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Effectiveness of Xenon as a Fire Suppressant Under Microgravity Combustion Environment

机译：氙在微重力燃烧环境下作为灭火剂的功效

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The 'FLame EXtinguishment' (FLEX) program conducted by NASA on board the International Space Station (ISS) has been assisting in developing fire-safety protocols for low gravity applications through microgravity droplet combustion experiments. A wide range of fuels, including alcohols and alkanes, have been studied in different ambient conditions that also encompass the use of various diluent species and concentrations. A prime focus of the work has been to observe the relative effectiveness of atmospheric composition and pressure changes on fire suppression under 'reduced' gravity conditions. Here, detailed numerical simulations are performed to investigate the combustion and extinction characteristics of isolated sphero-symmetric 1.0-2.0-mm diameter methanol droplets burning in xenon (Xe)-enriched environments. Comparisons of diluent behaviors under identical conditions using argon (Ar), carbon dioxide (CO2), and helium (He) as the alternative diluent to nitrogen are also reported. The predictions are compared against ISS experiments with good agreement and with less satisfactory agreement with the results published earlier by Shaw and Wei (2012). Xenon as diluent rather than nitrogen results in reduced burning rate, larger extinction diameter and counter intuitively, and prolonged burning time. The limiting oxygen index (LOI) for xenon is found to decrease significantly from that found with argon, carbon dioxide, or helium. The numerical analyses indicate that the lower thermal diffusivity of xenon is the principal factor responsible for the remarkably lower LOI. Water accumulation within the methanol droplet and its relevance to the extinction process is also discussed. It is concluded that the combined observation of elevated peak gas temperature, its slow decay due to minimal diffusive heat loss, and the exceptionally lower LOI value associated with xenon as a diluent all detract from its utility for suppressing fire concerns in reduced gravity applications.
机译：美国宇航局在国际空间站（ISS）上实施的“火焰熄灭”（FLEX）计划一直在通过微重力液滴燃烧实验协助开发低重力应用的防火协议。在不同的环境条件下研究了多种燃料，包括醇和烷烃，其中还包括各种稀释剂种类和浓度的使用。这项工作的主要重点是观察“减少”重力条件下大气成分和压力变化对灭火的相对有效性。在这里，进行了详细的数值模拟，以研究在富氙（Xe）的环境中燃烧的直径为1.0-2.0mm的球对称对称甲醇液滴的燃烧和消光特性。还报道了在相同条件下使用氩气（Ar），二氧化碳（CO2）和氦气（He）作为氮气的替代稀释剂的稀释剂行为的比较。将这些预测与国际空间站实验进行比较，结果吻合得很好，而与肖和韦（2012）早些时候发表的结果却不太令人满意。氙作为稀释剂而不是氮气会导致燃烧速度降低，消光直径变大，直觉相反，燃烧时间延长。发现氙的极限氧指数（LOI）较氩，二氧化碳或氦气的极限氧指数显着降低。数值分析表明，较低的氙热扩散率是导致LOI显着降低的主要原因。还讨论了甲醇滴中的水积聚及其与消光过程的关系。结论是，结合观察到的峰值气体温度升高，由于最小的扩散热损失而导致的缓慢衰减以及与氙作为稀释剂相关的异常低的LOI值，均不利于其在降低重力应用中抑制火灾的效用。

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