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Droplet Combustion Experiments Aboard the International Space Station

机译:国际空间站上的液滴燃烧实验

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This paper summarizes the first results from isolated droplet combustion experiments performed on the International Space Station (ISS). The long durations of microgravity provided in the ISS enable the measurement of droplet and flame histories over an unprecedented range of conditions. The first experiments were with heptane and methanol as fuels, initial droplet droplet diameters between 1.5 and 5.0 mm, ambient oxygen mole fractions between 0.1 and 0.4, ambient pressures between 0.7 and 3.0 atm and ambient environments containing oxygen and nitrogen diluted with both carbon dioxide and helium. The experiments show both radiative and diffusive extinction. For both fuels, the flames exhibited pre-extinction flame oscillations during radiative extinction with a frequency of approximately 1 Hz. The results revealed that as the ambient oxygen mole fraction was reduced, the diffusive-extinction droplet diameter increased and the radiative-extinction droplet diameter decreased. In between these two limiting extinction conditions, quasi-steady combustion was observed. Another important measurement that is related to spacecraft fire safety is the limiting oxygen index (LOI), the oxygen concentration below which quasi-steady combustion cannot be supported. This is also the ambient oxygen mole fraction for which the radiative and diffusive extinction diameters become equal. For oxygenitrogen mixtures, the LOI is 0.12 and 0.15 for methanol and heptane, respectively. The LOI increases to approximately 0.14 (0.14 O_2/0.56 N_2/030 CO_2) and 0.17 (0.17 O_2/0.63 N_2/0.20 CO_2) for methanol and heptane, respectively, for ambient environments that simulated dispersing an inert-gas suppressant (carbon dioxide) into a nominally air (1.0 atm) ambient environment. The LOI is approximately 0.14 and 0.15 for methanol and heptane, respectively, when helium is dispersed into air at 1 atm. The experiments also showed unique burning behavior for large heptane droplets. After the visible hot flame radiatively extinguished around a large heptane droplet, the droplet continued to burn with a cool flame. This phenomena was observed repeat-ably over a wide range of ambient conditions. These cool flames were invisible to the experiment imaging system but their behavior was inferred by the sustained quasi-steady burning after visible flame extinction. Verification of this new burning regime was established by both theoretical and numerical analysis of the experimental results. These innovative experiments have provided a wealth of new data for improving the understanding of droplet combustion and related aspects of fire safety, as well as offering important measurements that can be used to test sophisticated evolving computational models and theories of droplet combustion.
机译:本文总结了在国际空间站(ISS)上进行的孤立液滴燃烧实验的第一个结果。 ISS中提供的长微重力持续时间使得能够在前所未有的条件范围内测量液滴和火焰历史。最初的实验是以庚烷和甲醇为燃料,初始液滴的液滴直径在1.5到5.0 mm之间,环境氧气的摩尔分数在0.1到0.4之间,环境压力在0.7到3.0 atm之间,以及含有氧气和氮气的二氧化碳和二氧化碳稀释的周围环境。氦。实验显示了辐射消散和扩散消光。对于这两种燃料,火焰在辐射消光过程中均以约1 Hz的频率显示出消光前的火焰振荡。结果表明,随着环境氧摩尔分数的降低,扩散消光液滴直径增大,辐射消光液滴直径减小。在这两个极限消光条件之间,观察到准稳态燃烧。与航天器着火安全有关的另一个重要测量指标是极限氧指数(LOI),低于该极限氧指数就无法支持准稳态燃烧。这也是环境的氧摩尔分数,辐射消散直径和扩散消散直径相等。对于氧气/氮气混合物,甲醇和庚烷的LOI分别为0.12和0.15。对于模拟分散惰性气体抑制剂(二氧化碳)的周围环境,甲醇和庚烷的LOI分别增加到大约0.14(0.14 O_2 / 0.56 N_2 / 030 CO_2)和0.17(0.17 O_2 / 0.63 N_2 / 0.20 CO_2)。进入标称空气(1.0 atm)的周围环境。当氦气在1个大气压下分散到空气中时,甲醇和庚烷的LOI分别约为0.14和0.15。实验还显示了大庚烷液滴的独特燃烧行为。在可见的热火焰围绕大庚烷小滴辐射熄灭后,小滴继续在冷火焰中燃烧。在广泛的环境条件下可重复观察到这种现象。这些冷火焰对于实验成像系统而言是不可见的,但是它们的行为是由可见火焰熄灭后持续的准稳态燃烧来推断的。通过实验结果的理论和数值分析,建立了这种新燃烧方式的验证。这些创新的实验提供了丰富的新数据,可增进对液滴燃烧和消防安全相关方面的理解,并提供可用于测试复杂的不断发展的计算模型和液滴燃烧理论的重要测量值。

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