Numerical Modeling of Flows Related to Gas Turbine Combustors

Sammanfattning: The thesis focuses on the numerical simulation of the flow field in gas turbine burners. Due to tougher and tougher requirements on low levels of emission of pollutants there is a need for new combustion technologies. In gas turbines one method to attain low pollutant levels is the use of lean premixed combustion. However, in lean combustion, problems related to the flame stability can arise. Swirling jets can be used to assure flame holding by forming an internal recirculation zone which helps to re-ignite the eventually extincted flame. Additionally, the swirl is enhancing the mixing process. This is important not only for flame stability and efficiency of combustion, but also the NOx levels have been proved to be highly dependent on the local mixture composition. Swirling flows, however, are difficult (but not impossible) to examine experimentally because of the high sensitivity of the flow field to external conditions. In addition, experimental approaches cannot be used to study the effects of individual parameters. Consequently, the use of accurate numerical methods is preferred for analysis and design. Of course, the accuracy of the numerical tools and the design must be verified by experiments. Here, the flow field downstream of a burner providing three swirling coaxial jets is studied numerically. The turbulence is accounted for by the Large Eddy Simulation (LES) approach because RANS-based models have been proved to fail to account accurately for some characteristics of the swirling flows, like large streamline curvature, flow reversal and flow dynamics. Since only limited experimental data exists for the given combustion chamber, the computational approach is tested first on a test case involving a single swirling jet. Further, the flow fields obtained by altering different parameters (Reynolds number, swirl number, inlet velocity profile, confinement) are compared with the base case in order to evaluate the importance of each parameter in the present set-up. These parameters are important for an efficient flow control. A study of the turbulent mixing in the combustion chamber is also presented. The effect of Schmidt number has been studied and the presence of counter gradient diffusion is established. The acoustic field generated by the swirling jet has been evaluated in order to identify resonant frequencies of the flow.