The Effects of Converging and Diverging Axial Taper on the Rotordynamic Coefficients of Liquid Annular Pressure Seals: Theory Versus Experiment

1995 ◽  
Author(s):  
W. Todd Lindsey ◽  
Dara W. Childs
Keyword(s):  
Turbulent Flow ◽  
Experimental Results ◽  
Running Speed ◽  
Rotordynamic Coefficients ◽  
Rotation Rates ◽  
Damping Coefficients ◽  
Shaft Rotation ◽  
Theoretical Predictions ◽  
Theory Versus ◽  
Direct Stiffness

Abstract Experimental results are compared to predictions for turbulent flow, short (D = 76.2mm, L/D = .17), smooth annular seals with converging and diverging axial taper. Results are presented for four geometries: two convergent, two divergent, and a constant-clearance. Measurements were taken at seal pressure differentials and shaft rotation rates ranging from 1.34 to 3.54 MPa and 10,200 to 24,600 rpm, respectively. Measurements parameters include leakage, direct stiffness, cross-coupled stiffness, and direct damping coefficients. Results show that direct stiffness generally increases with converging axial taper and decreases with diverging axial taper. Direct damping and cross-coupled stiffness were shown to decrease with increasing convergent or divergent taper. Measured damping values increase with increased running speed and decreasing average clearance. Theoretical predictions for rotordynamic coefficients are in good overall agreement with measured results. The theory consistently underpredicts leakage by ranges of 10 ∼ 30%. The accuracy of predictions for leakage and rotordynamic coefficients was not influenced by running speed.

The Effects of Converging and Diverging Axial Taper on the Rotordynamic Coefficients of Liquid Annular Pressure Seals: Theory Versus Experiment

1995 ◽  
Vol 122 (2) ◽  
pp. 126-131 ◽  
Author(s):  
W. Todd Lindsey ◽  
Dara W. Childs
Keyword(s):  
Turbulent Flow ◽  
Qualitative Agreement ◽  
Running Speed ◽  
Taper Angle ◽  
Rotordynamic Coefficients ◽  
Rotation Rates ◽  
Damping Coefficients ◽  
Theoretical Predictions ◽  
Theory Versus ◽  
Direct Stiffness

Experimental results are compared to predictions for turbulent flow, short (D=76.2mm,L/D=.17), smooth annular seals with converging and diverging axial taper. Results are presented for four geometries with the same minimum clearances: two convergent, two divergent, and a constant-clearance. Measurements were taken at seal pressure differentials and shaft rotation rates ranging from 1.34 to 3.54 MPa and 10,200 to 24,600 rpm, respectively. Measurements parameters include leakage, direct stiffness, cross-coupled stiffness, and direct damping coefficients. Results show that direct stiffness generally increases with converging axial taper and decreases with diverging axial taper; however, direct stiffness decreases in the first increase in the taper angle, contrary to predictions. Direct damping and cross-coupled stiffness were shown to decrease with increasing convergent or divergent taper. Measured damping values increase with increased running speed and decreasing average clearance. Theoretical predictions for rotordynamic coefficients are in reasonable qualitative agreement with measured results. The theory consistently underpredicts leakage by ranges of 10∼30 percent. The accuracy of predictions for leakage and rotordynamic coefficients was not influenced by running speed. [S0739-3717(00)70102-4]

scholarly journals Theory Versus Experiment for the Rotordynamic Coefficients of Annular Gas Seals: Part 2—Constant-Clearance and Convergent-Tapered Geometry

1986 ◽  
Vol 108 (3) ◽  
pp. 433-437 ◽  
Author(s):  
C. C. Nelson ◽  
D. W. Childs ◽  
C. Nicks ◽  
D. Elrod
Keyword(s):  
Experimental Test ◽  
Experimental Results ◽  
Test Facility ◽  
Rotordynamic Coefficients ◽  
Theory Versus ◽  
Direct Stiffness ◽  
Gas Seals ◽  
Predicted Values

An experimental test facility is used to measure the leakage and rotordynamic coefficients of constant-clearance and convergent-tapered annular gas seals. The results are presented along with the theoretically predicted values. Of particular interest is the prediction that optimally tapered seals will have significantly larger direct stiffness than straight seals. The experimental results verify this prediction. Generally the theory does quite well, but fails to predict the large increase in direct stiffness when the fluid is prerotated.

