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Journal ArticleDOI

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

01 May 2016-Microgravity Science and Technology (Springer Netherlands)-Vol. 28, Iss: 2, pp 87-94
TL;DR: In this article, the authors investigated flame spread and extinction over a PMMA in purely opposed and concurrent flows by conducting systematical experiments in a narrow channel apparatus and found that the opposed flame is much faster than the concurrent flame at a given flow velocity.
Abstract: 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 low-velocity 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 only. The lowest oxygen concentration allowing concurrent flame spread in forced flow is estimated to be approximately 14 % O-2, substantially below that for opposed spread (18.5 % O-2).
Citations
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Journal ArticleDOI
TL;DR: In this article, the authors revisited the problem of opposed fire spread under limited and excessive oxygen supply and reviewed various near-limit fire phenomena, as recently observed in flaming, smoldering, and glowing spread under various environment and fuel configurations.

40 citations

Journal ArticleDOI
TL;DR: In this article, the transition between two steady states was studied, despite that it often happens to many fire events, and the experimental data on the transitional behavior of wind-assisted flame spread, which helps evaluate the fire hazard under the sudden change of environmental conditions.

18 citations

Journal ArticleDOI
TL;DR: In this article, a microgravity combustion experiment conducted aboard the SJ-10 satellite of China, focusing on the structure and dynamics of diffusion flames spreading over a thick PMMA in low-velocity opposed flows, is reported.

16 citations

Journal ArticleDOI
TL;DR: In this article, the authors used the MSU Narrow Channel Apparatus (MSU-NCA) to investigate opposed flow flame spread over samples of thermally thick polymethylmethacrylate (PMMA).

13 citations

Journal ArticleDOI
TL;DR: In this article, the authors studied the opposed flame spread over polyethylene (PE) in microgravity with varying flow velocity and oxygen concentration, and found that LDPE is more flammable than HDPE.
Abstract: Thermoplastics are melted and often dripped down during the flame spread over them in normal gravity. The flame spread behaviors, therefore, could be quite different from those in microgravity because they involve the dripping. However, no studies have addressed the flame spread over thermoplastics to be dripped in microgravity. This work then studied the opposed flame spread over polyethylene (PE) in microgravity with varying flow velocity and oxygen concentration. Two different PEs, a semi-transparent low-density polyethylene (LDPE) and an opaque high-density polyethylene (HDPE), were tested. Microgravity experiments were conducted in parabolic flights which provided a microgravity environment of 10−2 g for 20 s. Experimental results showed that the limiting oxygen concentration (LOC) of LDPE was 20% and 1% lower than that of HDPE. The flame spread of LDPE was faster than that of HDPE too. These indicate that LDPE is more flammable than HDPE, which well agrees with the literatures on the flame spread over PE-insulated wires. Flame spread rates of both LDPE and HDPE increased with flow velocity and oxygen concentration. The flame length also increased with flow velocity, but the preheating length showed an opposite dependence. The effects of flow velocity and oxygen concentration on flame spread rate, flame length, and preheating length are discussed via a simplified flame-spread model. This study’s findings help ensure fire safety in spacecraft because a flame spreads without melted materials being dripped in a spacecraft environment.

10 citations

References
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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


"Flame Spread and Extinction Over a ..." refers background in this paper

  • ...Flame spread over a thermally-thick solid fuel in oxidizer flow has been studied extensively during the past decades (e.g., de Ris 1969; Fernandez-Pello et al. 1981; Bhattacharjee et al. 1996)....

    [...]

