Analysis of low-pressure turbine flutter for different shroud interfaces

This paper reports the findings of a flutter investigation on a low-pressure turbine rotor having an integrally machined tip shroud with different type of constraints. Two types of tip shroud constraints, namely fully constrained and tangentially free, were used, representing two extreme conditions: (a) a typical integrated shroud design with a tight interlocking and no wear on contact surface; and (b) an extremely smooth contact surface design or the most severe wear of a fully constrained interface, or changes in the manufacturing process that result in almost no friction across the shroud surface. The tangentially free constraint is unusual in that it seeks to explore how sensitively the contact constraint could cause blades to response. The mode shapes and corresponding frequency characteristic are presented for both shroud constraints using a standard finite element analysis. The flutter analysis was firstly undertaken by considering all vibration modes of interest in a single calculation using a whole-annulus model of the rotor. It was found that the removal of the tip constraints in the tangential direction was responsible for introducing the unstable first flap family under condition of zero mechanical damping. Of considerable interest was the fact that instability in the first flap mode occurred in forward-travelling nodal diameter modes, which is considered as somewhat different from classical low-pressure turbine flutter where instability exists in backward-travelling nodal diameter modes. The flutter mechanism was verified by undertaking a detailed investigation on the forward-travelling nodal diameter modes of the first flap family using a single-passage analysis. It was concluded that tip shroud constraints are highly sensitive for turbine blade interlock designs and unusual response could be excited under extremely severe wear condition.