Abstract
A numerical study has been conducted to explore the effect of a pressure-side winglet on the flow and heat transfer over a blade tip. Calculations are performed for both a flat tip and a squealer tip. The winglet is in the form of a flat extension, and is shaped in the axial chord direction to have the maximum thickness at the chord location, where the pressure difference is the largest between the pressure and suction sides. For the flat tip, the pressure-side winglet exhibits a significant reduction in the leakage flow strength. The low heat transfer coefficient “sweet-spot” region is larger with the pressure-side winglet, and lower heat transfer coefficients are also observed along the pressure side of the blade. For the flat tip, the winglet reduces the heat transfer coefficient locally by as much as 30%, while the average heat transfer coefficient is reduced by about 7%. In the presence of a squealer, the role of the winglet decreases significantly, and a 5% reduction in the pressure loss coefficient is achieved with the winglet with virtually no reduction in the average heat transfer coefficient. On the other hand, the suction-side squealer with constant width winglet shows lower heat transfer (reduction of 5.5%) and pressure loss coefficient (reduction of 26%) than its baseline counterpart.
Highlights
In the development of modern high-performance gas turbines, reduction of aerodynamic losses and thermal loading to the blades are two key areas targeted for improvement
A higher value of the pressure ratio corresponds to a lower static pressure and vice versa. In both the baseline and the winglet-tip cases, the pressure ratio distribution clearly demonstrates the tip leakage flow pattern, that is, the flow is from the lower value of Pt, in/P on the pressure side towards the higher value of Pt, in/P on the suction side
Numerical calculations have been performed for a flat tip and a squealer tip turbine blade at a pressure ratio of 1.73, and the role of a pressure-side winglet in reducing the leakage flow and the tip heat transfer is explored
Summary
In the development of modern high-performance gas turbines, reduction of aerodynamic losses and thermal loading to the blades are two key areas targeted for improvement. Such efforts directed toward reducing the leakage flows have been termed tip desensitization Example of such desensitization studies include (i) addition of tip winglets (Harvey and Ramsdan [16]; Dey and Camci [17]); (ii) modification of the tip blade profile (Bindon and Morphis [18]); (iii) incorporating a nonuniform tip gap in the direction normal to the blade surface (Tallman and Lakshminarayana [19]); (iv) normal/angled injection of fluid into the squealer tip regions (Chen et al [20]); and (v) generation of high intensity turbulence upstream of the rotor for weakening the tip vortex (Staubach et al [21]).
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