Evaluation of FDS V.4: Upward Flame Spread

NIST’s Fire Dynamics Simulator (FDS) is a powerful tool for simulating the gas phase fire environment of scenarios involving realistic geometries. If the fire engineer is interested in simulating fire spread processes, FDS provides possible tools involving simulation of the decomposition of the condensed phase: gas burners and simplified pyrolysis models. Continuing to develop understanding of the capability and proper use of FDS related to fire spread will provide the practicing fire engineer with valuable information. In this work three simulations are conducted to evaluate FDS V.4’s capabilities for predicting upward flame spread. The FDS predictions are compared with empirical correlations and experimental data for upward flame spread on a 5 m PMMA panel. A simplified flame spread model is also applied to assess the FDS simulation results. Capabilities and limitations of FDS V.4 for upward flame spread predictions are addressed, and recommendations for improvements of FDS and practical use of FDS for fire spread are presented.     

Further Validation of a Numerical Model for Prediction of Pyrolysis of Polymer Nanocomposites in the Cone Calorimeter

Nanocomposites have been increasingly used, as an alternative to traditional fire retardants, to improve the strength and fire retardancy of polymeric materials. A number of studies using the cone calorimeter showed that the nanoparticles used in small quantities (e.g., 3 wt%) reduce significantly the heat release rate (HRR). The formation of a surface layer on top of the unpyrolysed material is generally considered responsible for the reduced HRR. In a previous study, the global effects of the surface layer were examined by the present authors and a methodology was subsequently developed to predict pyrolysis of a polyamide nylon (PA6) nanocomposite in good agreement with the experimental data. This work presents further validation of the methodology for two more nanocomposites, namely polybutylene terephthalate and ethylene-vinyl acetate. Furthermore, the existing model is extended to explain the effects of change in the nanofiller loading on the HRR, and the modified model is applied to the experimental data obtained for a PA6 nanocomposite by Morgan et al. (Fire and polymers: materials and solutions for hazard prevention. American Chemical Society, Washington, DC, 2001, pp 9–23).     

Finite element modelling of the pyrolysis of wet wood subjected to fire

The modelling of the pyrolysis of wet wood provides more realistic fire scenarios for structural fire design by taking into account variable thermal properties of wood which are beyond the scope of conventional structural fire design codes. The proposed numerical methodology has been written in MATLAB environment. A 2D nonlinear finite element analysis (FEA) is performed to model the pyrolysis of wet wood subjected to high temperature. The varying of thermal proprieties of wood are discussed from the point of view of changes of structure and chemical composition under fire condition. The validity of the model is established by comparing the predicted results with results from fire resistance tests presented in literature. Qualitatively, the model provides good agreement with the experimental data. It is shown that the model can handle layers of a wooden composite structure. Temperature profiles at different points in the wood sample and the two-dimensional charring depth of Laminated Veneer Lumber (LVL) panels are calculated and compared with experimental data.     

Experimental and numerical study on water mist suppression system on room fire

Full-scale experiment and numerical simulations are carried out on a room fire to study water mist suppression system with heat release rate of 6 MW. A computational fluid dynamics (CFD) model of fire-driven fluid flow, FDS (Fire Dynamics Simulator), is used to solve numerically a form of the Navier–Stokes equations for fire. A fire experiment without water mist is performed and the temperatures are measured to validate the predictions of FDS code against the experimental data. Then a fire experiment with water mist suppression system is performed and the temperatures and extinguishing time are measured. The validated numerical model is used to simulate the experiment; the temperatures, oxygen concentration and extinguishing time are compared and studied. In numerical simulations, the cell size sensitivity is analyzed. The experimental results of temperatures and extinguishing time are compared with the results of numerical simulations. It appears that the numerical results are in good agreement (qualitatively) with the experimental data in temperature fields. These useful data can be helpful in accomplishing the design of water mist suppression system and the design regulations for fire safety management.     

