The Effects of Imposed Pre-Swirl on the Static and Rotordynamic Performance for Smooth Annular Liquid Seals
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The tested annular seals were smooth with a length-to-diameter ratio of 0.50, a diameter of 102.0064 mm (4.016 inches), and a nominal radial clearance of 0.2032 mm (0.008 inches). The tests were conducted for angular shaft speeds, w, of 2, 4, 6, and 8 krpm, axial pressure drops, deltaP, of 2.1, 4.13, 6.21, and 8.27 bar (30, 60, 90, and 120 psi), eccentricity ratios, e0, of 0.00, 0.27, 0.53, and 0.8, with three pre-swirl inserts to target zero, medium, and high (0.0, 0.4, and 0.8) pre-swirl ratios for a set of pre-determined operating conditions with ISO VG 2 oil at 46.1°C (115°F). Circumferential fluid velocity components were measured at single locations upstream and downstream from the seal’s inlet and exit. Most of the data presented in this thesis takes place in the transitional regime. The static performance of the seals was evaluated by measuring volumetric leakage rate, Q, inlet and exit circumferential fluid velocity, and the rotor’s eccentric position. Comparisons are made between measured values and predictions. The measured vector Reynolds number ranged from approximately 1180 to approximately 4350, with only occasional points in the laminar and turbulent regimes. Pitot tubes were used to measure the average circumferential velocity at the inlet and outlet of the seal were used to calculate pre-swirl ratio, PSR, and post-swirl ratio, OSR, respectively. The PSR tended to converge to a value of approximately 0.4-0.5 as w increased for all tested pre-swirl inserts. There was little correlation between the inlet pre-swirl ratio and post-swirl ratio, OSR. This is the first known attempt to measure OSR. Rotordynamic coefficients analyzed include direct and cross-coupled: stiffness, damping, and virtual mass. The rotordynamic coefficients were generally unaffected by changing e0 E [0.00, 0.53]. When e0 = 0.80, the measured Kxx values were generally negative, while Kyy increased in magnitude by a factor of approximately 1.5 from the measured values at e0 = 0.53. The results indicated that when higher PSRs were experienced by the seal, the seal was more destabilizing. The direct damping values, Cxx and Cyy, were normally positive, and equal. When e0 = 0.80 the measured cross-coupled damping, Cxy and Cyx, decreased by a factor of 5, causing both to become negative; changing the damping from gyroscopic to dissipative. The whirl frequency ratio, WFR, indicated that the cross-coupled virtual mass terms were too small in magnitude to affect overall rotordynamic stability. WFR was generally within ~0.3-0.6 and was unaffected by changing either e0 or delta P except at w = 2 krpm. When w = 2 krpm for the zero pre-swirl insert, WFR increased in magnitude by a factor of approximately 4 when e0 increased from 0.00 to 0.80. The effective damping results showed that of the tested seals, the most stable seal configuration was w = 2 krpm for the zero pre-swirl insert particularly at high delta P’s. Both the WFR and effective damping showed that as w increases, the effect of the pre-swirl insert diminishes. For the zero pre-swirl insert, the seals were generally less stable than predicted with a few exceptions..
Bullock, Joshua Thomas (2019). The Effects of Imposed Pre-Swirl on the Static and Rotordynamic Performance for Smooth Annular Liquid Seals. Master's thesis, Texas A&M University. Available electronically from