EcoSmart Fire as Structure Ignition Model in Wildland Urban Interface: Predictions and Validations

EcoSmartFire is a Windows program that models heat damage and piloted ignition of structures from radiant exposure to discrete landscaped tree fires. It calculates the radiant heat transfer from cylindrical shaped fires to the walls and roof of the structure while accounting for radiation shadowing, attenuation, and ground reflections. Tests of litter burn, a 0.6 m diameter fire up to 250 kW heat release under a Heat Release Rate (HRR) hood, with Schmidt-Boelter heat flux sensors in the mockup wall receiving up to 5 kW/m² radiant flux, in conjunction with Fire Dynamic Simulator (FDS) modeling verified a 30% radiant fraction, but indicated the need for a new empirical model of flame extinction coefficient and radiation temperature as function of fire diameter and heat release rate for use in ecoSmartFire. The radiant fluxes predicted with both ecoSmartFire and FDS agreed with SB heat flux sensors to within a few percent errors during litter fire growth. Further experimental work done with propane flame heating (also with 30% radiant fraction) on vertical redwood boards instrumented with embedded thermocouples validated the predicted temperature response to within 20% error for both models. The final empirical correlation for flame extinction coefficient and temperature is valid for fire diameters between 0.2 and 7.9 m, with heat release rates up to 1000 kW. From the corrected radiant flux the program calculates surface temperatures for a given burn time (typically 30 s) and weather conditions (typically dry, windy, and warm for website application) for field applications of many trees and many structural surfaces. An example was provided for a simple house exposed to 4 burning trees selected on a Google enhanced mapping that showed ignition of a building redwood siding. These temperatures were compared to damage or ignition temperatures with output of the percentage of each cladding surface that is damaged or ignited, which a homeowner or a landscaper can use to optimize vegetation landscaping in conjunction with house exterior cladding selections. The need for such physics-based fire modeling of tree spacing was indicated in NFPA 1144 for home ignitability in wildland urban interface, whereas no other model is known to provide such capability.     

Ellipsoidal Solid Flame Model for Structures Under Localized Fire

This paper presents a model to evaluate the thermal energy transfer between a localized fire and the surfaces exposed to it, without the flame impinging the ceiling of the semi-open compartment. Although this type of fire may not have significant consequences for the structure as a whole, it is capable of triggering other disasters such as explosions and larger fires, which is why its study becomes increasingly important. Currently, this accident is analyzed using either sophisticated or semi-empirical numerical models available in the literature. The former uses computational fluid dynamics (CFD), which acceptably reproduces the fire, although with high computational cost. In turn, the semi-empirical models generally present conservative results. The proposed model presents variants in classic simple models available in the literature with the aim of being a tool that allows designers to estimate the thermal fields resulting from this type of fires at the preliminary structure design stage. In this model, the thermal analysis is performed using a finite element program, considering relevant parameters that characterize the fire such as: heat release rate, location and equivalent diameter of the fire source, among others. Through subroutines, the finite element model considers (a) a modification of hot gases temperature field based in a classic simple model and (b) proposition of a new geometry of the flame. The estimated radiative heat flux employs a solid ellipsoidal flame whose height changes according to the heat release rate. The convective heat flux is evaluated using a model for localized fire. Efficiency and accuracy of the methodology are checked by comparing the simulation results with those obtained by sophisticated models developed in fire dynamic simulator (FDS). The cases studied consider: (a) the replication of the experimental test conducted at Luleå University and (b) an offshore platform deck under localized fire action. The results of the first case confirm that the FDS replicates the experimental measurements with high accuracy. Finally, the results show that the proposed model allows to realistically represent the temperature fields generated by the fire, with relatively low computational cost compared to the CFD models for cases (a) and (b), therefore it is possible to use it to develop preliminary analyses in other fire scenarios.     

