In many combustion systems, flames are required to be stabilized in flows with velocities higher than the flame speed. Bluff-bodies can be placed in the flow to create a low speed region to enable flame stabilization. Bluff-body stabilized flames are used in many applications including afterburners for aircraft engines. In afterburner flames several additional features complicate a comprehensive understanding of the flame stabilization process: (1) the flow is pre-heated due to combustion within the engine so that the temperature and oxygen levels affect the flame, (2) fuel is added to the flow close to the flame so that is does not have time to fully mix, and (3) the afterburner is enclosed in a duct open at one end (the jet exit) so that acoustic resonance can cause oscillations in the flame. Previous research has primarily studied flame stabilization without these complications.
The proposed work will use a recently constructed combustor rig at the University of Connecticut that provides high speed flows with elevated temperatures and reduced oxygen levels to simulate conditions in an aircraft engine. The rig is capable of creating bluff-body stabilized flames with control over the temperature, oxygen level, fuel/air mixing, and acoustic level to address each of these three complications separately. The flames are stabilized within a test section with windows that permit laser-based measurements and imaging of the flame. Measurements of the flame and flow field will be made via particle imaging velocimetry and planar laser induced fluorescence. In addition, high-speed imaging of unsteady flames will be accomplished with the support of Pratt & Whitney through equipment and personnel exchange. These measurements recover the local velocity and flame shape so that the cause of flame de-stabilization can be quantitatively examined. These measurements will be applied as the three chosen features of aircraft afterburner flames are parametrically varied. The resulting experiments are anticipated to provide a more complete and quantitative description of the flame stabilization process for bluff-body flames than presently available. The quantitative data sets from numerous cases will be made publicly available. The results of this study are directly applicable to aircraft afterburners, but will also be broadly applicable to new combustor designs with alternative fuels, which may require enhanced bluffbody stabilization for fuel flexible operation.
Broader Impacts
The proposed work will lead to a better fundamental understanding of flame propagation in flows with non uniform fuel/air mixing including the effects of elevated temperature and reduced oxygen level. This has direct impact on the design of devices such as afterburners for aircraft engines, but also expands understanding of basic flame behavior in flows that could be broadly applicable to improved combustor design for new fuels. The project will support two graduate students and one undergraduate student each year, and will provide opportunities for these students to be trained in a broad range of optical measurement techniques. The students and PIs will collaborate directly with Pratt & Whitney on the selections of test conditions and analysis of afterburner data to provide good coupling between the results from the studies in the UConn rig and larger full scale tests. Graduate students will have the opportunity for internships at P&W. The data sets measured in this program will be shared broadly to provide a basis for future model comparisons.
