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.

Figure 1. The Dual-Cavity Flow Path in which flow moves from left to right.
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.

Figure 2. The predicted CFD Temperatures for the dual-cavity run. The spark plug in the bottom cavity was simulated using a heat source on the cavity wall. The figure shows the highest temperatures occur in the top cavity, as expected. However, there is still a temperature increase in the bottom cavity. These results support the experimental data showing the temperature rise in both cavities, not just the top.

The final objective was to analyze the overall advantages and disadvantages of the two flow path configurations and determine if the dual-cavity flame holder may be a viable option for future scramjet engines. The single-cavity flow path has been more extensively studied in the past and is known to provide sufficient combustion under most conditions. This research verified operability of the single-cavity flow path between equivalence ratios of approximately 0.53 to 0.95. The operability window of the dual-cavity flow path was smaller than that of the single cavity as the equivalence ratio was increased. However, the dual-cavity did provide increased overall performance shown by the stream thrust and pressure ratio results.

The analysis conducted in this study suggests the dual-cavity flame holder flow path provides significant advantages over the baseline and would be a viable option for future scramjet engines. However, the flight conditions and equivalence ratios could provide limitations to its capabilities. Operational equivalence ratios range from approximately 0.3 to 1.2. Fuel-rich conditions are useful during periods of acceleration where maximum thrust is desired. The inability to operate with these high equivalence ratios could increase acceleration time, thereby reducing cruise speed, flight time, and range.

The dual-cavity flame holder showed a significant overall increase in performance through higher temperatures, pressure ratios, and stream thrusts. The operability was slightly reduced due to an increase in pre-combustion shock train position. CFD and flow path visualization were used to verify these results.

This work was done by MacKenzie J. Collatz of the Air Force Institute of Technology. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Physical Sciences category. AFRL-0141


This Brief includes a Technical Support Package (TSP).
Performance and Operability of a Dual-Cavity Flame Holder in a Supersonic Combustor

(reference AFRL-0141) is currently available for download from the TSP library.

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This article first appeared in the April, 2010 issue of Defense Tech Briefs Magazine.

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