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.     

The use of low power carbon monoxide sensors to provide early warning of fire

At present, smoke, heat and light detectors generally used for early warning of fire all have limitations. Carbon monoxide (CO) sensors offer an alternative means of detecting fire and gave good overall results in a trial consisting of six standard fires. However, current CO sensors operate at temperatures of 300°C or above and have power requirements that make them unsuitable for general applications.

Harwell Laboratory was commissioned to determine whether sensors could be fabricated which were sensitive to CO in the range 10–100ppm and insensitive to other gases. CO-sensitive devices with high surface area were dusted with an optimum distribution of precious metal particles. The addition of the metal reduced the effect of moisture which is usually apparent and a response to CO was even seen at 20°C. Sensors based on this principle appear to have potential as low power fire detectors.     

Fire detection, location and heat release rate through inverse problem solution. Part I: Theory

A proposed method of detecting, locating and sizing accidental fires, based on the solution of an inverse heat transfer problem, is described. The inverse heat transfer problem to be solved is that of the convective heating of a compartment ceiling by the hot plume of combustion gases rising from an accidental fire. The inverse problem solution algorithm employs transient temperature data gathered at the ceiling of the compartment to determine the location and heat release rate of the fire. An evaluation of the proposed fire detection system, demonstrating the limits on the accuracy of the inverse problem solution algorithm, is presented. The evaluation involves operating the inverse problem solution algorithm on transient temperature data from computer simulated compartment fires. The simulated fire data are generated assuming fires with quadratic growth rates, burning in a 20 m wide by 20 m deep by 3 m high enclosure with a smooth, adiabatic ceiling. The accuracy of the inverse problem solution algorithm in determining the location of a fire is shown to be insensitive to the errors in the fire model used in the forward problem solution, but sensitive to errors in the measured temperature data. The accuracy of the heat release rate of the fire is sensitive to both errors in the fire model and errors in the temperature data. The validity of the use of computer simulated data in the evaluation is verified with a second evaluation using fire data interpolated from published measurements taken in large-scale compartment fire burns.     

Species Transport from Post-Flashover Fires

A study was performed to determine the use of an equivalence ratio to predict gas levels (CO, CO₂, O₂, and unburned hydrocarbons) transported to locations remote from a post-flashover compartment fire. A series of tests were conducted in a reduced-scale facility to measure the evolution of post-flashover compartment fire gases flowing down a hallway. Test variables included air entrainment into gases in the hallway, stoichiometry of the compartment fire gases entering the hallway, mass flow rate of compartment fire gases, and the presence of a vitiated smoke layer accumulated in the hallway. In cases with no layer accumulated in the hallway, species yields in the hallway were found to correlate with a control volume equivalence ratio. The control volume equivalence ratio is the ratio of the mass loss rate of fuel inside the compartment to the air flow into the compartment plus the air entrained into compartment fire gases flowing along the hallway. Layers that accumulate in the hallway were determined to limit oxidation, which in some cases resulted in CO yields transported to remote locations being 20% higher than those inside the compartment. Based on the experimental data, a methodology was developed for predicting species levels transported to remote locations.     

Experimental study of fire growth in a reduced-scale compartment under different approaching external wind conditions

In order to clarify the fire growth process in compartments under external wind conditions, detailed fire tunnel experiments were conducted in a reduced-scale compartment. The approaching external wind velocity was set to 0.0, 1.5 and 3.0 m/s, and the location of the fire source was changed between the downwind corner, upwind corner and center of the compartment. The experiments considered the effect of wind on a through-ventilation situation. The temperatures of the air and the wall surfaces in the compartment and the temperatures of the flames ejected from the opening were measured. The fuel mass loss rate and the heat flux from the opening were also recorded. Different fire growth characteristics are shown under different wind and fire source conditions. The temperature rises faster and burnout time is reduced under windy conditions. It is found that external wind has two opposing effects. One is to promote combustion within the compartment and thus raise the temperature, the other is to blow away and dilute the combustible gases in the compartment and decrease the temperature, or hasten its extinction. When the approaching wind velocity is high, the external plume is greatly inclined to the downwind side, and the flame becomes larger, thus increasing the risk of the fire spreading to neighboring buildings. The dimensionless temperature of the external flame was a little lower than the results indicated by Yokoi's experiments without wind.     

