Flutter of Grouped Turbine Blades

Abstract

An analysis is presented to predict flutter in a wheel of turbine blades which are connected together into a number of identical groups. The natural frequencies and mode shapes of a group are assumed to be known. The unsteady aerodynamic coefficients for free-standing blades are assumed to be known from an unsteady aerodynamic program, and FINSUP is used here. The work fed into the vibration by the aerodynamic forces is then calculated. This is illustrated by two examples of low pressure steam turbine blade rows GR-1 and GR-2. On GR-1 the three modes considered are all found to be stable, but on GR-2 the lowest frequency mode shows some instability. Tying the blades together in groups is found to be stabilizing. Blade response, measured by a Blade Vibration Monitor at two different installations, is shown for a range of operating conditions. The measured responses indicate the GR-1 blade is stable whereas the GR-2 blade shows, at the lowest frequency, high response that is dependent on turbine operating conditions.