Highly Reliable Plasma Igniter for Ram/Scramjet

冲压/超燃冲压发动机是航天飞机推进系统中最有前途的候选发动机之一，为了实现冲压/超燃冲压发动机，超声速气流中燃料的可靠点火是一个重要课题。等离子体火炬已经在直连式燃烧器以及小型发动机中进行了开发和测试。然而，这些测试的早期结果表明，等离子点火器的耐久性和可靠性不足。本课题旨在通过实验、理论和数值方法，明确等离子体点火器故障的原因，研制高可靠性的等离子体点火器。火炬的原料由氧改为氮。这种变化显著地抑制了电极和喷枪喷嘴的损耗。此外，火炬的冷却剂由气体改为水。这一变化减少了火炬的热负荷。这些变化大大提高了等离子炬的耐用性。实验和理论研究证实，氮气火炬的点火性能与氧气火炬一样好。采用氮气/氢气混合物作为原料，提高了火炬的最小输入功率，提高了点火性能。点火性能的提高主要是由于氢在等离子体射流中的燃烧。实验和数值模拟结果表明，在等离子体炬上游注入燃料时，喷射器横向和横向位置的选择对燃料射流与等离子体射流的碰撞非常重要。流向位置和注入角度对其影响较小。当燃料被注入等离子炬的下游时，流向位置也变得很重要。气流马赫数的增加使火炬的点火性能变差。在本实验中，气流压力的作用并不是直接的。

In order to realize the ramjet/scramjet engine which is one of the most promising candidates of the propulsion system of aerospace planes, reliable ignition of fuel in supersonic airstream is an important subject. Plasma torches have been developed and tested in direct-connect combustors as well as in a sub-scale engine. However, early results of these tests showed insuffficient durability and reliability of the plasma igniter. The present research project has been carried out to clarify the cause of malfunction of the igniter and to develop highly reliable plasma igniter with experimental, theoretical and numerical approaches.The feedstock of the torch was changed from oxygen to nitrogen. Detrition of the electrodes and the nozzle of the torch was significantly suppressed by this change. In addition, the coolant of the torch was changed from gas to water. This change lessened thermal load of the torch. Durability of the plasma torch was greatly improved by these changes.Experimental and theoretical studies confirmed that the ignition performance of the nitrogen torch was as good as that of the oxygen torch. Improvement of the ignition perfomance was achieved by use of nitrogen/hydrogen mixture as the feedstock, though the minimum input power for the torch was increased. The improvement of ignition perfomance was caused by combustion of hydrogen in the plasma jet.When fuel was injected upstream the plasma torch, experiment and numerical simulation showed that it is important to select the lateral and transverse locations of the injector to make collision of the fuel jet and the plasma jet. The streamwise location and the angle of injection showed minor influence. When the fuel was injected downstream the plasma torch, the streamwise location became important, too.Increase of airstream Mach number made ignition performance of the torch worse. The effect of the pressue of the airstream was not straight forward in the present experiment.