Research on Combustion Fluctuation and Compound Choking in SCRAM Jet Engines

适用于高超声速运输机和空天飞机的超声速燃烧冲压式喷气发动机（SCRAM）最重要的问题是热堵塞和复合堵塞现象。在整体式SCRAM发动机中，沿机身方向发展的边界层流随主流被吸入发动机。边界层流管与主流流管的总温度相同，但总压和马赫数不同。当它们在发动机中共存时，它们对发动机的横截面积变化和每个流管内的热量增加表现得非常奇怪。在超声速条件下，由于边界层流动的存在，使两流管之间的静压保持平衡，导致主流管容易堵塞。这种现象被称为复合窒息。本研究的目的是研究短时间记忆存储器发动机中呛气现象的细节，并找出短时间记忆存储器发动机稳定运行所必需的控制规则。为了实现SCRAM发动机的节流，在马赫数2.0、温度最高为2000K的污迹空气驱动下，将一个30mm×32.6mm前横截面的发动机模块置于100mm×100mm横截面的超音速风洞中。通过向发动机模块注入氢燃料实现了自燃。从发动机壁面测得的压力信号可以看出，在低氢喷射压力下实现了超声速燃烧。但当喷射压力增加时，发动机内部压力增加，表明亚音速燃烧。在这种情况下，观察到高压区域在发动机内依次由下游向上游延伸，最终进入不启动状态。

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.