Research on Combustion Fluctuation and Compound Choking in SCRAM Jet Engines

对于适用于高超声速运输机和空天飞机的超燃冲压发动机（SCRAM）来说，最重要的问题是热阻塞和复合阻塞现象。在集成紧急停堆发动机的情况下，沿着飞行器机身形成的附面层气流与主流一起被吸入发动机。边界层测流管和主流测流管具有相同的总温，但总压和马赫数不同。当它们在发动机中共存时，它们的行为与发动机中横截面积的变化和每个流管中的热量增加相当奇怪。在超声速条件下，由于边界层流动的存在，主流管容易被阻塞，其中两个流管之间保持静压平衡。本文的目的是研究紧急停堆发动机中阻塞现象的细节，找出紧急停堆发动机稳定工作所必需的控制规律，以便在紧急停堆发动机中实现阻塞，将一个前截面为30mm×32.6mm的发动机模块置于截面为100mm×100mm的超声速风洞中，以温度不稳定的空气为驱动力2000K（最大值），马赫数2.0。通过向发动机模块内喷射氢燃料实现了自燃。从测量的发动机壁面上的压力信号，它被认为是实现超声速燃烧时，氢气的喷射压力低。但是当喷射压力增加时，发动机内的压力增加，表明亚音速燃烧。在这种情况下，观察到的高压区域在发动机中从下游到上游依次延伸，并最终落入不起动状态。

The most important problems in supersonic combustion ram jet engines (SCRAM) which are suitable for hypersonic transports and space planes are the phenomena such as thermal choking and compound choking. In the case of integrated SCRAM engines, the boundary layer flow which is developed along the vehicle fuselage is sucked into engines with the main flow. The flow tube of the boundary layer and that of the main flow have the same total temperature but different total pressures and Mach numbers. When they coexist in the engine they behave quite queerly against the cross-sectional area change in the engine and heat addition in each flow tube. The main flow tube is easily choked at supersonic conditions because of the existence of the boundary layer flow, where the static pressure balance is kept between two flow tubes. This phenomenon is called compound choking.The purpose of the present research is to investigate the details of choking phenomena in SCRAM engines and to find out the control rule which is necessary for stable operation of SCRAM engines.In order to realize choking in SCRAM engines, an engine module of 30mm×32.6mm frontal cross section was put in the supersonic wind tunnel of 100mm×100mm cross section driven by the vitiated air of temperature 2000K (maximum) at Mach number 2.0. Self-ignition was realized by injecting hydrogen fuel into the engine module. From the pressure signals measured on the engine wall, it was recognized that supersonic combustion was realized while the injection pressure of hydrogen was low. But when the injection pressure was increased the pressure inside the engine increased indicating subsonic combustion. In such circumstances the region of high pressure was observed to extend in sequence from downstream to upstream in the engine and it finally fell into the unstart condition.