Modeling fire growth in a combustible corner

A fire growth model was developed to predict the flame spread and total heat release rate of a fire in a corner configuration with a combustible lining. Input data for the combustible lining were developed using small-scale test data from the ASTM E1354 cone calorimeter and ASTM E1321 LIFT. The fire growth model includes a flame spread model linked with a two zone compartment fire model, CFAST Version 3.1.2. At a user selected time interval, the flame spread model uses the gas temperature from CFAST to predict the heat release rate of the fire at that time interval, and then provides CFAST with a new heat release rate to predict conditions during the next time step. The flame spread model is an improved version of the flat wall flame spread model previously developed for the US Navy. The model is capable of predicting flame spread in a variety of configurations including a flat wall, a corner with a ceiling, flat wall with a ceiling, unconfined ceiling, and parallel walls. The model has been validated against ISO 9705 test data and was used in this study to simulate conditions that develop in three open corner tests each with a different lining material. The model was able to predict the heat release rate of the fire and provide a reasonable estimate of the flame fronts and flame lengths during the growing fire.     

Quantitative comparison of FDS and parametric fire curves with post-flashover compartment fire test data

This paper presents a comparison of two parametric fire modelling techniques (Eurocode 1, and the BFD curve method) and one field model (fire dynamics simulator) against large-scale post-flashover test data. Using a method of the product moment correlation coefficient, it is shown that the BFD curve predictions are most closely representative of reality. For the computational test data, two grid resolutions are adopted in the FDS field model, the finer of which having comparable results in terms of regression analysis to the BFD method. Both the field model and the BFD curve method were found to give better predictions compared to the Eurocode method over the duration of the test. However, a direct comparison of the maximum gas temperatures shows the field model to be poorer in its predictive capability than the parametric methods, under-predicting the maximum gas temperatures. In addition, a more in-depth analysis of the FDS predictions indicates that by considering simply average compartment temperatures the more inaccurate spatially specific temperature predictions were disguised. This study provides useful quantitative data on the three techniques presented and discusses more general issues concerning fire modelling.     

Effects of nanoclay and fire retardants on fire retardancy of a polymer blend of EVA and LDPE

The effects of nanoclay (organoclay) and fire retardants (aluminium tri-hydroxide and magnesium hydroxide) on the fire retardancy of a polymer blend of ethylene-vinyl acetate (EVA) and low-density polyethylene (LDPE) were assessed using thermogravimetric analysis (TGA) and the cone calorimeter. TGA measurements were conducted in nitrogen and air atmospheres at different heating rates (1–20 °C/min), whilst in the cone calorimeter square samples were tested under various external heat fluxes (15–60 kW/m²). The TGA results indicate that the nanoclay (NC) alone has little effect on the degradation of the polymer blend, whereas aluminium tri-hydroxide (ATH) and magnesium hydroxide (MH), used as fire retardants (FRs), generally decrease the onset degradation temperature and also reduce the peak mass loss rate. However, it was found in the cone calorimeter that, though having negligible effect on ignition, the nanoclay reduces the heat release rate (HRR), and increases smoke and CO yields. In comparison, FRs (ATH or MH) were found to delay ignition owing to loss of water at lower temperatures, reduce the HRR, and have similar smoke and CO yields compared to the polymer blend. The reduced HRRs for both the nanoclay and FRs can be attributed to the formation of a surface layer (a nano layer for nanoclay and a ceramic-like layer of Al₂O₃/MgO for FRs), which acts as mass and heat barriers to the unpyrolysed material underneath. The global effect of the surface layer for the polymer blend nanocomposite was examined using a previously developed numerical model, and a methodology for predicting the mass loss rate was subsequently developed and validated.     

