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Journal ArticleDOI

Study of vertical upward flame spread on charring materials—Part II: Numerical simulations

01 Aug 2011-Fire and Materials (John Wiley & Sons, Ltd)-Vol. 35, Iss: 5, pp 261-273

Abstract: Simulation results, obtained by means of application of an enthalpy-based pyrolysis model, are presented. The ultimate focus concerns the potential of the model to be used in flame spread simulations. As an example we discuss vertically upward flame spread over a charring material in a parallel plate configuration. First, the quality of the pyrolysis model is illustrated by means of cone calorimeter results for square (9.8 cm × 9.8 cm exposed area), 1.65 cm thick, horizontally mounted MDF samples. Temperatures are compared at the front surface and inside the material, for different externally imposed heat fluxes (20, 30 and 50 kW/m2), for dry and wet samples. The mass loss rate is also considered. Afterwards, vertically upward flame spread results are reported for large particle board plates (0.025 m thick, 0.4 m wide and 2.5 m high), vertically mounted face-to-face, for different horizontal spacings between the two plates. The simulation results are compared to experimental data, indicating that, provided that a correct flame height and corresponding heat flux are applied as boundary conditions, flame spread can be predicted accordingly, using the present pyrolysis model. Copyright © 2010 John Wiley & Sons, Ltd.
Topics: Flame spread (68%), Charring (53%), Heat flux (52%)

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This item is the archived peer-reviewed author-version of:
Study of vertical upward flame spread on charring materials-Part II: Numerical simulations
Wasan, S.R., Rauwoens, P., Vierendeels, J. and Merci, B.
In: Fire and Materials 35 (5), 261-273, 2011.
To refer to or to cite this work, please use the citation to the published version:
Wasan, S.R., Rauwoens, P., Vierendeels, J. and Merci, B (2011). Study of vertical upward
flame spread on charring materials-Part II: Numerical simulations. Fire and Materials 35 (5)
261-273.

Study of Vertical Upward Flame Spread on Charring Materials
Part II: Numerical Simulations
S.R. Wasan, P. Rauwoens, J. Vierendeels and B. Merci
Ghent University UGent, Department of Flow, Heat and Combustion Mechanics
Corresponding author: Bart.Merci@UGent.be

2
Abstract
Simulation results, obtained by means of application of an enthalpy based pyrolysis
model, are presented. The ultimate focus concerns the potential of the model to be used in
flame spread simulations. As an example we discuss vertically upward flame spread over
a charring material in a parallel plate configuration. Firstly, the quality of the pyrolysis
model is illustrated by means of cone calorimeter results for square (9.8 cm x 9.8 cm
exposed area), 1.65cm thick, horizontally mounted MDF samples. Temperatures are
compared at the front surface and inside the material, for different externally imposed
heat fluxes (20 kW/m
2
, 30 kW/m
2
and 50 kW/m
2
), for dry and wet samples. The mass
loss rate is also considered. Afterwards, vertically upward flame spread results are
reported for large particle board plates (0.025 m thick, 0.4 m wide and 2.5 m high),
vertically mounted face-to-face, for different horizontal spacing between the two plates.
The simulation results are compared to experimental data, indicating that, provided that a
correct flame height and corresponding heat flux are applied as boundary conditions,
flame spread can be predicted accordingly, using the present pyrolysis model.

3
1. Introduction
In part I [1], the outcome of an experimental campaign was reported. In the present paper,
we apply a simple pyrolysis and evaporation model, based on enthalpy [2], to the same
configurations.
Firstly, we discuss the one-dimensional cone calorimeter configurations.
Afterwards, vertically upward flame spread in a parallel plate configuration is considered.
By no means, it is our intention to introduce a (semi-empirical) flame spread model, to be
used for other configurations than the specific one considered here. The only objective is
to illustrate that the developed pyrolysis model is ready-to-use for such configurations
and that reasonably accurate results can be obtained, provided an appropriate value for
incoming heat flux onto the solid material is provided. This heat flux could stem from
CFD (Computational Fluid Dynamics) in the gas phase, where the turbulent combustion
is simulated. However, we do not use CFD in the present paper, as uncertainties in CFD
would distract the attention from our objective as mentioned. Rather, we use
experimental data [1] to estimate the heat fluxes that serve as boundary conditions for the
simulations. In this sense, the set-up is somehow a sophism, but this suffices for the sake
of the present paper, as explained above. The major advantage is that the strong
sensitivity of flame heat fluxes to e.g. fuel sootiness [3, 4] is avoided. To summarise,
expressions as developed in [3] for a similar set-up as the one under study in the present
paper, are not applied here, but the present paper is not intended to provide an alternative
for such relationships.

