Ignition and Combustion of Metals as Fuel of Breathing Propulsion in CO_2 Planet Atmospheres

一种以金属为燃料的吸入二氧化碳涡喷发动机有可能用于在火星和金星大气中工作的飞机发动机，这种发动机由CO_2组成。几种可能的金属是镁、锂、铝和B，因为它们与CO_2的反应热很高。特别是火焰温度较高的镁和铝，是比锂和B更有吸引力的燃料。根据热力学循环分析对发动机性能进行评估，评估了发动机的可行性。分析表明，虽然发动机消耗了大量的金属燃料，但它提供了足够的推力。对金属在撞击纯CO2气流中的着火和燃烧进行了基础实验研究。结果表明，锂、镁、铝在水蒸气中有自燃现象。Li和Mg的自燃温度远低于Al。Li的着火过程分为两个不同的步骤，首先是表面反应，然后是气相反应。点火后，锂首先在气相中燃烧，然后在金属表面燃烧。另一方面，在蒸汽中点火时，镁表面覆盖了一层保护膜，以防止进一步氧化。当镁的初始温度高于某一临界温度时，在反应初期形成的膜不再具有保护性，然后发生气相反应，导致着火。点火后，镁始终以汽相燃烧。铝的点火过程与镁的点火过程相似。然而，到目前为止，Al进程的细节还不清楚。在这四种金属中，只有B在CO_2气流中不能点火，但在表面发生了轻微的反应，表面出现了熔融的BO_2。

A carbon dioxide breathing turbojet engine using metals as fuel has potential for an airplane engine which can work in Mars and Venus atmospheres, consisting of CO_2. Several possible metals for this purpose are Mg, Li, Al and B, because of their high heats of reaction with CO_2. In particular, Mg and Al which have higher flame temperatures, are more attractive fuels than Li and B. The feasibility of the engine was assessed by estimating the engine performance according to thermodynamic cycle analysis. The analysis indicates that although the engine consumes a large amount of metal fuel, it gives the sufficient value of thrust.A fundamental study was performed experimentally on the ignition and combustion of the metals in an impinging pure C0_2 gas stream. It was found that Li, Mg and Al could ignite spontaneously in t-he stream. The spontaneous ignition temperatures of Li and Mg were much lower than that of Al. The ignition process of Li had two distinct steps, first a surface reaction and then a gas phase reaction. After,ignition, Li burned in vapor-phase at first and then on the metal surface. On the other hand, at the ignition in the stream, the Mg surface was covered with a protective film against further oxidation. When initial temperatures of Mg were higher than a Critical temperature, the film formed in early reaction stages became non-protective and then gas phase reactions occurred, leading to ignition. After ignition, Mg burned in vapor-phase throughout. The ignition process of Al was found to be similar to that of Mg. However, details of the process of Al have not been made clear so far. Among these four metals, only B could not ignite in the C0_2 stream, but the reactions occurred appreciably on the surface, and the molten boron oxide appeared on its surface.