Ram-Scram Transition and Flame/Shock-Train Interactions in a Model Scramjet Experiment

The behavior of a ram-scram transition was examined using a direct-connect model scramjet experiment along with pressure measurements and high-speed laser interferometry. The work quantifies the sudden change in the wall static pressure profile and flame position that occurs as the downstream boundary condition abruptly changes when the flow becomes unchoked. Transition was studied in three ways; as a quasi-steady phenomenon due to slow decreases in fuel flow rate, or as caused by rapid variations in either fuel flow rate or test-section wall temperature. A regime diagram was constructed that plots the measured ram-scram transition boundary. Under certain conditions some periodic low-frequency oscillations of the flame position occur and they are shown to be correlated with oscillations of the upstream precombustion pseudoshock. A self-sustaining shear-layer instability, associated with the flameholding cavity, is believed to be the mechanism causing this behavior. The relevant time scales associated with the ram-scram transition and the flame-shock interactions are discussed.