Heeger, Christof (2012)
Flashback investigations in a premixed swirl burner by high-speed laser imaging.
Buch, Erstveröffentlichung

Kurzbeschreibung (Abstract)

In this thesis flame flashback in a lean premixed swirl burner with central bluff-body was investigated using high speed multi-parameter laser imaging diagnostics. Starting with the fundamentals, the theoretical background of fluid dynamics was presented. This included turbulence, swirl and flows in boundary layers. Regarding the involved chemistry, the oxidation of methane was detailed and six mechanisms of nitric oxides formation together with reduction strategies were pictured. Lean premixed combustion as the primary reduction strategy for gas turbine combustors and the inherent flashback problem were discussed. Different mechanisms of flashback due to thermo-acoustic instabilities, propagation in boundary layers and in the core flow were classified. The swirl burner under investigation was described with an emphasis on the movable block swirl generator and the modifications with respect to the original design. The measurement principles for the various experimental combustion diagnostics were explained along with their limitations in investigating unpredictable transient processes. The measurement techniques included Chemiluminescence, Planar Laser-Induced Fluorescence of OH radicals, two- as well as three-component Particle Image Velocimetry and wall pressure measurements on the bluff-body. The extension to temporally correlating data acquisition by using high speed lasers and cameras was described. The interdependency of the spatial and temporal scales was investigated and a method to visualize the camera performance with respect to the scales of interest was developed. Furthermore, the concept of quasi four-dimensional data acquisition by laser sheet scanning was exemplified. The recent work using planar multi-parameter imaging was summarized and multi-dimensional data conditioning was presented as a technique to statistically compare unpredictable transient events. Multi-parameter measurements were applied in three different experimental setups. The results showed three different stabilization points of the flame base. Stability maps of the burner's parameter field with respect to various flow and flame parameters were generated. Comparison of the dataset to a calibrated flashback model by Konle et al. revealed good agreement. Frequency analysis of the measurements proved the existence of large scale coherent flow structures. Further correlations of flame structure showed that limit cycle oscillations were a subsequent result of upstream flame propagation. The analysis of the global flame tip location revealed propagation speeds against the oncoming flow that exceeded the estimated turbulent flame speed. The existence of a negative recirculation zone bubble upstream of the flame tip was observed based on the conditioned data. The flame was following the bubble upstream at a velocity of approximately 1.5 m/s. Based upon these findings a flame propagation prototype was formulated and a hypothesis for flashback in swirled flows with central bluff-body was postulated. For the first time, simultaneous acquired and temporally resolved experimental data on the velocity field and scalar information close to the wall inside a burner nozzle was acquired. It appears that the recorded data of flashback phenomenon supports theories based on aerodynamics and baroclinity.

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