Ignition Transients in a Scramjet Engine with Air Throttling Part 1: Nonreacting Flow

Achieving efficient ignition and stable combustion in a high-speed environment has long been a serious concern in the development of scramjet engines. In the engine startup stage, the low chamber pressure and unsettled fuel-air mixing tend to blow off the flame, even if a flameholding device such as a cavity is employed. The problem may be circumvented by modulating the flow structures in the isolator and combustor through air throttling downstream of the flameholder. In experiments, compressed air is introduced in a controlled manner into the combustor to generate a precombustion shock train in the isolator. The resultant increases in the temperature and pressure of the airstream in the combustor, along with the decrease in the flow velocity, lead to smooth and reliable ignition. The incidentally formed separated flows adjacent to the combustor sidewall improve fuel-air mixing as a result of enhanced flow distortion and increased residence time. Because insufficient reaction heat release often leads to an unstable shock train, and exceedingly large heat release may cause severe flow spillage or even inlet unstart, dynamic optimization of the throttling operation is needed to ensure the creation of flow conditions conducive to efficient ignition. The present work establishes an integrated theoretical/numerical framework, within which the influences of all known effects on the engine ignition transient and flame development are studied systematically. Part 1 of the study focuses on nonreacting flow development and fuel-air mixing under the influence of air throttling.