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 paper , the effect of air gap distance on flame spread along sample edge is characterized by two correction factors, one of which describes solid surface curvature at sample edge, the other of which is the effective flame temperature considering the heat loss from the spreading flame to the metal wall.
Abstract: Flames propagating along the sample edges (edge flame) will transform two-dimensional flame spread into three-dimensional flame spread, causing a significant interference with results of material flammability test. A potential method to minimize above effect is utilizing an inert wall to inhibit the side surface of sample with a designed air gap distance. In this work, the downward flame spread along the sample edge with various air gap distances is systematically studied. Cast PMMA with thicknesses from 0.4 to 2.5 mm were used as samples. The general trend of flame spread rate (FSR) at edge with respect to air gap distance can be summarized into three regimes: when the air gap distance is less than 2 mm, the edge flame is quenched by wall heat loss, and the overall flame spread behavior is two-dimensional. Then, the edge flame appears after the air gap distance exceeds 2 mm, resulting in a sharp increase in FSR. This critical air gap distance for the onset of edge flame is less sensitive to the sample thickness. Finally, as the air gap distance continues to increase, the FSR gradually approaches the asymptotic value of the uninhibited sample. With the increase in air gap distance, thermally-thin assumption loses validity as a result of the reduction in residence time of solid phase (larger FSR). To interpret the general trend of FSR with respect to air gap distance, a modified flame spread model based on conventional thermal theory is developed. The effect of air gap distance on flame spread along sample edge is characterized by two correction factors, one of which describes solid surface curvature at sample edge, the other of which is the effective flame temperature considering the heat loss from the spreading flame to the metal wall. The new model well reproduces the experimental results, and further supports the significance of heat loss in the trend of flame spread rate. This work helps to understand the flame spread mechanism under effects of solid surface curvature and heat loss, while also providing scientific support for optimization of material flammability testing standards.
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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