Experimental investigation into thermo-acoustic instability in pre-mixed, pre-vaporized bluff-body stabilized flames

Date of Award

2015

Degree Name

Ph.D. in Mechanical Engineering

Department

Department of Mechanical and Aerospace Engineering

Advisor/Chair

Advisor: Reza Kashani

Abstract

An experimental campaign was undertaken to investigate the thermo-acoustic properties of a bluff-body stabilized flame in an atmospheric pressure facility at the Air Force Research Laboratory. Of particular interest were the possible interactions between the acoustic properties of the test rig, the vortex shedding due to the presence of the bluff-body, and the unsteady heat release within the chamber. An analysis of the vortex shedding modes due to the bluff-body and the acoustic modes indicated that there are regions in the operating envelope where the two mode types share similar frequencies given an operating condition, creating a scenario where feedback might be possible. Further investigation into the fluctuating velocity components in the wake of the bluff-body indicated that the Strouhal number is not single-valued, and that vortices of varying sizes, and accompanying characteristic frequencies, are shed from a single bluff-body. With previous research indicating that lean blow-off is preceded by local extinctions within the reaction zone, and blow-off being closely related to the ratio of chemical and fluidic time scales, an experiment was conducted to determine whether or not flames undergoing thermo-acoustic instability also exhibit regions of decreased residence time. This experiment concluded that the regions of acoustically-coupled flames which undergo large-scale oscillations do, in fact, correlate with decreased residence time. This conclusion links both lean static stability and near-stoichiometric dynamic stability to simple time scales prescribed by vortex behavior in the wake of a bluff-body. An investigation was conducted which utilized simultaneous high-speed particle image velocimetry (PIV), planar laser-induced fluorescence (PLIF) and pressure measurements in the near-wake region of a bluff-body stabilized flame. In addition to the simultaneous measurements listed, high-speed broadband chemiluminescence was also collected. The 2-D nature of these measurements led to their analysis through the utilization of proper orthogonal decomposition (POD). The decomposition of the highly-complicated data sets allowed the dominant features to be extracted. These dominant features, in an acoustically-coupled flame, show remarkable symmetry that is not readily apparent in uncoupled flames. Further analysis of an objective measure of the flame symmetry as a function of equivalence ratio indicated that the fluctuations in the axial component of velocity best correlate with overall sound pressure level. This correlation indicates that the feedback interactions are based around the longitudinal acoustic modes of the combustion chamber.

Keywords

Combustion chambers Acoustic properties, Combustion chambers Thermal properties, Vortex shedding Effect of heat on, Mechanical Engineering, Thermo-Acoustics, Instability, Bluff-Body, POD

Rights Statement

Copyright © 2015, author

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