On Burning Rate and Flame Extinction for Toluene Droplets in Low Gravity

Toluene (C6H5CH3, boiling point of 384 K) is a monocyclic aromatic compound found in transportation fuels, which plays an important role in soot formation during combustion. To understand its combustion and sooting dynamics, an attractive experimental platform is the spherically symmetric droplet flame. In this work, droplet combustion experiments carried out on the International Space Station are reported to study near spherically symmetric burning of untethered or free-floating toluene droplets at atmospheric pressure over a wide range of initial droplet diameters (D0 ∼ 1 to 7 mm). Spherically symmetric burning conditions and one-dimensional gas transport provide a useful environment to reveal the burning characteristics of this complex fuel, including the influence of D0 on radiative transport, fuel burning rate, and flame extinction. Results showed that droplets burned to completion without experiencing radiative extinction for D0 D0, which is consistent with trends previously reported for other hydrocarbon fuels. The instantaneous droplet burning rate (K) was initially steady as combustion developed and then decreased with time, accompanied by a significant reduction in flame radiance when the flame radiatively extinguished. A scaling analysis by an energy balance at the droplet interface revealed that the estimated flame temperature dropped sharply from approximately 1400 to 560 K shortly after radiative extinction and then gradually decreased toward ambient levels. Finally, an averaged K (Kavg) was obtained for each D0 in the visible flame regime and correlated by a power law relation Kavg ∼ D0–n.

甲苯（Toluene，分子式C₆H₅CH₃，沸点384 K）是一种存在于运输燃料中的单环芳香族化合物，在燃烧过程中对碳烟生成具有关键作用。为阐明其燃烧与碳烟生成动力学特性，球对称液滴火焰是极具研究价值的实验平台。本研究报道了在国际空间站开展的液滴燃烧实验，用以研究常压下、初始液滴直径D₀约1~7 mm宽范围工况内，无系留或自由漂浮甲苯液滴的近球对称燃烧行为。
球对称燃烧工况与一维气体输运环境，为揭示该复杂燃料的燃烧特性提供了理想条件，涵盖初始液滴直径D₀对辐射输运、燃料燃烧速率及火焰熄火的影响规律。
实验结果表明，在部分初始液滴直径工况下，甲苯液滴可完全燃烧且未发生辐射熄火，该规律与此前其他烃类燃料的报道趋势一致。瞬时液滴燃烧速率（K）在燃烧发展初期保持稳定，随后随时间逐渐降低；当发生辐射熄火时，火焰辐射强度会显著下降。
通过液滴界面处的能量平衡开展尺度分析可知，辐射熄火后不久，估算得到的火焰温度从约1400 K骤降至560 K，随后逐渐趋近于环境温度水平。
最终，针对可见火焰阶段的各初始液滴直径D₀，得到了平均燃烧速率（Kavg），并通过幂律关系Kavg ∝ D₀^(-n)对其进行了关联拟合。