A comparison of extinction limits and spreading rates in opposed and concurrent spreading flames over thin solids
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.
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TL;DR: In this paper, the authors introduce fire safety standards for flammability evaluation of solid material intended for use in a spacecraft habitat, and the difference between the limiting value in microgravity and the indices given by the standard test methods on the ground is discussed.
Abstract: This paper introduces fire safety standards for flammability evaluation of solid material intended for use in a spacecraft habitat. Two types of existing standards include material evaluation by pass/fail criteria corresponding to Test 1 of NASA STD 6001B and evaluation by a flammability index such as maximum oxygen concentration (MOC) corresponding to the improved Test 1. The advantage of the latter is the wide applicability of the MOC index to different atmospheres in spacecraft. Additionally, the limiting oxygen index (LOI) method is introduced as a potential alternative index for the evaluation using the improved Test 1 method. When criteria based on an index such as MOC or LOI are applied for material screening, the discrepancy of the index to the actual flammability limit in microgravity such as minimum limiting oxygen concentration (MLOC) is essential information for guaranteeing fire safety in space because material flammability can be higher in microgravity. In this paper, the existing research on the effects of significant parameters on material flammability in microgravity are introduced, and the difference between the limiting value in microgravity and the indices given by the standard test methods on the ground is discussed. Finally, on-going efforts to develop estimation methods of material flammability in microgravity according to normal gravity tests are summarized.
54 citations
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TL;DR: In this article, a fully coupled 2D fluid-solid direct numerical simulation (DNS) approach is used to simulate co-flow flame spread over poly(methyl methacrylate) (PMMA) at different angles of inclination.
Abstract: A fully coupled 2D fluid–solid direct numerical simulation (DNS) approach is used to simulate co-flow flame spread over poly(methyl methacrylate) (PMMA) at different angles of inclination. Comparison of simulations and experimental measurements are conducted over a range of flame spread rates. Results show that the heat flux to the preheating region varies considerably in time — contradicting often employed assumptions used in established flame spread theories. Accounting for the time dependent behavior is essential in accurate predictions of flame spread, however, a universal characterization in terms of easily defined parameters is not found. Alternatively, a reaction progress variable based embedded flame model is developed using mixture fraction, total enthalpy and surface temperature. State maps of the gas-phase properties and surface heat flux are constructed and stored in pre-computed lookup tables. The resulting model provides a computationally efficient and a local formulation to determine the flame heat flux to the surface resulting in excellent agreement to DNS and experiments for predictions of flame spread rate and position of the pyrolysis front.
47 citations
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Glenn Research Center1, Case Western Reserve University2, University of California, Berkeley3, University of Maryland, College Park4, University of Paris5, University of Bremen6, Moscow State University7, Hokkaido University8, European Space Research and Technology Centre9, University of Edinburgh10
TL;DR: In this paper, a large-scale flame spread experiment was conducted inside an orbiting spacecraft to study the effects of microgravity and scale and to address the uncertainty regarding how flames spread when there is no gravity and if the sample size and the experimental duration are, respectively, large enough and long enough to allow for unrestricted growth.
Abstract: For the first time, a large-scale flame spread experiment was conducted inside an orbiting spacecraft to study the effects of microgravity and scale and to address the uncertainty regarding how flames spread when there is no gravity and if the sample size and the experimental duration are, respectively, large enough and long enough to allow for unrestricted growth. Differences between flame spread in purely buoyant and purely forced flows are presented. Prior to these experiments, only samples of small size in small confined volumes had been tested in space. Therefore the first and third flights in the experimental series, called “Saffire,” studied large-scale flame spread over a 94 cm long by 40.6 cm wide cotton-fiberglass fabric. The second flight examined an array of nine smaller samples of various materials each measuring 29 cm long by 5 cm wide. Among them were two of the same cotton-fiberglass fabric used in the large-scale tests and a thick, flat PMMA sample (1-cm thick). The forced airflow was 20–25 cm/s, which is typical of air circulation speeds in a spacecraft. The experiments took place aboard the Cygnus vehicle, a large unmanned resupply spacecraft to the International Space Station (ISS). The experiments were carried out in orbit before the Cygnus vehicle, reloaded with ISS trash, re-entered the Earth's atmosphere and perished. The downloaded test data show that a concurrent (downstream) spreading flame over thin fabrics in microgravity reaches a steady spread rate and a limiting length. The flame over the thick PMMA sample approaches a non-growing, steady state in the 15 min burning duration and has a limiting pyrolysis length. In contrast, upward (concurrent) flame spread at normal gravity on Earth is usually found to be accelerating so that the flame size grows with time. The existence of a flame size limit has important considerations for spacecraft fire safety as it can be used to establish the heat release rate in the vehicle. The findings and the scientific explanations of this series of innovative, novel and unique experiments are presented, analyzed and discussed.
39 citations
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TL;DR: In this article, the heat transfer in horizontal flame spreading over the surface of construction materials was experimentally investigated at two different altitude conditions (the Lhasa plateau 3658m, and the Hefei plain, 50m).
