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The Effect of Microgravity on Flame Spread over a Thin Fuel

01 Dec 1987-
TL;DR: In this article, a flame spreading over a thermally thin cellulose fuel was studied in a quiescent microgravity environment, and two different extinction limits were found in microgravity for the two thicknesses of fuel.
Abstract: A flame spreading over a thermally thin cellulose fuel was studied in a quiescent microgravity environment. Flame spread over two different fuel thicknesses was studied in ambient oxygen-nitrogen environments from the limiting oxygen concentration to 100 percent oxygen at 1 atm pressure. Comparative normal-gravity tests were also conducted. Gravity was found to play an important role in the mechanism of flame spread. In lower oxygen environments, the buoyant flow induced in normal gravity was found to accelerate the flame spread rate as compared to the microgravity flame spread rates. It was also found to stabilize the flame in oxidizer environments, where microgravity flames in a quiescent environment extinguish. In oxygen-rich environments, however, it was determined that gravity does not play an important role in the flame spread mechanism. Fuel thickness influences the flame spread rate in both normal gravity and microgravity. The flame spread rate varies inversely with fuel thickness in both normal gravity and in an oxygen-rich microgravity environment. In lower oxygen microgravity environments, however, the inverse relationship breaks down because finite-rate kinetics and heat losses become important. Two different extinction limits were found in microgravity for the two thicknesses of fuel. This is in contrast to the normal-gravity extinction limit, which was found to be independent of fuel thickness. In microgravity the flame is quenched because of excessive thermal losses, whereas in normal gravity the flame is extinguished by blowoff.
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Journal ArticleDOI
TL;DR: In this paper, a flame spread model is formulated in three dimensions to simulate opposed flow flame spread over thin solid fuels, coupled with a three-dimensional gas radiation model to assess the role of radiation and effect of dimensionality on the prediction of the flame spread phenomena.
Abstract: In this work a flame-spread model is formulated in three dimensions to simulate opposed flow flame spread over thin solid fuels. The flame-spread model is coupled to a three-dimensional gas radiation model. The experiments [1] on downward spread and zero gravity quiescent spread over finite width thin fuel are simulated by flame-spread models in both two and three dimensions to assess the role of radiation and effect of dimensionality on the prediction of the flame-spread phenomena. It is observed that while radiation plays only a minor role in normal gravity downward spread, in zero gravity quiescent spread surface radiation loss holds the key to correct prediction of low oxygen flame spread rate and quenching limit. The present three-dimensional simulations show that even in zero gravity gas radiation affects flame spread rate only moderately (as much as 20% at 100% oxygen) as the heat feedback effect exceeds the radiation loss effect only moderately. However, the two-dimensional model with the gas radi...

35 citations


Cites background or methods or result from "The Effect of Microgravity on Flame..."

  • ...Gravity No gas radiation S2 S4 S6 S8 Experimental Values [1]...

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  • ...The need for a three-dimensional model was found to be indispensable for consistently describing the zero gravity flame-spread experiments [1] (including flame spread rate and flame size) especially at high oxygen levels (>30%)....

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  • ...Geometry and computational domain Olson [1] carried out experiments on thermally thin fuel specimens of dimension 3 cm × 15 cm....

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  • ...The computed values are compared with simulations without gas radiation model experimental values [1]....

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  • ...be noted that the experiments [1] were carried out with fuel specimen of width 3 cm....