Compartment fire near-field entrainment measurements

A widely accepted consensus on entrainment models for large fires in compartments does not yet exist. To obtain further information on such entrainment rates, 20 full-scale, near-field experiments were conducted. Near-field entrainment occurs when hot layer interface heights are beneath the burner mean flame height so that cold layer entrainment occurs only near the burner surface. A durable compartment, similar to the standard fire test compartment, was designed and used in conjunction with a 0·61 m × 1·22 m porous surface propane burner to produce compartment fires with heat release rates from 330 to 980 kW. Entrainment rates of 0·74–0·98 kg/s were calculated from temperature measurements made within the compartment and in the doorway. The entrainment rates determined here were correlated with values from the literature. This correlation led to two curve fits which modify Zukoski's far-field offset model and can be used to estimate near-field entrainment rates. An offset for the near-field model of Thomas was also developed. The fire plume model of Baum and McCaffrey was found to compare favorably with the entrainment rates determined here.     

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.     

A comprehensive study of externally venting flames —: Part II: Plume envelope and centre-line temperature comparisons,secondary fires,wind effects and smoke management system

In this part, comparisons are presented between predicted and experimental plume characteristics in terms of plume envelope and centre-line temperatures of venting flames during full-scale flashover fires. Window cracking times and heat transfer from the external plume are discussed in relation to the possibility of a secondary fire. Surface and line approximations are developed to represent the change in the external plume temperature with height, width and depth within the plume above the window opening.     

Analysis of experimental hydrocarbon localised fires with and without engulfed steel members

Localised fires can represent an important hazard to structural safety of buildings where a fully generalised fire cannot develop or when it is at its early stage. Plume correlations given in the codes are valid for undisturbed plume and it is not known whether the presence of a structural element engulfed into the localised fire can affect the validity of such correlations. In structural design, this may lead to highly conservative assumptions or, even, to possible misuses of the correlations. In order to provide insight into this issue, a comprehensive experimental programme aimed at providing data on hydrocarbon localised fires with and without engulfed vertical steel members was performed. In detail, a series of 22 tests of circular hydrocarbon pool fires in well-ventilated conditions of diameters ranging from 0.6 m to 2.2 m were performed with diesel and heptane. The particular aspect of these tests is that they were performed by means of a system that controlled the fuel flow and thus the rate of heat release (RHR) of the fire. The flame length and the temperatures of the fire plume measured experimentally were compared with existing plume correlations, data in the literature and the Eurocode correlations. The results show that: the presence of the column contributed to “straighten” the flame; although pool fires with same diameters were characterised by the same RHR, the flame length was different depending on the fuel type; experimental gas temperatures were lower than the temperature correlation given in the Eurocodes. In sum, the correlations included in the Eurocodes provided reasonable predictions in terms of flame length and of fire plume temperature rise around a steel vertical element located along the centreline of the localised fire.     

Tenability conditions and filling times for fires in large spaces

Motivated by recent demands on regulatory reform, closed form solutions are developed for the filling times and upper layer temperatures for fires in large spaces including the volume expansion term that was neglected in previous similar efforts. The solutions evolve from (a) utilizing the air entrainment to a buoyant plume from a point source having the same convective heat release as the fire and (b) applying an energy balance for the hot layer. Heat losses to the surfaces of the enclosure and provisions for smoke control by natural ventilation are also considered in an approximate way. Although analytic solutions for the filling times exist in the literature if the volume expansion term is neglected, this work is the first to (a) present analytic solutions for the upper layer temperature including the volume expansion term and (b) incorporate heat losses and smoke control by natural ventilation. The present predictions agree with recent numerical results (Fire Sci. Technol. 19(1) (1999) 27), which agree with experimental data and consequently, the present results in turn agree well with experimental data (Fire Sci. Technol. 19(1) (1999) 27). They are also corroborated by asymptotic analysis worked out in Appendix A. For certain large spaces, the results show that critical times for evacuation or rescue operations from fire brigade depend on the upper layer temperature reaching high enough values to cause harm by radiation to occupants or fire fighting rescuers. Thus, critical times in large spaces do not result from the smoke layer descending below a critical height (e.g. 2.1 m from the floor), as they do for small spaces. The present results for large spaces having high ceiling clearance do not agree with CFAST calculations because the entrainment equation for the fire plume in CFAST is different from the one in this work.     

