The results show that the homogeneous temperature assumption does not hold well in post-flashover compartment fires. Instead, a rational statistical approach to fire behaviour could be used in fire safety and structural engineering applications.     

空气幕对列车车厢火灾烟气的影响规律研究

为研究防烟空气幕对列车车厢火灾烟气的影响规律,建立了CRH2A动车组一节车厢的内部模型,利用FDS模拟软件对列车车厢火灾时期的烟气流动规律进行数值模拟.依据给定火灾场景下烟气水平运动速度设置空气幕水平切向速度,通过改变空气幕安装角度,研究车厢内部火灾时,在空气幕作用下车厢空间各区域烟气层高度的变化,温度及CO浓度的分布...     

浅谈城市地下综合管廊防火分区设计

城市地下综合管廊,是一种相对密闭的地下构筑物。廊体内很容易沉积大量的热空气和有害气体,发生火灾事故的概率较高。一旦发生火灾事故,其产生的危害与损失也难以估量。根据我国相关法律法规,城市地下综合管廊必须要进行防火分区的设计。文章重点针对城市地下综合管廊的防火分区设计进行详细的分析,旨在提升城市地下综合管廊运营的安全性与稳定性,以供参考。     

Impact of Air Velocity on the Detection of Fires in Conveyor Belt Haulageways

A series of large-scale experiments were conducted in an above-ground fire gallery using three different types of fire-resistant conveyor belts and four air velocities for each belt. The goal of the experiments was to understand and quantify the effects of air velocity on the detection of fires in underground conveyor belt haulageways and to determine the rates of generation of toxic gases and smoke as a fire progresses through the stages of smoldering coal, flaming coal, and finally a flaming conveyor belt. In the experiments, electrical strip heaters, imbedded approximately 5 cm below the top surface of a large mass of coal rubble, were used to ignite the coal, producing an open flame. The flaming coal mass subsequently ignited 1.83-m-wide conveyor belts located approximately 0.30 m above the coal surface. Gas samples were drawn through an averaging probe for continuous measurement of CO, CO₂, and O₂ as the fire progressed. Approximately 20 m from the fire origin and 0.5 m below the roof of the gallery, two commercially available smoke detectors, a light obscuration meter, and a sampling probe for measurement of total mass concentration of smoke particles were placed. Two video cameras were located upstream of the fire origin and along the gallery at about 14 m and 5 m in order to detect both smoke and flames from the fire. This paper discusses the impact of ventilation airflow on alarm times of the smoke detectors and video cameras, CO levels, smoke optical densities and smoke obscuration, total smoke mass concentrations, and fire heat release rates, examining how these various parameters depend upon air velocity and air quantity, the product of air velocity, and entry cross-section.     

Numerical study of under-ventilated fire in medium-scale enclosure

In an enclosure, as all the air inflow is consumed in burning with the excess fuel, the internal fire enters the decay phase, and such process is said flame exhaust. The complicated multistage process from an initial fire growth up to a flame exhaust followed by an external burning is investigated by means of a Large-Eddy-Simulation (LES). Turbulent combustion process is modelled by an Eddy Break-Up concept by using two sequential, semi-global steps for CO prediction. The numerical model solves three dimensional, time-dependent Navier–Stokes equations, coupled with submodels for soot formation and thermal radiation transfer. The critical fuel supply rate needed for flame to exhaust and the time period from the fuel ignition to the appearance of an external flaming in medium-scale facilities are previously obtained experimentally by Chamchine AV, Graham TL, Makhviladze GM, et al. [Experimental studies of under-ventilated combustion in small and medium-scale enclosures. In: Proceedings of the fourth international seminar on fire and explosion hazards; 2003. p. 97–107.], and the general trends predicted by the numerical model follow closely their experimental observation. This model is capable of adequately describing the essential simultaneous phenomena (flame height, soot generation, CO production, convection and radiation) occurring in a room fire. The distinct transient stages of fire development prior to flame exhaust and scenarios of the exhaust are analysed. An external burning is followed after the flame exhaust inside enclosure, and the flame height, H_f, past the ceiling is approximately in an order of the opening height. Even though the flame exhaust takes place under the critical conditions, the heat transferred from the hotter gases and the external fire source poses significant threat to people inside enclosure, and potentially induces an ignition of fuel package exposed near the opening of an enclosure.