WP 2 - Unsteady Aerodynamics M odel Detailing: Tip Shroud Sensitivity of Forced Response Predictions Tobias Gezork, KTH Heat and Power Technology /10 TURBO POWER Program Conference 2014

WP4 HCF Life WP2 Aerodynamic Damping and Forcing WP3 Structural Damping WP2 WP3 WP4 X WP1 Synthesis 2

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Background CFD calculations should match reality, yet calculations are done on simplified models using as much detailing as necessary for fast but reliable or accurate analysis predictions (detailing: e.g. mesh or geometry features (tip gap, cavity flow, fillets, mid-span shrouds, …)) Uncertainties arise from both the physical model and assumptions The necessity to quantify the influence of these features on unsteady aerodynamics is given 4

Unsteady aerodynamics Turbomachinery aeromechanics comprises blade row excitation and forcing and flutter (aeromechanic stability) 5 (Giles, 1991)

This talk Today state of the art forcing and damping calculations are performed with 3D calculation tools including tip clearances Conservative modelling approaches with unknown uncertainty inhibit optimum design Unsteady forcing can change by 20% when neglecting a feature such as a tip shroud leakage cavity 6

Detailing feature: Cavity flow Tip clearance or tip leakage flow for shrouded blades Cavity leakage flow between disks and stages Cavities are at present not included in most aeromechanical calculations Test case: KTH test turbine

Detailing feature: Cavity flow Unsteady rotor blade loading at 90% span shows significant local influence, especially on the suction side For a blade prone to vibration with large deflections close to the tip this effect has to be accounted for 8 cavity case

Detailing feature: Cavity flow Distinct differences in forcing (20%) is visible in forcing for two operating points 9 circumferential forces axial forces

Detailing feature: Cavity flow Changes in loading and forcing originate from vortex interactions cavity case base case streamlines at cavity inlet colored by time difference in first excitation harmonic unsteady pressure on rotor blade compared to base case first excitation harmonic unsteady pressure on rotor outlet compared to base case 10

Other influence features to investigate in future work Mesh convergence Detailing features Leakage flow Changes/uncertainty in operating/boundary conditions Flow path geometry (cold to hot, manufacturing tolerances) Correct modal representation for damping calculations De-coupled/coupled fluid structure interaction … all of these have an influence on prediction accuracy for flutter and forcing 11

Synthesis The work done in this work package improves the understanding and requirements of accurate unsteady CFD predictions for modelling forced response or flutter The aim is to identify sensitivity of different parameters or features to determine with what penalty they can be omitted or added as a quantifiable uncertainty Results of shroud cavity study published in peer reviewed conference paper GT Future investigations will focus on test cases with available high quality data (compressor test rig/turbine test case) 12

AROMA prediction accuracy 13 design influence and accuracy knowledge increase/decrease forcing affect accuracy slightly/significantly feature: