Impact of Turbine Center Frame Wakes on Downstream Rows in Heavy Duty Free Power Turbine

In this work, the effects of Turbine Center Frame (TCF) wakes on the aeromechanical behavior of the downstream Free Power Turbine (FPT) blades are numerically investigated and compared with experimental data. The FPT of a small industrial gas turbine has been selected as a test case. It is composed by a TCF followed by two stages and a Turbine Rear Frame (TRF) before the exhaust plenum. Full annulus unsteady computations of the whole low-pressure module have been performed. Two operating conditions, full (100%) and partial (50%) load, have been investigated with the aim of highlighting the impact of TCF wakes convection and diffusion through the downstream rows. Attention was paid to the harmonic content of rotors’ blades. From an aerodynamic point of view, the results show a slower decay of the wakes through the downstream rows in off-design conditions as compared to the design point. The wakes generated by the struts at partial load persist throughout the domain outlet, while they are chopped and circumferentially transported by the rotors motion. This is due to the strong incidence variation at which the TCF works that induces the growth of wide regions of separated flow on the rear part of the struts. Nevertheless, the analysis of the rotors’ frequency spectrum reveals that moving from design to off-design conditions, the effect of the TCF does not change significantly, thanks to the filtering action of the first FPT stage movable Nozzle Guide Vane (NGV). From unsteady calculations the harmonic contribution of all turbine components has been extracted, highlighting the effect of statoric parts on the last FPT bucket. Anyhow the TCF harmonic remains the most relevant from an aeromechanic point of view as per experimental evidence, and it is considered for a Force Response Analysis (FRA) on the last FPT bucket itself. Finally, aerodynamic and aeromechanic predictions have been compared with the experimental data to validate the numerical approach. In the final part of this paper some general design solutions, that can help mitigation of the TCF wakes impact, are discussed.