| dc.description.abstract | The thesis presents measured results for rotordynamic coefficients and leakage of a long annular smooth seal tested with a mixture of silicone oil and air. Due to the difficulties in (a) making 2-phase homogeneous mixtures with gas volume fraction (GVF) in a range of 10 to 92% and (b) operating the test rig, the tests are carried out for two operation ranges: (1) pure- and mainly-oil and (2) pure- and mainly-air conditions. Leakage mass flow rate and rotordynamic coefficients are measured, and the effect of changing inlet GVF/LVF, pressure drop/pressure ratio (PD/PR), w, and inlet preswirl are studied. The test results are compared with predictions from a bulk-flow-model developed in 2011.
Under pure- and mainly-oil conditions, the test seal is centered, the seal exit pressure is maintained at 6.89 bar-g while the fluid inlet temperature is controlled within 37.8-40.6oC. The test seal is tested with 3 inlet preswirl inserts (zero, medium, and high), 3 speeds (3, 4, and 5 krpm), 6 inlet GVFs (0%, 2%, 4%, 6%, 8%, and 10%), and 3 PDs (27.6, 34.5, and 41.4 bars) for the zero-preswirl insert, and 2 PDs (20.7 and 27.6 bars) for the medium- and high-preswirl inserts.
Both predictions and measurements show a significant drop in measured and predicted direct stiffness K as a result of injecting air into the oil flow at PD=41.4 bar for the zero preswirl and at PD=27.6 bar for the medium and high preswirls. Since the flows are in a transitional regime, an increase of friction factor with increasing Reynolds number (Re), due to a GVF increment, potentially causes a reverse Lomakin effect, causing a drop in K. A drop in K could decrease the rotor’s critical speed, reduce the onset speed of instability, and cause dynamic instability. As predicted and measured, increasing inlet preswirl significantly increases cross coupled stiffness k and whirl-frequency ratio and decreases effective damping Ceff, which decrease the seal’s stabilizing properties. The results obtained from K and k potentially relate to the instability issue (subsynchronous vibration) on the high-differential-pressure helico-axial multiphase pump, reported in 2013.
Under pure- and mainly-air conditions, due to the rig-stator’s dynamic instability, tests were only conducted for the zero-preswirl insert. The test seal is centered, and inlet pressure is maintained at 62.1 bar-g. The supplied oil temperature is maintained within 35 to 37.8oC. The seal is tested with 3 PRs (0.6, 0.5, and 0.4 - corresponding to PD of 24.8, 31.1 and 37.3 bar), 3 speeds (5, 10, and 15 krpm), and 5 inlet LVFs (0%, 2%, 4%, 6%, and 8%).
Predictions show no change at PR=0.4 and a drop at PR=0.5 and 0.6. With representative operating conditions for a compressor balance-piston seal, measured Ceff is significantly higher than predicted. As inlet LVF increases, predictions show a drop of the seal’s stabilizing capacity (predicted Ceff decreased). However, measurements show the lowest measured Ceff is at inlet LVF=2%.
Smooth seals are not used in centrifugal compressors. The data presented here are mainly for examining the previously developed model. From the same rig, test results for tooth-on-stator labyrinth seals were recently reported. Future testing, under mainly-gas conditions, could consider different seal types, such as, hole-pattern and pocket damper seals which are used widely in compressors. | |
