Performance and Operability of a Dual-Cavity Flame Holder in a Supersonic Combustor

Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio

Thursday, April 01 2010

This technology enables stabilization and enhancement of supersonic combustion in scramjets.

Supersonic combustion has been of interest for many years in order to support future Air Force hypersonic missions. The current generation of hydrocarbon-fueled scramjet combustors typically requires a flame-holding device to facilitate flame ignition and stable combustion. The amount of time available for fuel injection, fuel-air mixing, and combustion is very short — on the order of 1 millisecond. This short dwell time, along with the relatively long ignition delay times of hydrocarbon fuels, makes the flow path and flame holder design extremely important. This study investigates the perormance and operability of using a symmetric dual-cavity flame holder flow path to stabilize and enhance supersonic combustion.

Testing of this flow path configuration, as well as a baseline single cavity flow path, was conducted. Multiple flight conditions, equivalence ratios, and fueling schemes were studied. Performance and operability of the flow paths were determined through analysis of wall pressures, temperatures, pressure ratios, stream thrusts, combustion efficiencies, computational fluid dynamics (CFD), and visualization.

The first objective was to investigate the dual-cavity performance and determine the advantages and disadvantages of using a dual-cavity versus a single-cavity flame holder. This objective was accomplished by studying wall pressures, temperatures, pressure ratios, stream thrusts, combustion efficiencies, CFD, and visualization. Peak pressure and combustor exit pressures were studied, and the dual-cavity consistently showed higher ratios for both. The increase in pressure is a result of additional heat being released from the combustion process. This result suggests the dual-cavity flow path provides better combustion and performance than the single cavity.

Stream thrust was the next performance parameter studied. Each case showed a stream thrust significantly higher for the dual-cavity than for the single-cavity. The dual flow path had an average of 34% higher values over all of the cases. The increase of stream thrust with the dual-cavity is further evidence that adding the additional flame holder provides better performance.

The second objective was to investigate the operability of the dual-cavity flow path over a range of equivalence ratios and fuel injection schemes. Each run with the dual-cavity configuration had a shock position farther upstream in the isolator than the single-cavity flow path. The dual-cavity had a smaller range of operability and proved to be more likely to unstart as the equivalence ratio was increased to 1.0 or higher. Including bottom side injection also moved the pre-combustion shock train further upstream and decreased the operability range of the dual-cavity.