Inherently unsteady flame spread to extinction over thick fuels in microgravity

Altenkirch, Robert A.; Tang, Lin; Sacksteder, Kurt R.; Bhattacharjee, Subrata; Delichatsios, Michael

doi:10.1016/s0082-0784(98)80103-3

Cited by 39 publications

(21 citation statements)

References 8 publications

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“…For this reason, a number of theoretical and experimental studies [1][2][3][4] have investigated flame spread over thermally thin cellulosic materials with external flows in a microgravity environment. Many researchers [1][2][3][4] have observed the flame spread behavior in a two-dimensional configuration with a line-shape flame across the sample width and compared this with calculations based on two-dimensional models. Because of the inherent 2-D nature of the models , severa] important physical phenomena could not be captured.…”

Section: Introductionmentioning

confidence: 99%

Effect of wind velocity on flame spread in microgravity

Prasad¹,

Nakamura²,

Olson³

et al. 2002

Proceedings of the Combustion Institute

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A three-dimensional, time-dependent model is developed describing ignition and subsequent transition to flame spread over a thermally thin cellulosic sheet heated by external radiation in a microgravity environment. A low Mach number approximation to the Navier Stokes equations with global reaction rate equations describing combustion in the gas phase and the condensed phase is numerically solved. The effects of a slow external wind (1-20 cm/s) on flame transition are studied in an atmosphere of 35% oxygen concentration. The ignition is initiated at the center part of the sample by generating a line-shape flame along the width of the sample. The calculated results are compared with data obtained in the lOs drop tower. Numerical results exhibit flame quenching at a wind speed of 1.0 cm/s, two localized flames propagating upstream along the sample edges at 1.5 cm/s, a single line-shape flame front at 5.0 cm/s, three flames structure observed at 10.0 cm/s (consisting of a single line-shape flame propagating upstream and two localized flames propagating downstream along sample edges) and followed by two line-shape flames (one propagating upstream and another propagati ng downstream) at 20.0 cm/s. These observations qualitatively compare with experimental data. Three-dimensional visualization of the observed flame complex, fuel concentration contours, oxygen and reaction rate isosurfaces, convective and diffusive mass flu x are used to obtain a detailed understanding of the controlling mechanism. Physical arguments based on lateral diffusive flux of oxygen, fuel depletion, oxygen shado w of the flame and heat release rate are constructed to explain the various observed flame shapes .2

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Section: Introductionmentioning

confidence: 99%

Effect of wind velocity on flame spread in microgravity

Prasad¹,

Nakamura²,

Olson³

et al. 2002

Proceedings of the Combustion Institute

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“…The sample was assumed to be thermally thin, and radiative heat losses from the surface were included. However, radiative heat transfer from flame to the sample was not significant for thermally thin samples [7], and also, the color of edge flames shown in Fig. 1 was blue.…”

Section: Theoretical Modelmentioning

confidence: 90%

“…For this reason, many flame spread experiments over combustible solid surfaces have been conducted in microgravity environments: for example, over a thermally thin cellulosic sample with external flows [1,2]; at various ambient pressures in a quiescent environment [3,4]; in a three-dimensional spread pattern from a localized spot ignition [5]; and over a thermally thick polymethylmethacrylate (PMMA) sample in quiescent, high oxygen concentration environments [6,7]. All these experiments were conducted over the center part of the sample to avoid the effects of the sample edges as much as possible.…”

Section: Introductionmentioning

confidence: 99%

Flame spread along free edges of thermally thin samples in microgravity

Mell

Olson

Kashiwagi

2000

Proceedings of the Combustion Institute

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The effects of imposed flow velocity on flame spread along open edges of a thermally thin cellulosic sample in microgravity were studied experimentally and theoretically. In this study, the sample was ignited locally at the middle of the 4 cm wide sample, and subsequent flame spread reached both open edges of the sample along the direction of the flow. The following flame behaviors were observed in the experiments and predicted by the numerical calculation, in order of increased imposed flow velocity: (1) ignition but subsequent flame spread was not attained, (2) flame spread upstream (opposed mode) without any downstream flame, and (3) the upstream flame and two separate downstream flames traveled along the two open edges (concurrent mode). Generally, the upstream and downstream edge flame spread rates were faster than the central flame spread rate for an imposed flow velocity of up to 5 cm/s. This was due to greater oxygen supply from the outer free stream to the edge flames and more efficient heat transfer from the edge flames to the sample surface than the central flames. For the upstream edge flame, flame spread rate was nearly independent of, or decreased gradually with, the imposed flow velocity. The spread rate of the downstream edge, however, increased significantly with the imposed flow velocity.

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“…However, the limiting flow velocity for steady concurrent flame spread to occur over a thin fuel is finite and does not go to zero. On the contrary, the microgravity experiments on thermally-thick fuels (PMMA plates) in a quiescent, 50 % or 70 % O 2 , 1 atm environment produced unsteady flames that extinguished by itself eventually Altenkirch et al 1998). Furthermore, the only microgravity experiment (Olson et al 2004) where the opposed-flow flame spread over thermally-thick fuels in moving atmospheres was examined showed that, although the flame extinguished at 35% O 2 for a flow velocity of 1 cm/s, steady flame spread rates were achieved at 50 % and 70 % O 2 for the same flow velocity.…”

Section: Introductionmentioning

confidence: 99%

Flame Spread and Extinction Over a Thick Solid Fuel in Low-Velocity Opposed and Concurrent Flows

Zhu

Wang

2015

Microgravity Sci. Technol.

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Flame spread and extinction phenomena over a thick PMMA in purely opposed and concurrent flows are investigated by conducting systematical experiments in a narrow channel apparatus. The present tests focus on lowvelocity flow regime and hence complement experimental data previously reported for high and moderate velocity regimes. In the flow velocity range tested, the opposed flame is found to spread much faster than the concurrent flame at a given flow velocity. The measured spread rates for opposed and concurrent flames can be correlated by corresponding theoretical models of flame spread, indicating that existing models capture the main mechanisms controlling the flame spread. In low-velocity gas flows, however, the experimental results are observed to deviate from theoretical predictions. This may be attributed to the neglect of radiative heat loss in the theoretical models, whereas radiation becomes important for low-intensity flame spread. Flammability limits using oxygen concentration and flow velocity as coordinates are presented for both opposed and concurrent flame spread configurations. It is found that concurrent spread has a wider flammable range than opposed case. Beyond the flammability boundary of opposed spread, there is an additional flammable area for concurrent spread, where the spreading flame is sustainable in concurrent mode

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Inherently unsteady flame spread to extinction over thick fuels in microgravity

Cited by 39 publications

References 8 publications

Effect of wind velocity on flame spread in microgravity

Effect of wind velocity on flame spread in microgravity

Flame spread along free edges of thermally thin samples in microgravity

Flame Spread and Extinction Over a Thick Solid Fuel in Low-Velocity Opposed and Concurrent Flows

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