Turbine Blade Vibration Due to Nozzle Wakes

Abstract

Turbine blade vibrations due to resonance with the nozzle passing frequency have been measured and discussed with primary emphasis on the relationships between the resonant stresses and the stage flow parameters. Vibratory stresses were measured in a full admission, single-stage, impulse steam turbine test facility. The blading was shrouded into 6-blade groups, designed to bring the out-of-phase tangential modes into resonance. Some comparisons are made between measured and predicted frequencies and mode shapes. The variation of stresses, with such parameters as stage torque, velocity ratio, and mass flow, was studied by testing at eleven different operating conditions. It was found that stresses increased with torque, but less than proportionately so. It was found that stresses varied with velocity ratio in a complicated fashion, reaching a maximum and then a minimum as velocity ratio is increased. These empirical results are interpreted qualitatively in the light of available theory. It is concluded that the apparent dependence on velocity ratio may also reflect a dependence on Mach number and a dependence on the number of wakes in simultaneous engagement with a given blade. The same rotor blading was tested with two sets of nozzle (stator) blades of different sizes, and some data were also obtained at various values of axial spacing between rotating and stationary blades. The interrelationship between the effects of relative sizes and spacing between the two blade rows is discussed.