Comparison of Mode-Transition Phenomena in Axisymmetric and Rectangular Scramjet Flowpaths

Abstract

Dual to scramjet mode-transition phenomena in rectangular and axisymmetric scramjet geometries are compared through three-dimensional, unsteady Reynolds Averaged Navier-Stokes simulations. The rectangular case geometry is taken from previous work on the HIFiRE-2 ground test, whereas the axisymmetric geometry is a hypothetical case which reformulates the cross sectional area ratios of the HIFiRE-2 into a round configuration to examine the effects of corner separation (exists in the rectangular but not axisymmetric configuration) on the mode-transition process. After describing the previously validated computational methodology, including meshing, theoretical model, and numerical procedure, the mode transition simulations are described for both the rectangular and round geometries. The adverse pressure gradient induced boundary layer separation forward of the primary injectors that dominates dual-mode operation in the rectangular geometry is muted in the axisymmetric geometry. However, the mode transition event in both cases is triggered by a loss of primary injector interaction induced boundary layer separation, i.e., the collapse of the flameholding from the primary injectors. Differences in flame holder and combustion patterns are described.