Thursday, May 16, 2013
Mechanical and Civil Engineering Seminar
Progress and Challenges in Liquid Rocket Combustion Stability
Venke Sankaran, Senior Scientist/ST, AFRL/RQ, Edwards AFB
Combustion instabilities have been observed in nearly every major liquid rocket engine development effort, including the most recent development programs. They are caused by the coupling of the natural acoustic modes of the combustion chamber with the dynamics of the heat release, which can in turn lead to catastrophic damage of the internal components of the rocket engine. Rayleigh's criterion states that combustion instabilities are driven when the pressure waves and the heat release are in phase and that the instabilities are damped when they are out of phase. Despite the simplicity of this relationship, the prediction of the occurrence of combustion instabilities has proven to be an enduring challenge because of the inherent complexities in the physics of multiphase turbulent flames. The Air Force Research Lab (AFRL)'s Advanced Liquid Rocket Engine Stability Technology (ALREST) program is a coordinated effort that involves both modeling and experimental components at various universities, small business, industry and in-house. The overall approach is to conduct data-centric, multi-fidelity combustion stability model development. "Data-centric" means that all model development is directed at experimental data sets. "Multi-fidelity model development" means that the most effective way to advance modeling capability is to do it simultaneously at multiple levels of fidelity. The talk will focus particularly on the modeling efforts of two experimental datasets obtained at Purdue University, which involve self-excited high-amplitude acoustic instabilities in longitudinal-mode and transverse-mode rocket chambers. In both cases, Detached Eddy Simulations of the turbulent reacting flowfield are shown to be effective in predicting the instability phenomena and the associated trends. In addition, the talk will also provide a general overview of rocket propulsion activities at AFRL.