For an improvement of the performance of turbomachinery, decrease of leakage flows in labyrinth seals is very effective. But it can cause subsyncrhonous vibration because of increasing the fluid destabilization forces in labyrinth seals. Seal fluid force has been investigated numerically and experimentally by many researchers and prediction accuracy of seal fluid force under full admission has become reasonable by applying CFD technique in these days. However there is little knowledge about seal fluid force under partial admission such as control stage of conventional steam turbines because phenomenon becomes more complex than full admission and research examples for dynamic characteristics are very less. Partial admission can increase instability of a rotor system therefore high accuracy prediction method for seal fluid force under partial admission is required.  
In this study the experiment which measures a reaction force acting on labyrinth seal with changing admission condition at seal inlet was conducted in order to compare the swirl velocity distribution and seal fluid force under full and partial admission. 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 lot of swirl nozzles were installed at seal inlet so that admission condition and inlet swirl velocity can be controlled by changing a number of effective nozzles.  
According to the experimental results it was found that (1) average swirl velocity in a circumferential direction increased under partial admission compared with full admission and destabilization force (cross-coupled stiffness) also increased, (2) seal fluid force under partial admission had anisotropy because it was varied periodically by the instant rotor position. A relationship between average swirl velocity and destabilization force became almost same even if admission condition is different.  
Effect of swirl brakes on destabilization force under full and partial admission was also confirmed in the experiment. Average swirl velocity at 1st fin decreased around 0.3 times by swirl brakes and destabilization force also decreased almost proportionally to swirl velocity. Furthermore it was found that the relationship between average swirl velocity and destabilization force became almost same despite the existence of swirl brakes.  
Transient calculation and steady calculation of CFD analysis were conducted in order to confirm the effectiveness of CFD analysis for prediction of seal fluid force under partial admission. In partial admission condition, uneven pressure distribution in a circumferential direction is occurred even if there is no rotor eccentricity. Therefore it is necessary to calculate both eccentric and concentric case in order to estimate a change of pressure distribution caused by eccentricity. It was confirmed that experimental results and both CFD analysis results under partial admission were in good agreement.  
Also it was found that CFD analysis could predict the effect swirl brakes on reduction of swirl velocity and seal fluid force under full admission with reasonable accuracy. In the case with swirl brakes, mesh density in a circumferential direction was determined by grid sensitivity analysis results of 1 pitch model　with periodic boundary condition. Frozen rotor interface was applied at swirl brakes region in order to estimate the damping coefficients as well as stiffness coefficients approximately by using steady state CFD (Whiling Rotor method).


Effect of Partial Admission and Swirl Brakes on Destabilization Force of Labyrinth Gas Seal

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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