Abstract: The heat transfer in horizontal flame spreading over the surface of construction materials was experimentally investigated at two different altitude conditions (the Lhasa plateau 3658 m, and the Hefei Plain, 50 m). The experiments were carried out with the charring material Whitewood and the thermoplastic material the extruded polystyrene (XPS) over a wide range of sample widths. The temperature profile in solid phase was measured. Different flame spread behaviors were found from the temperature profiles of the XPS and Whitewood. The XPS undergoes the melting stage in the preheated stage while the Whitewood undergoes the char burning stage after the pyrolysis stage. The heat transfer analysis suggests that the gas phase heat transfer is dominant for whitewood and XPS, even though the XPS is thermally thick. The flame spread rate and the surface heat flux with various sample widths were measured. They both dropped firstly and then rose with the increasing sample width. Furthermore, the spread rate was correlated with a pseudo-property Ф, which is the product of the surface heat flux and the preheated length. A good linear relationship was found between the spread rate and Ф, which agrees well with Quintiere’s model. Moreover, the sample width effects on flame spread rate were well explained by the effects on heat transfer. The horizontal flame spread behaviors with sample width were dominated by two different regimes: the spread rate drops for small sample width in convection regime but rises for large sample width in radiation regime.
39 citations
References
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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.
21,638 citations
"A comparison of extinction limits a..." refers methods in this paper
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TL;DR: In this paper, the methode des ordonnees discretes discretees au calcul numerique du transfert radiatif de chaleur dans une enceinte rectangulaire bidimensionnelle remplie d'un milieu gris absorbant, emissif and diffusant isotrope.
Abstract: Utilisation de la methode des Sn ordonnees discretes au calcul numerique du transfert radiatif de chaleur dans une enceinte rectangulaire bidimensionnelle remplie d'un milieu gris absorbant, emissif et diffusant isotrope. Presentation des resultats pour les approximations S 2 , S 4 et S 6 et comparaison aux solutions exactes obtenues par la methode numerique des zones de Hottel
560 citations
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TL;DR: In this paper, a theoretical analysis of diffusion flame extinction in the stagnation point region of a condensed fuel has been made including radiative heat loss from the fuel surface, and the extinction boundary consists of a blowoff and a radiative extinction branch.
Abstract: A theoretical analysis of diffusion flame extinction in the stagnation point region of a condensed fuel has been made including radiative heat loss from the fuel surface. In addition to the blowoff limit normally found when flame stretch rate is large, an extinction limit has been identified when the flame stretch rate becomes sufficiently small. This limit occurs as a result of flame temperature reduction when the rate of radiative loss becomes substantial compared with the rate of combustion heat release. A flammability map using oxygen mass fraction and stretch rate as coordinates shows that the extinction boundary consists of a blowoff and a radiative extinction branch. The merging point of the two branches defines a fundamental low oxygen flammability limit.
176 citations
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TL;DR: In this paper, the steady-state flame spread over a thermally thin solid fuel is investigated, and qualitative agreement is obtained with experimental results in the near-extinction limit region.
Abstract: A theory for the steady-state flame spread over a thermally thin solid fuel is developed in this study. The model considers a laminar diffusion flame in a uniform opposed flow and includes the two-dimensional, elliptic, gas-phase energy, and species equations with one-step overall chemical reaction and second-order, finite-rate Arrhenius kinetics. The unsteady, solid-fuel equations neglect heat conduction ahead of the flame but include transient heating and Arrhenius pyrolysis kinetics and are coupled to the quasisteady gas phase. The equations are solved in the laboratory coordinate system. In this study the two-dimensional distributions of temperature and species are obtained, including the low reactivity zone in the flame region. The solid-fuel surface profiles indicate a region of almost uniform temperature (vaporization temperature) during pyrolysis for some parameter values; however, the value is not universally constant for the fuel and does depend on the ambient parameters (pressure, oxygen mass fraction, and magnitude of opposed velocity). The dependence of the flame-spread rate on the ambient parameters is investigated, and qualitative agreement is obtained with experimental results in the near-extinction-limit region. Quantitative agreement with experimental data depends on the choice of parameter values, especially the gas-phase kinetics model parameters, which are generally unknown. The flame-extinction limits due to increased opposed velocity, reduced pressure, and reduced ambient oxygen mass fraction are all obtained in the results calculated from this theory.
173 citations
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TL;DR: A critical, historical review of the flame spread literature is given in this article, beginning with the first systematic studies of opposed-flow flame spread, including qualitative, simplified, and comprehensive numerical modeling.
Abstract: A critical, historical review of the flame spread literature is given, beginning with the first systematic studies of opposed-flow flame spread. Important modeling effects are described, including qualitative, simplified, μg and comprehensive numerical modeling. A brief discussion of subjects with the potential for further development is also given. Although this review focuses on flame-spread theory the emphasis is on the logical development, not the detailed mathematics.
168 citations
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