An Investigation of the Effect of Swirl Vane Angle on Fuel Concentration and Velocity Fields in Gas Turbine Mixers

Abstract

The flow fields of two different Siemens-Westinghouse gas turbine mixers were studied experimentally in an effort to better understand fuel-air mixing in confined swirling flows found in industrial applications. The mixers consist of an annular flow region and mixing is achieved using swirl vanes, the pressure side of which is used to inject the fuel. The difference between the two mixers studied is the degree of swirl imparted on the flow by the swirl vanes (45° vs. 55°). Velocity (both axial and azimuthal) and fuel concentration profiles were obtained for non-reacting, atmospheric pressure flows at several axial and radial locations downstream of the swirl vanes by the use of LDV and infrared laser light absorption techniques, respectively. The fuel used in this work was a methane/air mixture, which was injected at a momentum flux ratio comparable to that under operational conditions. Results show that flow uniformity, as evidenced by velocity and fuel concentration profiles, is reached further downstream of the swirl vanes for the 45° mixer than for the 55° mixer. This indicates a lesser mixing performance in the 45° mixer. The axial and azimuthal RMS velocities were consistently higher for the 55° degree mixer and this was a likely contributor to its superior mixing performance. High velocity and fuel concentration gradients are common for both mixers and present in the near-field region close to the swirl vanes. The data obtained indicates that the flow behavior in the region near the swirl vanes strongly influences the mixing of the fuel and air. Frequency analysis of the fuel concentration data shows that some turbulent structures prevail throughout the mixing region in both mixers, revealing that some large scale flow features emanating from the swirl vanes are not dissipated even in the high degree of swirl hardware. Lastly, unmixedness levels in both mixers tested are calculated and compared with a discussion on how they might impact NOx emission levels.