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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 article, the authors investigated the heat feedback amount of a small pool flame experimentally under normal to partial gravity conditions; using the drop tower at Hirosaki University in Japan to obtain arbitrary partial gravity condition, which varied from 1 G to 0.55 G.
Abstract: The height of the pool fire depends on the amount of heat feedback from the flame to the fuel. In order to predict flame height in a partial gravity environment, we investigated the heat feedback amount of a small pool flame experimentally under normal to partial gravity conditions; using the drop tower at Hirosaki University in Japan to obtain arbitrary partial gravity condition, which varied from 1 G to 0.55 G. We performed the measurement of the flame shape with a digital camera. Based on the experiment result, we expected the amount of fuel vapor from the amount of heat feedback of the pool flame calculated and to establish the prediction formula of the flame height in the partial gravity environment.

1 citations

Book ChapterDOI
01 Jan 2015
TL;DR: In this paper, the effects of gravity on flame spread along a thermally thin combustible solid for different sample orientations (−20° downward, horizontal, and +20° upward) was experimentally investigated by changing the ambient oxygen concentration and gravity level.
Abstract: In order to secure fire safety over the entire period of a manned space mission, gaining a systematic understanding of the effects of gravity on flame spread is important. In this study, opposed-flow flame spread along a thermally thin combustible solid for different sample orientations (−20° downward, horizontal, and +20° upward) was experimentally investigated by changing the ambient oxygen concentration and gravity level. The flame spread rate decreases with decreasing oxygen concentration under normal gravity, and its rate at 18 % oxygen concentration is equivalent to that at 21 % oxygen concentration under microgravity. The downward flame spread rate decreases with an increase in gravity. In contrast, the horizontal and the +20° upward flame spread rates clearly increase as the gravity level increases. The flame spread rate varies remarkably with sample orientation in a supergravity environment. To clarify the effect of gravity on flame spread, the relation between the non-dimensional flame spread rate and the Rayleigh number was examined. The Ra number both for horizontal and upward flame spread increases with increasing gravity, while the Ra number for downward flame spread decreases slightly with a decrease in gravity. The non-dimensional flame spread rate is almost constant under normal and supergravity conditions for Ra numbers less than 103 and is equivalent to that under microgravity. When the Ra number is greater than 103, the non-dimensional flame spread rate increases with increasing Ra number and is proportional to Ra1/3.
Journal ArticleDOI
TL;DR: In this paper, a radiative absorption model was proposed to estimate radiative heat transfer of small-scale pool fires under conditions of normal to partial gravity; using the drop tower at Hirosaki University in Japan to obtain arbitrary partial gravity condition, which varied from 1 G to 0.55 G.
Abstract: The flame characteristics of pool fires such as their height vary depending on gravity. To improve our understanding of the effects of gravity on flame characteristics, we experimentally investigated small-scale pool fires under conditions of normal to partial gravity; using the drop tower at Hirosaki University in Japan to obtain arbitrary partial gravity condition, which varied from 1 G to 0.55 G. We performed the measurement of the temperature distribution with a thermocouple and that of the flame shape with a digital camera. Based on these data, we estimated radiative heat feedback using our new model “The radiative absorption model”. It becomes easy to estimate radiative heat transfer using this model if flames have complicated shapes and time variability. From these analyses, we made clear that the radiative heat feedback of small-scale pool fires decreases under partial gravity environment.
Book ChapterDOI
01 Jan 2017
TL;DR: In this article, a model of flame spread over an array of fuel sheets of finite width size has been modeled and numerically investigated for opposed, low convective flows in microgravity, and steady flame spread rates were observed for all separation distances up to the separation distance of flame extinction.
Abstract: Flame spread over an array of fuel sheets of finite width size has been modeled and numerically investigated for opposed, low convective flows in microgravity. As opposed to the previous studies based on 2D models, steady flame spread rates were observed for all separation distances up to the separation distance of flame extinction. The flame spread rate increased with decrease in separation distance up to a point where it was maximum, further reduction in separation distance, reduced the flame spread rate. The flammability map as a function of separation distance was also obtained for different fuel widths. While the extinction map qualitatively matches with the flammability map obtained from the 2D model, the flame extinguished at higher oxygen levels with the decrease in fuel width due to radiation heat losses.
30 Aug 2013
TL;DR: A recent survey of results, the available set of reduced gravity facilities, and plans for experimental capabilities in the Space Station era can be found in this paper, where the authors introduce the promise of microgravity combustion research by way of a brief survey.
Abstract: The promise of microgravity combustion research is introduced by way of a brief survey of results, the available set of reduced gravity facilities, and plans for experimental capabilities in the Space Station era. The study of fundamental combustion processes in a microgravity environment is a relatively new scientific endeavor. A few simple, precursor experiments were conducted in the early 1970's. Today the advent of the U.S. space shuttle and the anticipation of the Space Station Freedom provide for scientists and engineers a special opportunity, in the form of long duration microgravity laboratories, and need, in the form of spacecraft fire safety and a variety of terrestrial applications, to pursue fresh insight into the basic physics of combustion. The microgravity environment enables a new range of experiments to be performed since buoyancy-induced flows are nearly eliminated, normally obscured forces and flows may be isolated, gravitational settling or sedimentation is nearly eliminated, and larger time or length scales in experiments become permissible. The range of experiments completed to date was not broad, but is growing. Unexpected phenomena have been observed often in microgravity combustion experiments, raising questions about the degree of accuracy and completion of our classical understanding and our ability to estimate spacecraft fire hazards. Because of the field's relative immaturity, instrumentation has been restricted primarily to high-speed photography. To better explain these findings, more sophisticated diagnostic instrumentation, similar to that evolving in terrestrial laboratories, is being developed for use on Space Station Freedom and, along the way, in existing microgravity facilities.