A Computational Study on Opposed Flow Flame Spread Over Thin Solid Fuels with Side-Edge Burning
18 Aug 2010-Combustion Science and Technology (Taylor & Francis Group)-Vol. 182, Iss: 9, pp 1321-1340
TL;DR: In this article, a steady-state flame spread model has been used to study the effect of side-edge burning on flame spread over thin solid fuel strips of finite width, and simulations have been carried out for fuel strips with both inhibited (by metallic strips) and uninhibited side edges.
Abstract: A steady-state flame spread model has been used to study the effect of side-edge burning on flame spread over thin solid fuel strips of finite width. Simulations have been carried out for fuel strips with both inhibited (by metallic strips) and uninhibited side edges. The effect inhibition on both normal- and microgravity flame spread along with several intermediate gravity levels has been investigated. Such a study is important for understanding the physiochemical processes controlling the flame spread in low gravity where human experience is limited. Although simulations have shown an overall increase in spread rate for uninhibited cases for both normal- and microgravity flames, some effects such as flame spread variation with external imposed velocity and flame extinction limits show different behavior for microgravity and normal gravity flames. The heat and mass transport processes in the flame have been discussed in detail to explain the observed trends.
Citations
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TL;DR: In this paper, a 2D numerical model based on OpenFOAM is used to simulate the flame spread in a natural convective environment under normal gravity, and the model is validated with detailed experimental data involving spatial distributions of temperature and species, and flame spread rates.
Abstract: In the present study, downward flame spread over multiple parallel fuel sheets is investigated experimentally and numerically to understand the mechanism that controls the flame spread process and compare that with the flame spread over a single fuel sheet. A 2D numerical model, based on OpenFOAM is used to simulate the flame spread in a natural convective environment under normal gravity. The model is validated with detailed experimental data involving spatial distributions of temperature and species, and flame spread rates. Flame spread rates on central fuel sheet have been measured in 3 parallel fuel sheets configurations, considering spacing (s) between the fuel sheets in the range of 0.5 cm–3 cm. The flame spread rate varies non-monotonically with spacing, with a peak spread rate at 1 cm spacing between the fuel sheets. Conduction is the dominant mode of heat transfer to the single fuel sheet case. However, in the case of multiple fuel sheets, radiation contributes almost to the same order as that of conduction. The non-monotonic trend in the flame spread rate with spacing is attributed to two opposing effects, namely, increase in oxygen availability and decrease in heat transfer to the fuel, with an increase in the spacing between the fuel sheets.
4 citations
TL;DR: In this paper, the authors derived an analytical expression for the flame front speed in downward combustion of thin solid fuels based on a model that consists of a preheating region plus a pyrolyzing one.
Abstract: We derive a new analytical expression for the flame front speed in the downward combustion of thin solid fuels based on a model that consists of a preheating region plus a pyrolyzing one. The solid phase is modeled by means of a simple but physically realistic behavior of the main variables in these two regions, whereas the gas phase follows classical reaction–diffusion–convection equations for the fuel mass fraction, oxygen mass fraction, and gas temperature. Both fuel and oxygen mass fractions are expressed in terms of the gas temperature by using a simplified vertical flame model. This approach allows us to reduce the combustion model into a single reaction–diffusion–convection equation of a single variable (gas temperature) for both regions. Matching conditions at the flame leading edge gives us an analytical expression for the burning rate that depends on the kinetics parameters of the combustion reaction and the opposed flow. In contrast with de Ris's classical formula, our analytical expression pre...
2 citations
Cites methods from "A Computational Study on Opposed Fl..."
...58 10(9)m(3) kg 1 s 1 that are similar to those adopted in recent studies (Kumar et al., 2003; Kumar and Kumar, 2010)....
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...By analyzing values used in several models, West et al. (1992) propose activation energy value E¼ 116800 Jmol 1 and preexponential factor value B¼ 1.58 109m3 kg 1 s 1 that are similar to those adopted in recent studies (Kumar et al., 2003; Kumar and Kumar, 2010)....
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01 Jan 2015
TL;DR: In this paper, an experimental study on the characteristics of downward flame spread of extruded polystyrene (XPS) is presented, where the effects of sample thickness (d), sidewalls and atmospheric pressure (p), and the combined effects of these factors on the flame spread are studied.
Abstract: An experimental study on the characteristics of downward flame spread of extruded polystyrene (XPS) is presented. The parameters investigated include average mass loss rate per unit of thickness ( _ m 0 ), average flame height (Hf), average flame spread rate (vf), and mass growth rate ( _ m1) of molten XPS. The effects of sample thickness (d), sidewalls and atmospheric pressure (p), and the combined effects of these factors on the flame spread are studied. The larger sample thickness corresponds to larger _ m
2 citations
TL;DR: In this article , the effect of the internal angle on downward flame spread over a thick folded plate is experimentally investigated, and a simple model is developed considering change of heated volume against the flame and change of the induced flow velocity due to the geometry.
