Aeromechanical Optimisation of a Winglet-Squealer Tip for an Axial Turbine

The possibility of reducing the over tip leakage loss of unshrouded axial turbine rotors has been investigated in an experiment using a linear cascade of turbine blades and by using CFD. A numerical optimisation of a winglet-squealer geometry was performed. The optimisation involved the structural analysis alongside the CFD. Significant effects of the tip design on the tip gap flow pattern, loss generation and mechanical deformation under centrifugal loads were found. The results of the optimisation process were verified by low speed cascade testing. The measurements showed that the optimised winglet-squealer design had a lower loss than the flat tip at all of the tested tip gaps. At the same time, it offered a 37% reduction in the rate of change of the aerodynamic loss with the tip gap size. The optimised tip geometry was used to experimentally assess the effects of the opening of the tip cavity in the leading edge part of the blade and the inclination of the pressure side squealer from the radial direction. The opening of the cavity had a negligible effect on the aerodynamic performance of the cascade. The squealer lean resulted in a small reduction of the aerodynamic loss at all the tested tip gaps. It was shown that a careful consideration of the mechanical aspects of the winglet is required during the design process. Mechanically unconstrained designs could result in unacceptable deformation of the winglet due to centrifugal loads. An example winglet geometry is presented that produced a similar aerodynamic loss to that of the optimised tip but had a much worse mechanical performance. The mechanisms leading to the reduction of the tip leakage loss were identified. Using this knowledge, a simple method for designing the tip geometry of a shroudless turbine rotor is proposed. Numerical calculations indicated that the optimised low-speed winglet-squealer geometry maintained its aerodynamic superiority over the flat tip blade with the exit Mach number increased from 0.1 to 0.8.