Rotordynamic Characterization of Labyrinth Seals in Steam Turbines: Effects of Thermal and Mechanical Loads

Over the last few decades, the increasing demand on efficiency and performance for steam turbines has resulted in OEMs operating machines near critical conditions of their structural and thermal capabilities. Consequently, a more accurate estimation of the dynamic behavior of the machine has become mandatory as well as the stability assessment. Steam turbines are subjected to high temperatures, high pressures and centrifugal forces that could change the nominal geometry, especially the clearance profile in correspondence of the sealing components, occasionally generating a convergent or divergent annulus. In this paper, a new thermo-elasto bulk-flow model for labyrinth seals has been introduced. The model includes the bulk-flow model for estimating the dynamic coefficients, heat transfer model for evaluating the temperature distribution in the rotating and stationary parts and structural-mechanics model for calculating the radial growth. By considering a staggered labyrinth seal installed in the balancing drum of a steam turbine, different inlet pre-swirl ratios, as well as the stability of the seal are investigated in this paper. The model can be extremely useful for the dynamic characterisation of a wide class of labyrinth seals considering the effect of the surrounding environment on the rotordynamic coefficient prediction.