EFFECTS OF TURBINE BLADE TIP PRESSURE SIDE WINGLET AND CAVITY RIM ON AERODYNAMIC PERFORMANCE

Turbine blade tip leakage flows occur between the unshrouded rotor and stationary casing. Such flow is one of the main sources of aerodynamic loss in a turbine. Many different methods have been proposed to reduce this loss, including the squealer tip and winglet design. In this study, numerical analysis showed that a large fraction of the leakage flow occurs after 50% of the axial chord. Thus, an additional rim was installed at the mid-point of the cavity, in an attempt to block the leakage flow to the suction side and redirect it back to the pressure side. The effect of an extended pressure-side winglet was also investigated and compared with the base squealer tip. The passage velocity field between the blades of a linear cascade was measured at 0, 25, 50, 75, and 100% of the axial chord using a 5-hole probe to assess the development of flow structures responsible for the tip leakage loss. The total pressure loss coefficient distribution was measured downstream of the cascade. Performance results from the different tip geometries were experimentally compared with each other, and also used to validate the numerical results. The winglet design showed the best performance. This design does not have a pressure-side rim, and thus the tip leakage flow has less hindrance passing over the suction-side rim, which creates a strong coherent tip leakage vortex compared to that of the squealer tip. This leads to an increase in loss beyond 90% of the span, but the counter-rotating tipwall passage vortex underneath interacts with the tip leakage vortex to reduce the loss in the 70 - 80% span region by a greater extent. Therefore, the pressure-side winglet design has an overall loss that is 28.3% smaller than that of the baseline squealer tip. The additional cavity rim did not show any noticeable improvements, possibly due to the angle at which it was placed, and thus needs further investigation.