Abstract: The effect of the internal angle on downward flame spread over a thick folded plate is experimentally investigated in this study. The lateral side of a thermoplastic plate is cut at desired angles and two of these plates are welded to form a single folded sample. All the faces other than the front and two 5-mm areas of the front face from each side were coated to inhibit the combustion. Experiments are conducted for various folded plates of different angles, 60°, 90°, 120°, and 180°. Following ignition at the upper edge of the sample, the opposed flame spread on the front face is observed and the flame spread rate at various positions is quantified by image analysis. The flame spreads more rapidly at the folding edge, while the other parts then accelerate to catch up with the corner spread and, eventually, all the parts across the width achieve the same flame spread rate as the folding edge. This result indicates that the flame spread rate at the folding edge is the characteristic value which represents the steady-state of the system of interest. A simple model is developed considering change of heated volume against the flame and change of the induced flow velocity due to the geometry. The former effect is formulated from geometrical consideration in the vicinity of the edge, while the latter is formulated based on an experimental fact that the flame height is inversely proportional to the internal angle. A prediction formula of the flame spread rate at the folding edge is established by modifying the conventional one for the flat plate. The calculated flame spread rate shows reasonably good agreement with the experimental data. This study helps fundamental understanding of the flame spread behavior of practical combustibles such as pillars or rods with various cross-sectional shapes.
1 citations
Dissertation•
14 Jul 2014
TL;DR: In this article, the effects of the sides to the vertically downward flame spread over a thin solid sample were investigated, as well as the effect of having multiple parallel samples burning simultaneously, and flame spreading over horizontal and downward inclined samples.
Abstract: Flame spread over solid samples has been studied from many points of view, as it is key for fire safety, yet it is a complex phenomenon that involves processes occurring in both the solid and the gas phases. This Ph.D. thesis studies the flame spread over thin solid samples in processes more complex than the classical cases where a flame spreads horizontally or downward over a vertical solid sample. In particular, this thesis deals with three different situations: the effects of the sides to the vertically downward flame spread over a thin solid; the effects of having various parallel samples burning simultaneously, and flame spread over horizontal and downward inclined samples. For all these situations a complete experimental study is made and a model that explains the obtained results is developed
Cites background or methods or result from "A Computational Study on Opposed Fl..."
...Although the above data may suggest that the extinction limit for the one inhibited edge case occurs at lower XO2 , Kumar and Kumar [3] found that by doing simulations with and without side-burning, the extinction point is at XO2 = 20% for the inhibited case and XO2 = 20....
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...On the other hand, the comprehensive gas phase governing equations can be expressed as [3]...
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...The previous equations are used in numerical simulations [3], although for typical configurations the solid can be considered infinitely wide and only two-dimensional equations (x, y) need to be considered [2, 10, 11]....
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...1) and the one extracted from the 3-D simulations carried out by Kumar and Kumar [3] both using cellulosic type fuels with width w = 2 cm (see other sample details in Table 3....
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...These equations are used in complex numerical simulations [3]....
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References
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Book•
01 Jan 1980
TL;DR: In this article, the authors focus on heat and mass transfer, fluid flow, chemical reaction, and other related processes that occur in engineering equipment, the natural environment, and living organisms.
Abstract: This book focuses on heat and mass transfer, fluid flow, chemical reaction, and other related processes that occur in engineering equipment, the natural environment, and living organisms. Using simple algebra and elementary calculus, the author develops numerical methods for predicting these processes mainly based on physical considerations. Through this approach, readers will develop a deeper understanding of the underlying physical aspects of heat transfer and fluid flow as well as improve their ability to analyze and interpret computed results.
21,858 citations
"A Computational Study on Opposed Fl..." refers methods in this paper
...The system of coupled elliptic partial differential equations for the flow and combustion in the gas phase is solved numerically by SIMPLER algorithm Patankar (1980)....
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...The system of coupled elliptic partial differential equations for the flow and combustion in the gas phase is solved numerically by SIMPLER algorithm Patankar ( 1980 )....
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01 Jan 1991
241 citations
"A Computational Study on Opposed Fl..." refers background in this paper
...The transport properties are modeled following Smooke and Giovangigli (1991). l ¼ T0:7; j=cp ¼ T0:7; qDi ¼ T0:7; i ¼ F; O2; CO2; H2O; N2 ð1Þ where j is gas thermal conductivity (1.93 10 4 cal=cm=s=K), q is reference gas density (2.75 10 4 g=cm3), q1 is ambient gas density (1.15 10 3 g=cm3), l is…...
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TL;DR: In this paper, the near-limit characteristics of a spreading flame are considered, where the flame is extinguished by increasing the heat loss, reducing the total pressure, or reducing the oxygen mole fraction in the environment.