4
2. Numerical simulations set-up
2.1 Model description
In [2, 5], the model, along with the solution procedure, is extensively described and
applied to some basic configurations. The reader is referred to those references for all
details. We only recall here that the model relies on a fixed computational mesh, which
can be relatively coarse. On this mesh, the energy equation is solved numerically.
Pyrolysis (and evaporation) is modelled as an infinitely fast irreversible process, taking
place at an infinitely thin front at ‘pyrolysis’ (resp. ‘evaporation’) temperature. Thus,
fronts are moving through the solid material. As the evaporation front passes, wet virgin
material becomes dry virgin material. As the pyrolysis front passes, dry material becomes
char. In the present simulations, the water vapour and pyrolysis gases are assumed to
leave the solid instantaneously. They are in thermal equilibrium with the solid. More
advanced pyrolysis modelling (e.g. [6]) is possible, but this is beyond the scope of the
present paper.
2.2 Cone calorimeter set-up
We first discuss the results for the cone calorimeter set-up of [1]. We consider one-
dimensional heat transfer in the solid [1]. The computational mesh in the solid, with
thickness 1.65 cm, contains 33 cells. The time step size is set to 0.5s. It has been verified
that the results presented do not vary when more cells or smaller time steps are chosen.
The externally imposed heat flux
''
ext
q
equals 20 kW/m
2
, 30 kW/m
2
or 50 kW/m
2
. The
experiments were performed in open atmosphere, so that flames appear when the
volatiles are ignited with a spark ignition placed above the retainer frame. These flames
provide additional heat flux to the solid during pyrolysis. In the simulations, this is

Citations
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Abstract: In this paper, experimental methods and theoretical analysis are employed to investigate effects of parallel curtain wall on downward flame spread characteristics of insulation materials on building facade. As the curtain wall spacing ( D ) rises, the front surface of facade flame becomes more irregular. For small spacing, the flame stretching phenomenon is obvious and periodical change of the flame height is observed. The average flame height ( H f ) first drops and then rises as the spacing increases. The variation of H f is significant as D ≤ 11.5 cm while indistinctive change is observed for D > 11.5 cm. The average maximum flame temperature first rises and then drops as D increases. There is a power function relationship between internal surface temperature of the curtain wall and D . The total heat feedback from the curtain wall to the facade decreases exponentially with the rising of D . A formula is proposed to predict the radiant heat feedback from the curtain wall, which is more dominant than the convective heat feedback. As D rises, the radiant heat feedback decreases, while the ratio of convective heat feedback to the total first rises and then drops. With the increasing of D , the average flame spread rate first rises and then drops, which is attributed to the competition of negative effect and positive effect of the curtain wall.

23 citations


Journal ArticleDOI
Canjun Liang1, Canjun Liang2, Xudong Cheng2, Hui Yang2  +2 moreInstitutions (2)
01 Apr 2016-Fire and Materials
Abstract: Summary Experiments of vertically upward flame spread over polymethyl methacrylate slabs were conducted in Hefei (with an altitude of 29.8 m) and Lhasa (with an altitude of 3658.0 m). Measurements were taken for the flame heights, the flame heat flux to the fuel surface and the flame spread rate. Two regions were identified for the dependence of the flame height on the heat release rate per unit width . When is less than 22 kW/m, the flame height scales as while it scales as , when is greater than 22 kW/m. The flame heights in Lhasa are approximately 1.34 and 1.25 times, respectively, of those in Hefei for these two regions. The flame heat flux to the fuel surface decreases significantly from the pyrolysis front to the flame tip, whereas it decreases slowly above the flame tip. In both regions, it can be correlated reasonably well with (x − xp)/(xf − xp) using the form of . The flame heat flux to the fuel surface in Lhasa is approximately 0.75 times of that in Hefei. The flame spread process can be divided into three stages, which correspond to a flow region of laminar, transitional, and turbulent, respectively. The transition to a turbulent flow is delayed in Lhasa compared with Hefei. The flame spread rate in Lhasa is about half of that in Hefei because of the lower flame heat flux caused by the lower ambient pressure. Copyright © 2015 John Wiley & Sons, Ltd.

