Variable geometry turbines are widely employed to improve the off-design performance of gas turbine engines; however, there is a performance penalty associated with the vane-end partial gap required for the movement of variable vanes. This paper is a continuation of the previous work and aims to understand the leakage flow and loss mechanisms under the influence of the pivoting axis. Experimental investigations with a variable geometry turbine linear cascade have been conducted for tip gap heights of 1.1% and 2.2% blade spans as well as setting angles of À6 , 0 , and 6 , so as to reveal the three-dimensional clearance flow characteristics associated with partial gaps. Besides, numerical predictions are also carried out to better understand the experimental results. Pressure measurements were performed on the tip endwall as well as on the vane surface, and three-dimensional clearance flow fields downstream of the variable cascade were measured with a five-hole probe. The results show that as the vane setting angle is changed from design to closed, the vane loading increases and tends to be more aft-loaded, thus increasing the tip leakage loss, and vice versa. There are strong interactions between the flow around the pivoting axis and the leakage flow in the vane tip rear part, which leads to a low-pressure region on the tip endwall. The leakage vortex core is made up of the leakage flow in the vane tip rear part at both two tip gap heights, and the leakage vortex core formation process is different from the one in the rotor blade. The present results can provide useful references for the vane-end clearance design of variable geometry turbines.
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.
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