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.

Measured Static and Rotordynamic Characteristics of a Smooth-Stator/Grooved-Rotor Liquid Annular Seal

2018 ◽  
Vol 140 (10) ◽  
Author(s):  
J. Alex Moreland ◽  
Dara W. Childs ◽  
Joshua T. Bullock
Keyword(s):  
Radial Clearance ◽  
Test Results ◽  
Dynamic Features ◽  
Axial Pressure ◽  
Pressure Drops ◽  
Rotordynamic Coefficients ◽  
Annular Seal ◽  
Direct Stiffness ◽  
Static Eccentricity ◽  
Dynamic Variables

Electric submersible pumps (ESPs) utilize grooved-rotor/smooth-stator (SS/GR) seals to reduce leakage and break up contaminants within the pumped fluid. Additionally, due to their decreased surface area (when compared to a smooth seal), grooved seals decrease the chance of seizure in the case of rotor-stator rubs. Despite their use in industry, the literature does not contain rotordynamic measurements for smooth-stator/circumferentially grooved-rotor liquid annular seals. This paper presents test results consisting of leakage measurements and rotordynamic coefficients for a SS/GR liquid annular sdeal. Both static and dynamic variables are investigated for various imposed preswirl ratios (PSRs), static eccentricity ratios (0–0.8), axial pressure drops (2–8 bars), and running speeds (2–8 krpm). The seals' static and dynamic features are compared to those of a smooth seal with the same length, diameter, and minimum radial clearance. Results show that the grooves reduce leakage at lower speeds (less than 5 krpm) and higher axial pressure drops, but does little at higher speeds. The grooved seal's direct stiffness is generally negative, which would be detrimental to pump rotordynamics. As expected, increasing preswirl increases the magnitude of cross-coupled stiffness and increases the whirl frequency ratio (WFR). When compared to the smooth seal, the grooved seal has smaller effective damping coefficients, indicative of poorer stability characteristics.

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 High Pressure Honeycomb Annular Gas Seal at Eccentric Positions

2003 ◽  
Vol 125 (2) ◽  
pp. 422-429 ◽  
Author(s):  
Mark Weatherwax ◽  
Dara W. Childs
Keyword(s):  
Back Pressure ◽  
Test Fluid ◽  
Test Results ◽  
Eccentricity Ratio ◽  
Cell Width ◽  
Rotordynamic Coefficients ◽  
Annular Seal ◽  
Theory Versus ◽  
A Cell ◽  
San Andres

Test results for leakage and rotordynamic coefficients are presented for a honeycomb-stator/smooth-rotor annular seal for eccentricity ratios out to 0.5 using air as the test fluid. Tests were conducted at supply pressures up to 70 bars and running speeds up to 20200 rpm. The seal has a diameter of 115 mm, a cell width of 0.79 mm, and a cell depth of 3.10 mm. Tests were conducted for the back-pressure ratios of 0.35, and 0.5. Comparisons are made to predictions from an analysis due to San Andres. The test results show a minimal sensitivity of either leakage or rotordynamic coefficients to changes in the eccentricity ratio. The predictions generally agree well with measurements.

Measured Static and Rotordynamic Characteristics of a Smooth-Stator/Grooved-Rotor Liquid Annular Seal

2017 ◽  
Author(s):  
J. Alex Moreland ◽  
Dara W. Childs ◽  
Joshua T. Bullock
Keyword(s):  
Frequency Ratio ◽  
Dynamic Performance ◽  
Radial Clearance ◽  
Test Results ◽  
Axial Pressure ◽  
Pressure Drops ◽  
Rotordynamic Coefficients ◽  
Damping Coefficients ◽  
Annular Seal ◽  
Direct Stiffness

