Preswirl and Mixed-Flow (Mainly Liquid) Effects on Rotordynamic Performance of a Long (L/D= 0.75) Smooth Seal

摘要

Measured results are presented for rotordynamic coefficients and mass leakage rates of a long smooth annular seal (length-to-diameter ratio LID = 0.75, diameter D = 114.686 mm, and radial clearance C, = 0.200 mm) tested with a mixture of silicone oil (PSFScSt) and air. The test seal is centered, the seal exit pressure is maintained at 6.9 bars-g while the fluid inlet temperature is controlled within 37.8-40.6 ℃. It is tested with three inlet-preswirl inserts, namely, zero, medium, and high (the preswirl ratios (PSRs), i.e., the ratio between the fluid's circumferential velocity and the shaft surface's velocity, are in ranges of 0.10-0.18, 0.30-0.65, and 0.65-1.40 for zero, medium, and high preswirls, respectively), six inlet gas-volume fractions GVFj (0%, 2%, 4%, 6%, 8%, and 10%), four pressure drops PDs (20.7, 27.6, 34.5, and 41.4 bars), and three speeds w (3, 4, and 5 krpm). The targeted test matrix could not be achieved for the medium- and high-preswirl inserts at PD ≥ 27.6 bars due to the test-rig stator's dynamic instability issues. Spargers were used to inject air into the oil, and GVF_1 values higher than 0.10 could not be consistently achieved because of unsteady surging flow downstream from the sparger mixing section. Leakage mass flow rate m and rotordynamic coefficients are measured, and the effect of changing inlet preswirl and GVFj is studied. The test results are then compared with predictions from a two-phase, homogeneous-mixture, bulk-flow model developed in 2011. Generally, both measurements and predictions show little change in m as inlet preswirl changes. Measured m remains unchanged or slightly increases with increasing GVFj, but predicted m decreases. Measured m is comparable to predicted values but consistently lower. Dynamic-stiffness coefficients are measured using an ensemble of excitation frequencies and curve-fitted well by frequency-independent stiffness K_(ij), damping C_(ij), and virtual mass M_(ij) coefficients. Planned tests with the medium- and high-preswirl inserts could not be accomplished at PD = 34.5 and 41.4 bars because the seal stator became unstable with any finite injection of air. The test results show that the instability arose because the seal's direct stiffness K became negative and increased in magnitude with increasing GVF_i The model predicts a drop in K as GVF_i increases, but the test results dropped substantially more rapidly than predicted. Also, the model does not predict the observed strong tendency for K to drop with an increase in preswirl in moving from the zero-to-medium and medium-to-high preswirl inserts. The authors believe that the observed drop in K due to increasing GVF_i is not explained by either (a) a reverse Lomakin effect from operating in the transition flow regime or (b) the predicted drop in K at higher GVF_i values from the model. A separate and as yet unidentified two-phase flow phenomenon probably causes the observed results. The negative K results due to increasing GVF_i and moving from the zero to medium, and medium to high preswirl observed here could explain the instability issue (sudden subsyn-chronous vibration) on a high-differential-pressure helico-axial multiphase pump (MPP), reported in 2013. Effective damping C_(eff) combines the stabilizing effect of direct damping C, the destabilizing effect of cross-coupled stiffness k, and the influence of cross-coupled mass m_q. As predicted and measured, increasing inlet preswirl significantly increases k and decreases C_(eff), which decreases the seal's stabilizing properties. C_(eff) increases with increasing GVF_i—becomes more stable.