Flame Spread On Charring Materials: Numerical Predictions And Critical Conditions
01 Jan 1994-Fire Safety Science (International Association for Fire Safety Science)-Vol. 4, pp 457-468
TL;DR: In this article, a detailed flame spread computer code has been applied to predicting flame spread and fire growth for the upward wall orientation on charring materials, and the results showed that flame spread stops when the flame height is nearly equal to the length of pyrolysis region.
Abstract: A detailed flame spread computer code has been applied to predicting flame spread and fire growth for the upward wall orientation on charring materials. The flammability properties for the pyrolysis rate of charring materials have been measured by performing pyrolysis experiments and surface temperature measurements in inert atmospheres. An extensively validated integral pyrolysis model for charring materials was employed in a flame spread code: the present flame spread code is an improvement and modification of a code presented in a previous paper. The improvements include: a) calculation of the burnout front, b) incorporation of convective heat loss from the front and back surface and c) profiles for the heat flux distribution from the flame to the wall. The flame spread code can also routinely handle preheating of the wall by externally imposed heat fluxes prior to ignition. Predictions from the model are compared with various experiments for upward flame spread performed at NIST and in Japan; the agreement is good. In addition, the model and the experiments verify that flame spread stops when the flame height is nearly equal to the length of pyrolysis region. This maximum flame spread length is simply related to the properties of the charring material as E,-(q:a, AH,/(AH,),)' where q:, is the net heat flux from the flames (equal to flame heat flux minus reradiation losses at the pyrolysis temperature), x,.AH, is the actual heat of combustion and (AH,),, is the heat of gasification of the material corrected for char conductivity effects.
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TL;DR: In this article, a simple and practical pyrolysis model was developed to describe the response of the solid fuel, which was first tested against the Cone Calorimeter data for both charring and non-charring materials under different irradiance levels and then coupled to CFD calculations.
77 citations
TL;DR: In this paper, a lateral diffusion throughout the flame width was proposed to cause thicker flame along its centerline for wider flames and enhance combustion efficiency, and a power value of 0.35 existed between the flame spread rate and width.
66 citations
TL;DR: Compared to flames without sidewalls, the existence of sidewalls lengthened flame heights and generally reduced heat feedback along the central lines of the flames, resulting in higher flame spread rates for narrower flames and lower flame spread rate for wider flames.
53 citations
TL;DR: In this article, an experimental program was intended to analyse the heat feedback from a spreading wall fire to its unburned surface, one of the two important parameters determining its spread rate.
Abstract: An experimental programme was intended to analyse the heat feedback from a spreading wall fire to its unburned surface, one of the two important parameters determining its spread rate. The heat flux from PMMA fires was observed to be higher than those from previous investigations as plotted with normalised flame height (X/Xf). However, very good consistency was performed with another study as a parameter " wo q& is introduced with heat flux plotted in another normalisation. The averaged heat flux, an important parameter used in upward flame spread modelling, was discussed and suggested to be around 15–20 kW/m. This is much lower than the commonly accepted 25-30 kW/m.
47 citations
Cites background or methods from "Flame Spread On Charring Materials:..."
...[11] 30 Delichatsios and Delichatsios [12] 25 Delichatsios and Chen [13] 25...
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...Delichatsios MA and Chen Y, Flame spread on charring materials: numerical predictions and critical conditions, Fire Safety Science- Proceedings of the Fourth International Symposium, pp. 457-468, 1995 14....
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...However, to simplify the problem, most of these models [9-18] assume that " w q& is constant over the preheating region, becoming zero at heights above Xf....
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...However, 25 kW/m(2) was used in their projects concerning PE/PVC cables [12] and charring materials [13]....
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TL;DR: In this article, a series of upward flame spread tests on different wood products was carried out on 2.4 m and 7.2 m high wall assemblies, where the ignition source was a 1.2m wide and 0.1 m deep propane diffusion burner applied usually at a rate of 100 kW.
Abstract: A series of upward flame spread tests on different wood products was carried out on 2.4 m and 7.2 m high wall assemblies. The ignition source was a 1.2 m wide and 0.1 m deep propane diffusion burner applied usually at a rate of 100 kW. The measurements included gas and surface temperatures, heat fluxes to the specimen, rate of heat release, etc. After an initial growing fire, a period of decay was observed until the lower parts of the wall started to burn through. If the rear side of the board was insulated, a second phase of increased flame spread was observed, whereas in the case of a conducting substrate the intensity of the fire remained low. A thermal flame spread model was successfully applied to simulate the rate of heat release as a function of time.
31 citations
Cites background from "Flame Spread On Charring Materials:..."
...have pursued another kind of thermal model with more detailed physics included [9,10]....
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References
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TL;DR: In this article, a linear integral equation of the Volterra type is derived for the spread rate of turbulent flames along thermally thick vertical sheets for both noncharring and charring fuels.