Wall flame heights with external radiation

An existing flame heat transfer fire testing apparatus was used to study the upward flame spread potential of two kinds of wall materials: (1) PMMA (Polymethylmethacrylate) and (2) Douglas Fir Particle Board. PMMA is noncharring whereas Douglas Fir Particle Board is a charring material. Various levels of external radiant heat flux ranging from 1.8 W/cm² to 3.4 W/cm² were imposed onto the wall samples in order to measure the flame heights as a function of energy release rate. Flame height measurements were established visually by a review of video recordings. The results for these wall flames correlate flame height to the 2/3 power of energy release rate per unit sample width. The wall results are generally higher than data from gas burner line fires against a wall for a range of 10 to 200 kW/m.Note: This paper is a contribution of NIST and is not subject to copyright.     

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.     

Flame Spread Modelling of Complex Textile Materials

Flame spread in textile materials was modelled using two different simulation programs: the semi-empirical area-based code ConeTools, and the computational fluid dynamics, CFD, code Fire Dynamics Simulator, FDS, (version 5). Two textile products developed within the EU-project Flexifunbar were selected for study. The two products show a large difference in composition and application area, one material is developed to function as a protecting layer for the underlying structure in case of fire while the other is an insulating material with no requirements on fire performance. The products represent materials for which fire test results indicate a classification on either end of the rating scale for wall materials according to EN 13501. Two FDS-models were developed for the simulations. The first FDS model was a relatively simple model of the small scale Cone Calorimeter test (ISO 5660) which served the purpose of a first preliminary validation of the model for pyrolysis of the material. In the second FDS model, a model of the intermediate scale Single Burning Item, SBI, test method (EN 13823), the fire scenario was expanded to simulate flame spread over a surface. The work included determination of the necessary material properties. In ConeTools, the option to predict an SBI test was used. The results from the two simulation methods were compared to real SBI tests. Neither model was able to fully predict the heat release rate for these complex products. However, the results from both codes were accurate enough to correctly predict the fire rating class for wall linings according to EN13501.     

Combustibility Parameters for Enclosure Lining Materials Obtained During Surface Flame Spread Using a Reduced Scale Ignition and Flame Spread Technique

A reduced scale ignition and flame spread technique, RIFT, was implemented in the cone calorimeter system to obtain thermocombustibility properties of enclosure lining materials during flame spread over the sample surface. Previously, a thermal model of ignition and opposed flow flame spread was used to analyze flame spread data obtained using RIFT. Here, a framework is discussed for deducing critical material combustibility parameters from the measured heat release and mass loss rates as the spreading flame proceeds to the point of flame extinction. The nature of the data and analytical framework allows users to deduce spreading flame flux from the heat release rate (HRR) and mass loss rate (MLR) data relatively economically and directly. The anomalies highlighted by comparing flame spread data in the RIFT system compared to data from the BS 476 Part 7 apparatus indicates that the RIFT system is well-suited for developing and refining models describing ignition, flame spread, and mass burning.     

Thermal Radiation from Fire Whirls: Revised Solid Flame Model

The conventional solid flame model for calculating the radiation from pool fires involves a constant flame surface emissive power. In this paper, in terms of the fact that the flame shape of fire whirl can be simulated by combination of a cylinder and a cone, a new formula is proposed to characterize the vertical profile of the flame surface emissive power of fire whirl, by which a revised solid flame model is presented for calculating the radiant heat flux from the flame, in the horizontal and vertical directions. In comparison to the conventional solid flame model, the revised model agrees better with the experimental data of radiant heat flux, especially for heat release rates below a certain critical range. By the revised model it is indicated that the profile of radiant heat flux significantly varies with the distance away from the pool centerline. The effect of plume radiation and flame pulsation are suggested to be considered for further improving the radiation model of fire whirl.     

