The Influence of Groove Size on the Static and Rotordynamic Characteristics of Short, Laminar-Flow Annular Seals

Test results are presented for a smooth seal and three centrally-grooved seals that are representative of buffered-flow oil seals in centrifugal compressors. The seals are short (L/D ≅ .21), with a diameter of 117 mm and a nominal radial clearance of 0.085 mm, netting the clearance-to-radius ratio .0015. The grooves have groove depth to clearance ratios (D_g/C_r) of 5, 10, and 15. Test conditions include three shaft speeds from 4000 to 10000 rpm, three inlet oil pressures from 24 to 70 bars, and seal eccentricity ratios from 0 (centered) to 0.7. Dynamic results include stiffness, damping, and added-mass coefficients; static results include stator position, attitude angles, and seal leakage. Stiffness, damping, and mass coefficients plus leakage are compared for the seal geometries. Results show that all rotordynamic coefficients consistently decrease with increasing seal groove depths, and seal leakage is largely unchanged. Comparisons are also made between experimental results and predictions from a computer program based on a Reynolds + energy equation model. The model includes the assumption that a groove is large enough to create separate lands within the seal, creating a zero or negligible pressure perturbation within the groove. Test results show that even the deepest groove depth tested is not deep enough to satisfy this assumption.