Transient, isolated n-alkane droplet combustion is simulated at elevated pressure for helium-diluent substituted-air mixtures. We report the presence of unique quasi-steady, three-stage burning behavior of large sphero-symmetric n-alkane droplets at these elevated pressures and helium substituted ambient fractions. Upon initiation of reaction, hot-flame diffusive burning of large droplets is initiated that radiatively extinguishes to establish cool flame burning conditions in nitrogen/oxygen “air” at atmospheric and elevated pressures. However, at elevated pressure and moderate helium substitution for nitrogen (X_He > 20%), the initiated cool flame burning proceeds through two distinct, quasi-steady-state, cool flame burning conditions. The classical “Hot flame” (∼1500 K) radiatively extinguishes into a “Warm flame” burning mode at a moderate maximum reaction zone temperature (∼ 970 K), followed by a transition to a lower temperature (∼765 K), quasi-steady “Cool flame” burning condition. The reaction zone (“flame”) temperatures are associated with distinctly different yields in intermediate reaction products within the reaction zones and surrounding near-field, and the flame-standoff ratios characterizing each burning mode progressively decrease. The presence of all three stages first appears with helium substitution near 20%, and the duration of each stage is observed to be strongly dependent on helium substitutions level between 20–60%. For helium substitution greater than 60%, the hot flame extinction is followed by only the lower temperature cool flame burning mode. In addition to the strong coupling between the diffusive loss of both energy and species and the slowly evolving degenerate branching in the low and negative temperature coefficient (NTC) kinetic regimes, the competition between the low-temperature chain branching and intermediate-temperature chain termination reactions control the “Warm” and “Cool” flame quasi-steady conditions and transitioning dynamics. Experiments onboard the International Space Station with n-dodecane droplets confirm the existence of these combustion characteristics and predictions agree favorably with these observations.

对于氦稀释剂替代空气混合物，在升高的压力下模拟了瞬态孤立的正构烷烃液滴燃烧。我们报告了在这些升高的压力和氦气置换的环境分数下，大型球对称正构烷烃液滴存在独特的拟稳定，三阶段燃烧行为。反应开始后，会开始大液滴的热火焰扩散燃烧，这种燃烧会通过辐射熄灭，在大气压和高压下在氮气/氧气“空气”中建立凉爽的火焰燃烧条件。但是，在高压和中等程度的氦气替代氮气（X _He > 20％），则在两个不同的准稳态冷火焰燃烧条件下，开始进行的冷火焰燃烧。经典的“热火焰”（约1500 K）在中等最高反应区温度（约970 K）下辐射熄灭为“暖焰”燃烧模式，然后过渡到较低温度（约765 K），近似于稳定的“凉爽的火焰燃烧状况。反应区（“火焰”）温度与反应区和周围的近场内的中间反应产物的收率明显不同有关，并且表征每种燃烧模式的火焰隔离率逐渐降低。这三个阶段的出现首先出现在氦替代率接近20％的情况下，并且观察到每个阶段的持续时间都强烈依赖于氦替代率在20％至60％之间。对于大于60％的氦气置换，仅在较低温度的冷火焰燃烧模式之后才熄灭热火焰。除了能量和物质的扩散损失与低和负温度系数（NTC）动力学方案中缓慢发展的简并分支之间的强耦合之外，“暖”和“冷”火焰准稳态条件和过渡动力学。国际空间站上正十二烷液滴的实验证实了这些燃烧特性的存在，并且预测与这些观察结果吻合良好。