Effect of oxygen on flame heat flux in horizontal and vertical orientations

A systematic empirical and analytical study was conducted to directly quantify the effect of enhanced ambient oxygen concentration on flame heat flux at bench scale and its ability to represent large-scale flame heat flux of well-ventilated fires. The Advanced Flammability Measurements Apparatus was used to conduct bench scale horizontal and single wall vertical orientation experiments for black polymethylmethacrylate, propylene gas and black polyoxymethylene. The key aspect of this study was direct experimental measurements of flame heat flux back to the burning surface for 20.9–40% ambient oxygen concentrations over a range of applied heat flux. The total flame heat flux, as well as the radiative and convective components, was experimentally measured with various gages. To gain more insight into the effects of oxygen, the flame emissivity, flame height and flame temperature were measured and used to calculate the radiative and convective components of the flame heat flux. Gas burner experiments were conducted to decouple the solid and gas phase effects of the ambient oxygen. Large scale tests of black polymethylmethacrylate were conducted in a horizontal orientation and literature data was used for single wall vertical orientations for comparison to the bench scale, enhanced oxygen results. The main conclusion is that the flame heat flux in enhanced ambient oxygen bench scale does not simulate large-scale flame heat flux in horizontal orientations but simulates a more severe large-scale geometry (parallel wall) in vertical orientations and is useful for evaluation of materials’ vertical flame spread potential.     

Modeling pool-like gas flames of propane

Turbulent, buoyant pool-like propane flames with heat release rates of 15.8, 22.9 and 37.9 kW are numerically modeled. The model assumes a parabolic flow field, κ–ε turbulence model, and an eddy-dissipation concept for the interaction of the chemistry and turbulence. Radiative heat transfer is incorporated by the flux model with the absorption and emission coefficients evaluated using a temperature-weighted gray gas model. Predictions are made for the flame shapes, axial velocity, axial mean temperature and various scalar properties along the centerline, the radial distribution of temperature and velocity at various axial heights, and the air entrainment behavior. The overall agreement between the predicted and experimental flame behavior is seen to be good; however, the radial expansion of the flame is underestimated in the combustion zone.     

On the flame height definition for upward flame spread

Flame height is defined by the experimentalists as the average position of the luminous flame and, consequently is not directly linked with a quantitative value of a physical parameter. To determine flame heights from both numerical and theoretical results, a more quantifiable criterion is needed to define flame heights and must be in agreement with the experiments to allow comparisons. For wall flames, steady wall flame experiments revealed that flame height may be defined by a threshold value on the wall heat flux. From steady wall flame measurements, three definitions of flame height from wall heat flux are retained: the first is based on the continuous flame while the two others are based on threshold values of 4 kW/m² and 10 kW/m². These definitions are applied to determine flame heights from a two-dimensional time-dependent CFD model used to describe flame spread on a vertical slab of PMMA. Results show that the predicted flame heights are consistent with the available data of the literature. Defining flame height by threshold values on the wall heat flux of 4 and 10 kW/m² allows the correlation of the wall heat flux in terms of (x−x_p)/(x_fl−x_p), which is the dimensionless characteristic length scale for upward flame spread. It is also found that the continuous flame is not a characteristic length for the heat transfer to the unburnt fuel and is not really appropriate to define flame height in upward flame spread.     

油池火下横隔梁对多肋钢筋混凝土T梁桥温度场的影响

基于油池火焰蔓延特性,探究了火灾下横隔梁对多肋钢筋混凝土T梁桥温度场的影响。理论分析了T梁底火焰蔓延总长度与热释放速率间的无量纲关系,用FDS建立4种火灾场景的流体计算模型,分析T梁边界温度时空分布规律,将FDS计算结果加载到T梁桥有限元模型上,计算截面内部温度场。结果表明:在开放空间,T梁底火蔓延与无量纲热功率之间的线性比值大于封闭空间的2.58;横隔梁有效降低了火焰邻近区域的温度,在顺桥向将T梁边界温度场分成4个区域,各分区间温度比无横隔梁时分别降低9.7%、41%、56.8%。说明横隔梁限制了热传递,使火源直接作用的梁构件温度梯度梁肋中部提高33%,梁底提高13.3%,翼板底提高5%。     

Large-scale fire suppression modeling of corrugated cardboard boxes on wood pallets in rack-storage configurations