真实火灾下单层门式刚架抗倒塌性能试验研究

设计完成了一个单层单跨门式刚架厂房的足尺火灾试验,得到了主要构件的温度及位移发展规律,分析了真实火灾下门式刚架厂房结构的受力响应。结果显示：真实火灾下门式刚架的温升曲线与标准升温曲线有较大差别,燃烧室中的上部构件达到较高温度而提前失效,下部构件温度较低;在火灾下,未做防火保护的钢结构很短时间内就会发生垮塌,在火场及构件到达峰值温度前结构已产生较大位移。试验研究发现,受火柱的柱顶出现了热膨胀伸长、轴向压缩、轴向破坏三个阶段,且受顶部热烟气聚集的影响,各柱的柱顶轴向位移均大于柱中位移。试验成果可为门式刚架结构抗火数值模拟研究及结构防火设计提供参考。     

Comparison of FDS Prediction of Smoke Movement in a 10-Storey Building with Experimental Data

In this study, the Fire Dynamics Simulator (FDS), a computational fluid dynamics (CFD) model developed by National Institute of Standards and Technology (NIST) is used to simulate fire tests conducted at the National Research Council of Canada (CNRC). These tests were conducted in an experimental 10-storey tower to generate realistic smoke movement data. A full size FDS model of the tower was developed to predict smoke movement from fires that originate on the second floor. Three propane fire tests were modelled, and predictions of O₂, CO₂ concentrations and temperature on each floor are compared with the experimental data. This paper provides details of the tests, and the numerical modelling, and discusses the comparisons between the model results and the experiments. The 10-storey experimental tower was designed to simulate the centre core of high-rise buildings. It includes a compartment and corridor on each floor, a stair shaft, elevator shaft and service shafts. Three propane fire tests were conducted in 2006 and 2007 to study smoke movement through the stair shaft to the upper floors of the building. The fire was set in the compartment of the 2nd floor. Thermocouples and gas analyzers were placed on each floor to measure temperature and O₂, CO₂ and CO concentrations. Comparisons in the fire compartment and floor of fire show that the FDS model gives a good prediction of temperature and O₂ and CO₂ concentrations. In the stair shaft and upper floors there are some small differences which are due to the effect of heat transfer to the stairs that was not considered in the model. Overall the study demonstrates that FDS is capable of modelling fire development and smoke movement in a high rise building for well ventilated fires.     

Heat feedback to the fuel surface of a pool fire in an enclosure

As a part of an effort to determine the energy balance at the pool fire surface in compartments, a series of fire experiments were conducted to study heat flux of the flame in a vitiated environment formed with air and combustion products gases. This paper presents experimental results of the burning behaviour of a heptane pool fire in a reduced scale compartment equipped with a mechanical ventilation network. Measurements of heat fluxes, fuel mass loss rate, oxygen concentration and temperature are performed for heptane fires of 0.26 and 0.3 m diameter pans at different ventilation flow rates. An original method to separate effects of the radiant heat flux of the flame and of the external heat feedback to the fuel surface is developed. This was achieved by using an additional heat flux measurement located under the pool fire. A correlation was also developed to determine the temperature rise on the plume centerline in the compartment as a function of the heat release rate. The results indicate a decrease in the fuel mass loss rate, flame temperature and heat fluxes to the fuel surface as the oxygen concentration measured near the fuel decreases by varying the air refresh rate of the compartment. The flame radiation fraction shows a similar behaviour, whereas the convective fraction of the flame heat flux increases when oxygen concentration decreases. Based on these experimental findings, it was discussed that any classification of the burning regime of a pool fire should consider both the effects of pan diameter and the burning response to vitiated air.     

