PREDICTION OF UNSTEADY FORCE FOR AXIAL TURBINE BUCKETS (EFFECTS OF NOZZLE-BUCKET AXIAL GAP LENGTH AND BLADE COUNT RATIO)

The purpose of this study is to investigate the unsteady force acting on steam turbine buckets which is induced by a potential field and a wake. The authors have already proposed a new method that can separate the unsteady force into a potential field interaction and a wake interaction from the unsteady viscous flow computation: It is understood from the previous work that simple expressions may be written for the relationships between the nozzle-bucket axial gap length and the unsteady force, which means the amplitude and the phase of the unsteady force with the nozzle passing frequency component. This paper adopts the blade count ratio as the factor to study the unsteady force of the bucket sections with different bucket heights in the same stage. Four points are seen from the results; points (a)-(c) do not depend on the blade count ratio. (a) The amplitudes of the unsteady force of each interaction can be approximated by an exponential or a power function of axial gap length. (b) The amplitudes of each interaction tend to become large when bucket turning angle is large. (c) The phase of the unsteady force of each interaction with the nozzle passing frequency component has a linear relationship with axial gap length. (d) The amplitudes of each interaction become the maximum value when the blade count ratio is 1.5.