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21. Flame spread: Effects of microgravity and scale

Author

Urban, David L., Ferkul, Paul, Olson, Sandra, Ruff, Gary A., Easton, John, T'ien, James S., Liao, Ya-Ting T., Li, Chengyao, Fernandez-Pello, Carlos, Torero, Jose L., Legros, Guillaume, Eigenbrod, Christian, Smirnov, Nickolay, Fujita, Osamu, Rouvreau, Sébastien, Toth, Balazs, and Jomaas, Grunde

Published
2019
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34. Fundamentals of Premixed Flames

Author

Jomaas, Grunde, Hurley, Morgan J., editor, Gottuk, Daniel, editor, Hall, John R., Jr., editor, Harada, Kazunori, editor, Kuligowski, Erica, editor, Puchovsky, Milosh, editor, Torero, José, editor, Watts, John M., Jr., editor, and Wieczorek, Christopher, editor

Published
2016
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44. Results of Large-Scale Spacecraft Flammability Tests

Author

Ferkul, Paul, Olson, Sandra, Urban, David L, Ruff, Gary A, Easton, John, T'ien, James S, Liao, Ta-Ting T, Fernandez-Pello, A. Carlos, Torero, Jose L, Eigenbrand, Christian, Legros, Guillaume, Smirnov, Nickolay, Fujita, Osamu, Rouvreau, Sebatien, Toth, Balazs, and Jomaas, Grunde

Subjects
Space Transportation And Safety, Engineering (General)
Abstract

For the first time, a large-scale fire was intentionally set inside a spacecraft while in orbit. Testing in low gravity aboard spacecraft had been limited to samples of modest size: for thin fuels the longest samples burned were around 15 cm in length and thick fuel samples have been even smaller. This is despite the fact that fire is a catastrophic hazard for spaceflight and the spread and growth of a fire, combined with its interactions with the vehicle cannot be expected to scale linearly. While every type of occupied structure on earth has been the subject of full scale fire testing, this had never been attempted in space owing to the complexity, cost, risk and absence of a safe location. Thus, there is a gap in knowledge of fire behavior in spacecraft. The recent utilization of large, unmanned, resupply craft has provided the needed capability: a habitable but unoccupied spacecraft in low earth orbit. One such vehicle was used to study the flame spread over a 94 x 40.6 cm thin charring solid (fiberglasscotton fabric). The sample was an order of magnitude larger than anything studied to date in microgravity and was of sufficient scale that it consumed 1.5 of the available oxygen. The experiment which is called Saffire consisted of two tests, forward or concurrent flame spread (with the direction of flow) and opposed flame spread (against the direction of flow). The average forced air speed was 20 cms. For the concurrent flame spread test, the flame size remained constrained after the ignition transient, which is not the case in 1-g. These results were qualitatively different from those on earth where an upward-spreading flame on a sample of this size accelerates and grows. In addition, a curious effect of the chamber size is noted. Compared to previous microgravity work in smaller tunnels, the flame in the larger tunnel spread more slowly, even for a wider sample. This is attributed to the effect of flow acceleration in the smaller tunnels as a result of hot gas expansion. These results clearly demonstrate the unique features of purely forced flow in microgravity on flame spread, the dependence of flame behavior on the scale of the experiment, and the importance of full-scale testing for spacecraft fire safety.

Published
2017

46. Flammability Behaviour of Polydimethylsiloxane (PDMS) Membranes Under Normoxic Conditions for Spacecraft Applications

Author

Rojas-Alva, Ulises, Møller-Poulsen, Frederik, and Jomaas, Grunde

Abstract

Two types of polydimethylsiloxane PDMS membranes with two thicknesses, 0.125 mm and 0.25 mm, were studied experimentally for a series of environmental conditions in a closed chamber. The conditions were changing oxygen concentration, ambient pressure and normoxic conditions. The results for piloted ignition are linearly dependent on oxygen concentration and ambient pressure via chemistry. A non-monotonic dependency was found in normoxic conditions, but the dominant mechanisms are not clear. The flame spread has a linear dependence on oxygen concentration via chemistry and thermal transfer. Two dependency regimes (linear and asymptotic) emerge for flame spread as a function of ambient pressure. The mechanisms are due to thermal transfer (linear regime) and a combination of thermal transfer and kinetics (asymptotic regime). Flame lengths are exponentially dependent on oxygen concentration via oxygen/fuel supply and soot oxidation. On the contrary, ambient pressure affects flame length non-monotonically via fuel supply (increasing regime) and an excess of oxidiser (decreasing regime). Under normoxic conditions, increasing oxygen concentration had a stronger influence than decreasing pressure with respect to both flame spread and flame length. The extinction conditions at low pressure are due to a combination of increased radiative losses (similar to those of microgravity conditions) and kinetic effects. Furthermore, extinction at low pressures occurred well below normoxic or hypoxic conditions. Thus, the two PDMS materials tested are more flammable under normoxic conditions, which needs to be considered when assessing the fire risk associated with spacecraft design and operation. It remains unclear how these results directly translate to microgravity, where other phenomena might dominate, so future experiments in microgravity are recommended.

Published
2021

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