Enclosure smoke filling revisited

Building fires go through a series of stages. They start with a fire plume/ceiling jet period during which buoyant fire gases rise to the ceiling and spread radially beneath the ceiling. A second stage, the enclosure smoke-filling period, follows; this second stage is the subject of this paper. It has been more than 20 yr since Zukoski first addressed the smoke filling stage of enclosure fires in terms of thermodynamic control volume concepts and fire plume entrainment, yet his analysis remains pertinent. This paper reviews and extends fire modeling concepts related to enclosure smoke filling developed by Zukoski. The mass-based analysis of Zukoski is recast in terms of the volumetric flow rates typically used for ventilation system design; it is extended to consider global average temperature rise and the effects of oxygen consumption on the maximum global average temperature rise that can be achieved in a closed-room fire. A spreadsheet template is developed to compare hand calculations based on a global analysis with numerical smoke filling calculations. Results of this comparison suggest that there is little difference in conditions predicted with the global hand calculations and the numerical smoke filling calculations; consequently, the hand calculations are suitable for preliminary fire hazard analyses.     

Toxicity and the smoke problem

The production and toxicological effects of selected, major fire gases are reviewed. These gases include carbon monoxide, hydrogen cyanide, carbon dioxide, irritants and oxygen depletion, among others. The measurement of the toxicity of smoke is discussed relative to the parameters which need to be measured, in addition to some selected, existing test methods. These test methods are reviewed relative to their relevance to “real” fires, utility and appropriateness as standard tests.     

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.     

Mixing effect and combustion efficiency in compartment fires

A new model for compartment fires is proposed in which the new concept of combustion efficiency based on the mixing process of the fuel gas and air has been considered. This new concept was formulated by the mixing parameter, μ. It was defined as μ1 − exp(−τ^*) and τ^* was related to the residence time t_R and mixing time t_M, that is, τ^* = t_R/t_M.

A simple one zone model was used in order to demonstrate the effect of the mixing process. Theoretical results were in good agreement with the experimental results of methanol and PMMA compartment fires, and especially the scale effect of the compartment was predicted successfully. Further, the similarity law for this scale effect was investigated, and the upper and lower limits of flashover were defined using a new number F. This F number was found to be the key parameter for the prediction of the compartment fire behavior.     

中庭的形状系数与烟羽方程

分析了我国中庭空间几何形状的实地终统计调查结果，提出了形状系数的概念，并根据形状系数大小对中庭进行了分类。针对国内较普遍的瘦高型中庭(形状系数小于0．4)，按1／8比例建造了中庭火灾相似模型实验台，开展了烟气填充与机械排烟实验研究，得到了适用于这类中庭的稳态火灾烟气填充方程与反映受限烟羽特点的烟羽质量流量方程。     

A quasi-steady-state model for predicting fire suppression in spaces protected by water mist systems

A quasi-steady-state model was developed to predict the effectiveness of a water mist system for extinguishing fuel spray and pool fires. The model was developed for obstructed fires where extinguishment primarily occurs as a result of a reduction in oxygen concentration due to the consumption of oxygen by the fire and due to dilution of the oxygen with water vapor. Interactions between the mist and the flame are neglected resulting in limiting case predictions. The model is based on conservation of energy and requires the following input parameters: fire size, compartment geometry, vent area, and water flow rate. The steady-state temperatures and oxygen concentrations predicted by the model can be used to determine the smallest fire that can be extinguished. The predictions made by the model compared favorably to the results of three full-scale test series conducted for the US Coast Guard. These tests were conducted in shipboard machinery spaces with compartment volumes ranging from 100 to 500 m³ with a wide range of ventilation rates and openings. The model was able to accurately predict the compartment temperatures during the tests where steady-state conditions were produced. The model was also able to accurately predict the extinguishment times for a wide range of fire sizes and was used to identify the smallest fire that could be extinguished for a given set of conditions.     

Time-dependent smoke yield and mass loss of pool fires in a reduced-scale mechanically ventilated compartment

Technical and pure grades of the combustibles heptane and dodecane were used in a series of small-scale fire tests conducted in a 1 m³ compartment that was mechanically ventilated at 5 and 8 air changes per hour (ACH). Combustible mass loss rates, soot mass concentrations, soot size distributions, several gas species concentrations, and compartment temperatures were measured during the fire. Results for the two pure-grade hydrocarbons were compared with results obtained for their respective technical grades. Technical-grade dodecane produced the highest soot emissions; pure n-heptane produced the lowest. Soot size distributions of all four combustibles attained a steady profile whose modal diameter was about 200 nm. Underventilated fires showed higher carbon monoxide yields than soot yields. Both compartment ventilation rates produced similar results, although the fire self-extinguished earlier for 5 ACH.