The flow in turbomachinery is inherently three-dimensional unsteady and turbulent. Unsteady factors due to the viscous wakes and potential effects of blades, affect the blade surface pressure distribution, which leads to blade vibrations by periodic pulsating flow forces. While the aerodynamic excitation frequency equals to the natural frequency of blades, critical blade vibrations are excited, which could lead to a reduction of lifetime or even a destruction of rotor blades.
Although a large number of investigations about turbine unsteady flow fields and unsteady excitation forces have been carried out, these investigations do not focus on how to reduce the turbine blade unsteady forces. In this article, the turbine stators were redesigned by forward-swept and positive-leaned vanes, in order to reduce the blade excitation forces. And, the swept angles and leaned angles are determined respectively, that is, the vane is forward swept 3 degree, and positive leaned 13 degree, with the purpose of ensuring that while the hub exit flow of a vane aligns with the leading edge of one downstream rotor blade, the tip exit flow could align with the leading edge of another rotor blade that is adjacent to the above rotor blade pressure side. Comparative investigations into the turbine unsteady flow fields between the redesigned and original cases are performed by using a three-dimensional Navier-Stokes viscous solver. Emphasis is placed on how the vane modification reduces the turbine blade unsteady forces.
The results indicate that the static pressure fluctuation at vane trailing edge region is reduced compared to the baseline, and high loss region inside vane passages is also reduced. Besides, blade-to-blade entropy distribution of the turbine stage shows that, the vane wake width is reduced by its shape modification, especially at 5% and 95% span. Due to these, the temporal-spatial profile of rotor blade static pressure coefficient shows that, the unsteady fluctuation of rotor blade becomes weak obviously for the redesigned case. Overall, compared to the baseline, on the condition that the mass flowrate and expansion ratio are nearly the same, the torque fluctuation is reduced from 12.78% to 6.92%, and the axial force fluctuation is reduced from 8.82% to 6.51%, which not only is good for reducing the stream exciting force, but also can make the output power more stable. And, the turbine stage time-averaged efficiency increases slightly by 0.18%. Detailed results about static pressure coefficient distributions, entropy distributions, torque and axial forces are presented and discussed in the paper.
Aerothermodynamic Effects and Modeling of the Tangential Curvature of Guide Vanes in an Axial Turbine Stage