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Abstract: We analyze the flame front speed in the downward combustion of multiple parallel samples of thermally thin fuels at normal gravity and far from extinction conditions. In contrast with the single sample case, where conduction through the gas-phase is the dominant heat transfer mechanism, in the multiple parallel samples case, radiative heat fluxes may become very relevant, which compromises the application of the well-known formula of de Ris for determining the burning rate. Here we study the downward combustion of multiple parallel sheets by (1) obtaining new experimental data at different oxygen atmospheric levels; (2) generalizing a previous comprehensive energy balance model now expected to be valid for a wide range of scenarios; and (3) deriving an analytical approximation for the burning rate that generalizes the classical de Ris formula for those cases where radiative effects cannot be neglected. The comparison with own as well as with external data reveals the strengths and weaknesses of these type...

6 citations


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Journal ArticleDOI
Rongliang Pan1, Guoqing Zhu1, Guowei Zhang1, Weiguang An1  +2 moreInstitutions (2)
Abstract: To study the effects of spacing on the downward flame spread over polymethyl methacrylate (PMMA), an experiment was conducted by pure PMMA (with 200 mm height, 50 mm width, and 1 mm thickness) with spacings of 7 mm, 10 mm, 13 mm, 16 mm, 19 mm, 22 mm, and 25 mm to observe the flame height, pyrolysis spread rate of fuels, and heat feedback from the wall. The heat feedback received by PMMA was used to analyze the influencing mechanism of wall spacing on flame spread. The results are as follows: (1) The average flame height decreases with the increase in distances ( $$\delta$$ ). This decrease in average flame height cycles through two stages: a fast drop stage and a slow drop stage. (2) The average pyrolysis spread rate first increases with the increase in distance, and a maximum pyrolysis spread rate occurred in the 13 mm spacing scenario. Then, the average pyrolysis spread rate decreases monotonously when the distance between wall and sample exceeds 13 mm. (3) The heat flux received by the sample consists of both heat flux from the flame and heat feedback from the wall. With the increase in distance, the heat feedback from the wall follows a downward trend, while the heat flux from the flame first increases and then remains constant. Because of the effects of heat flux from flame and heat feedback from the wall, the heat flux received by the sample first increases and then decreases with the increase in distance.

5 citations



References
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Journal ArticleDOI
Abstract: A generalized pyrolysis model (Gpyro) is applied to simulate the oxidative pyrolysis of white pine slabs irradiated under nonflaming conditions. Conservation equations for gaseous and solid mass, energy, species, and gaseous momentum (Darcy’s law approximation) inside the decomposing solid are solved to calculate profiles of temperature, mass fractions, and pressure inside the decomposing wood. The condensed phase consists of four species, and the gas that fills the voids inside the decomposing solid consists of seven species. Four heterogeneous (gas/solid) reactions and two homogeneous (gas/gas) reactions are included. Diffusion of oxygen from the ambient into the decomposing solid and its effect on local reactions occurring therein is explicitly modeled. A genetic algorithm is used to extract the required material properties from experimental data at 25 kW/m 2 and 40 kW/m 2 irradiance and ambient oxygen concentrations of 0%, 10.5% and 21% by volume. Optimized model calculations for mass loss rate, surface temperature, and in-depth temperatures reproduce well the experimental data, including the experimentally observed increase in temperature and mass loss rate with increasing oxygen concentration.