Abstract: Mechanisms and rates of upward spread of turbulent flames along thermally thick vertical sheets are considered for both noncharring and charring fuels. By addressing the time dependence of the rate of mass loss of the burning face of a charring fuel, a linear integral equation of the Volterra type is derived for the spread rate. Measurements of spread rates, of flame heights and of surface temperature histories are reported for polymethylmethacrylate and for Douglas-fir particle board for flames initiated and supported by a line-source gas burner, with various -rates of heat release, located at the base of the fuel face. Sustained spread occurs for the synthetic polymer and not for the wood. Comparisons of measurements with theory aid in estimating characteristic parameters for the fuels.
158 citations
TL;DR: In this article, an integral thermal pyrolysis model was developed and evaluated by comparison of the results with exact solutions, and the purpose for the development of this simple model has been the desire for predicting pyroolysis histories of materials exposed to pre heat fluxes by using equivalent properties tailored to the present model and common flammability test measurements.
Abstract: A new integral thermal pyrolysis model for the transient pyrolysis of charring and on-charring materials has been developed and evaluated by comparison of the results with exact solutions. The purpose for the development of this simple model has been the desire for predicting pyrolysis histories of materials exposed to pre heat fluxes by using “equivalent” properties tailored to the present model and common flammability test measurements. The pyrolyzing material is divided into a char layer and a unpyrolyzed (virgin) layer where the material has not yet pyrolyzed. These two layers are separated by an isothermal interface which is at a pyrolysis temperature (characteristic of the material). At this interface, heat is transferred to the virgin layer, causing further pyrolysis of the material (namely a thermal pyrolysis model is used). A one-dimensional transient heat conduction model is used to predict the heat transfer within the material. Exponential temperature profiles were assumed for the heat conducti...
74 citations
TL;DR: In this article, a method for predicting the heat release rate of wood for different thicknesses, moisture contents, and exposure conditions is described, and a model has been set up and calculations have been made on a microcomputer Heat release rates and effective heats of combustion were measured as a function of time and external radiant flux on 125 mm thick dry vertical specimens of Douglas fir particle board.
Abstract: A method for predicting the heat release rate of wood for different thicknesses, moisture contents, and exposure conditions is described A model has been set up and calculations have been made on a microcomputer Heat release rates and effective heats of combustion were measured as a function of time and external radiant flux on 125 mm thick dry vertical specimens of Douglas fir particle board The calculated and measured curves are similar in shape and amplitude but differ significantly in time scale The initial results with the model are promising INTRODUCTION The model described in this paper embodies most of the features of the mass loss rate model originally developed by Kung [lJ and modified by Taminini [2J and Atreya [3J The chief distinguishing features of the present model over that of Atreya are: (1) it takes char shrinkage parallel and normal to the surface into account, (2) the thermal properties are a function of temperature and degree of char, (3) it allows for several components of the wood each with different decomposition rate constants, and (4) it takes the change in the heat of combustion of the volatiles for each of the components as a function of its individual degree of char into account This paper will describe the model and show some calculations and comparisons with the heat release rate and effective heat of combustion measured in the cone calorimeter [4J For a more detailed discussion of the model and the experimental procedures see reference [5J HEAT RELEASE RATE MODEL The heat release rate model (1) breaks the specimen down into thin slices parallel to the surface as seen in figure 1, (2) calculates the mass loss rate for each slice, (3) multiplies this rate by the local heat of combustion of the volatiles generated, and (4) sums these contributions over the depth of the specimen to obtain the total heat release rate assuming complete combustion of the volatiles leaving the front surface The boundaries of each slice move as the specimen shrinks during the burning period so that no solid material crosses a boundary The rear surface is impervious to mass flow Flow into the cooler region of the specimen followed by condensation and subsequent evaporation is not treated here All of the FIRE SAFETY SCIENCE-PROCEEDINGS OF THE FIRST INTERNATIONAL SYMPOSIUM 207 Copyright © International Association for Fire Safety Science
58 citations
52 citations
Dissertation•
01 Jan 1992
TL;DR: In this paper, a thermal theory of wind-aided flame spread on thick solids is examined and solutions are given for flame spread velocities under ceilings and in wall-ceiling intersections.
Abstract: An extensive research program, 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 mathematical models where material properties derived from standardised bench-scale tests are used as input data. The models predict fire growth in the full or 1/3 scale tests, and consist 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 for flame spread velocities under ceilings and in wall-ceiling intersections. Flame extensions under a ceiling, associated with these processes, are discussed and the behaviour of the solutions analysed. The results from the models are compared with experiments on 22 materials tested in the full scale room and 13 materials tested in the 1/3 scale room for Scenario A. Comparisons for Scenario B are made with 4 materials in full scale and 10 materials in 1/3 scale. The results show reasonably good agreement for most materials between the model and the experiments. (Less)
44 citations