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LINEAR COMBUSTION STABILITY ANALYSIS OF OXIDIZER-RICH STAGED COMBUSTION ENGINES
Thesis Defense
Christopher Lioi
Thursday, Nov. 27th, 2018
MK 317, 12:30-3:00 pm.
Adviser: Dr. Vigor Yang
Committee members: Drs. Krish Ahuja, Lakshmi Sankar, Tim Lieuwen, and Rob Funk
Oxidizer-Rich Staged Combustion (ORSC) engines have gained increased attention recently for their potential as high-thrust first stage devices. In this class of engine, a small amount of fuel is combusted in one or more oxidizer-rich preburners, the exhaust from which is used to drive the main turbopump assembly. The remaining fuel is burned with the oxidizer-rich products of combustion in the main chamber. As part of the domestic maturation of this technology, risk mitigation analysis is required, a main subheading of which is an analysis of the propensity of a design to combustion induced acoustic oscillations, or combustion instabilities. A broad literature has emerged on this subject which nonetheless leaves remaining questions as to many of the physics in real systems with high turbulence levels and complex geometry.
In this thesis, a comprehensive framework for analyzing the linear acoustic stability of a candidate engine is presented, using a model based on the Russian RD-170 engine as a case study. Both the main combustion chamber (MCC) and preburner (PB) are studied separately. A linearized acoustic wave equation is derived with source terms accounting for various system inhomogeneities. Boundary and internal damping effects are captured by theoretical impedance models. Spatially distributed fields for mean velocity components and heat release are extracted from LES results for the main chamber. The combustion response, represented by a Flame Transfer Function (FTF), is based on a POD analysis of the heat release snapshots; spatially distributed gain and phase fields are computed for the nominal chamber eigenfrequencies and then utilized in a stability analysis.
The combustion response for three different injector configurations with three different recess lengths are investigated. An adjoint sensitivity analysis is conducted on a reduced set of scalar parameters which characterize the distributed combustion fields. These parameters are chosen to capture the strength of the combustion-acoustic interaction as well as the extent of the non-compactness of the fields. It is found that distributing the heat release response over a greater (especially axial) extent may have noticeable effects on the stability.
