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, the effects of geometrical effects on the flame spread over thermally thick combustibles in a narrow channel were experimentally investigated and two series of experiments were conducted with two kinds of fuel geome...
Abstract: Geometrical effects on the flame spread over thermally thick combustibles in a narrow channel were experimentally investigated. Two series of experiments were conducted with two kinds of fuel geome...
9 citations
Cites result from "A Computational Study on Opposed Fl..."
...The side-wall effect has also been confirmed by other researchers numerically and experimentally (Comas et al., 2015; Comas and Pujol, 2012; Kumar and Kumar, 2010; Zhang and Yu, 2011)....
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TL;DR: In this paper, the effects of wall spacing on the downward flame spread over polymethyl methacrylate (PMMA) were studied, and the authors found that the average flame height decreases with the increase in distances.
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.
7 citations
Journal Article•
TL;DR: In this paper, the effects of sample width and sidewalls on downward flame spread over extruded polystyrene (XPS) slabs were investigated in a series of laboratory-scale experiments.
Abstract: To study the effects of sample width and sidewalls on downward flame spread over extruded polystyrene (XPS) slabs, a series of laboratory-scale experiments were conducted. Flame shape, flame spread rate, mass loss rate and temperature were recorded. For XPS without sidewalls, the average maximum flame height (H) and average flame area per unit of width (A/w) rise linearly with an increase in sample width (w) and mass loss rate per unit of width. When sidewalls are absent, flame spread rate first drops and then rises with an increase in width. This trend is determined by gas-phase heat transfer. When sidewalls are present, flame spread rate increases with a rise in width, and solid-phase heat conduction determines the trend. Sidewall effects are comprised of four aspects: oxygen concentration near the sidewalls and gypsum board is low, which leads to reduced flame heat flux; upward and front air flow is intensified; the flame is stretched, and the surface flame is weakened; and molten XPS mass decreases. For narrow samples, H and A/w with sidewalls are higher than those without sidewalls, while the reverse was observed in wider samples. The mass loss rate, preheating length and average flame spread rate with sidewalls are smaller than those obtained without sidewalls. Flame spread acceleration with sidewalls occurs at a broader width than that without sidewalls. The experimental results agree well with the theoretical analysis.
5 citations
01 Jan 2019
TL;DR: In this article, Carmignani et al. investigated several aspects of opposed-flow flame spread and found that the interaction between fuel and flow field is more important than the dependence on fuel thickness.
Abstract: Author(s): Carmignani, Luca | Advisor(s): Seshadri, Kalyanasundaram; Bhattacharjee, Subrata | Abstract: Several aspects of opposed-flow flame spread are experimentally investigated because of their relevance in fire safety studies. Different burning regimes based on the intensity of the opposed flow velocity are identified for acrylic fuels. In downward flame spread, where the flow around a flame is only naturally induced by gravity, the spread rate is highly dependent on fuel size and geometry. The fuel cross-sectional shape is experimentally varied, and a formula which takes into account geometrical effects is proposed by extending previous solutions for two-dimensional flames.The burning region of a solid fuel shows a consistent slope due to the competition between flame spread and surface regression. The angle at the vertex of the pyrolysis region, called burn angle, can be used to indirectly calculate the fuel burning rate. The burn angle depends on fuel thickness; a numerical model and a scale analysis are used to explore the reasons for this behavior.Next, the effect of a forced flow is investigated. The extreme case of blow-off extinction over thin fuels is considered, with flames extinguishing at locations determined by the flow velocity. Results suggest that the interaction between fuel and flow field is more important than the dependence on fuel thickness. The evolution of flame structure and pyrolysis also appear to be driven by flow interactions. A scale analysis is used to explore these dependencies.Finally, previous microgravity experiments are used to explore differences and similarities with ground-based results. By suppressing the buoyant flow, flame radiation becomes essential for the flame spread process. The experimental conditions are simulated numerically to describe the importance of a developing boundary layer in this regime.A numerical parametric study of the radiative emission of flames in microgravity, inspired by the experimental data, shows its dependence on flame area, mass burning rate and flame temperature by changing the burning conditions. For these small flames, soot does not seem to dominate flame radiation, although its generation increases with fuel thickness, oxygen concentration and flow velocity.The experiments in microgravity considered in this work showed flame extinction in a quiescent environment. However, two acrylic cylinders at higher oxygen concentrations from a previous investigation can burn vigorously. To clarify whether these flames are stable, a scale analysis is used to study the influence of surface curvature on radiation losses.