Material fire properties and predictions for thermoplastics

Ignition and burning rate data are developed for nylon 6/6, polyethylene, polypropylene and black polycast PMMA in a cone calorimeter heating assembly. The objective is to examine a testing protocol that leads to the prediction of ignition and burning rate for thermoplastics from cone calorimeter data. The procedure consists of determining material properties, i.e. thermal inertia, specific heat, thermal conductivity, ignition temperature, heat of gasification and flame heat flux from cone data, and utilizing these properties in a model to predict the time to ignition and transient burning rate. The procedure is based on the incident flame heat flux being constant in the cone calorimeter which occurs for flames above the top of the cone heater. A constant net flame heat flux of approximately 20 kW/m² for nylon 6/6, 19 kW/m² for polyethylene, 11 kW/mP² for polypropylene and 28 kW/m² for black PMMA is obtained for irradiation levels ranging from 0 to 90 kW/m². The burning rate model is shown to yield good accuracy in comparison to measured transient burning in the cone assembly.     

Pyrolysis and spontaneous ignition of wood under transient irradiation: Experiments and a-priori predictions

Wood is a material widely used in the built environment, but its flammability and response to fire are a disadvantage. Therefore, it is essential to have substantial knowledge of the behavior of wood undergoing external heating such as in a fire. The majority of studies in the literature use constant irradiation. Although this assumption simplifies both modelling and experimental endeavors, it is important to assess the behavior of materials under more comprehensive heating scenarios which might challenge the validity of solid-phase ignition criteria developed previously. These criteria are evaluated here for the spontaneous ignition under transient irradiation by combining experimental measurements and a-priori predictions from a model of heat transfer and pyrolysis. We have applied a two-step transient irradiation in the cone calorimeter in the form of a growth curve followed by constant irradiation. We use white spruce samples of size 100×100 mm and thickness of 38 mm . We measure the temperature at different depths and the mass loss. A one dimensional model written in the open source code Gpyro is used to predict the pyrolysis behavior. The model has a chemical scheme in which the components of wood (hemicellulose, cellulose, lignin) become active, then decompose in two competing reactions: one reaction to char and gas, and one reaction to tar. The kinetic parameters, as well as the thermal properties of the wood and char are taken from the literature, while ρ and moisture content are measured experimentally. A priori predictions of the temperature, made prior to the experiments, show excellent agreement with the measurements, being within the experimental uncertainty range. The mass loss rate (MLR) predictions are qualitatively similar to the measurements, but there is a large uncertainty in the measurements. For a-posteriori simulations, certain parameters are changed after having access to the measurements to improve the simulations. Also, we perform an evaluation of the solid phase ignition criteria used in the literature, and find that neither criteria is a consistent indicator of ignition. These results help understand the spontaneous ignition of wood subjected to transient irradiation and identify strengths and gaps in the topic.     

Multi-scale modeling of the thermal decomposition of fire retardant plywood

Due to the complexity and costs of full scale fire-test experiments, numerical simulations provide a useful alternative when investigating the fire behavior of new materials. The mass loss rate of the solid is one of the most important parameters in assessing fire behavior as it is directly linked with the pyrolysis gas flow rate and represents the initial factor of the combustion process. In this paper, fire retardant plywood is investigated with a focus on the solid mass loss rate modeling. A multi-scale approach is followed in order to establish the kinetic mechanism of thermal degradation. A combination of small scale and large scale tests were completed to fully develop and validate the proposed kinetic mechanism. For small scale testing, experiments are conducted by using thermo-gravimetric analysis coupled to gas analysis with FTIR technique under nitrogen and air atmospheres. These experiments were completed at several heating rates. Thermo-gravimetric results are used to propose a kinetic mechanism for the thermal decomposition of the solid and the kinetic parameters are calculated by using the genetic algorithms method. For larger-scale testing, experiments were carried out in a cone calorimeter coupled to a FTIR gas analyzer. The experiments were completed in air atmosphere in order to validate the kinetic mechanism developed from small-scale testing. The kinetic model developed is implemented into the general Gpyro model which takes into account both thermal and mass transfer phenomena inside the solid. The results showed good agreement between the model calculations and the experimental data.     

