4/29/2023 0 Comments Shroud steam![]() The case of the shortest axial distance between the swirl breaker and rotor shroud increased turbine stage efficiency by 0.7% compared to the conventional cavity geometry. By decreasing the axial distance between the swirl breaker and rotor shroud, the tangential velocity and the mixing region in the tip side which is influenced by the rotor shroud leakage flow were decreased and the stage efficiency was increased. ![]() Based on CFD results, the verification tests were conducted in a 1.5-stage air model turbine. Compared to a conventional single-stage CFD analysis, by conducting an additional single-rotor analysis with the modified shear stress transport (SST) model coefficient, the prediction accuracy for typical improvements in stage efficiency was increased in comparison to the single-stage analysis with the default SST model. In this Part II of the study, five cases of swirl breaker geometry with different axial distances between the swirl breaker and rotor shroud, which covered a range for the stage axial distance of actual high and intermediate (HIP) pressure steam turbines, were investigated using computational fluid dynamics (CFD) analysis and tests. When the swirl breaker is installed in the circulating region of leakage flow at the rotor shroud exit cavity, the axial distance between the swirl breaker and rotor shroud is a crucial factor to trap the leakage flow into the swirl breaker cavity. The basic design concept and typical performance of the proposed swirl breaker are presented in the first paper Part I, Design Concept and Typical Performance of a Swirl Breaker, and the efficiency improvement effect of the swirl breaker when applied to a real steam turbine is discussed in Part II – Effect of Axial Distance between a Swirl Breaker and Rotor Shroud on Efficiency Improvement.Abstract: The basic principle of a distinct idea to reduce an aerodynamic mixing loss induced by the difference in tangential velocity between mainstream flow and rotor shroud leakage flow is presented in “Part I – Design Concept and Typical Performance of a Swirl Breaker” The design concept offers an effective geometry for improving steam turbine stage efficiency. The proposed swirl breaker was judged to be an effective way to achieve highly efficient steam turbines because it not only reduces the mixing losses but also improves the incidence angle distribution onto the downstream blade row. Test results showed that the axial fin and the swirl breaker raised turbine stage efficiency by 0.2% and 0.7%, respectively. In addition to the conventional cavity geometry, three types of shroud exit cavity geometries were designed, manufactured and tested using a 1.5-stage air model turbine with medium aspect ratio blading. One idea was an axial fin placed from the shroud downstream casing to reduce the axial cavity gap, and the other was a swirl breaker placed in the rotor shroud exit cavity to reduce the tangential velocity of the leakage flow. In order to reduce this mixing loss, two distinct ideas for rotor shroud exit cavity geometries were investigated using computational fluid dynamics (CFD) analyses and experimental tests. Shroud leakage flow with a large tangential velocity creates a significant aerodynamic loss due to mixing with the mainstream flow. In high and intermediate pressure (HIP) steam turbines with shrouded blades, it is well known that shroud leakage losses contribute significantly to overall losses.
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