Rotordynamic-Coefficient and Leakage Characteristics for Hole-Pattern-Stator Annular Gas Seals—Measurements Versus Predictions

2004 ◽  
Vol 126 (2) ◽  
pp. 326-333 ◽  
Author(s):  
Dara W. Childs ◽  
Jonathan Wade
Keyword(s):  
Flow Model ◽  
Control Volume ◽  
Test Results ◽  
Damping Coefficients ◽  
Supply Pressure ◽  
Static Data ◽  
Leakage Characteristics ◽  
Gas Seals ◽  
Stiffness And Damping ◽  
Rotordynamic Coefficient

Selected test results are presented for an annular gas seal using a smooth rotor and a hole-pattern-roughness stator for a supply pressure of 70 bar, three pressure ratios, three speeds up to 20,000 rpm, two clearances, and three preswirl ratios. Dynamic data include frequency-dependent direct and cross-coupled stiffness and damping coefficients. Static data include leakage and upstream and downstream pressures and temperatures. Very good agreements are found between measurements and predictions from a two-control-volume bulk-flow model.

Experimental Results for Labyrinth Gas Seals With Honeycomb Stators: Comparisons to Smooth-Stator Seals and Theoretical Predictions

1989 ◽  
Vol 111 (1) ◽  
pp. 161-168 ◽  
Author(s):  
Larry Hawkins ◽  
Dara Childs ◽  
Keith Hale
Keyword(s):  
Flow Model ◽  
Control Volume ◽  
Labyrinth Seal ◽  
Experimental Results ◽  
Test Results ◽  
Labyrinth Seals ◽  
Model Predictions ◽  
Theoretical Predictions ◽  
Gas Seals ◽  
Stiffness And Damping

Experimental measurements are presented for the rotordynamic stiffness and damping coefficients of a teeth-on-rotor labyrinth seal with a honeycomb stator. Inlet circumferential velocity, inlet pressure, rotor speed, and seal clearance are primary variables. Results are compared to (a) data for teeth-on-rotor labyrinth seals with smooth stators, and (b) analytical predictions from a two-control-volume compressible flow model. The experimental results show that the honeycomb-stator configuration is more stable than the smooth-stator configuration at low rotor speeds. At high rotor speeds, the stator surface does not affect stability. The theoretical model predicts the cross-coupled stiffness of the honeycomb-stator seal correctly within 25 percent of measured values. The model provides accurate predictions of direct damping for large clearance seals; however, the model predictions and test results diverge with increasing running speed. Overall, the model does not perform as well for low clearance seals as for high clearance seals.

scholarly journals Rotordynamic Coefficient and Leakage Test Results for Interlock and Tooth-on-Stator Labyrinth Seals

1988 ◽  
Author(s):  
Dara W. Childs ◽  
David A. Elrod ◽  
Keith Hale
Keyword(s):  
Labyrinth Seal ◽  
Damping Coefficient ◽  
Test Results ◽  
Labyrinth Seals ◽  
Rotordynamic Coefficients ◽  
Damping Coefficients ◽  
Leakage Test ◽  
Stiffness And Damping ◽  
Rotordynamic Coefficient

Test results (leakage and rotordynamic coefficients) are presented for an interlock and tooth-on-stator labyrinth seals. Tests were carried out with air at speeds out to 16,000 cpm and supply pressures up to 7.5 bars. The rotordynamic coefficients consist of direct and cross-coupled stiffness and damping coefficients. Damping-coefficient data have not previously been presented for interlock seals. The test results support the following conclusions: (a) The interlock seal leaks substantially less than labyrinth seals. (b) Destabilizing forces are lower for the interlock seal. (c) The labyrinth seal has substantially greater direct damping values than the interlock seal. A complete rotordynamics analysis is needed to determine which type of seal would yield the best stability predictions for a given turbomachinery unit.

Measurement Versus Predictions of Rotordynamic Coefficients of a Hole-Pattern Gas Seal With Negative Preswirl

2012 ◽  
Author(s):  
Philip D. Brown ◽  
Dara W. Childs
Keyword(s):  
Control Volume ◽  
Pressure Ratio ◽  
Excitation Frequency ◽  
Ideal Gas ◽  
Rotor Speed ◽  
Rotordynamic Coefficients ◽  
Damping Coefficients ◽  
The Cross ◽  
Gas Seals ◽  
Stiffness And Damping