Generalized pyrolysis model for combustible solids

This paper presents a generalized pyrolysis model that can be used to simulate the gasification of a variety of combustible solids encountered in fires. The model, Gpyro, can be applied to noncharring polymers, charring solids, intumescent coatings, and smolder in porous media. Temperature, species, and pressure distributions inside a thermally stimulated solid are determined by solving conservation equations for the gaseous and condensed phases. Diffusion of species from the ambient into the solid is calculated with a convective–diffusive solver, providing the capability to calculate the flux and composition of volatiles escaping from the solid. To aid in determining the required material properties, Gpyro is coupled to a genetic algorithm that can be used to estimate the model input parameters from bench-scale fire tests or thermogravimetric (TG) analysis. Model calculations are compared to experimental data for the thermo-oxidative decomposition of a noncharring solid (PMMA), thermal pyrolysis of a charring solid (white pine), gasification and swelling of an intumescent coating, and smolder in polyurethane foam. Agreement between model calculations and experimental data is favorable, especially when one considers the complexity of the problems simulated.     

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.     

Flammability assessment of fire-retarded Nordic Spruce wood using thermogravimetric analyses and cone calorimetry

Experimental techniques such as the cone calorimeter, representing realistic fire conditions, and the thermogravimetric analyser (TGA) combined with evolved gas analysis (EGA) can be used to determine flammability and degradation properties of materials. The desire is to correlate the flammability properties measured in the cone calorimeter for samples of size 100 mm×100 mm with those measured or deduced from TGA combined with EGA for milligram samples. Such an achievement will allow the design of fire-safe materials by quickly assessing (a) the fire safety of materials in their earliest milligram formulation and (b) the dependence of their flammability properties on the molecular structure of the material. In the present study, a cone calorimeter and TGA investigation is conducted for commercial Nordic Spruce wood impregnated by mono-ammonium phosphate (fire retardant, FR) through a vacuum pressure method. Experiments in the cone calorimeter with increasing FR concentrations indicated that (a) the char yields increased, (b) the apparent ignition temperature increased, (c) time to piloted ignition increased, (d) the total amount of heat released was reduced, (e) the peak heat release rate was reduced and (f) the carbon monoxide and smoke yields increased especially before ignition occurred. By comparison, char yields also increased with FR content in the TGA degradation experiments in nitrogen. The increase in the char yield with FR content explains quantitatively the decrease in the heat release in the cone calorimeter. By contrast, the onset temperatures measured in TGA decreased, whereas the ignition temperature deduced in the cone calorimeter increased with FR content. This difference is attributed to reduced yield of levoglucosan (reported in recent literature using TGA/EGA) with increased char yield as well as to the presence of phosphorous containing moieties in the volatiles, which both can quench piloted ignition. Finally, the TGA measurements showed that the FR concentrations decreased for milligram samples at different distances from the surface of the wood used in the cone calorimeter measurements. The variation of FR retardant with depth needs to be considered when using TGA data to interpret cone calorimeter measurements and the fire performance of the FR wood in approval tests such as the single burning item (SBI).     

Modeling carbon black trace in building fire and its validation

The carbon black trace is an important characteristic in a building fire accident and becomes crucial evidence in fire investigation. Based on the particle deposition theory, the mathematical model is established for the carbon black trace in a building fire. The numerical model of the carbon black trace is implemented into the Fire Dynamics Simulator (FDS) software. The total mass of the carbon black particle deposited on the wall surface can be calculated quantitatively and be simulated visually. The proposed model is applied into a fire accident as a validation. A numerical model is used to simulate the fire accident. In numerical simulations, the grid size resolution is analyzed. The accident reconnaissance data, accident interview record and accident scene photo/video are compared with the results of numerical simulations. It shows that the simulation results have a good agreement with those in the fire accident, which validates the mathematical model in this study. The proposed method can provide useful data for fire reconstruction and fire investigation.     

