Validation of CFD Analysis Method for Seal Dynamic Coefficients With Various Labyrinth Seal Types

Abstract

Seal flows in labyrinth seals can produce fluid destabilization forces, which in turn can cause subsyncrhonous vibration in turbomachinery such as steam turbines and centrifugal compressors. In this study, an experiment measuring the dynamic coefficients of several types of labyrinth seal is conducted. Analytical results are obtained by computational fluid dynamics (CFD) and are compared with experimental results. For the experiment, a 500mm diameter rotor was used so that size of the labyrinth seal can be close to the large-scale steam turbine. A rotor was excited in a circular whirl orbit at the natural frequency of the parallel mode of the rotor by an electromagnetic actuator. The stator reaction force was measured with load cells and the dynamic force of the stator was corrected with measurements from accelerometers. The tests are conducted at an operating point of 8000rpm with airflow corresponding to a 0.2MPa pressure difference as rated condition. Labyrinth seal types include a staggered labyrinth seal with an abradable coating and a stepped labyrinth seal with straight fins and with slanted fins. For the CFD calculation, a steady state calculation can estimate not only the stiffness coefficients but also the damping coefficients of the seals by solving in a rotating frame of reference during rotor whirl. It was found that both the CFD analysis results and the experimental results were in reasonable agreement for all types of seal. Therefore a CFD analysis method for obtaining seal dynamic coefficients has been validated with various labyrinth seal types.