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Description:
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An analysis is developed for a compressible bulk -flow model of the leakage path
between a centrifugal compressor's impeller shroud and housing along the front and back
side of the impeller . This is an extension of analysis performed first by Childs (1989 ) for
a shrouded pump impeller and its housing considering an incompressible fluid , and then
later by Cao (1993 ) using a compressible bulk flow model for the shroud of a cryogenic
fluid pump . The bulk -flow model is used to develop a reaction force and moment model
for the shroud of a centrifugal compressor by solving the derived governing equations
and integrating the pressure and shear stress distribution . Validation is done by
comparing the results to published measured moment coefficients by Yoshida et al .
(1996 ) . The comparison shows that the shroud casing clearance flow and the fluid force
moment can be simulated by the bulk flow model fairly well . An Iwatsubo -based
labyrinth seal code developed by Childs and Scharrer (1986 ) is used to calculate the
rotordynamic coefficients developed by the labyrinth seals in the compressor . Tangential
force and transverse moment components acting on the rotor are found to have a
destabilizing influence on the rotor for a range of precession frequencies . Rotordynamic
coefficients are derived for a single stage of a multistage centrifugal compressor , and a
comparison is made to stability predictions using Wachel's coefficient using the XLTRC
(rotordynamic FEA code ) . For the model employed , Wachel's model predicts a slightly
lower onset speed of instability . The results also show that leakage that flows radially
inwards on the back shroud has a greater destabilizing influence than leakage flow that is
radially outwards . Seal rub conditions are simulated by increasing the clearance and
simultaneously decreasing the tooth height , which increased the leakage and the swirl tothe eye seal inlet ; and therefore reduced stability . Calculated results are provided for
different seal clearances and tooth height , for seal and shroud forces and moments . |