Dense core response to forced acoustic fields in oxygen-hydrogen rocket flames

Youhi Morii ^a,b,*, Scott Beinke ^c, Justin Hardi ^c, Taro Shimizu ^a, Hideto Kawashima ^d, Michael Oschwald ^c,e

a. Research and Development Directorate, JAXA, Sagamihara, 252-5210, Japan
         b. Institute of Fluid Science, Tohoku University, Sendai, 980-8577, Japan
         c. Institute of Space Propulsion, DLR, Lampoldshausen, 74239, Germany
         d. Research and Development Directorate, JAXA, Tsukuba, 305-0047, Japan
         e. Institute of Jet Propulsion and Turbomachinery, RWTH Aachen, 52062, Germany

Abstract: Oscillatory combustion representative of thermo-acoustic instability in liquid rockets is simulated by experiment and LES calculation to investigate the flame behavior in detail. In particular, we focus on how the velocity and pressure fluctuations affect the behavior of the dense oxygen jet, or ‘LOx core’. The test case investigated is a high pressure, multi-injector, oxygen-hydrogen combustor with a siren for acoustic excitation. First, the LES calculation is validated by the resonant frequencies and average flame topology. A precise frequency correction is conducted to compare experiment with LES. Then an unforced case, a pressure fluctuation case, and a velocity fluctuation case are investigated. LES can quantitatively reproduce the LOx core shortening and flattening that occurs under transverse velocity excitation as it is observed in the experiments. On the other hand, the core behavior under pressure excitation is almost equal to the unforced case, and little shortening of the core occurs. The LOx core flattening is explained by the pressure drop around an elliptical cylinder using the unsteady Bernoulli equation. Finally, it is shown that the shortening of the LOx core occurs because the flattening enhances combustion by mixing and increase of the flame surface area.

Keywords: Liquid rocket engine; Combustion instability; Computational fluid dynamics (CFD); Large eddy simulation (LES); Supercritical fluid