Variable geometry turbines are widely used to improve the part-load performance of gas turbine engines. However, there is a performance penalty associated with the vane-end clearance required for the movement of variable vanes. Especially for variable geometry turbines with high casing-endwall angles, greater vane-end clearances are necessary due to annulus slope, and then high endwall leakages would occur, which further deteriorates turbine efficiency.
The variable geometry design of the first stage stator vane in a four-stage power turbine featuring very high endwall angles has been carried out by proposed stepped spherical endwall concept. The vane endwalls are spherically shaped so as to maintain constant endwall clearance at all turning angles. And, downstream of the spherical endwall an endwall step is introduced, in order to match the original S-shaped endwall contour and to reduce the leakage loss. Meantime, the rotating shaft is inclined upstream to further match the original endwall contour, and cavity tip design has been used to further reduce the leakage loss. An efficient numerical method has been employed to validate the variable geometry design as mentioned, and the effect of a rotating shaft has been included in the calculations. Then, the four-stage variable geometry power turbine characteristics are evaluated.
Results show that the proposed stepped spherical endwall concept can be applied to the variable geometry design of the power turbine featuring very high endwall angles, and compared to the fixed geometry turbine, the efficiency of the new-designed variable geometry power turbine keeps nearly unchanged. Detailed results from this investigation are well presented and discussed in this paper.
