Design Considerations for Axial Steam Turbine Rotor Inlet Cavity Volume and Length Scale

In this paper we examine the interaction between the cavity and main flows of three different rotor cavities. For each of the three rotor cavities, the cavity inlets differ in their axial cavity lengths, which are modified by extending the upper casing stator platform. The three cavity volumes are comprised of a baseline case, along with a 14% and a 28% volume reduction relative to the baseline case. Measurements show that there is an increase in efficiency of 0.3% for the 14% cavity volume reduction case (relative to the baseline case), whereas a further volume reduction of 28% (relative to the baseline case) decreases the efficiency. Computational analysis highlights the breakup of a toroidal vortex within the cavity as the primary factor explaining the changes in efficiency. The dominant cavity vortex originally present in the baseline case firstly broken up into two smaller vortices for the 14% cavity volume reduction case and secondly, completely replaced with a strong radial jet for the 28% volume reduction case. From a design perspective, reducing the cavity volume by extending the upper casing stator platform yields improvements in efficiency provided that the cavity vortex is still present. The design considerations, analysis and the associated aerodynamics are discussed in detail within this paper.