99 citations


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Journal ArticleDOI
Abstract: There are various methodologies available for the determination of flammability properties of non-charring polymeric materials (Int. J. Transp. Phenomena 1 (1999) 79; Proceedings of the Third International Symposium on Fire Safety Science, 1991, p. 167; Methodology and validation for material flammability properties, Combust. Flame, August 2002, accepted for publication). This paper presents a demonstration, validation and extension of a distinct methodology for obtaining the flammability properties of charring materials. Australian radiata pine was used for the study wherein ignition and pyrolysis characteristics were obtained using a cone calorimeter. The deduced properties were applied for the prediction of ignition and pyrolysis histories of wood. These predictions were compared with the data obtained from the cone calorimeter at different imposed heat fluxes and wood thickness and a close agreement was observed between the theoretical and experimental results.

86 citations


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  • ...Note that the same value was reported for the experiments in [8] on pine samples....

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Journal ArticleDOI
Abstract: 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 (C 3 H 8 ), and propylene (C 3 H 6 ). 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 C 3 H 8 flames were twice those from CO flames, whereas the peak heat fluxes from C 3 H 8 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.

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Additional excerpts

  • ...fuel sootiness [3, 4] is avoided....

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Journal ArticleDOI
Abstract: In a simulation of a developing fire, flame spread must be properly accounted for. The pyrolysis model is important in this respect. To that purpose, we develop a simplified enthalpy-based pyrolysis model that is extendable to multi-dimensional solid-phase treatments. This model is to be coupled to gas phase turbulent combustion simulations. The description of the pyrolysis process is simplified in order to acquire short simulation times. In this paper, first, the basic thermodynamic description of pyrolysis phenomena is revisited for charring and non-charring materials, possibly containing moisture. The heat of pyrolysis is defined and its relation to the formation enthalpies of individual constituents is explained. Solving only one equation for enthalpy on a fixed computational mesh, provides a useful description of the transport of heat and the pyrolysis process inside the solid material. Models for e.g. char oxidation or complex transport of the pyrolysis gases or water vapour inside the solid material can be coupled to the present model. Next, numerical issues and implementation are discussed. We consider basic test cases with imposed external heat flux to a solid material that can be dry or contain moisture. We illustrate that continuous pyrolysis gases mass flow rates are obtained when a piecewise linear representation of the temperature field is adopted on the fixed computational mesh. With constant temperature per computational cell, discontinuities, with sudden drops to zero, are encountered, as reported in the literature. We show that the present model formulation is robust with respect to numerical aspects (cell size and time step) and that the model performs well for variable external heat fluxes. For charring and non-charring materials, we validate the model results by means of numerical reference test cases and experimental data. By means of a numerical test case, we show that the model, when coupled to CFD calculations, is able to simulate flame spread.

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"Study of vertical upward flame spre..." refers background or methods in this paper

  • ...Important model parameters are the heat of pyrolysis and the pyrolysis temperature [2]....

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  • ...In the present paper, we apply a simple pyrolysis and evaporation model, based on enthalpy [2], to the same configurations....

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  • ...Model description In [2, 6], the model, along with the solution procedure, is extensively described and applied to some basic configurations....

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Journal ArticleDOI
Abstract: Flame heights and flame heat-flux distributions are measured for a wide range of fuels burning between two parallel panels. The flame heat flux levels are very sensitive to fuel sootiness. The heat flux distributions are obtained from the transient temperature rise of thermocouples peened into the steel parallel panel sidewalls. The measured flame heights imply an actual heat release rate per unit flame volume, 1110 = ′ ′ ′ q� kW/m 3 , consistent with literature values. This heat release rate per unit volume is independent of fuel type and fire scale. The heat flux distributions are integrated to obtain the net total heat transfer () 0 p Qq ′′ � � to the panels above an arbitrarily specified panel heat loss rate, 0 q� ′ ′ . The integration is performed only over areas for which 0 0 ≥ ′ ′ − ′ ′ q q f � � to obtain the net heat transfer, needed by fire growth models. The results are described by a simple theoretical model that assumes heat transfer occurs only by radiation. The model gives the net heat transfer p

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"Study of vertical upward flame spre..." refers result in this paper

  • ...Therefore, we set the convection coefficient to h = 15 W/(m 2 K) here, in line with the value reported in [3]....

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  • ...To summarise, expressions as developed in [3] for a similar set-up as the one under study in the present paper, are not applied here, but the present paper is not intended to provide an alternative for such relationships....

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