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Acoustically Forced Coaxial Hydrogen / Liquid Oxygen Jet Flames

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Conference Paper

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Air Force Research Laboratory (AFMC) AFRL/RQRC Edwards AFB United States

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Combustion instabilities can pose serious problems in the development of liquid rocket engines. In order to understand andpredict them, it is necessary to understand how representative liquid rocket injector flames react to acoustic waves. In thisstudy, a representative coaxial gaseous hydrogen liquid oxygen LOX jet flame is visualized for both reacting andnonreacting cases. The jet flame was studied unforced, without acoustics, and forced, with transverse acoustic waves in apressure node and a pressure antinode configuration. For unforced flames, reactions are found to cause a significantly moreexpanded plume due to the vaporization and expansion of the LOX. Flame holding is established at the lip with a particularlydominant LOX recirculation zone. Nonreacting convective structures propagate downstream at relatively constant velocity,while reacting structures start at a slow speed and gradually accelerate with downstream distance. These structures never reachthe velocity of the nonreacting structures. Reactions shift the spectral content to lower frequencies, consistent with trendsobserved in the linear stability literature. For forced flames, acoustics do not appear to affect the flame holding. Dynamic modedecomposition detects jet response not only at the fundamental frequency but at higher harmonics as well. Reactions produceinconsistent trends in the harmonics reactions promote harmonics at a pressure antinode while they damp harmonics at apressure node.

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