Investigation of Turbine Shroud Distortions on the Aerodynamics of a One and One-Half Stage High-Pressure Turbine

As part of a proactive effort to investigate the ability of computational fluid dynamic (CFD) tools to predict time-accurate surface-pressure histories, a combined experimental/computational investigation was performed examining the effect of rotor shroud (casing) out-of-roundness on the unsteady pressure loading for the blade row of a full-stage turbine. The casing out-of-roundness was idealized by designing a casing ring with a sinusoidal variation. This casing ring was used to replace a flat casing for an existing turbine and direct comparisons were made between the time-accurate pressure measurements and predictions obtained using the flat and “wavy” casings. For both casing configurations, predictions of the unsteady pressure loading for many locations on the blade and vane were obtained using Numeca’s FINE/Turbo code and the General Electric TACOMA code. This paper will concentrate on the results obtained for the “wavy” casing, but the results for the flat casing are presented as a baseline case. The time-accurate surface-pressure measurements were acquired for the vane and blade of a modern, 3-D, stage and 1/2 high-pressure turbine operating at the design corrected speed and stage pressure ratio. The research program utilized an un-cooled turbine stage for which all three airfoil rows are heavily instrumented at multiple spans to develop a full dataset. The vane-blade-vane count for this machine is 38-72-38. The number of waves in the distorted shroud “wavy wall” is approximately 1.5-times the number of vanes. The resulting changes in aerodynamic surface-pressure measurements were measurable at all blade span wise locations. Variations in time-average surface pressure of up to 5% of the flat casing values were observed. In addition, the frequency content of the time-resolved blade data for the “wavy” casing changed substantially from that measured using the flat casing, with changes in both amplitudes and frequencies. Imposing the casing irregularity changed the fundamental physics of the problem from a single frequency and its harmonics to a multi-frequency problem with mixed harmonics. The unsteady effects of this type of problem can be addressed using the harmonic method within Numeca’s FINE/Turbo code, which is designed to handle multiple blade passing frequencies and harmonics for one blade row. A more traditional approach is included in the paper by employing the TACOMA code in a linearized mode that produces results for a single frequency. These results show that casing irregularity can have a significant influence on the blade surface-pressure characteristics. Further, it is demonstrated that the FINE/Turbo code experienced difficulty predicting the unsteady pressure signal attributed to the “wavy” casing configuration, while at the same time capturing the unsteady signal attributed to the vane passing due to limitations in the current methodology.