Aeroelastic Stability of Welded-in-Pair Low Pressure Turbine Rotor Blades: A Comparative Study Using Linear Methods

The aerodynamic damping of a modern low pressure turbine bladed-disk with interlock rotor blades is compared for the first time to that obtained when the rotor blades are welded in pairs through the lateral face of the shroud. The damping is computed solving the linearized Reynolds averaged Navier-Stokes equations on a moving grid. First the basics of the stabilizing mechanism of welding the rotor blades in pairs is investigated using two-dimensional analyses and the Panovsky and Kielb method. It is concluded that the stabilizing effect is due to the suppression of unsteady perturbations in one out of the two passages providing for the first time a physical explanation to engine data. Three-dimensional effects are then studied using the actual mode shapes of two bladed disks differing solely in the shroud boundary conditions. It is concluded that the increase in the aerodynamic damping, due to the modification of the mode shapes caused by welding the rotor blades in pairs, is smaller than that due to the overall raise of the reduced frequencies of a bladed disk with an interlock design. The modification of the flutter boundaries due to mistuning effects is assessed using the reduced order model known as the Fundamental Mistuning Model. A novel extension of the critical reduced frequency stability maps accounting for mistuning effects is derived and applied for both, the freestanding and welded-in-pair airfoils. The stabilizing effect of mistuning is clearly seen in these maps. Finally, the effect of mistuning on low-pressure-turbine bladed disks is studied. It is shown that the modification on the stability limit of the interlock bladed disk is negligible, while for the welded-in-pair configuration a 0.15% increase of the damping relative to the critical damping is found. This qualitative difference between both configurations had not been reported before.