Theory Versus Experiment for the Rotordynamic Characteristics of a Smooth Annular Gas Seal at Eccentric Positions

1995 ◽  
Vol 117 (1) ◽  
pp. 148-152 ◽  
Author(s):  
C. R. Alexander ◽  
D. W. Childs ◽  
Z. Yang
Keyword(s):  
Frequency Ratio ◽  
Pressure Ratio ◽  
Experimental Results ◽  
Test Results ◽  
Eccentricity Ratio ◽  
Rotordynamic Coefficients ◽  
Theory Versus ◽  
Direct Stiffness ◽  
Static Eccentricity ◽  
Independent Model

Experimental results are presented for the rotordynamic coefficients of a smooth gas seal at eccentricity ratios out to 0.5. The effects of speed, inlet pressure, pressure ratio, fluid prerotation, and eccentricity are investigated. The experimental results show that direct stiffness KXX decreases significantly, while direct damping and cross-coupled stiffness increase with increasing eccentricity. The whirl-frequency ratio, which is a measure of rotordynamic instability, increases with increasing eccentricity at 5000 rpm with fluid prerotation. At 16,000 rpm, the whirl-frequency ratio is insensitive to changes in the eccentricity. Hence, the results show that eccentric operation of a gas seal tends to destabilize a rotor operating at low speeds with preswirled flow. At higher speeds, eccentric operation has no significant impact on rotordynamic stability. The test results show that the customary, eccentricity-independent, model for rotordynamic coefficients is only valid out to an eccentricity ratio of 0.2~0.3. For larger eccentricity ratios, the dependency of rotordynamic coefficients on the static eccentricity ratio needs to be accounted for. Experimental results are compared to predictions for static and dynamic characteristics based on an analysis by Yang (1993). In general, the theoretical results reasonably predict these results; however, theory overpredicts direct stiffness, fails to indicate the decrease in KXX that occurs with increasing eccentricity, and incorrectly predicts the direction of change in KXX with changing pressure ratio. Also, direct damping is substantially underpredicted for low preswirl values and low supply pressures, but the predictions improve as either of these parameters increase.

scholarly journals Theory Versus Experiment for the Rotordynamic Coefficients of an Interlocking Labyrinth Gas Seal

1995 ◽  
Author(s):  
David A. Elrod ◽  
Joseph M. Pelletti ◽  
Dara W. Childs
Keyword(s):  
Control Volume ◽  
Labyrinth Seal ◽  
Back Pressure ◽  
Experimental Results ◽  
Rotordynamic Coefficients ◽  
Inlet Guide Vanes ◽  
Stabilizing Effect ◽  
Theory Versus ◽  
High Fluid ◽  
Direct Stiffness

Experimental results for the rotordynamic coefficients of an interlocking, compressible flow, labyrinth seal are presented. Tests were conducted at supply pressures out to 18.3 bars and rotor speeds out to 16,000 rpm. Seal back pressure was controlled to provide four pressure ratios at all supply pressures. Inlet guide vanes were used to provide fluid prerotation at the seal inlet. The experimental results are compared to the predictions of the bulk-flow, turbulent, one-control-volume, perturbation analysis of Scharrer (1988). The results show that the direct stiffness is negative but small, and is predicted well by theory. At high rotor speeds, the experimental cross-coupled stiffness is negative (stabilizing for forward whirl) for all values of fluid prerotation. Theory predicts positive (destabilizing for forward whirl) cross-coupled stiffness for high fluid prerotation, and overpredicts the direct damping of the seal. In general, the net stabilizing effect of the seal, as indicated by the whirl frequency ratio, is predicted well.

scholarly journals Annular Honeycomb Seals: Test Results for Leakage and Rotordynamic Coefficients; Comparisons to Labyrinth and Smooth Configurations

1989 ◽  
Vol 111 (2) ◽  
pp. 293-300 ◽  
Author(s):  
D. Childs ◽  
D. Elrod ◽  
K. Hale
Keyword(s):  
Labyrinth Seal ◽  
Test Results ◽  
Rotordynamic Coefficients ◽  
Damping Coefficients ◽  
Stiffness Coefficients ◽  
Shaft Rotation ◽  
Honeycomb Seal ◽  
Cell Dimensions ◽  
Honeycomb Seals ◽  
Rotordynamic Coefficient

