Date of Completion
In bluff body-stabilized flames, a variety of physical phenomena contribute to the flame destabilization as lean blowoff is approached. These effects include increased strain on the flame shear layers, decreased attenuation of Bénard-von Kármán vortex shedding, and the presence of thermoacoustic instabilities. Lean, bluff body-stabilized flames were studied in an enclosed rectangular-duct, turbulent combustion rig with a triangular flame holder under vitiated conditions with both symmetric and asymmetric fuel distributions. Air and fuel flows within the rig were characterized using a PIV system and a continuous emissions gas analyzer, respectively.
High-speed videos of these flames undergoing blowoff were taken to serve as the primary data source for analysis. To examine the effects of Bénard-von Kármán vortex instabilities and local strain-induced extinctions on the lean blowoff process, proper orthogonal decomposition (POD) algorithms were applied to the frames of the high-speed videos. POD mode shapes representing each of these phenomena were extracted and the relative contributions of these mode shapes were plotted over time as the flame approaches blowoff. Time-series analysis was also performed on a trace of the pressure oscillations and POD mode coefficients for a preliminary examination of the relevant acoustic influence on the blowoff event.
For future application to the turbulent combustion rig, a tunable diode laser absorption spectroscopy (TDLAS) system was developed. A TDL with a center wavelength in the near infrared region was simultaneously scanned across absorption lines of H2O and CO2 to obtain temperature and species concentration data. A McKenna Flat Flame burner was used to create a calibration environment for these measurements. Both direct and wavelength-modulated spectroscopy methods are examined, and their respective viability for use in the combustion rig is discussed.
Jensen, Trevor, "An Optical Analysis of the Blowoff Behavior for Bluff Body-Stabilized Flames in Vitiated Flow" (2011). Master's Theses. Paper 141.