Test results are presented for rotordynamic coefficients of a hole-pattern annular gas seals at supply pressures to 84 bar and running speeds to 20,200 RPM. The principal test variable of interest was negative preswirl. Preswirl signifies the circumferential fluid flow entering a seal, and negative preswirl indicates a fluid swirl in a direction opposite to rotor rotation. The influences of pressure ratio and rotor speed were also investigated. Measured results produce direct and cross-coupled stiffness and damping coefficients that are a function of excitation frequency Ω. Changes in pressure ratio had only small effects on most rotordynamic coefficients. Cross-coupled stiffness showed slightly different profiles through the mid-range of Ω values. Increasing rotor speed significantly increased cross-coupled stiffness and cross-coupled damping. At 10,200 RPM, high negative inlet preswirl produced negative cross-coupled stiffness over an excitation frequency range of 200–250 Hz. Negative preswirl did not affect direct stiffness and damping coefficients. Effective damping combines the stabilizing effect of direct damping and the destabilizing effect of cross-coupled stiffness. The cross-over frequency is the precession frequency where effective damping transitions from a negative value to a positive value with increasing frequency. At 20,200 RPM with a pressure ratio of 50%, peak effective damping was increased by 50%, and the cross-over frequency was reduced by 50% for high-negative preswirl versus zero preswirl. Hence, reverse swirl can greatly enhance the stabilizing capacity of hole-pattern balance-piston or division-wall seals for compressors. A two-control-volume model that uses the ideal gas law at constant temperature was used to predict rotordynamic coefficients. The model predicted direct rotordynamic coefficients well, but substantially under predicted cross-coupled rotordynamic coefficients especially at high negative preswirls.

Rotordynamic-Coefficients and Static (Equilibrium Loci and Leakage) Characteristics for Short, Laminar-flow Annular Seals

2005 ◽  
Vol 128 (2) ◽  
pp. 378-387 ◽  
Author(s):  
Dara W. Childs ◽  
Luis E. Rodriguez ◽  
Vito Cullotta ◽  
Adnan Al-Ghasem ◽  
Matthew Graviss
Keyword(s):  
Laminar Flow ◽  
Groove Depth ◽  
Test Results ◽  
Clearance Ratio ◽  
Industrial Practice ◽  
Rotordynamic Coefficients ◽  
Damping Coefficients ◽  
Static Data ◽  
Deep Groove ◽  
Stiffness And Damping

Test results are presented for laminar-flow seals that are representative of buffered-flow oil seals in centrifugal compressors. The seals are short (L∕D≅0.21), with a diameter of 117mm and a clearance-to-radius ratio 0.0007. A smooth seal, a seal with one central groove, and a seal with three grooves were tested. Groove geometries employed are representative of industrial practice for compressors with a groove-depth to clearance ratio on the order of 6. Tests were conducted at 4000, 7000, and 10,000rpm shaft speed with delta pressures across the seals of 21, 45, and 69bars. For all cases, the flow was laminar. The seals were tested from a centered position out to an eccentricity ratio of 0.7. Static data included leakage and equilibrium loci for a range of loads. Direct and cross-coupled stiffness and damping coefficients and direct mass coefficients were determined from dynamic tests. For the smooth seal, comparisons between measurements and predictions were reasonable for the direct and cross-coupled stiffness and damping coefficients; however, measured added mass coefficients were roughly ten times larger than predicted. Predictions for the grooved seals from a “deep-groove” model that assumed zero pressure oscillations in the grooves greatly over predicted the influence of the grooves. In a centered position, smooth and grooved seals had comparable leakage performance. At higher eccentricity ratios, the grooved seals leaked modestly more. For eccentricity ratios less than approximately 0.3, the grooved seals and the smooth seal had qualitatively similar static and dynamic characteristics. In terms of cross-coupled stiffness coefficients, the grooved seals were less stable than the smooth seal at eccentricity ratios greater than approximately 0.5 but had significantly lower cross-coupled coefficients at reduced eccentricity ratios between zero and 0.3. A grooved centered seal is more stable than a smooth centered seal. The smooth seal had higher damping than the grooved seals and had moderately better centering capabilities.

A Comparison of Rotordynamic-Coefficient Predictions for Annular Honeycomb Gas Seals Using Three Different Friction-Factor Models

2002 ◽  
Vol 124 (3) ◽  
pp. 524-529 ◽  
Author(s):  
Rohan J. D’Souza ◽  
Dara W. Childs
Keyword(s):  
Friction Factor ◽  
Flow Model ◽  
Factor Model ◽  
Control Volume ◽  
Bulk Flow ◽  
Shear Stresses ◽  
Steam Turbines ◽  
Frequency Range ◽  
Rotordynamic Coefficients ◽  
Rotordynamic Coefficient

A two-control-volume bulk-flow model is used to predict rotordynamic coefficients for an annular, honeycomb-stator/smooth-rotor gas seal. The bulk-flow model uses Hirs’ turbulent-lubrication model, which requires a friction factor model to define the shear stresses at the rotor and stator wall. Rotordynamic coefficients predictions are compared for the following three variations of the Blasius pipe-friction model: (i) a basic model where the Reynolds number is a linear function of the local clearance, fs=ns Rems (ii) a model where the coefficient is a function of the local clearance, and (iii) a model where both the coefficient and exponent are functions of the local clearance. The latter models are based on data that shows the friction factor increasing with increasing clearances. Rotordynamic-coefficient predictions shows that the friction-factor-model choice is important in predicting the effective-damping coefficients at a lower frequency range (60∼70 Hz) where industrial centrifugal compressors and steam turbines tend to become unstable. At a higher frequency range, irrespective of the friction-factor model, the rotordynamic-coefficient predictions tend to coincide. Blasius-based Models which directly account for the observed increase in stator friction factors with increasing clearance predict significantly lower values for the destabilizing cross-coupled stiffness coefficients.