Test results are presented for leakage and rotordynamic coefficients for seven honeycomb seals. All seals have the same radius, length, and clearance; however, the cell depths and diameters are varied. Rotordynamic data, which are presented, consist of the direct and cross-coupled stiffness coefficients and the direct damping coefficients. The rotordynamic-coefficient data show a considerable sensitivity to changes in cell dimensions; however, no clear trends are identifiable. Comparisons of test data for the honeycomb seals with labyrinth and smooth annular seals shows the honeycomb seal had the best sealing (minimum leakage) performance, followed in order by the labyrinth and smooth seals. For prerotated fluids entering the seal, in the direction of shaft rotation, the honeycomb seal has the best rotordynamic stability followed in order by the labyrinth and smooth. For no prerotation, or fluid prerotation against shaft rotation, the labyrinth seal has the best rotordynamic stability followed in order by the smooth and honeycomb seals.

Theory Versus Experiment for Short (L/D = 1/6) Honeycomb and Smooth Annular Pressure Seals

1993 ◽  
Author(s):  
Dara W. Childs ◽  
George F. Kleynhans
Keyword(s):  
Pressure Ratio ◽  
Surface Friction ◽  
Computer Code ◽  
Cell Width ◽  
Rotordynamic Coefficients ◽  
Test Parameters ◽  
Theoretical Predictions ◽  
Theory Versus ◽  
Honeycomb Cell ◽  
Parameter Ranges

Abstract A study which compares theoretical predictions of experimental rotordynamic and leakage results is presented for short (L/D = 1/6) honeycomb and smooth annular pressure seals. A computer code used in this comparison has been developed from a theory that employs a perturbation analysis of the governing equations flow and uses Moody’s pipe friction relationship for the surface friction of the rotor and stator. This study was undertaken to investigate how well an existing code could predict these characteristics with input provided from recorded test data and independent flat-plate tests. The results examine the effect that the following independent test parameters have on the experimental measurements and theoretical predictions: inlet preswirl, rotor speed, inlet pressure, pressure ratio across seal, seal clearance, and honeycomb cell width. Experimental results show that leakage is reduced by decreasing the honeycomb cell width. Rotordynamically, the short seals are stabilizing over all test parameter ranges. However, the short seals did not perform as favorably as longer (L/D = 1/3) seals. In general, the theory overpredicts rotordynamic coefficients and leakage.

An Entrance Region Friction Factor Model Applied to Annular Seal Analysis: Theory Versus Experiment for Smooth and Honeycomb Seals

1989 ◽  
Vol 111 (2) ◽  
pp. 337-343 ◽  
Author(s):  
D. Elrod ◽  
C. Nelson ◽  
D. Childs
Keyword(s):  
Friction Factor ◽  
Factor Model ◽  
Experimental Results ◽  
Entrance Region ◽  
Analysis Theory ◽  
Annular Seal ◽  
Theory Versus ◽  
Direct Stiffness ◽  
Honeycomb Seals

A friction factor model is developed for the entrance-region of a duct. The model is used in an annular gas seal analysis similar to Nelson’s (1984). Predictions of the analysis are compared to experimental results for a smooth-stator/smooth-rotor seal and three honeycomb-stator/smooth-rotor seals. The model predicts leakage and direct damping well. The model overpredicts the dependence of cross-coupled stiffness on fluid prerotation. The model predicts direct stiffness poorly.

A Comparison of Experimental Results and Theoretical Predictions for the Rotordynamic Coefficients of Short (L/D = 1/6) Labyrinth Seals

1991 ◽  
Author(s):  
Joseph M. Pelletti ◽  
Dara W. Childs
Keyword(s):  
Flow Model ◽  
Control Volume ◽  
Pressure Ratio ◽  
Experimental Results ◽  
Rotor Speed ◽  
Labyrinth Seals ◽  
Rotordynamic Coefficients ◽  
Supply Pressure ◽  
Test Conditions ◽  
Theoretical Predictions

Abstract Experimental results for the rotordynamic coefficients of short (L/D = 1/6) teeth-on-stator and teeth-on-rotor labyrinth seals are presented. The effects that pressure ratio (fluid density), rotor speed, fluid pre-swirl and seal clearance have on these coefficients are studied. Tests were run out to speeds of 16000 rpm with a supply pressure of 17.3 bar and seal clearances ranging from 0.229–0.419 mm. The experimental results are compared with theoretical predictions of a two control volume compressible flow model. The experimental results show that decreases in pressure ratio and increases in rotor speed are stabilizing while increases in fluid pre-swirl and seal clearance are destabilizing for both seal configurations. The theoretical model correctly predicts the effects of pressure ratio, rotor speed and fluid pre-swirl on the cross-coupled stiffness. It also predicts reasonable values for direct damping for all test conditions. However, the theory incorrectly predicts the effect of seal clearance on these coefficients. Consequently the theoretical predictions are much better for the large clearance seals.

