Aerodynamics of a Scramjet Cavity Flameholder at On- and Off-Design Conditions

Supersonic combustion ramjet (scramjet) engines offer the promise of making hypersonic aircraft an operational reality, lowering the costs of space access and enabling a responsive high-speed flight capability for national defense. Particle image velocimetry (PIV) is a technique ideal for the aerodynamic characterization of the scramjet flameholding process due to its ability to provide instantaneous and ensemble measurements at high spatial resolution across a planar measurement region. This work applied PIV to two configurations of a DMSJ cavity flameholder, characterizing its turbulent aerodynamics at conditions representative of operational design points as well as departures from them. An examination of replicated inlet-generated distortion found that the extent of recirculating flow in the cavity, which governs entrainment and residence time of fuel and air, was strongly dependent on the impingement location of an oblique shock wave. Impingement upstream of the cavity diminished the size of the recirculation region, whereas impingement directly on the cavity increased it. The aerodynamics of fuel injection upstream of the cavity were replicated with wall-normal sonic air injection, which thickened the shear layer separating cavity recirculation from main-duct flow and reduced spatial variation of fluctuating velocity magnitudes.

Past measurements have been challenged by the tendency of conventional metal oxide velocimetry tracers to inhibit their own imaging by adhering to flowpath windows. To resolve this limitation, the novel application of graphite flakes as PIV tracers in a high-speed flow was shown to provide acceptable flow tracking while maintaining compatibility with flowpath windows and persistence through the reaction zone. Subsequent measurements examined flameholder operation with premixed hydrocarbon fueling at both steady-state conditions as well as during the lean-blowout transient. Air injection downstream of the combustor was used to maintain the pre-combustion shock system independent of the amount of heat release.

It was found that mean velocity in the spanwise center plane of the cavity flameholder was governed primarily by the length of the shock system, whereas turbulent fluctuations were most strongly influenced by whether or not combustion was present, indicating turbulent aerodynamics of dual-mode operation cannot be replicated by artificial blockage. Comparison of PIV results with corresponding measurements of the OH and \ce{CH^*} radicals demonstrated how main-duct combustion is dependent on turbulent exchange with the cavity, and three-component PIV measurements at the boundary of the cavity and main duct indicated that the flow is largely spanwise-uniform at this interface. Turbulence intensity at the combustor entrance was found to be positively correlated with prior measurements of flame front angle, demonstrating how turbulent fluctuations drive flame spreading rate in the premixed regime. Integral length scales at this location were found to be constant with a change in fueling rate from one operational limit to the other, providing further evidence that turbulent aerodynamics are dependent on the presence of heat release and not its magnitude.

Examination of the lean blowout transient with kHz-rate PIV found that changes in cavity aerodynamics were confined to a period prior to blowout two orders of magnitude smaller than that of the collapse of the pre-combustion shock system, suggesting the cessation of flameholding was due to the cavity mixture falling below flammability limits, rather than an alteration of the turbulent exchange between the cavity and the remainder of the combustor. This result suggests current flight-proven velocimetry techniques could be used in a flight vehicle to predict imminent blowout and enable operation near the lean limit.