Journal ArticleDOI
01 Jan 1981
TL;DR: In this article, the velocity of flame propagation over the surface of thick PMMA and thin paper sheets has been measured as a function of the velocity and oxygen concentration of a forced gas flow opposing the direction of the flame propagation.
Abstract: The velocity of flame propagation over the surface of thick PMMA and thin paper sheets has been measured as a function of the velocity and oxygen concentration of a forced gas flow opposing the direction of flame propagation. It is shown that although for thin fuels the flame spread rate always decreases as the opposed flow velocity increases, for thick fuels the dependence of the spread rate on the gas velocity is also a function of the ambient oxygen concentration. For low oxygen concentrations the flame spread rate decreases as the velocity of the gas flow increases. For high oxygen concentrations, however, the spread rate increases with the flow velocity, reaches a maximum and then decreases as the gas velocity increases. The velocity of the opposed flow at which the maximum occurs is a function of the oxygen concentration, decreasing as the concentration decreases. Following phenomenological considerations and simplified descriptions of the primary mechanisms occurring during the flame spread process, the experimental results are correlated by two non-dimensional parameters, one describing the gas phase kinetic effects and the other describing the process of heat transfer from the flame to the fuel. Such a correlation provides a powerful means of predicting the flame spread prcess as well as physical insight into the mechanisms controlling the propagation of the flame.

173 citations


"Flame Spread and Extinction Over a ..." refers background in this paper

  • ...The velocity of opposed flow is defined as positive, and concurrent flow as negative For opposed flame spread, attempts have been made to correlate the spread rate with the Damköhler number, and a Damköhler number correlation has been successful for both moderate-velocity flow (thermal) regime and high-velocity (kinetic) regime (Fernandez-Pello et al. 1981; Bhattacharjee et al. 1996)....

    [...]

  • ...Figure 9 plots the flammability boundaries for flames spreading over PMMA under purely opposed and purely concurrent flow conditions, by integrating the results from the present experiments and available published data on opposed flame spread from other groups (Olson et al. 2004; Fernandez-Pello et al. 1981)....

    [...]

  • ...6 for opposed flame, together with the data of Fernandez-Pello et al. (1981) for high flow velocities, are presented versus the Damköhler number of Bhattacharjee et al., DaEST....

    [...]

  • ...…flame spread, attempts have been made to correlate the spread rate with the Damköhler number, and a Damköhler number correlation has been successful for both moderate-velocity flow (thermal) regime and high-velocity (kinetic) regime (Fernandez-Pello et al. 1981; Bhattacharjee et al. 1996)....

    [...]

  • ...…the flammability boundaries for flames spreading over PMMA under purely opposed and purely concurrent flow conditions, by integrating the results from the present experiments and available published data on opposed flame spread from other groups (Olson et al. 2004; Fernandez-Pello et al. 1981)....

    [...]

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

Journal ArticleDOI
TL;DR: In this article, the authors show that buoyancy influences the downward spread rate of flames consuming thermally thin fuel beds, and that a small change in orientation with respect to the vertical is equivalent to a change in the magnitude of gravity in the direction of spread.

131 citations


"Flame Spread and Extinction Over a ..." refers background in this paper

  • ...Note that the effect of the sample width on flame spread rate measurements has been extensively observed for thin fuels (Altenkirch et al. 1980; Shih and T’ien 2003; Zhang and Yu 2011)....

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Journal ArticleDOI
TL;DR: A survey of the characteristics and predictions of the different theoretical models of the spread of flames over the surface of a solid combustible in opposed or concurrent oxidizing flows is presented in this paper.
Abstract: A survey of the characteristics and predictions of the different theoretical models of the spread of flames over the surface of a solid combustible in opposed or concurrent oxidizing flows shows that, at present, there is a good understanding of what are the controlling mechanisms of the flame spread process andl of what is the necessary formulation to develop a rigorous analysis of the phenomenon. It also shows, however, that the problem is very complicated and difficult to solve mathematically particularly if an analytical solution is sought, and that this complexity is what has prevented so far the development of an analysis capable of describing accurately the flame spread process under realistic conditions where material properties, finite rate kinetics, turbulence and radiation effects can determine the characteristics of the process. Although some of the analyses presently available are capable of predicting quantitatively or at least qualitatively rates of flame spread under certain limit...

89 citations