Investigation of Partially Premixed Combustion Instabilities through Experimental, Theoretical, and Computational Methods.

Abstract

Partially premixed combustion has the merits of lower emission as well as higher efficiency. However, its practical application has been hindered by its inherent instabilities. This work is a study of instabilities in partially premixed combustion, through a combination of numerical simulation, theoretical modeling, and experimental investigation, with the hope of furthering our understanding of the underlying physics. Specifically, a Flamelet/Progress Variable (FPV) combustion model in the context of Large Eddy Simulation (LES) is extended to simulate a piloted (partially) premixed jet burner (PPJB). The ability and shortcomings of this state-of-the-art high fidelity combustion model are assessed. Furthermore, a Modular Reduced-order Model Framework (MRMF) is developed to integrate a range of elementary models to describe the instabilities that may occur in combustors utilizing partially premixed combustion technologies. A multi-chamber Helmholtz analysis is implemented, which is shown to be an improvement over previous single-chamber analyses. The assumptions and predictions of the proposed model are assessed by pressure and simultaneous Particle Image Velocimetry (PIV)–formaldehyde (CH2O) Planar Laser Induced Fluorescence (PLIF) measurements on a Gas Turbine Model Combustor (GTMC) at a sustained rate of 4 kHz. The proposed model is shown to be able to predict the instability frequency at experimental conditions. It also explains the trends of the variation of instability frequency as mass flow rates and burner geometry are changed, as well as the measured phase shift between different chambers of the burner. Finally, under the current framework an explanation of the dependence of the existence of combustion instability on equivalence ratio is provided.