CFD-BASED PREDICTION OF ROTORDYNAMIC PERFORMANCE OF SMOOTH STATOR-GROOVED ROTOR (SS-GR) LIQUID ANNULAR SEALS
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Copyright 2017 ASME. Circumferentially grooved, annular liquid seals typically exhibit good whirl frequency ratios and leakage reduction, yet their low effective damping can lead to instability. The current study investigates the rotordynamic behavior of a 15 stage groove-on-rotor annular liquid seal by means of CFD, in contrast to previous studies which focused on a groove-on-stator geometry. The seal dimensions and working conditions have been selected based on experiments of Moreland and Childs. The precessional frequency ratios as high as 4 have been studied. The CFD model replicates the whirling motion imposed by the 2D shaker apparatus in Moreland and Childs experimental setup. Implementation of pressure-pressure inlet and outlet conditions obviates the need for loss coefficients at the entrance and exit of the seal. A computationally efficient quasi-steady approach is used to obtain impedance curves as functions of excitation frequency . The effectiveness of steady-state CFD approach is validated by comparison with the experimental results of Moreland and Childs. Results show good agreement in terms of leakage, pre-swirl ratio and rotordynamic coefficients. Leakage is shown to decrease with spin rotational speed , whirl speed and surface roughness . The variation of pre-swirl ratio (PSR) and outlet-swirl ratio (OSR) with these parameters is presented. It was found that PSR will be about 0.3-0.4 at the entrance of seal in the case of radial injection and OSR always converges to values near 0.5 for current seal and operational conditions. The rotordynamic coefficients show negligible dependence on in agreement with experiments. The small negative value of direct stiffness coefficients, large cross-coupled stiffness coefficients and small direct damping coefficients explain the destabilizing nature of these seals. Finally, influence of surface roughness on leakage, PSR, OSR and stiffness coefficients is discussed.
