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Journal ArticleDOI

A numerical analysis of ignition to steady downward flame spread over a thin solid fuel

01 May 2003-Combustion Science and Technology (Taylor and Francis Ltd.)-Vol. 175, Iss: 5, pp 933-964
TL;DR: In this article, a numerical analysis using an unsteady combustion model is presented to study the ignition and subsequent downward flame spread over a thermally thin solid fuel in a gravitational field.
Abstract: A numerical analysis using an unsteady combustion model is presented to study the ignition and subsequent downward flame spread over a thermally thin solid fuel in a gravitational field. The solid-fuel temperature rises gradually in the heat-up stage and the pyrolysis becomes more intense. Ignition, including the induction period and thermal runaway, occurs as soon as a flammable mixture is formed and the gas-phase temperature, heated by the solid fuel, becomes high enough. During the induction period, the reactivity and temperature in the gas phase are mutually supportive. The thermal runaway consists of a burning premixed flame as the flow moves with the flame front. This is followed by a transition from a premixed flame into a diffusion flame. The flame front extends along and toward the upstream virgin fuel as the diffusion flame is formed. Finally, steady flame spread takes place as burnout appears. The ignition delay time is found to be controlled mainly by the time required to form the flammable mi...
Citations
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Journal ArticleDOI
TL;DR: In this paper, the amplitude dependence of the flame response to inlet velocity forcing is investigated using turbulent combustion CFD and a modified laminar flamelet approach with an algebraic representation of flame surface density.

121 citations

Journal ArticleDOI
TL;DR: In this paper, a comparison between the downward flame spread rate for thermally thin samples with one or two inhibited edges is done in multiple situations, and the effects of atmospheric composition as well as the width and thickness of a cellulosic-type fuel are tested experimentally.
Abstract: A comparison between the downward flame spread rate for thermally thin samples with one or two inhibited edges is done in multiple situations. The effects of atmospheric composition as well as the width and thickness of a cellulosic-type fuel are tested experimentally. We have found that the normal velocity to the inclined flame front in a side-edge burning is very similar to the downward flame front speed when the sample is inhibited by both edges. Also, the effect of locating a sidewall close to the free edge of the sample is investigated. All these results may be important in order to validate or refute possible models of downward flame spread that take into account side effects.

27 citations


Cites result from "A numerical analysis of ignition to..."

  • ...In comparison with other values found in the literature, our ks coincides within the error band with the value used by di Blasi (1994), although other authors employ thermal conductivities for cellulose samples that are 20% higher (see e.g., Chen, 1990; di Blasi et al., 1988; Frey and T’Ien, 1979; Wu and Chen, 2003)....

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  • ...…other values found in the literature, our ks coincides within the error band with the value used by di Blasi (1994), although other authors employ thermal conductivities for cellulose samples that are 20% higher (see e.g., Chen, 1990; di Blasi et al., 1988; Frey and T’Ien, 1979; Wu and Chen, 2003)....

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Proceedings ArticleDOI
08 Jan 2007
TL;DR: In this paper, a 2D CFD-based computer modeling of opposed flow flame spread over thick samples of polymethylmethacrylate (PMMA) is presented.
Abstract: This paper reports 2D CFD-based computer modeling of opposed flow flame spread over thick samples of polymethylmethacrylate (PMMA). Model predictions are compared with experimental data from normal-gravity experiments at multiple forced flow velocities and KC-135 parabolic flight microgravity experiments. For the normal gravity experiments, good agreement between the model predictions and experimental data is obtained at one oxygen level, but flame spread rates at other oxygen levels are not well predicted. Of the four microgravity data points, the model underpredicts the spread rate of two of the data points by 35% or less. However, the model overpredicts the other two data points by almost a factor of two. Potential reasons for the discrepancies between the model predictions and the experimental data are discussed.

10 citations

Journal ArticleDOI
01 Jan 2021-Fuel
TL;DR: In this paper, the authors investigated the relationship between the flame spread and the gap length between fuel beds at different opposed-flow velocities, and the analytical and numerical results showed that the critical gap size is proportional to the reciprocal of opposed flow.

10 citations

Journal ArticleDOI
S.L. Olson1
01 Jan 2011
TL;DR: In this article, the authors studied the ignition delay of thermally-thin cellulose as a function of forced flow, oxygen concentration, and pressure in a low-speed flow tunnel mounted on a NASA Zero Gravity Research Facility drop vehicle.
Abstract: Piloted ignition delay of thermally-thin cellulose as a function of forced flow, oxygen concentration, and pressure was studied in a low-speed flow tunnel mounted on a NASA Zero Gravity Research Facility drop vehicle. Fuel heat-up times are very short and invariant due to the rapid heating of the pilot igniter and thermally-thin fuel used in this study. The effect of flow was found to be negligible for the test conditions studied, where mixing times were very rapid. Ignition delay in these tests was found to be dominated by the induction time, which was found to be inversely proportional to the oxygen concentration and the total pressure, or equivalently, the oxygen partial pressure. The reaction rate (the inverse of the induction time) is thus linear with the oxygen partial pressure, in agreement with kinetic theory for simple thermal reactions.

10 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 numerical analysis of ignition to..." refers methods in this paper

  • ...The numerical scheme adopts the SIMPLE algorithm (Patankar, 1980)....

