Measurement of combustion response to transverse modes at high pressure
A two dimensional transverse combustor capable of producing and modulating an unstable flowfield was tested at Purdue. The combustor used an array of coaxial ox-centered shear injector elements. The oxidizer posts were able to be discretely changed in length between tests to investigate different post resonance conditions. A single Study element was placed in the middle of the driven unsteady flowfield so its response could be measured using pressure measurement and high speed imagery. This approach was supported by past testing of the longitudinal Continuously Variable Resonance Combustor and past transverse chambers. Mean flow rates and pressures were fixed across the series of tests. Phase lag data between pressure measurement locations support a simple linear post-to-chamber interaction mechanism. Phase lags between chamber measurements were unaffected by the change in oxidizer post length. Scaling was established between amplitude levels and post lengths between the longitudinal and transverse combustors, which further supported the proposed post-to-chamber response mechanism. High speed imaging of OH* chemiluminesence was used as a marker of heat release. Mean flame behavior showed broad distribution and flame shortening at high forcing amplitude and compact, elongated flame shape at low forcing amplitude. Multiple image analysis techniques were utilized to process these data. Proper orthogonal decomposition and dynamic mode decomposition showed spatial and temporal character of image time slices sorted by descriptive content and frequency, respectively. These analyses both identified significant behavior at the same frequencies as chamber pressure perturbations. A systematic code was developed to select relevant time slices to generate flame describing functions of image magnitude and phase lag as a function of pressure amplitude. The magnitude flame describing functions showed low amplitude linear behavior with a divergence at high amplitude. For phase lag flame describing functions, a convergence towards phase locked behavior was noted as amplitude increased.
Anderson, Purdue University.
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