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As global warming becomes a cause of serious concern worldwide, stricter and stricter emission regulations are being imposed on gas turbine engines. Flameless combustion is a novel combustion technique that offers a significant reduction in NOx and CO emissions. The presence of a flameless flame is indicated by uniform temperature distribution in the combustor, which leads to simultaneous reductions in NOx and CO emissions. Nick Overman conducted tests on a swirl stabilized flameless burner in the GDPL lab at the University of Cincinnati. A well-distributed flameless flame was observed for an overall equivalence ratio of 0.36. But as the equivalence ratio was increased, the flame in the combustor switched to a diffusion flame, and non-uniform temperature distribution was observed, which led to an increase in NOx emissions. The work in this thesis aims to improve the operational range of flameless combustion by modifying the swirl stabilized setup used by Nick Overman to include a cavity upstream of the swirler. ANSYS Fluent is used to numerically investigate the performance of such a cavity-swirler setup. The k-epsilon realizable and Laminar Finite Rate model is used to model turbulence and combustion, respectively. Multiple cavity designs which lead to a final successful design are described in detail in this thesis. The final successful design consists of 8 fuel injectors surrounded by 24 air injectors introducing fresh reactants to the cavity. Modifications were then made to this design to include 8 injectors in the second stage of the swirler. The cavity injectors aligned at an angle to the cavity also possess a swirl angle to impart a tangential component of velocity to the reactants being introduced in the cavity. The performance of two designs, the Swirler Reduced air and Swirler Fuel cases, are investigated at different equivalence ratios. Parameters such as temperature, OH distribution, NOx, CO, CO2, H2O, and combustion efficiency are used to compare the two designs. The swirler-cavity setups were able to extend the range of flameless combustion to an equivalence ratio of 0.55. At 0.65, transitional behavior was observed, and the flame switched back to a diffusion flame at an equivalence ratio of 0.70. For the swirler-cavity F=0.55 cases (Swirler Reduced air and Swirler Fuel), uniform temperature distribution was observed in the combustor with a simultaneous reduction in NOx and CO emissions. The Swirler Fuel case exhibited an overall improved performance in the flameless regime as compared to the Swirler Reduced Air cases. The Swirler Fuel case shows a lower centerline peak temperature, lower centerline OH peak, reduced NOx, and higher combustion efficiency than the Swirler Reduced Air case. Even though both the Swirler Reduced air and Swirler Fuel cases were able to extend the range of flameless combustion to 0.55, the Swirler Fuel case clearly is the better candidate to operate in the flameless regime. |
