Air Model Tests of Labyrinth Seal Forces on a Whirling Rotor

Abstract

Steam-excited whirl of high pressure turbine rotors is caused by shroud and shaft labyrinth seal forces and by flow forces on the blades. Accurate test results are needed to guide development of a valid method for calculating labyrinth seal whirl forces and to verify the method. This paper describes apparatus for accurate measurement of labyrinth seal forces on a whirling model rotor. The effects of some system parameters on the whirl excitation constant (whirl force/whirl amplitude.) and radial stiffness of a model seal are shown. The seal whirl force is destabilizing for some conditions and stabilizing for others. A method is given for predicting the net seal and blade-row excitation constant that will cause self-excited whirl of rotors having specified shaft and bearing parameters.