Numerical modeling of Swirling Flows in Gas Turbine Burners

Abstract: The present thesis focuses on the numerical simulation of the flowfield in gas turbine burners. Due to the higher and higher requirements regarding pollutant emissions there is a need for new combustion technologies. In gas turbines one method to obtain low pollutant levels is the use of lean premixed or partially premixed combustion. However, in lean combustion problems regarding the flame stability can arise. Swirling jets can be used to assure flame stabilization by forming an internal recirculation zone which helps to re-ignite the eventually extincted flame. Additionally, the swirl is enhancing the mixture quality. This is important not only for flame stability and efficiency of combustion, but also the $NO_x$ levels have been proved to be highly dependent on the local mixture composition. Swirling flows, however, are difficult to examine experimentally because of the high sensitivity of the flowfield to external conditions. In addition, experimental approaches demand larger resources and are limited in studying effects of individual parameters. Consequently, the use of numerical methods is preferred for analysis and design. Of course, the design should be validated by experiments. Here, the flowfield downstream of a burner providing three swirling coaxial jets is studied numerically. The turbulence is accounted for by the Large Eddy Simulation approach because simpler methods has been proved to fail to account accurately for some characteristics of the swirling flows, like large streamline curvature and flow reversal. The spatial derivatives are discretized with high-order schemes ($3rd$ and $4th$ order). Because no experimental data exist for the given combustion chamber, the computational approach is tested first on a testcase involving a single swirling jet. Later, the flowfields obtained by altering different parameters (Reynolds number, swirl number, inlet velocity profile, confinement) are compared with a base case in order to evaluate the importance of each parameter in the present set-up. A study of the turbulent mixing in the combustion chamber is also presented. The effect of Schmidt number will be studied and the presence of counter gradient diffusion is established.