Robust Flow Structures of Turbulent Premixed Swirl Flames Experiencing Self-Excited Thermoacoustic Instabilities at Elevated Power Densities

Ryan B Griffin, Purdue University

Abstract

Historically one of the most expensive aspects of a gas turbine engine development enterprise has been large-scale experimental testing to quantify operating limits, and performance. To defray these costs, computational fluid mechanics and additional model-based design tools have been incorporated into the engine design process to better predict these characteristics of interest. However, even the most powerful supercomputers cannot resolve the interacting physics across the broad range of length-and temporal-scales present in actual engines. Therefore the computational cost must be reduced with modeling techniques that require a series of assumptions that rely on prior studies and similar empirical data sets. As a result, high-fidelity, temporally and spatially resolved measurements are required both to establish the proper boundary conditions as well as to validate the model parameters and results. This work expands upon prior studies of a well-characterized gas turbine model combustor (GTMC) by expanding operation to elevated pressure and thermal power. To characterize the velocity field, three-component, two dimensional velocity fields were acquired via stereoscopic particle image velocimetry (SPIV) at 6 kHz. The turbulent flame dynamics were simultaneously imaged via Planar Laser Induced Fluorescence and Chemiluminescence also at 6 kHz. In addition, acoustic measurements were obtained simultaneously with pressure measurements from an array of sensors throughout the combustor to allow the linking of the heat release, pressure, and velocity field fluctuations. These measurements were captured at two operating conditions: one with a flame demonstrating stable operation, and one flame exhibiting self-excited thermoacoustic oscillations. Decomposition techniques were applied to these two cases in order to elucidate the mechanisms driving the physical phenomena in each flame. The data was then phase conditioned according to the results of these decomposition techniques to construct volumetric representations of the physical phenomena. This reconstruction process revealed a precessing vortex core (PVC) during unstable operation of the burner that is not present during stable operation of the burner. This PVC operates within the inner shear layer between the central recirculation bubble (CRB) and incoming reactant jets. Over the course of the thermoacoustic cycle the CRB grows and shrinks in volume by 10% and the reactant jets change in size by upwards of 75%. These changes in volume correspond with the combustion chamber pressure fluctuations associated with the thermoacoustic fluctuations. Increases in the azimuthal vorticity and strain magnitude along the surface of the CRB correspond to increased heat release fluctuations computed from the chemiluminescence images and indicate an increased transport within the inner shear layer facilitated by the PVC.

Degree

M.S.A.A.

Advisors

Slabaugh, Purdue University.

Subject Area

Aerospace engineering|Mechanical engineering

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