In this study, the fire growth and suppression models in FireFOAM were validated for rack-storage commodities consisting of two solid fuels, namely corrugated cardboard boxes and hardwood pallets, which are referred to as Class 2 commodity. Validation experiments included two fire-growth and two fire-suppression configurations with different rack-storage array heights (3 and 5 tiers). In the suppression study, standard-response upright ceiling sprinklers (K-factor of 160 lpm/bar^1/2) were used. The time-resolved chemical heat release rates obtained from the experiments were used to validate the fire growth model. The observed sprinkler activations and fire-spread patterns were used to validate the suppression model. This study identified that lateral flame spread is primarily enabled by flames impinging on the commodity's bottom surfaces. This study also showed that obstructions, such as wood pallets, can significantly impede convective and radiative heat transfer to the underside of the commodity, reducing the lateral flame spread rate. Fire-suppression modeling revealed that both surface water transport and lateral flame spread rates are important when predicting fire-suppression behavior. Therefore, as the rack-storage array height increases, so does the water transport time, which results in the fire becoming more difficult to control. Likewise, as the lateral spread rate increases, e.g., as occurs in the absence of wood pallets, fire-suppression also becomes more difficult.     

Flame Heights and Heat Transfer in Façade System Ventilation Cavities

The design of buildings using multilayer constructions poses a challenge for fire safety and needs to be understood. Narrow air gaps and cavities are common in many constructions, e.g. ventilated façade systems. In these construction systems flames can enter the cavities and fire can spread on the interior surfaces of the cavities. An experimental program was performed to investigate the influence of the cavity width on the flame heights, the fire driven upward flow and the incident heat fluxes to the inner surfaces of the cavity. The experimental setup consisted of two parallel facing non-combustible plates (0.8 × 1.8 m) and a propane gas burner placed at one of the inner surfaces. The cavity width between the plates ranged from 0.02 m to 0.1 m and the burner heat release rate was varied from 16.5 kW to 40.4 kW per m of the burner length. At least three repeated tests were performed for each scenario. In addition, tests with a single plate were performed. The flame heights did not significantly change for Q′/W < 300 kW/m² (where Q′ is the heat release rate per unit length of the burner and W is the cavity width). For higher Q′/W ratios flame extensions up to 2.2 times were observed. When the distance between the plates was reduced or the heat release rate was increased, the incident heat fluxes to the inner surface increased along the entire height of the test setup. The results can be used for analysing methodologies for predicting heat transfer and fire spread in narrow air cavities.     

不同燃料液面深度的变压器套管油池火燃烧特性数值模拟

以某典型变压器油浸纸套管火灾事故为对象,利用FDS研究了燃料液面深度对套管油池火燃烧速率和火焰高度的影响。结果表明：随着燃料液面深度的增加,油池火的质量损失速率和热释放速率呈指数衰减,燃烧效率呈先减小后增大的变化规律,同时提出了耦合套管尺寸和燃料液面深度的无量纲火焰高度预测模型。     

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.     

Facade flame height ejected from an opening of fire compartment under external wind

This paper presents an experimental study and analysis on the facade flame height ejected from an opening of fire compartment under external wind. Experiments are carried out in a reduced-scale model consisting of a cubic fire compartment with a vertical facade wall. An opening is designed at the center of one sidewall of the fire compartment at the facade side and subjected to external wind (normal to the opening) provided by a wind tunnel. The facade flame heights are measured by a CCD camera for five different openings at various fuel supply heat release rates and wind speeds. It is found that the facade flame height decreases with increasing external wind speed. A scaling analysis is performed to interpret this behavior based on the change of air entrainment into the flame from both the facing-facade and parallel-facade directions caused by the external wind flow. A global model incorporating the external wind speed, the two characteristic length scales of the opening as well as the dimensionless excess heat release rate is developed for describing the facade flame height of various conditions. The proposed model correlates the experimental data well.     

