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Low-temperature multistage warm diffusion flames

机译：低温多级温扩散火焰

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We report on experimental evidence of the existence of a new self-sustaining low-temperature multistage warm diffusion flame, existing between the cool flame and hot flame, at atmospheric pressure in the counterflow geometry. The structure of multistage warm diffusion flames was examined by using thermometry, laser-induced fluorescence, and chemiluminescence measurements. It was found that the warm diffusion flame has a two-staged double flame structure, with a leading diffusion cool flame stage on the fuel side and a second intermediate stage on the oxidizer side, with strong heat release in the second stage that can be comparable to that of the first stage. The results demonstrate that the spatially-distinct multistage character is due to the low-temperature fuel reactivity that allows for the production of reactive intermediates in a leading cool flame. These intermediates are then oxidized, on the oxidizer side, in a second stage via intermediate-temperature chemistry. In the case of dibutyl ether, the low-temperature peroxy branching pathway supports the first cool flame oxidation stage and produces intermediates such as alkyl and carbonyl radicals. The alkyl and carbonyl radicals then react with the hydroperoxyl radical and molecular oxygen to form the second oxidation stage. A detailed analysis revealed that ozone addition in the oxidizer promotes the second stage oxidation by increasing both the radical pool population and the flame temperature, but does not fundamentally change the multistage flame structure. Furthermore, the analysis revealed that with the increase of fuel concentration, a single-stage cool flame can ignite to a warm flame or a hot flame. Moreover, a warm flame can extinguish into a cool flame or ignite to a hot flame when the fuel concentration is substantially reduced or increased, respectively. Finally, under certain conditions, a hot flame can extinguish directly into either a warm flame or a cool flame. Hence, the results suggest that the multistage warm flame can act as a critical bridge between cool flames and hot flames and that it is a fundamental burning mode characteristic of low-temperature non-premixed combustion. The multistage warm diffusion flame is particularly relevant to combustion in highly turbulent flow fields and in microgravity environments, owing to the possibility of long residence times.
机译：我们报告的实验证据表明，在大气压下，在逆流几何形状中，存在一种新的自持式低温多级温扩散火焰，存在于冷火焰和热火焰之间。通过使用测温法，激光诱导的荧光和化学发光测量来检查多级暖扩散火焰的结构。已经发现，热扩散火焰具有两级双火焰结构，在燃料侧具有领先的扩散冷火焰级，在氧化剂侧具有第二中间级，在第二级具有强大的热释放能力，可与之媲美。到第一阶段。结果表明，空间上不同的多级特征是由于低温燃料反应性，该反应性允许在领先的冷火焰中产生反应性中间体。然后，这些中间体在第二阶段通过中间温度化学反应在氧化剂侧被氧化。在二丁醚的情况下，低温过氧支化途径支持第一冷火焰氧化阶段并产生中间体，例如烷基和羰基。然后，烷基和羰基与氢过氧基和分子氧反应形成第二氧化阶段。详细的分析表明，在氧化剂中添加臭氧可通过增加自由基库数量和火焰温度来促进第二阶段的氧化，但不会从根本上改变多级火焰的结构。此外，分析表明，随着燃料浓度的增加，单级冷火焰可点燃成热火焰或热火焰。此外，当燃料浓度显着降低或升高时，温暖的火焰可熄灭为冷火焰或点燃为热火焰。最后，在某些条件下，热火焰可直接熄灭为热火焰或冷火焰。因此，结果表明，多级温暖火焰可以充当冷火焰和热火焰之间的关键桥梁，并且它是低温非预混燃烧的基本燃烧模式特征。由于存在长停留时间的可能性，多级暖扩散火焰尤其与高度湍流场和微重力环境中的燃烧有关。

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