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

以金属为燃料的二氧化碳吸气式涡轮喷气发动机具有在火星和金星大气中工作的潜力。用于此目的的几种可能的金属是Mg、Li、Al和B，因为它们与CO_2的反应热高。特别地，具有较高火焰温度的Mg和Al是比Li和B更有吸引力的燃料。通过热力循环分析估算发动机性能，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值。对金属在纯CO_2气流中的点火和燃烧进行了基础性的实验研究。发现Li、Mg和Al在水蒸气中能自燃。Li和Mg的自燃温度远低于Al的自燃温度，Li的自燃过程分为两个明显的步骤，首先是表面反应，然后是气相反应。点火后，Li首先在气相中燃烧，然后在金属表面上燃烧。另一方面，在流中点燃时，Mg表面覆盖有防止进一步氧化的保护膜。当Mg的初始温度高于临界温度时，在早期反应阶段形成的膜变得不保护，然后发生气相反应，导致着火。点火后，Mg始终以气相燃烧。Al的点火过程与Mg的点火过程相似。不过，到目前为止，Al的过程细节还不清楚。在这四种金属中，只有B在CO_2气流中不能点燃，但反应在表面上发生，并且熔融的氧化硼出现在其表面上。

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.