Characteristics of Combined-Cycle Inlet During Mode Transition in Off-Design State

Abstract

Mode transition is essential for the combined-cycle engine inlet. During actual inlet operation, mode transition may occur in the off-design state. An experiment was performed with a generic over/under combined-cycle engine inlet mode to investigate the characteristics of the inlet mode transition in the off-design state. The transition Mach number was set to Mach 3.5 by design, while it was 2.9 during the experiment. Oscillatory flow is obtained during both down- and up-rotation processes of the splitter. Results show that the size and location of the separation bubble vary with the splitter position. The dominant frequency of the flowfield oscillation is always below 200 Hz. Combining the analysis of schlieren and dynamic pressure data, it is found that the main factor affecting the flowfield oscillation features is the self-excited oscillation of the shock train, while the acoustic resonance frequency is also reflected near the unstart point. Comparison of the pressure curves of the two processes shows that the hysteresis interval accounts for 17.8% of the entire rotation time of the splitter and that the hysteresis stems from the presence of the separation bubble on the splitter during the up-rotation of the splitter. The analysis of the unstart and restart points shows that they deviate from the isentropic and Kantrowitz limits, respectively. The new unstart and restart boundaries should consider the combined effect of the separation bubble at the duct entrance, the internal wave system, and the throat Mach number.