Evaluating Models for Predicting Full-Scale Fire Behaviour of Polyurethane Foam Using Cone Calorimeter Data

Two models that can be used to predict full-scale heat release rates of polyurethane foam slabs were evaluated in this study. Predictions were compared with results of furniture calorimeter tests of 10 cm thick polyurethane foam specimens which were ignited in the centre or on the edge. Furniture calorimeter results indicated that peak heat release rates and fire growth rates were higher during centre ignition tests than edge ignition tests. For both situations, the growth phase of the heat release rate curves measured in the full-scale tests was successfully predicted using t ² design fires; the choice of a specific t ² fire depended on the surface area of the specimen and ignition location. A model originally developed during the European Combustion Behaviour of Upholstered Furniture (CBUF) project was also evaluated using heat release rate data from cone calorimeter tests and flame area burning rates measured using infrared video records of the furniture calorimeter tests. This model was able to successfully predict the initial growth phase of the fires and predictions of peak heat release rates were within 17% of measured values. The model had less success in predicting heat release rates later in the growth phase and during the decay phase of the fires, and did not appear to capture all of the physics of the full-scale tests, in particular foam melting and subsequent liquid pool burning. As the model did show promise, future work is planned to address these shortcomings and to develop improved flame spread models for polyurethane foam.     

A study of non-flashover and flashover fires in a full-scale multi-room building

Realistic fire environments in a prototype multi-room apartment in a multi-storey building are studied. The fires are designed as non-flashover and flashover types, using standard polyurethane mattresses as fuel. A comprehensive set of experimental data is presented. The measured results include flame spread velocity, mass release rate, gas temperature, radiation heat flux and gas analysis. A computational fluid dynamics (CFD) model, called a CESARE-CFD fire model, has been used to simulate these polyurethane slab fires. The CFD model is described by three-dimensional transport equations for mass, momentum and enthalpy. The turbulence flow was modelled using the k−ϵ model. A soot formation model and a flame spread model were incorporated into the CFD model. The flame spread velocity and the mass release rate of the polyurethane slab fires were predicted in this study. It was found that the CFD model provided reasonable predictions of the magnitude and trends for the experiments both in the non-flashover and flashover fire cases.     

Correlation equations on fire-induced air flow rates through doorway derived by large eddy simulation

Air flow rates through a doorway are important in modelling compartment fires. The ventilation factor is regarded as a key parameter and numerous efforts have been made on deriving the correlation of air flow rates with it. Most of the correlation expressions reported in the literature were derived empirically from experiments. The results might be different if the fire geometry, fuel type and ambient conditions are changed. Further, the heat release rates measured in most of the experiments were based on the mass loss rate of fuel, not by the oxygen consumption method. There might be some deviations from the actual heat release rates.Computational fluid dynamics (CFD) is now a practical tool in fire engineering. Aerodynamics through a doorway induced by a compartment fire can be simulated accurately. Factors which are difficult to control in experiments but affecting the doorway flow can be studied.The Fire Dynamics Simulator (FDS) developed by the National Institute of Standards and Technology, USA, is one of such CFD software. This is a product achieved from long-term research on developing a CFD model capable of carrying out fire simulations. This model is different from the others based on the Reynolds Averaging Navier–Stokes equations method. Physical processes occuring at small length and time scales were modelled by large eddy simulation (LES). Larger length scale on buoyancy-induced turbulence flow structure was computed directly from the set of equations with acoustic waves filtered out. The new version of this CFD package, FDS version 3.01, is now applied to derive equations on doorway flow rates induced by a fire. Results will be compared with those reported in the literature.     

Experimental investigation and numerical simulation of a furnished office fire

Experiments were conducted in a full-scale model room equipped with both movable and fixed fire loads to explore fire growth and spread via heat release rates, indoor air temperature and species concentration. The room space is a brick structure that measures 5.7 m in interior length, 4.7 m in width and 2.4 m in ceiling height. The northeast and southeast corners each feature a 2.1 m × 0.9 m open doorway. Numerical simulations with parameter adaptation were carried out using FDS software to predict the fire features and were compared with the experimental results. In this study, the material properties and oxygen limit settings in the FDS software were tested to explore their influence on the tendency of heat release rate and on the total amount of heat release. The results show that the heat release rate from the FDS simulations is comparable to the full-scale experiment results during the fire growth period. Temperature profile near ceiling can be modeled well. In the full-involvement burning and decaying periods, the qualitative trends were identical, although the simulated value differed greatly from the experimental result.