4 citations
Cites background or result from "A Computational Study on Opposed Fl..."
...They concluded that the enhanced heat transfer from the side flame and higher oxygen supply are responsible of the higher flame spread rate, in analogy with the previous works [58, 59]....
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...The microgravity environment, as well as the downward configuration, was also considered in the computational study of Kumar and Kumar [58], who quantified the spread rate with and without side burning over thin fuels....
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01 Jan 2015
TL;DR: In this paper, the authors explored the dynamic relationship between combustible condensed fuel surface and gas-phase flames in laminar boundary layers, representing the small scales in which materials are tested (where much of todays theoretical knowledge is also isolated) to realistic large-scale turbulent flames present in almost all unwanted fires, hybrid rocket motors and other similar combustion phenomena.
Abstract: Title of dissertation: A FUNDAMENTAL STUDY OF BOUNDARY LAYER DIFFUSION FLAMES Ajay Vikram Singh, Doctor of Philosophy, 2015 Dissertation directed by: Professor Michael J. Gollner Department of Fire Protection Engineering Modeling the realistic burning behavior of condensed-phase fuels has remained out of reach, in part because of an inability to resolve complex interactions at the interface between gas-phase flames and condensed-phase fuels. This interaction is even more complex as scales increase, because realistic fires occur under fully turbulent conditions which have yet to be fully replicated or understood at the bench scale, where detailed measurements can be conducted. The current research explores the dynamic relationship between combustible condensed fuel surface and gas-phase flames in laminar boundary layers, representing the small scales in which materials are tested (where much of todays theoretical knowledge is also isolated) to realistic large-scale turbulent flames present in almost all unwanted fires, hybrid rocket motors and other similar combustion phenomena. A thorough numerical and experimental investigation of laminar boundary-layer diffusion flames established over the surface of a condensed fuel is presented. By extension of the Reynolds Analogy, it is hypothesized that the non-dimensional temperature gradient at the surface of a condensed fuel is related to the local mass-burning rate through some constant of proportionality. First, this proportionality is tested by using a validated numerical model for a steady flame established over a condensed fuel surface, under free and forced convective conditions. Second, the relationship is tested by conducting experiments in a free and forced convective environment using methanol and ethanol as liquid fuels and PMMA as a solid fuel, where a detailed temperature profile is mapped during steady burning using fine-wire thermocouples mounted to a precision two-axis traverse mechanism. The results from the present study suggests that there is indeed a unique correlation between the mass burning rates of liquid/solid fuels and the temperature gradients at the fuel surface. The correlating factor depends upon the Spalding mass transfer number and gas-phase thermo-physical properties and works in the prediction of both integrated as well as local variations of the mass burning rate as a function of non-dimensional temperature gradient. Convective and radiative heat feedback from the flames were also measured both in the pyrolysis and plume regions by using temperature gradients near the wall. Additional results from precise measurements of the thermal field are also presented. A FUNDAMENTAL STUDY OF BOUNDARY LAYER DIFFUSION FLAMES
4 citations
Cites background from "A Computational Study on Opposed Fl..."
...A two-dimensional numerical model was developed by Kumar et al. [75] for investigation of opposed as well as concurrent flame spread over thin solids....
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...Later, Kumar and Kumar [76] investigated the effect of burning onto the side edges of a thin strip of solid on the spread rate....
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References
More filters
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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