Supercritical Liquid Fuel Combustion Mechanism

针对火箭发动机和高速柴油机的设计需要，提出了一种数值模拟方案，揭示了单个燃料液滴在超过燃料临界点的各种环境气体状态下浸泡在静止气体中的非定常蒸发和燃烧过程。本研究是在作者之前关于超临界液滴燃烧的理论工作的基础上，发现在混合物临界条件下，只有当相关的二元扩散系数在液气界面处消失时，才会发生从亚到超临界蒸发状态的过渡，提出了满足这一条件的扩散系数表达式，并在分析的基础上揭示了渐近过渡特征。液滴在其寿命期间经历转变的条件是根据环境气体温度和压力确定的。获得的其他重要知识如下。与实验结果一致，相变时液滴表面温度没有出现奇异行为。以高燃料浓度为特征的堆芯，即使在液滴失去液气界面后，仍保持了液滴的类液体性质。由于连续的相变发生在薄层中，通常的光学观测得到的图像就好像有表面的液滴仍然存在一样。一旦核心即将消失，浓度和中心温度就会突然变化，呈现气态。在高压下靠近液滴的火焰存在的情况下，过渡实现得快得多。过渡时期从汽化控制燃烧到扩散控制燃烧的变化，伴随着火焰温度的升高。

A numerical simulation scheme was developed to disclose the unsteady evaporation and combustion processes of a single fuel droplet immersed in an otherwise quiescent gas at a various ambient gas state beyond the critical point of the fuel, which is necessary for the design of rocket motors and high speed Diesel engines. This study was carried out, based on the author's previous theoretical work on supercritical droplet combustion wherein it was found that the transition from sub- to supercritical evaporation regime can take place only if the binary diffusion coefficient relevant vanishes at the liquid gas interface in a mixture critical condition, an expression for the diffusion coefficient which satisfies this condition was proposed and the asymptotic transition feature was revealed on an analytical basis.The condition under which the droplet experiences the transition during its lifetime was identified in terms of ambient gas temperature and pressure. Other important knowledge obtained is as follows. Consistent with experimental results no singular behavior is observed in the droplet surface temperature at the transition. The core characterized by high fuel concentration preserves the liquid-like property even after the droplet has lost its liquid-gas interface. Since continuous phase change takes place in a thin layer, usual optical observation will give images as if there still exists the droplet with surface. Once the core is about to disappear, the concentration as well as the central temperature changes abruptly to assume the gaseous state. The transition is achieved far more quickly in the presence of flame which is located closer to the droplet at higher pressure. The transition epochs the change from vaporization- to diffusion-controlled combustion, accompanying an increase in flame temperature.