Reduced Order Modeling Approach to Combustion Instabilities of Liquid Rocket Engines

Abstract

This paper describes one possible approach to a numerical framework for a reduced order tool that aims to simulate combustion instabilities in liquid rocket engines. The numerical framework relies on the projection of the pressure fluctuations on the eigenmodes of the system. The pressure fluctuations are solutions of the wave equation of the system. After projection on the eigenmodes, the wave equation takes the form of a series of second-order harmonic equations with source terms that drive combustion instabilities and damping terms that attenuate them. A test rig was developed to study cavity interactions, injector impedance, and damping effects. The damping rates that were measured on the test rig show a trend that is consistent with observations in liquid rocket engines. On the whole, the test rig can be used to validate simplified models of combustion instabilities. The global framework of the reduced order approach that was developed to predict combustion instabilities was first validated by comparing the data from simulations against experimental results from the test rig in a series of nonreacting experiments. Our approach was then applied to a case study of a full-scale rocket engine. This engine, under certain operating conditions, exhibits instabilities. Stable and unstable behaviors have been revealed by the temporal evolution of calculated pressure amplitudes.