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.
First of all, this paper presents the overall efficiency correlation to the balance hole diameter of the 3-stages air turbine test results. All 3 wheels have the same balance hole geometry, with total 5 holes in the wheel, and the variation of diameter is 0 mm, 16 mm, 25 mm, 30 mm, 40 mm, 50 mm, and 60 mm, respectively. The test results show that the lowest efficiency condition is the one without balance hole, and increases with the hole diameter until 25–30 mm, but decreases after it. Then, a single stage CFD model, including the first stage stator, moving blade, stator hub leakage, and wheel chamber, is built to simulate the stage efficiency trend under steady flow condition. The relative efficiency correlation of the simplified single stage CFD model matches very well with the test results. The CFD results show that, the maximum efficiency happens when there is minor gas suction from the main gas path, about 0.1%–0.2% compared with the main mass flow, but when the gas suction is above 0.2% of the main mass flow, the efficiency decreases, as less flow is passing through the moving blade to convert energy. Finally, the radial seal clearance’s influences in the wheel chamber between the diaphragm and the wheel is checked by CFD using the same geometry pattern. Five different radial seal clearances model, including 0 mm (ideal condition), 0.6 mm, 1.2 mm, 1.8 mm and 6 mm (without radial seal) are built, under two different balance hole diameter (14 mm and 25 mm), and three different hub seal clearances (0 mm, 0.6 mm and 1.2 mm) conditions. The CFD results show that the throttling effect is very small when the radial clearance is above or equal to 0.6 mm, with almost constant efficiency variation with the clearance up to 6 mm, under specified stator hub clearance and balance hole diameter. Very small deficiency (0.2%) is detected under small balance hole diameter (14 mm) and large stator hub clearance (1.2 mm) condition, as the hub leakage mass flow is much beyond the balance hole flow capability.
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