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.
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TL;DR: In this article, a coupled heat and mass transfer model including two-dimensional elliptical equations both in the gas phase and solid fuel was proposed to study the downward flame spread over PMMA surface.
Abstract: Downward flame spread over PMMA surface has been studied numerically by coupled heat and mass transfer model including two-dimensional elliptical equations both in the gas phase and solid fuel. Unlike the generally accepted approach based on the one-step macro-reaction for combustion, present model employs the two-step reactions mechanism in the gas phase. Solid fuel pyrolysis generates a gaseous product at the burning surface and, at the first reaction step, this relatively higher hydrocarbon decays into the lower-weight gas, which, in turn, reacts with surrounding oxidizer at the second reaction step, which is the combustion itself. The results showed that profiles of gas-phase temperature obtained through the two-step reaction model fit experimental data noticeably better than customary one-step reaction.
17 citations
TL;DR: In this paper, the downward flame spread over poly(methyl methacrylate) sheets with different dimensions was conducted in comparison with the downward flames spread over samples under infinite dimensions.
Abstract: The experiments of downward flame spread over poly(methyl methacrylate) sheets with different dimensions were conducted in this study. In comparison with flame spread over samples under infinite wi...
14 citations
01 Jan 2019
TL;DR: In this paper, the effect of flame propagation along vertical edges on the overall downward spread of flames using polymethyl methacrylate (PMMA) was measured and a MATLAB-based tool was used to calculate instantaneous spread rate for central and edge flames.
Abstract: This systematic experimental study measures the effect of flame propagation along vertical edges on the overall downward spread of flames using Polymethyl Methacrylate (PMMA). Samples with a wide range of regular cross-sections – from triangular through octagonal – as well as irregular ones, have been used to test a large variation of internal angles. A MATLAB-based tool was used to calculate instantaneous spread rate for central and edge flames. The edge flame is shown to significantly enhance the spread rate, as much as five times, in respect to samples with no edges. This amplification is shown to depend primarily on the internal angle at the edge (the smaller the angle, the faster the flame) and fuel thickness, and not on other factors such as aspect ratio or cross-sectional area. Using a phenomenological argument, the edge propagation rate is correlated to the spread rate over an equivalent cylindrical fuel (the limiting shape with infinite edges) with an effective radius obtained from the geometry of the edges and the diffusion length scale of the solid phase. A formula for flame spread over cylindrical fuel from the literature is used to link the new results to existing models. Both thick and thin limits are shown to encompass the wide range of three-dimensional spread rate data within the effective radius (the independent variable), which can be determined from the known parameters. Based on these results, different types of cross-sectional areas can be sorted in the order of their inherent fire safety characteristics.
12 citations
TL;DR: In this article, a detailed three-dimensional numerical model is used to compute the flow pattern and the flame behavior of thin solid fuels in a rectangular column that resembles a standard Limiting Oxygen Index (LOI) device.
Abstract: A detailed three-dimensional numerical model is used to compute the flow pattern and the flame behavior of thin solid fuels in a rectangular column that resembles a standard Limiting Oxygen Index (LOI) device. The model includes full Navier-Stokes equations for mixed buoyant-forced flow and finite rate combustion and pyrolysis reactions so that the sample LOI can be computed to study the effect of feeding flow rate, sample width and gravity levels. In addition to the above parameters, the sample location in the column and the column cross-sectional area are also investigated on their effect on the ambient air entrainment from the top.
11 citations
Cites background from "A Computational Study on Opposed Fl..."
...Also, it should be noted that the standard testing procedure requires fuel specimen to be placed in between metallic strips, however it was shown in [26] that in case the fuel is allowed to burn along the edges the LOI values can get affected drastically and even reverse the trend between 0ge and 1ge reported here....
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TL;DR: In this paper, a comprehensive study of thermal decomposition and combustion of a horizontally placed polyoxymethylene (POM) slab was performed, and the kinetic parameters of thermal degradation of POM in supposition of two parallel reactions were determined and were used for simulation of the flame spread over the POM.
Abstract: A comprehensive study of thermal decomposition and combustion of a horizontally placed polyoxymethylene (POM) slab was performed. The kinetic parameters of thermal degradation of POM in supposition of two parallel reactions were determined and were used for simulation of the flame spread over the POM. The following main characteristics of the POM slab's combustion were measured: the flame spread rate, the slab's mass loss rate, the width of the pyrolysis zone, the flame height, the temperature profile of the upper and lower surfaces of the slab, the temperature field and the fields of the main flame species concentrations over the burning slab, and the conductive heat flux from the flame onto the fuel surface. It was concluded from analysis of experimental data that two global gas phase reactions may be identified: the reaction of formaldehyde pyrolysis with light combustible gas formed (with the properties close to those of СО) and the subsequent reaction of its oxidation in the flame to the end combustion products (СО2+H2O). This approach was implemented as the coupled combustion model was modified, taking into account a two-step reaction in the gas phase. The results of the calculations made showed good agreement with the experimental data.
9 citations
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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