A comprehensive study of ignition transient in an ethylene-fueled scramjet combustor

Ignition and flame stability have long been a serious concern in the development of scramjet engines due to the difficulties of achieving efficient ignition and steady combustion in a high-speed environment. The situation becomes more challenging during the engine start-up stage during which the low chamber pressure and unsettled fuel/air mixing tend to blow off the flame, even when a flame holding device such as a cavity is employed. To circumvent this difficulty, one of the ignition aids is to modulate the flow structures in the isolator and combustor in order to reduce the local flow velocity and increase the pressure by means of air throttling downstream of the flame holder. The purpose was to establish a proper shock train in the isolator to facilitate ignition and flame stabilization. In experiments, compressed air was introduced in a controlled manner into the combustor to generate a pre-combustion shock train in the isolator. The resultant increases in the temperature and pressure of the air stream in the combustor, along with the decrease in the flow velocity, lead to smooth and reliable ignition. The shock train also gives rise to low-momentum regions and separated flows adjacent to the combustor side-walls, in which the fuel/air mixing process is considerably more efficient due to the shock-induced flow distortion and larger residence time. In general, air throttling is activated once steady fuel injection has been achieved and an ignition source, such as a spark plug, has been turned on. Air throttling is then terminated immediately after the flame is stabilized, in order to minimize the amount of throttling gas. A proper shock system in the isolator for sustaining combustion will be maintained if the subsequent heat release in the combustor is sufficiently high. Insufficient heat release often leads to an unstable shock train, and a premature removal of air throttling may result in flame blowout. It should also be noted that the shock train interacts with the inlet flowfield. Significant flow spillage or even inlet unstart may occur if the combustor is over pressurized. A dynamic optimization of the shock train is needed.