An experimental and numerical investigation into reacting vortex structures associated with unstable combustion

Citation

Kendrick, Donald William (1995) An experimental and numerical investigation into reacting vortex structures associated with unstable combustion. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/9QDG-AP23. https://resolver.caltech.edu/CaltechETD:etd-10122007-131523

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. An experimental and numerical investigation into reacting vortex structures shed from a rearward facing step flameholder was performed to gain insight into the fundamental reasons why certain acoustic modes of the laboratory dump combustor were excited for a given set of flow parameters. Cases examined used premixed [...] air mixtures [...], various duct heights (2.54, 5.08 and 7.62 cm) and dump plane speeds (21, 30 and 35 m/s). The above parameters permitted observing instabilities having either one or both of the longitudinal acoustic modes present (188 or 234 Hz) in their respective pressure and velocity spectra. Ignition of the vortex structures was found to be heavily dependent on geometry (i.e., duct height) and invariant to stoichiometric variations. This fact indicated the dominance of turbulent exchange processes over chemical effects for the pulsating flowfield. The coherent structures which typically convected at the local dump plane speed and exhibited high initial strain rates, were found to exhibit shorter burning times and more intense combustion for decreasing duct heights. Use of high-speed shadowgraph and chemiluminescent (CCD) imagery permitted a complete description of the typically nonuniform, reacting flowfield. Time-resolved vortex and floor temperature measurements as well as time-averaged floor heat flux measurements completed the quantitative description of the vortex structures. Culick's technique of expanding the acoustic filed into orthogonal modes was employed to confirm mode selection theories and suggest the importance of the shape of the average burner distribution. A nonlinear heat release model was formulated whereby the vortices were characterized as gaussian envelopes convecting at the local dump plane speed. The system of equations formulated was a set of two coupled oscillators with a nonlinear driving term. A final discussion was also undertaken to infer the geometrical implications into the mode selection process (what system acoustic mode was excited).

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