Fluid Dynamics and Performance of Partially and Fully Shrouded Axial Turbines

A unique comparative experimental and numerical investigation carried out on two test cases with shroud configurations differing only in the labyrinth seal path, is presented in this paper. The blade geometry and tip clearance is identical in the two test cases. The geometries under investigation are representative of an axial turbine with a full and partial shroud, respectively. Global performance and flow field data were acquired and analyzed. Computational simulations were carried out to complement the investigation and to facilitate the analysis of the steady and unsteady flow measurements. A detailed comparison between the two test cases is presented in terms of flow field analysis and performance evaluation. The analysis focuses on the flow effects reflected on the overall performance in a multi-stage environment. Strong interaction between the cavity flow and the blade tip region of the rotor blades is observed up to the blade mid span. A marked effect of this interaction can be seen in the downstream second stator where different vortex structures are observed. Moreover, in the partial shroud test case, a strong tip leakage vortex is developed from the first rotor and transported through the downstream blade row. A measurable change in the second stage efficiency was observed between the two test cases. In low aspect ratio blades within a multistage environment, small changes in the cavity geometry can have a significant effect on the mainstream flow. The present analysis has shown that an integrated and matched blade-shroud aerodynamic design has to be adopted to reach optimal performances. The additional losses resulting from small variations of the sealing geometry could result in a gain of up to one point in the overall stage efficiency.