Aerodynamic Optimization of a Winglet-Shroud Tip Geometry for a Linear Turbine Cascade

Abstract

This paper presents a continued study on a previously investigated novel winglet-shroud (WS) (or partial shroud) geometry for a linear turbine cascade. Various width of double-side winglets and different locations of a partial shroud are considered. In addition, both a plain tip and a full shroud tip are applied as the datum cases which were examined experimentally and numerically. Total pressure loss and viscous loss coefficients are comparatively employed to execute a quantitative analysis of the aerodynamic performance. The effectiveness of various width (w) of double-side winglets (DSW) involving 3%, 5%, 7% and 9% of the blade pitch (p) is numerically investigated. Skin-friction lines on the tip surface indicate that the different DSW cases do not alter flow field features including the separation bubble and reattachment flow within the tip gap region, even for the case with the broadest width (w/p = 9%). However, the pressure side extension of the DSW exhibits the formation of the separation bubble, while the suction side platform of the DSW turns the tip leakage vortex away from the suction surface. Meanwhile, the horse-shoe vortex near the casing is not generated even for the case with the smallest width (w/p=3%). As a result, both the tip leakage and the upper passage vortices are weakened and further dissipated with wider w/p in the DSW cases. Larger width of the DSW geometry is indeed able to improve the aerodynamic performance, but in a slight degree. With the w/p increasing from 3% to 9%, the mass-averaged total pressure loss coefficient over an exit plane is just reduced by 2.61%. Therefore, considering both the enlarged (or reduced) tip area and the enhanced (or deteriorated) performance compared to the datum cases, a favorable width of w/p=5% is chosen to design the WS structure. Three locations of the partial shroud (linkage segment) are devised, which are located near the leading edge, the middle and close to the trailing edge respectively. Results illustrates that all three cases of the WS have advantages in lessening the aerodynamic loss over the DSW arrangement, but with the linkage segment located in the middle having optimal effect. This conclusion verifies the feasibility of the previously studied WS configuration.