Effects of Design Variations of Rotor Entry Cavity Geometry on Shrouded Steam Turbine Performance

Abstract

This paper presents an experimental study of the effect of geometry variations of the rotor entry cavity on shrouded steam turbine performance. A series of experiments was carried out where different configurations of the geometry of the entry cavity were tested. Blade geometry and tip clearance remained unaltered for all cases examined. Interactions between cavity and main flow are carefully investigated and their consequences on shrouded steam turbine stage efficiency are examined. Geometry variations of the entry cavity were installed in a pre-existing ‘baseline’ case of high efficiency. Five different test cases were examined. For the first two of these cases a ring having a constant width of 2mm and 4mm in radial direction is used. The next two cases employ a non-uniform, wavy insert and for the last case a backwards slanted insert is used that covers most of the inlet to cavity area, maintaining a safety distance of 2mm from the downstream rotor. The cases are divided into two groups, based on the same inlet cavity volume. The first group of three cases has a cavity volume reduction of 14% compared to the baseline case, whereas in the second group two cases are examined which maintain a 28% cavity volume reduction compared to the baseline case. Stage performance and flow field data were acquired and analyzed. Strong interactions between cavity and main flow are observed for all cases, not only at the location where the variations were installed. An observed effect can also be seen downstream of the rotor affecting the stage performance. Measurements were performed with the use of miniature probes ensuring minimum blockage effects especially within the cavity, both at rotor inlet as well as downstream of the second rotor. The use of a uniform geometry variation for the inlet rotor cavity in both groups proved to be the best in terms of stage efficiency. Although more complex and non-uniform variations were also used, the simple design of uniform geometry caused the least disturbance in the flow downstream of the 2nd rotor, having at the same time a moderate positive influence at the exit of the 2nd stator. The use of a constant width insert ring (thickness = 2mm) showed an efficiency gain of at least 0.3% from cases with 14% cavity volume reduction, whereas in the cases with 28% cavity volume reduction the use of a uniform ring of 4mm width produced a marginal efficiency gain of 0.1% at the operational point.