Combustion of <em>n</em>-propylbenzene: Experiments and numerical modeling

The burning characteristics of n-propylbenzene (nP) were examined from the perspective of droplet flames burning in the standard atmosphere under conditions that promote a one-dimensional gas transport and spherical symmetry. The parameter was the initial droplet diameter (D0) in the range of 0.6 mm to about 6 mm and the droplet diameters, soot shell diameters and flame diameters were measured during droplet combustion. Experiments were carried out on the International Space Station (ISS) for D0>1 mm while ground based experiments in a drop tower were performed for D0=0.6 mm. Numerical simulations incorporated unsteady liquid and gas phase transport, a model for soot formation, radiation including both nonluminous and luminous components, a detailed nP kinetic mechanism that included formation of soot precursor species and a model for soot formation that incorporated processes of nucleation and aggregation to form macroscopic soot particles and ultimately a soot 'shell'. Results showed simulations that generally agreed with measurements of the evolution of droplet, flame, and soot shell diameters. Burning rates decreased with increasing D0 and were well predicted by the simulations. Flame extinction was observed for the large droplets, but cool flames were not observed under the conditions examined in keeping with the chemical structure of nP. Soot shells corresponded to the location of simulated maximum soot volume fraction. The importance of including a soot formation model in the simulations was demonstrated by simulations where the soot model was removed from the chemical mechanism, in which case the simulations were completely different from the experiments. The evolution of droplet diameter showed a very early extinction process not seen with the full model and with poor agreement of measurements compared to when soot was included.

从在促进一维气体传输和球对称的条件下于标准大气中燃烧的液滴火焰的角度研究了正丙苯（nP）的燃烧特性。参数是初始液滴直径（D0），范围从0.6毫米到约6毫米，并且在液滴燃烧过程中测量了液滴直径、炭黑壳直径和火焰直径。对于D0>1毫米的情况在国际空间站（ISS）上进行了实验，而对于D0 = 0.6毫米的情况在落塔中进行了地面实验。数值模拟包含了非稳态的液相和气相传输、一个炭黑形成模型、包括非发光和发光成分的辐射、一个详细的正丙苯动力学机制（其中包括炭黑前驱体物质的形成）以及一个炭黑形成模型（该模型包含成核和聚集过程以形成宏观炭黑颗粒并最终形成一个炭黑“壳”）。结果表明，模拟结果与液滴、火焰和炭黑壳直径的演变测量结果大体一致。燃烧速率随着D0的增加而降低，并且被模拟很好地预测。对于大液滴观察到了火焰熄灭，但在所研究的条件下，根据正丙苯的化学结构，未观察到冷焰。炭黑壳与模拟的最大炭黑体积分数的位置相对应。通过从化学机制中去除炭黑模型的模拟证明了在模拟中包含炭黑形成模型的重要性，在这种情况下，模拟结果与实验完全不同。液滴直径的演变显示出一个非常早期的熄灭过程，这在完整模型中未出现，并且与包含炭黑时相比，测量结果的一致性较差。