Electric submersible pumps utilize grooved-rotor seals to reduce leakage and break up contaminants within the pumped fluid. Additionally, due to their decreased surface area (when compared to a smooth seal), grooved seals decrease the chance of seizure in the case of rotor-stator rubs. Despite their use in industry, the literature does not contain measurements for smooth-stator/circumferentially-grooved-rotor liquid annular seals. This paper presents test results consisting of leakage measurements and rotordynamic coefficients for a smooth-stator/circumferentially-grooved-rotor liquid annular seal. Both static and dynamic performance for the grooved seal are investigated for various imposed pre-swirl ratios, static eccentricities, axial pressure drops, and running speeds. The grooved seals′ static and dynamic performance are compared to those of a smooth seal with identical length, diameter, and radial clearance. Results show that adding grooves reduces leakage at lower speeds (less than 5 krpm) and higher axial pressure drops, but does little at higher speeds. The grooved seal’s direct stiffness is generally negative, which would be detrimental to pump rotordynamics. Furthermore, increasing pre-swirl increases the magnitude of cross-coupled stiffness and increases the whirl frequency ratio. When compared to the smooth seal, the grooved seal has smaller effective damping coefficients, indicative of worse stability characteristics.

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.

scholarly journals Seal-Rotordynamic-Coefficient Test Results for a Model SSME ATD-HPFTP Turbine Interstage Seal With and Without a Swirl Brake

1991 ◽  
Vol 113 (1) ◽  
pp. 198-203 ◽  
Author(s):  
D. W. Childs ◽  
C. Ramsey
Keyword(s):  
Space Shuttle ◽  
Pressure Ratio ◽  
Tangential Velocity ◽  
Model Space ◽  
Design Guidelines ◽  
Test Results ◽  
Rotordynamic Coefficients ◽  
Direct Stiffness ◽  
Favorable Influence ◽  
Main Engine

Test results are presented and compared to theory for a model Space Shuttle Main Engine(SSME) Alternate Turbopump Development(ATD) High-Pressure Fuel Turbopump (HPFTP) with and without swirl brakes. Tests are conducted with supply pressures out to 18.3 bars and speeds out to 16,000 rpm. Seal back pressure is controlled to provide four pressure ratios at all supply pressures. Three inlet guide vanes are used to provide the following three fluid prerotation cases: (a) no pre-rotation, (b) moderate prerotation in the direction of rotation, and (c) high prerotation in the direction of rotation. Test results demonstrate the pronounced favorable influence of the swirl brake in reducing the seal destabilizing forces. Without the swirl brake, the cross-coupled stiffness k increases monotonically with increasing inlet tangential velocity. With the swirl brake, k tends to either be constant or decrease with increasing inlet tangential velocity. Direct damping either increases or remains relatively constant when the swirl brake is introduced. Direct stiffness is relatively unchanged. No measurable differences in leakage were detected for the seal with and without the swirl brake. Comparisons between Scharrer’s (1988) theory and measurements for the seal without a swirl brake indicate that the predictions can be used to provide design guidelines only. Specific predictions for rotordynamic coefficients should be treated cautiously, since systematic differences were observed between theory and experiment due to changes in running speed, supply pressure, and pressure ratio.

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.

Experimental Test Results for Four High-Speed, High-Pressure, Orifice-Compensated Hybrid Bearings

1994 ◽  
Vol 116 (1) ◽  
pp. 147-153 ◽  
Author(s):  
N. M. Franchek ◽  
D. W. Childs
Keyword(s):  
High Speed ◽  
Load Capacity ◽  
Pressure Ratio ◽  
Reynolds Numbers ◽  
Orifice Diameter ◽  
Test Results ◽  
Rotordynamic Coefficients ◽  
Geometric Configurations ◽  
Overall Performance ◽  
Mass Terms

In this study, four hybrid bearings having different geometric configurations were experimentally tested for their static and dynamic characteristics, including flowrate, load capacity, rotordynamic coefficients, and whirl frequency ratio. The four bearings included a square-recess, smooth-land, radial-orifice bearing (baseline), a circular-recess bearing, a triangular-recess bearing, and an angled-orifice bearing. Each bearing had the same orifice diameter rather than the same pressure ratio. Unique to these test results is the measurement of the added mass terms, which became significant in the present tests because of high operating Reynolds numbers. Comparisons of the results were made between bearings to determine which bearing had the best performance. Based on the parameters of interest, the angled-orifice bearing has the most favorable overall performance.

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.

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