High-Pressure Pocket Damper Seals: Leakage Rates and Cavity Pressures

2006 ◽  
Vol 129 (4) ◽  
pp. 826-834 ◽  
Author(s):  
Ahmed M. Gamal ◽  
Bugra H. Ertas ◽  
John M. Vance
Keyword(s):  
High Pressures ◽  
Prediction Models ◽  
Working Fluid ◽  
Dual Purpose ◽  
Rotordynamic Coefficients ◽  
Numerical Predictions ◽  
Theoretical Predictions ◽  
Mass Flow Rates ◽  
Direct Stiffness ◽  
Stiffness And Damping

The turbomachinery component of interest in this paper, the pocket damper seal, has the dual purpose of limiting leakage and providing an additional source of damping at the seal location. The rotordynamic coefficients of these seals (primarily the direct stiffness and damping) are highly dependent on the leakage rates through the seals and the pressures in the seals’ cavities. This paper presents both numerical predictions and experimentally obtained results for the leakage and the cavity pressures of pocket damper seals operating at high pressures. The seals were tested with air, at pressures up to 1000psi(6.92MPa), as the working fluid. Earlier flow-prediction models were modified and used to obtain theoretical reference values for both mass flow rates and pressures. Leakage and static pressure measurements on straight-through and diverging-clearance configurations of eight-bladed and twelve-bladed seals were used for code validation and for calculation of seal discharge coefficients. Higher than expected leakage rates were measured in the case of the twelve-bladed seal, while the leakage rates for the eight-bladed seals were predicted with reasonable accuracy. Differences in the axial pitch lengths of the cavities and the blade profiles of the seals are used to explain the discrepancy in the case of the twelve-bladed seal. The analysis code used also predicted the static cavity pressures reasonably well. Tests conducted on a six-bladed pocket damper seal to further investigate the effect of blade profile supported the results of the eight-bladed and twelve-bladed seal tests and matched theoretical predictions with satisfactory accuracy.

Rotordynamic Characteristics of a Flexure Pivot Pad Bearing With an Active and Locked Integral Squeeze Film Damper

2012 ◽  
Author(s):  
Jeff Agnew ◽  
Dara Childs
Keyword(s):  
Added Mass ◽  
Squeeze Film ◽  
Test Results ◽  
Squeeze Film Damper ◽  
Rotordynamic Coefficients ◽  
Damping Coefficients ◽  
Stiffness Coefficients ◽  
Direct Stiffness ◽  
Active Damper ◽  
Rotordynamic Coefficient

Measured rotordynamic coefficients are presented for a flexure-pivot-pad journal bearing (FPJB) in a load-between-pad configuration with: (1) an active, and (2) locked integral squeeze film damper (ISFD). Prior rotordynamic-coefficient test results have been presented for FPJBs (alone), and rotor-response results have been presented for rotors supported by FPJBS with ISFDs; however, these are the first rotordynamic-coefficient test results for FPJBs with ISFDs. A multi-frequency dynamic testing regime is employed. For both bearing configurations, quadratic curve fits provide good representation of the real portions of the dynamic-stiffness coefficients yielding a direct stiffness and a direct added-mass coefficient. The imaginary portions are well represented by linear curve fits, implying constant, frequency-independent direct-damping coefficients. Direct stiffness coefficients are ∼50% lower for the active-damper configuration, and direct damping coefficients are only modestly lower. The combination of ∼50% reduction in direct stiffness with a modest drop in direct damping indicates a very effective squeeze-film damper application. Added-mass coefficients are normally lower for the active-damper configuration, and all coefficient trends (for changes in loading and shaft speed) are “flatter” for the active flexure pivot-pad damper bearing. The measured rotordynamic coefficients are used to calculate the whirl frequency ratio and indicate high stability for both bearing configurations.

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