Prediction of Shrouded Turbomachinery Blade Forced Response by a Complex Mode Analysis

Abstract

Turbomachinery shrouded rotor blade design has been widely used in fans, compressors, and turbines. By using either tip or mid-span shroud, the blade structural damping and natural frequencies can be increased to reduce flutter or forced response problems. However, shrouded rotor blade design sometimes results in complex system modes with both bending and torsion present at the same time. Since shrouded rotors are rotationally periodic, the bending and torsion combined system mode can be decomposed into a real component and an imaginary component. Using both the real and imaginary mode shapes, coupled with the blade surface flow-field, a complex mode analysis was developed using a modal solution to determine the forced response of the system. This complex mode analysis was applied to both tip and mid-span shrouded rotor blade applications. Bending-dominated or torsion-dominated simple blade modes in general can be analyzed using an existing single mode approach. However, for the bending and torsion combined system modes, the single mode analysis can be misleading. By using the complex mode analysis, it was demonstrated that the combined bending and torsion system mode plays an important role in determining the blade forced response.