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Journal ArticleDOI
01 Jan 1969
TL;DR: In this article, a theoretical description of a laminar diffusion flame spreading against an air stream over a solid- or liquid-fuel bed is presented, where both a thin sheet and a semi-infinite fuel bed are considered.
Abstract: A theoretical description is presented for a laminar diffusion flame spreading against an air stream over a solid- or liquid-fuel bed. Both a thin sheet and a semi-infinite fuel bed are considered. The burning process is described as follows: The hot flame heats the unburned fuel bed, which subsequently vaporizes. The resulting fuel vapor reacts with the oxygen supplied by the incoming air, thereby producing the heat that maintains the flame-spread process. The formulated model treats the combustion as a diffusion flame, for which the details of the reaction kinetics can be ignored by assuming infinite reaction rates. The model includes the chemical stoichiometry, heat of combustion, gas-phase conductive heat transfer, radiation, mass transfer, fuel vaporization, and fuel-bed thermal properties. The radiation is mathematically treated as a heat loss at the flame sheet and a heat gain at the fuel-bed surface. The calculated flame-spread formulas are not inconsistent with available experimental data. These results reveal much of the physics involved in a spreading, flame. For instance, the flame-spread rate is strongly influenced by (1) the adiabatic stoichiometric flame temperature, and (2) the fuel-bed thermal properties, except for the fuel-bed conductivity parallel to the propagation direction.

356 citations


"A numerical analysis of ignition to..." refers background or result in this paper

  • ...This trend coincides with the thermal theories of de Ris (1969) and experiments by Lastrina et al. (1971) and Suzuki et al. (1994)....

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  • ...Note that the exponent,70.81, on the thickness correlation is not the same as 71, which was predicted by de Ris (1969) and Delichatsios (1986)....

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  • ...Figure 9 reports the flame-spread rates, as a function of the fuel halfthickness, predicted by this numerical model, the thermal theory of de Ris (1969), and the work of Delichatsios (1986), respectively....

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  • ...On the other hand, de Ris’ theory, (de Ris, 1969) predicted that flame-spread rate is independent of induced flow for thermally thin fuel....

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Journal ArticleDOI
TL;DR: In this paper, heat transfer and gas phase chemical kinetic aspects of the flame spread process are addressed separately for the spread of flames in oxidizing flows that oppose or concur with the direction of propagation.
Abstract: Recent advances in the experimental study of the mechanisms controlling the spread of flames over the surface of combustible solids are summarized in this work. The heat transfer and gas phase chemical kinetic aspects of the flame spread process are addressed separately for the spread of flames in oxidizing flows that oppose or concur with the direction of propagation. The realization that, in most practical situations, the spread of fire in opposed gas flows occurs at near extinction or non-propagating conditions is particularly significant. Under these circumstances, gas phase chemical kinetics plays a critical role and it must be considered if realistic descriptions of the flame spread process are attempted. In the concurrent mode of flame spread, heat transfer from the flame to the unburnt fuel appears to be the primary controlling mechanism. Although gas phase chemcial kinetics is unimportant in the flame spreading process, it is important in the establishment and extension of the diffusion ...

266 citations


"A numerical analysis of ignition to..." refers methods in this paper

  • ...Several experiments and models, such as those by Fernandez-Pello and Hirano (1983), Chen and Hou (1991), Olson (1987, 1991), and Sacksteder and T’ien (1994), indicated that the ambient oxygen concentration will alter flame behavior....

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Journal ArticleDOI
TL;DR: In this paper, the authors derived the time to achieve the ignition temperature as a function of the incident heat flux and the various thermophysical material parameters for thermally thick, thermally thin and thermally intermediate solid combustibles.
Abstract: Formulas are derived for the time to achieve the ignition temperature as a function of the incident heat flux and the various thermophysical material parameters for thermally thick, thermally thin and thermally intermediate solid combustibles. Predictions are compared with recent experimental data for various natural wood species and wood products, and to previous data for wood and thermoplastics. The correlations are excellent when (1) the physical parameters used as the axes of the plots are chosen consistent with those of the theoretical formulas and (2) the experiments and the materials do not violate any of the restrictions imposed by the theory. From these plots it is easy to estimate the minimum heat flux for ignition, which is of great importance both in practice and for making theoretical predictions.

162 citations


"A numerical analysis of ignition to..." refers background in this paper

  • ...Mikkola and Wichman (1989) developed a theoretical model to predict the ignition delay time using an energy balance....

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  • ...We define the gas-phase ignition as occurring as soon as its dimensional reaction rate ð _ oFÞ reaches 10(74) g=cm(3) s, according to Nakabe et al. (1994) and Ferkul and T’ien (1994)....

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Journal ArticleDOI
TL;DR: In this article, a flame spread map is presented which indicates three distinct regions where different mechanisms control the flame spread process: near-quenching region, very low characteristic relative velocities, a new controlling mechanism for flame spread - oxidizer transport-limited chemical reaction - is proposed.
Abstract: Microgravity tests varying oxygen concentration and forced flow velocity have examined the importance of transport processes on flame spread over very thin solid fuels. Flame spread rates, solid phase temperature profiles and flame appearance for these tests are measured. A flame spread map is presented which indicates three distinct regions where different mechanisms control the flame spread process. In the near-quenching region (very low characteristic relative velocities) a new controlling mechanism for flame spread - oxidizer transport-limited chemical reaction - is proposed. In the near-limit, blowoff region, high opposed flow velocities impose residence time limitations on the flame spread process. A critical characteristic relative velocity line between the two near-limit regions defines conditions which result in maximum flammability both in terms of a peak flame spread rate and minimum oxygen concentration for steady burning. In the third region, away from both near-limit regions, the flame spread behavior, which can accurately be described by a thermal theory, is controlled by gas-phase conduction.

142 citations


"A numerical analysis of ignition to..." refers methods in this paper

  • ...Several experiments and models, such as those by Fernandez-Pello and Hirano (1983), Chen and Hou (1991), Olson (1987, 1991), and Sacksteder and T’ien (1994), indicated that the ambient oxygen concentration will alter flame behavior....

    [...]