OPTIMIZATION DESIGNS OF FRONT STAGE NOZZLES IN A LP STEAM TURBINE

This paper presents the findings of the original nozzle of the fourth stage in a 7 stage LP steam turbine where an obvious geometric feature is the extremely thick leading edge (LE). A cascade test was carried out to investigate the mechanism of loss reduction. A detailed comparison study was carried out using a conventional thinner leading edge design and the original thicker LE profile. The studies reveal that the overall loss in the original design is significantly lower than the counterpart of the thinner LE option together with a much wider range of incidence for which the vane is of low loss. This design philosophy is then successfully cloned to the first stage and third stage nozzles in a seven stage LP steam turbine. The analysis indicates that the obvious advantages of the new designs over the conventional thinner option are on not only the reduction of the profile loss, but the reduction of the blade count which has a significant implication on manufacturing cost. The numerical studies reveal that the idea behind this thick LE design philosophy is to minimise the profile loss without incurring a significant penalty on diffusion loss or at the worst separation. A detail investigation on the stage 2 nozzle indicates that this concept only works for a reasonably high aspect ratio blading where the secondary loss is limited.