Scramjetto ramjet transition in a dual-mode combustor with fin-guided injection

A dual-mode scramjet combustor featuring a fin-guided hydrogen injector and a cavity flame holder was operated in a transitional regime characterized by frequent combustion mode hopping and large flame front movements. In this regime, which occurred at equivalence ratios between 0.19 and 0.24, the combustor operated in scramjet mode initially, but transitioned into ramjet mode with time. The experiments were performed using a direct-connect vitiated-air facility which generated a high-enthalpy Mach 1.9 flow at the isolator inlet. High-speed schlieren visualizations, OH* chemiluminescence, and wall pressure measurements were used to study the alternating combustion behavior that occurs throughout the scramjet to ramjet mode transition. The results showed that the transition process was characterized by a series of flame oscillations driven by mode hopping cycles, consisting of transient thermal choking, flame flashback, cavity flame holding and detachment. These mode oscillations were initially sporadic and short-lived events of upstream-downstream flame propagation, but as the transition advanced and the presence of the flame in the cavity increased, they transformed into longer and more stable cycles. The characteristic duration of the entire transition process was determined to range between 0.42-0.61 s using a statistical analysis based on image processing of the schlieren visualizations. Increasing the overall equivalence ratio decreased the transition duration, but had no significant effect on the dynamics of this process. The results suggest that thermal choking due to wall heating downstream of the cavity drives the scramjet to ramjet mode hopping, while flame quenching and blow-off at the cavity caused by its initially cold walls reinitializes scramjet operation. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.