Abstract: In this study the near-limit characteristics of a spreading flame are considered. Flame spreading rates and temperature profiles are measured as extinction conditions are approached. The flame is extinguished by increasing the heat loss, reducing the total pressure, or reducing the oxygen mole fraction in the environment. The gas phase temperature profiles are obtained with fine-wire thermocouple probes. The flame spreading results show that the power-law correlations of McAlevy and Magee [3] do not remain valid near the extinction limit. In all cases the slope of the Log (flame spread rate) vs. Log (total pressure) curves increase and approach vertical at extinction. Differences in vertical and horizontal flame spreading are discussed. The flame temperature profiles are examined for a near-limit flame, but the total pressure level is the only parameter changed. In the near-limit flame the maximum flame temperature is reduced slightly, but the flame is enlarged in physical size greatly. It is observed that near the pyrolysis front, heat transfer forward in the gas phase and normal to the fuel surface are of the same order of magnitude.
67 citations
TL;DR: In this paper, a two-dimensional, opposed-flow, flame-spread model, with flame radiation, has been formulated and solved numerically, and a comparison of flammability limits and flame-spreading rates between opposing and concurrent spreading flames are made; both models contain the same assumptions and properties.
Abstract: Flame-spread phenomena over thin solids are investigated for purely forced-opposing and concurrent flows. A two-dimensional, opposed-flow, flame-spread model, with flame radiation, has been formulated and solved numerically. In the first part of the paper, flammability limits and spread rates in opposed flow are presented, using oxygen percentage, free-stream velocity, and flow-entrance length as parameters. The comparison of the flammability boundaries and spread-rate curves for two different entrance lengths exhibits a cross-over phenomenon. Shorter entrance length results in higher spread rates and a lower oxygen-extinction limit in low free-stream velocities, but lower spread rates and a higher oxygen-extinction limit in high free-stream velocities. The entrance length affects the effective flow rate that the flame sees at the base region. This affects the radiation loss and gas residence-time in an opposing way to cause the cross-over. Radiation also affects the energy balance on the solid surface and is in part responsible for the solid-fuel non-burn-out phenomenon. In the second part of the paper, a comparison of flammability limits and flame-spreading rates between opposing and concurrent spreading flames are made; both models contain the same assumptions and properties. While the spread rate in concurrent spread increases linearly with free-stream velocity, the spread rate in opposed flow varies with free-stream velocity in a non-monotonic manner, with a peak rate at an intermediate free-stream velocity. At a given free-stream velocity, the limiting oxygen limits are lower for concurrent spread, except in the very low free-stream-velocity regime, where the spreading flame may be sustainable in opposed mode and not in concurrent mode. The cross-over disappears if the two spread modes are compared using relative flow velocities with respect to the flames rather than using free-stream velocities with respect to the laboratory.
58 citations
"A Computational Study on Opposed Fl..." refers background in this paper
...As for higher velocities, a previous study (Kumar et al., 2003) showed that the flammability limit for the microgravity flame (with inhibited edges) reverses trend with normal-gravity flame (i....
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...As for higher velocities, a previous study (Kumar et al., 2003) showed that the flammability limit for the microgravity flame (with inhibited edges) reverses trend with normal-gravity flame (i.e., microgravity flame extinction limits are lower compared to extinction limits for normal gravity flame)....
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01 Jan 2009
TL;DR: In this article, a 5.18-s drop tower with a thin cellulose fuel was used to investigate flame spread in both concurrent and opposed flow in a spacecraft, with a focus on pressure/oxygen combinations that result in earth-equivalent oxygen partial pressures (normoxic conditions).
Abstract: Flame spread experiments in both concurrent and opposed flow have been carried out in a 5.18-s drop tower with a thin cellulose fuel. Flame spread rate and flame length have been measured over a range of 0–30 cm/s forced flow (in both directions), 3.6–14.7 psia, and oxygen mole fractions 0.24–0.85 in nitrogen. Results are presented for each of the three variables independently to elucidate their individual effects, with special emphasis on pressure/oxygen combinations that result in earth-equivalent oxygen partial pressures (normoxic conditions). Correlations using all three variables combined into a single parameter to predict flame spread rate are presented. The correlations are used to demonstrate that opposed flow flames in typical spacecraft ventilation flows (5–20 cm/s) spread faster than concurrent flow flames under otherwise similar conditions (pressure, oxygen concentration) in nearly all spacecraft atmospheres. This indicates that in the event of an actual fire aboard a spacecraft, the fire is likely to grow most quickly in the opposed mode as the upstream flame spreads faster and the downstream flame is inhibited by the vitiated atmosphere produced by the upstream flame. Additionally, an interesting phenomenon was observed at intermediate values of concurrent forced flow velocity where flow/flame interactions produced a recirculation downstream of the flame, which allowed an opposed flow leading edge to form there.
56 citations
"A Computational Study on Opposed Fl..." refers background in this paper
...…flame lengths are shorter due to strong convective transport of heat and mass. Effect of Opposed Free-Stream Velocity The effect of free-stream velocity in the direction of opposing flame spread has been studied extensively both experimentally (Olson and Miller, 2009) and numerically (Kumar, 2004)....
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...The effect of free-stream velocity in the direction of opposing flame spread has been studied extensively both experimentally (Olson and Miller, 2009) and numerically (Kumar, 2004)....
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