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Journal ArticleDOI
TL;DR: In this paper, microgravity tests of flammability and flame spread were performed in a low-speed flow tunnel to simulate spacecraft ventilation flows, and the results showed that the 1g Upward Limiting Oxygen Index (ULOI) and 1g Maximum Oxygen Concentration (MOC) were greater than those in 0g, by up to 4% oxygen mole fraction.
Abstract: Microgravity tests of flammability and flame spread were performed in a low-speed flow tunnel to simulate spacecraft ventilation flows. Three thin fuels were tested for flammability (Ultem 1000 (General Electric Company), 10 mil film, Nomex (Dupont) HT90-40, and Mylar G (Dupont) and one fuel for flame spread testing (Kimwipes (Kimberly-Clark Worldwide, Inc.). The 1g Upward Limiting Oxygen Index (ULOI) and 1g Maximum Oxygen Concentration (MOC) are found to be greater than those in 0g, by up to 4% oxygen mole fraction, meaning that the fuels burned in 0g at lower oxygen concentrations than they did using the NASA Standard 6001 Test 1 protocol. Flame spread tests with Kimwipes were used to develop correlations that capture the effects of flow velocity, oxygen concentration, and pressure on flame spread rate. These correlations were used to determine that over virtually the entire range of spacecraft atmospheres and flow conditions, the opposed spread is faster, especially for normoxic atmospheres. The correlations were also compared with 1g MOC for various materials as a function of pressure and oxygen. The lines of constant opposed flow agreed best with the 1g MOC trends, which indicates that Test 1 limits are essentially dictated by the critical heat flux for ignition. Further evaluation of these and other materials is continuing to better understand the 0g flammability of materials and its effect on the oxygen margin of safety.

29 citations

Journal ArticleDOI
TL;DR: In this paper, a theoretical analysis of the downward flame spread over the thermally-thin solid fuel vertically in the gravitational field is presented, and the effect of Damkohler number (Da) as a function of gravity level (g) is investigated.
Abstract: A theoretical analysis is developed to study the downward flame spread over the thermally-thin solid fuel vertically in the gravitational field. The combustion model is basically similar to that of Chen (1990). The effect of Damkohler number (Da), as a function of gravity level (g), is under investigation. Beyond the blowoff limit, the flame spread rate is found to be proportional to (g)−1/3. A combination of the isotherm and velocity vector distributions in gas phase and the solid fuel pyrolysis are presented together to illustrate flame structures at normal gravity. Subject to the same Da, the flame is weaker than that in forced convection. Within the elevated gravity domain of experiment, the computed flame spread rate agree very well with the measurements obtained by Altenkirch el at. (1980).

24 citations

Journal ArticleDOI
01 Jan 2013
TL;DR: In this article, the effects of gravity on pool fire characteristics were investigated using a drop tower at Hirosaki University in Japan to obtain an arbitrary low gravity environment and particle track laser-sheet technique (PTLS) combined with a high-speed camera.
Abstract: The flame characteristics of pool fires such as oscillatory frequency, flame height, and so on differ depending on gravity. To improve our understanding of the effects of gravity on flame characteristics, we conducted an experimental investigation on small-scale pool fires under different gravity levels to normal. The drop tower facility at Hirosaki University in Japan was used to obtain an arbitrary low gravity environment. Our measurements include analyses of oscillatory frequency and flame height under low gravity environments and flow visualization by a particle-track laser-sheet technique (PTLS) combined with a high-speed camera. These measurements revealed that the puffing frequency and flame height declined with decreasing gravity. The puffing frequency was summarized by the relationship between the Strouhal and Froude numbers. The flame height showed a tendency to peak at a certain level of gravity. The behavior of the flame height under low gravity did not correlate to the scaling prediction previously presented, while the normal velocity component to the flame sheet declined with decreasing gravity. The chemical reaction in the flame may be restricted due to decreasing oxygen supply. Consequently, the convective heat feedback to the liquid surface falls, causing the flame height to decline with decreasing gravity.

22 citations

Journal ArticleDOI
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.

21 citations


Cites background or result from "The Effect of Microgravity on Flame..."

  • ...Experimental study on flame spread over thin solid fuels is generally carried over a finite-width fuel strip with the edges inhibited (Hirano and Saito, 1994; Frey and Tien, 1976; Olson 1987b)....

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  • ...In most experiments in opposed=downward flame spread (Frey and Tien, 1976; Hirano and Saito, 1994; Olson, 1987a) over thin fuel, the edges of sample are inhibited either by chemical treatment or by using a metal support strip....

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  • ...This is consistent with the previous studies (Olson, 1987b)....

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