The Effect of Turbine Cascade Pitch on the Incidence Losses

Abstract—

The results from studies of flat subsonic and transonic cascades with the convergence ratio C_r = 1.2–3.1 are considered. Series with three to four cascades in each with the same airfoil profile and different relative pitches were studied: nine series with a constant blade setting angle and eleven series with a constant effective outlet angle β_2eff. The pitch variation range in the series was approximately 25% with values larger and smaller than the optimal one. The cascades were mainly tested with the incidence angle ∆β₁ from –20° to 20° and flow outlet velocity λ₂ ≈ 0.6–0.9. The experiments have shown that, for cascades with a large convergence ratio C_r ≥ 2.9, the energy loss coefficient in the cascade ∆ζ = 0 and remains unchanged in varying the pitch and with all of the studied values of ∆β₁ and λ₂. In cascades with a smaller convergence ratio, with negative ∆β₁ and positive ∆β₁ ≤ 5°, it can be taken that the value of ∆ζ does not depend on the pitch. With ∆β₁ > 5°, increasing the pitch results in a growth of ∆ζ with decreasing the cascade convergence ratio and flow outlet velocity. The growth of losses is attributed to poorer streamlining of the airfoil profile suction side inlet section, in which the velocity peak increases. An analysis of losses has shown that the dependences $\Delta {{\zeta }}\left( {\bar {t}} \right)$ in the series can be taken to be linear. This made it possible to obtain a generalized dependence for the relative change of losses in varying the pitch as a function of C_r, ∆β₁, and λ₂. A study in which two cascade series with different pitches and the same airfoil profile and with airfoil profiles specially constructed for each pitch were compared using viscous flow numerical analyses for different specified flow conditions has shown that the influence of pitch on the incidence losses that has been revealed in the course of experiments can also be used in optimizing the cascades.