Effect of flame temperature ratio on bluff body wakes

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Lundrigan, Julia
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Safe operation of aircraft and aeroderivative industrial gas turbine engines relies heavily on the stability of the combustion process. Combustion instabilities are of interest because they may lead to harmful pressure oscillations and increases in temperatures within the engine that could be destructive to engine components. Fluid dynamics in the combustor directly affects combustion instabilities and is the subject of this research. The two types of hydrodynamic instabilities linked with combustion instabilities explored in this research are the Kelvin-Helmholtz and the Benard/von Karman instabilities. Previous research has shown that a high flame temperature ratio (the ratio between the temperature of the products and the temperature of the unburned reactants) suppresses the Von Karman street, allowing for the wake to be characterized by the higher frequency Kelvin-Helmholtz instability from which vortices are shed by the shear layers. However, at a low temperature ratio, the flame loses its ability to suppress the Von Karman instability. The result of the changing of wake structures is fluctuations in heat release and pressure, which can result in damaged hardware and loud audible tones. Research previously conducted in Georgia Tech's Ben T. Zinn Combustion Laboratory using chemiluminescence has given an approximation of the flow field disturbances caused by the Von Karman response by measuring flame response. The proposed research will utilize knowledge gained from previous flow field disturbance approximations and improve the approximations by using data acquisition hardware that will allow for the exact flow field to be measured. The purpose of this research is to demonstrate that hydrodynamic stability calculations can be used to predict the onset of the Benard/von Karman instability in bluff body flames.
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