The Reduction of Over Tip Leakage Loss in Unshrouded Axial Turbines Using Winglets and Squealers

The possibilities of reducing the over tip leakage loss of unshrouded rotors have been investigated using a linear cascade of turbine blades and computational fluid dynamics (CFD). The large-scale blade profile is the same as that of the tip profile of a low-speed high-pressure research turbine facility. The impact of various combinations of squealer and winglet geometries on the turbine performance has been investigated. The influence of the thickness of the squealers has also been assessed. It was found that a 22% reduction in loss slope was possible, when compared to the flat tip blade, using simple tip modifications. The results obtained with the suction side squealer and cavity tip agreed well with the work of other researchers. Three winglet-based tip geometries were tested. One was a plain winglet, the other two had squealers applied. A significant impact of the squealers and their shape on the tip gap flow pattern and loss generation was found. The physical processes occurring within the tip gap region for the tested geometries are explained using both numerical and experimental results. The impact of the flow pattern within the tip gap on the loss generation is described. Good agreement between CFD and the experimental data was found. This shows that CFD can be used with confidence in the design process of shroudless turbines.