Inlet buzz phenomenon driven by flow choking in high-enthalpy Mach 4.5 flows using circular scramjet models

Abstract

Inlet buzz driven by flow choking in circular dual-mode scramjets is experimentally investigated in high-enthalpy Mach 4.5 flows. Circular scramjet models with two different diverging combustors are used to investigate the correlation between the buzz frequency and flow choking driven by thermal and non-thermal loading. Flow and combustion dynamics are characterized by wall pressure histories, high-speed flame chemiluminescence, and flow luminosity to quantify and visualize the unsteady shockwave and flame propagations driven by fluid-combustion interactions. The circular geometry shows distinct physics from fluid/combustion dynamics due to the absence of corner boundary-layer effects that can distort flame propagation in rectangular geometries. It is observed that the 2° combustor can be thermally choked inside the diverging section while the 5° combustor can only be choked by additional non-thermal loading from its excessive area expansion. Correspondingly, different cyclic oscillations develop from the interactions between fluid and combustion during an unstart event due to the different characteristics of the choking. The 2° combustor is temporarily unchoked during a buzz cycle and the scramjet has a lower fundamental buzz frequency from the extended time to establish flow choking in the diverging section. In contrast, the 5° combustor is always choked during the buzz and the scramjet model has higher buzz frequencies that can be predicted by resonance modes of an acoustic duct with an open- and a closed-end.