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.

Dynamic Force Coefficients of a Multiple-Blade, Multiple-Pocket Gas Damper Seal: Test Results and Predictions

1999 ◽  
Vol 122 (1) ◽  
pp. 317-322 ◽  
Author(s):  
Jiming Li ◽  
Ramon Aguilar ◽  
Luis San Andre´s ◽  
John M. Vance
Keyword(s):  
Control Volume ◽  
Pressure Ratio ◽  
Flexible Structure ◽  
Dynamic Force ◽  
Test Results ◽  
Force Coefficients ◽  
Exit Pressure ◽  
Identification Technique ◽  
Gas Damper ◽  
Stiffness And Damping

Experimental rotordynamic force coefficients and leakage for a four-blade, two-four pocket gas damper seal are presented and compared to predictions based on a one control volume bulk-flow model. The test rig comprises a vertical shaft and a test seal housing and flexible structure suspended from a rigid centering frame. The experiments were conducted at increasing rotor speeds to 6000 rpm and inlet/exit pressure ratios from 1.0 to 3.0. The seal force coefficients are obtained from impact response measurements of the seal and flexible structure using a frequency domain parameter identification technique. Both measurements and predictions show the seal direct stiffness and damping coefficients are proportional to the inlet/exit pressure ratio and insensitive to rotor speed. The agreement between experimental results and analytical predictions is acceptable. Predicted cross-coupled stiffness coefficients are of small amplitude. However, the test results evidence cross-coupled stiffnesses without journal rotation due to a structural asymmetry induced by the external pressurization into the seal. [S0742-4787(00)04201-6]

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.

scholarly journals Multi-Objective Optimisation of an Aerostatic Pad: Design of Position, Number and Diameter of the Supply Holes

2020 ◽  
Vol 36 (3) ◽  
pp. 347-360
Author(s):  
F. Colombo ◽  
F. Della Santa ◽  
S. Pieraccini
Keyword(s):  
Genetic Algorithm ◽  
Load Capacity ◽  
Design Parameters ◽  
Objective Functions ◽  
Aerostatic Bearing ◽  
Multiobjective Optimisation ◽  
Damping Coefficients ◽  
Supply Pressure ◽  
Design Variables ◽  
Stiffness And Damping

ABSTRACTIn this paper, a rectangular aerostatic bearing with multiple supply holes is optimised with a multiobjective optimisation approach. The design variables taken into account are the supply holes position, their number and diameter, the supply pressure, while the objective functions are the load capacity, the air consumption and the stiffness and damping coefficients. A genetic algorithm is applied in order to find the Pareto set of solutions. The novelty with respect to other optimisations which can be found in literature is that number and location of the supply holes is completely free and not associated to a pre-defined scheme. A vector x associated with the supply holes location is introduced in the design parameters and given in input to the optimizer.

Rotordynamic Coefficients for a Tooth-on-Stator Labyrinth Seal at 70 Bar Supply Pressures: Measurements Versus Theory and Comparisons to a Hole-Pattern Stator Seal

2004 ◽  
Vol 127 (4) ◽  
pp. 843-855 ◽  
Author(s):  
Arthur Picardo ◽  
Dara W. Childs
Keyword(s):  
Control Volume ◽  
Labyrinth Seal ◽  
Test Results ◽  
Flow Models ◽  
Rotordynamic Coefficients ◽  
Volume Model ◽  
Supply Pressure ◽  
Theoretical Predictions ◽  
The One ◽  
Rotor Dynamic

Rotor dynamic and leakage coefficients are presented for a labyrinth seal that was tested at a supply pressure of 70 bar-a and speeds up to 20,200 rpm. Tests were conducted at clearances of 0.1 mm and 0.2 mm, pressure ratios of 0.10, 0.31, and 0.52, and three preswirls ratios. Comparisons are made between test data and predictions from one-control-volume and two-control-volume bulk-flow models. Generally, theoretical predictions agree poorly with the test results, with the one-control volume model giving better predictions. The one-control-volume model provides a conservative prediction for effective damping; i.e., this parameter is underestimated. Both models under predict leakage rates. Comparisons are also made between rotordynamic coefficients of labyrinth and hole-pattern seals.

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