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

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.

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

Topics:
Combustion and combustion processes Computational fluid dynamics Engines Fuel injection Fuel systems Jet engines Physical Sciences Powertrains Scramjet engines Simulation and modeling
This Brief includes a Technical Support Package (TSP).
Document cover
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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    Magazine cover
    Defense Tech Briefs Magazine

    This article first appeared in the April, 2010 issue of Defense Tech Briefs Magazine (Vol. 4 No. 2).

    Read more articles from this issue here.

    Read more articles from the archives here.

    Overview

    The document is a Master's thesis authored by First Lieutenant MacKenzie J. Collatz at the Air Force Institute of Technology, focusing on the performance and operability of a dual cavity flame holder in a supersonic combustor. The research was conducted from September 2008 to June 2009 and aims to address the challenges of achieving stable combustion in high-speed flight conditions, which is critical for the advancement of hypersonic technologies.

    The thesis begins with an introduction that outlines the motivation behind the research, the problem statement, and the research objectives. It emphasizes the importance of efficient combustion in supersonic environments, where traditional combustion methods may fail due to the extreme conditions. The study specifically investigates the design and functionality of cavity flame holders, which are essential for stabilizing flames in supersonic combustors.

    The theoretical framework includes a review of previous research on supersonic ignition, cavity flame holder design, and fuel injection techniques. The document details the experimental setup, including the test facility, instrumentation, and procedures used to gather data. A comprehensive data analysis process is also described, along with an uncertainty analysis to ensure the reliability of the results.

    The results section discusses various performance metrics, including equivalence ratios, pressure profiles, peak pressure ratios, and combustion efficiency. The findings indicate that the dual cavity flame holder design significantly enhances the operability of the combustor, leading to improved combustion stability and efficiency under supersonic conditions.

    Conclusions drawn from the research highlight the effectiveness of the dual cavity flame holder in achieving stable combustion, which is crucial for the development of future hypersonic vehicles. The thesis also provides recommendations for further research, suggesting areas where additional studies could enhance understanding and performance of combustion systems in high-speed applications.

    Overall, this thesis contributes valuable insights into the field of aerospace propulsion, particularly in the context of supersonic and hypersonic flight. It underscores the importance of innovative designs in combustion technology and sets the stage for future advancements in the area. The document is approved for public release, indicating its relevance and potential impact on ongoing research and development within the Air Force and broader aerospace community.

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