Particle image velocimetry measurements were made along the center plane of a scramjet cavity flameholder to analyze simulated inlet flow distortion in the direct-connect test environment. Mach 3 nonreacting tests examined an oblique shock impinging upon locations in-and upstream of the cavity, including cases with wall-normal air injection upstream of the cavity to simulate fuel injection. Addition of flow distortion altered the size and shape of the primary recirculation region within the cavity by deflecting the bounding shear layer: the recirculation region was compressed by shock impingement upstream of the cavity, and shock impingement on the cavity itself expanded it. Air injection upstream of the cavity thickened the shear layer and produced a stronger effect on velocity direction than magnitude, preventing the formation of a large-scale recirculation region in two of the three shock locations studied. Flow distortion and upstream air injection both increased flow unsteadiness, with the greatest increases occurring in the shear layer and above the cavity closeout ramp. Additionally, results suggest the formation of spanwise secondary flow patterns that may account for flow nonuniformities observed in prior studies. This work presents the first velocimetry characterization of a scramjet cavity flameholder under distorted-flow conditions.

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A large-eddy simulation / Reynolds-averaged Navier-Stokes (LES/RANS) methodology is used to simulate premixed ethylene-air combustion in a model scramjet designed for dual mode operation and equipped with a cavity for flameholding. A 22-species reduced mechanism for ethylene-air combustion is employed, and the calculations are performed on a mesh containing 93 million cells.Fuel plumes injected at the isolator entrance are processed by the isolator shock train, yielding a premixed fuel-air mixture at an equivalence ratio of 0.42 at the cavity entrance plane. A premixed flame is anchored within the cavity and propagates toward the opposite wall. Near complete combustion of ethylene is obtained. The combustor is highly dynamic, exhibiting a large-scale oscillation in global heat release and mass flow rate with a period of about 2.8 ms. Maximum heat release occurs when the flame front reaches its most downstream extent, as the flame surface area is larger. Minimum heat release is associated with flame propagation toward the cavity and occurs through a reduction in core flow velocity that is correlated with an upstream movement of the shock train.Reasonable agreement between simulation results and available wall pressure, particle image velocimetry, and OH-PLIF data is obtained, but it is not yet clear whether the system-level oscillations seen in the calculations are actually present in the experiment.

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