Numerical studies on heat release rate in a room fire burning wood and liquid fuel

Heat release rates of burning gasoline and wood fires in a room were studied by computational fluid dynamics (CFD). Version 5.5.3 of the software Fire Dynamics Simulator (FDS), which is the latest one available, was selected as the CFD simulation tool. Predicted results were compared with two sets of reported data from full-scale burning tests. In the two sets of experiments, the scenarios were set at gasoline pool fire and wood chipboard fire with gasoline respectively. The input heating rate of gasoline pool fire based on experimental measurements was used in the first set of experiments. Three scenarios G1, G2 and G3 with different grid systems were simulated by CFD. The grid system of scenario G2 gave more accurate prediction, which was then used to study the second set of experiments on wood chipboard with gasoline. The combustion model in FDS was used in wood chipboard fire induced by gasoline pool. The wood chipboard was allowed to burn by itself using the pyrolysis model in FDS. The effects of the boundary conditions on free openings for the same set of experiments were studied by three scenarios SOB1, SOB2 and SOB3. Boundary condition SOB2 gave more reliable prediction among the three boundary conditions. Two other scenarios on the effect of moisture content of wood were also studied. The predicted HRR curve was found to agree better with experiment in using SOB2.     

Models for calculating flame spread on wall lining materials and the resulting heat release rate in a room

An extensive research programme, dealing with fire growth on combustible wall lining materials, has been ongoing in Sweden over the last decade. Several lining materials were tested in bench-scale fire tests in order to derive basic material flammability parameters. The same materials were also tested in a full scale room test and a 1/3 scale room test for two different scenarios, A and B. Scenario A refers to the case where walls and ceiling are covered by the lining material, Scenario B where lining materials are mounted on walls only.

This study utilises the results from these experiments and presents a mathematical model where material properties derived from standardised bench-scale tests are used as input data. The model predicts fire growth in the full- or 1/3 scale tests, in two different scenarios (A and B), and consists of sub-models for calculating the rate of heat release, gas temperatures, radiation to walls, wall surface temperatures and flame spread on the wall lining material.

A thermal theory of wind-aided flame spread on thick solids is examined and solutions are given and analysed for flame spread velocities under ceilings. Both numerical and analytical solutions are discussed.

The analytical solutions can be used to evaluate the flame spread propensity of materials and thus, whether a certain material is likely to go to flashover or not in the Room Corner Test. More generally, the solutions can be used to estimate whether a material will spread flame in a variety of concurrent flow flame spread scenarios. Results from the analytical solutions are compared with experimental flashover data for 22 materials, showing a good agreement.

The numerical solutions are incorporated into a simple room fire model. The results from the numerical model are compared with experiments on 22 materials tested in the full scale room for Scenario A. Comparisons for Scenario B are made with 10 materials tested in the 1/3 scale room. The results show reasonably good agreement for most materials between the model and the experiments.     

Modelling thermal degradation of polymers using single-step first-order kinetics

A theoretical framework for characterising single-step Arrhenius degradation kinetics in terms of a characteristic temperature and temperature range is developed. It is demonstrated that for the purposes of practical calculation, the reaction order may be assumed to be unity and also that a first-order approximation to an nth-order TG curve remains a good approximation over an order-of-magnitude variation in heating rate. This fact implies that when the pyrolysis of much larger samples of material is modelled, the error involved in using first-order kinetics is small. The equivalent first-order approximation is then applied to a global in-depth model of polymer degradation in order to predict mass loss rates in bench-scale experiments such as the cone calorimeter test. The mass loss rate curves obtained from the equivalent first-order approximation are found to compare well with the full nth-order model. Finally, an estimate of the average or steady mass loss rate is developed which fully accounts for the interaction between the degradation kinetics, the external heat flux, the heat losses and the latent heat of vaporisation.     

An analysis of compartment fire doorway flows

A mathematical study is made to compute the doorway flow behavior due to fire in a room. Two approaches were taken, first a model attempting to include the effect of fire entrainment and vent mixing; second was a model based on an ideal point source plume fire—both in the zone model concept. Limiting analytic results were found for the latter to give insight into the physics. The results were compared to available flow data, and an approximate formula was developed to predict the doorway mass flow rate to within 20% for a wide range of fire conditions. CFD computations were also explored using FDS. Results are compared from FDS and the zone model with experimental data for a wide range of variables.