Abstract: Unstable vibration is likely to occur on turbine unit under partial admission mode. The computational fluid dynamics (CFD) model to calculate the uneven steam force of governor stage for a turbine unit is set up. Steam force at different valve sequence case is given. It is shown that difference of the magnitude and the angle of the uneven steam force at different partial admission mode is large. Journal eccentricity and load angle in bearing and bearing dynamics are changed a lot under the influence of steam force acting on rotor. Rotor bearing dynamic model to calculate the unbalance response and system stability during valve change process is set up using the finite element method. Rotor unbalance response and stability at difference partial admission mode is researched. The mechanism of the unstable vibration is explained on the base of the influence of uneven steam force. It is found that the unstable vibration under partial admission is influenced by the load of bearing near governor stage largely. The unstable vibration is likely to occur on lightly loaded bearing. A method to reduce the influence of partial admission is put forward. Elevation of bearing near governor stage should be raised properly on the premise that bearing temperature is not high.

Key words: turbine;partial admission;bearing;vibration