scholarly journals Theory Versus Experiment for the Rotordynamic Coefficients of Annular Gas Seals: Part 1—Test Facility and Apparatus

1986 ◽  
Vol 108 (3) ◽  
pp. 426-431 ◽  
Author(s):  
D. W. Childs ◽  
C. E. Nelson ◽  
C. Nicks ◽  
J. Scharrer ◽  
D. Elrod ◽  
...  
Keyword(s):  
Pressure Ratio ◽  
Excitation Frequency ◽  
Tangential Velocity ◽  
Test Facility ◽  
Rotordynamic Coefficients ◽  
Air Supply ◽  
Theory Versus ◽  
Leakage Characteristics ◽  
Gas Seals ◽  
A Current

A facility and apparatus are described for determining the rotordynamic coefficients and leakage characteristics of annular gas seals. The apparatus has a current top speed of 8000 cpm with a nominal seal diameter of 15.24 cm (6 in.). The air-supply unit yields a seal pressure ratio of approximately 7. The inlet tangential velocity can also be controlled. An external shaker is used to excite the test rotor. The apparatus has the capability to independently calculate all rotordynamic coefficients at a given operating condition with one excitation frequency.

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.

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 Coefficient of Labyrinth Gas Seals: Part II—A Comparison to Experiment

1988 ◽  
Vol 110 (3) ◽  
pp. 281-287 ◽  
Author(s):  
D. W. Childs ◽  
J. K. Scharrer
Keyword(s):  
Experimental Test ◽  
Damping Coefficient ◽  
Test Facility ◽  
Radial Clearance ◽  
Test Results ◽  
Rotordynamic Coefficients ◽  
Theory Versus ◽  
Gas Seals ◽  
Predicted Values ◽  
Rotordynamic Coefficient

An experimental test facility is used to measure the leakage and rotordynamic coefficients of teeth-on-rotor and teeth-on-stator labyrinth gas seals. The test results are presented along with the theoretically predicted values for the two seal configurations at three different radial clearances and shaft speeds to 16,000 cpm. The test results show that the theory accurately predicts the cross-coupled stiffness for both seal configurations and shows improvement in the prediction of the direct damping for the teeth-on-rotor seal. The theory fails to predict a decrease in the direct damping coefficient for an increase in the radial clearance for the teeth-on-stator seal.

scholarly journals Experimental Rotordynamic Coefficient Results for Teeth-on-Rotor and Teeth-on-Stator Labyrinth Gas Seals

1986 ◽  
Vol 108 (4) ◽  
pp. 599-604 ◽  
Author(s):  
D. W. Childs ◽  
J. K. Scharrer
Keyword(s):  
Experimental Test ◽  
Tangential Velocity ◽  
Operating Conditions ◽  
Test Facility ◽  
Rotordynamic Coefficients ◽  
Damping Coefficients ◽  
Gas Seals ◽  
First Time ◽  
Rotordynamic Coefficient

An experimental test facility is used to measure the rotordynamic coefficients of teeth-on-rotor and teeth-on-stator labyrinth gas seals. Direct damping coefficients are presented for these seals for the first time. The results are presented for the two seal configurations at identical operating conditions, and show that, in a rotordynamic sense, the teeth-on-stator seal is more stable than the teeth-on-rotor seal, for inlet tangential velocity in the direction of rotation.

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.

Theory Versus Experiment for the Rotordynamic Impedances of Two Hole-Pattern-Stator Gas Annular Seals

2001 ◽  
Vol 124 (1) ◽  
pp. 137-143 ◽  
Author(s):  
Christopher G. Holt ◽  
Dara W. Childs
Keyword(s):  
Control Volume ◽  
Pressure Ratio ◽  
Effective Stiffness ◽  
Cell Diameter ◽  
Rotordynamic Coefficients ◽  
Hole Area ◽  
Theory Versus ◽  
Density Factor ◽  
A Cell ◽  
Exit Temperature

Measured rotordynamic impedances are presented for two hole-pattern-stator seals and one smooth bore seal. These measured results are compared to predictions from a two-control-volume model and realized in the code ISOTSEAL (constant-temperature seal code). The hole-pattern seals have cell depths of 2.03 mm and 3.18 mm with a cell diameter of 1.59 mm. The hole-area density factor for both hole-pattern seals is 43 percent. The seal diameter is 114.71 mm with an L/D ratio of 0.75. Measured results for radial impedances and leakage were obtained. Test conditions involved three speeds out to 20,200 rpm, three inlet pressures out to 17.2 bar, and two exit-to-inlet pressure ratios of 40 percent and 54 percent. As predicted, the hole-pattern seals exhibit frequency-dependent rotordynamic coefficients K(Ω), k(Ω), C(Ω), c(Ω). Results of the tests show that the 3.18 mm hole-pattern seal has the highest average effective stiffness and lowest effective damping. Direct and effective stiffness were under-predicted in all cases; however, measured direct and effective damping are reasonably well predicted. Impedance predictions improve with increasing pressure ratio. Comparisons of leakage correlate extremely well with predictions; worse case deviations never exceed 10 percent. Results show that leakage decreases as cell depth increases. Results also show that the exit temperature increases substantially with increasing rotational speed.

Measurements Versus Predictions for the Dynamic Impedance of Annular Gas Seals: Part 1 — Test Facility and Apparatus

2001 ◽  
Author(s):  
Matthew P. Dawson ◽  
Dara W. Childs ◽  
Christopher G. Holt ◽  
Stephen G. Phillips
Keyword(s):  
Random Excitation ◽  
Supply System ◽  
Test Facility ◽  
Experimental Facility ◽  
Frequency Range ◽  
Labyrinth Seals ◽  
Dynamic Impedance ◽  
Air Supply ◽  
Gas Seals ◽  
Excitation Waveform

An experimental facility and apparatus are described for measuring the dynamic impedance and leakage characteristics of annular gas seals. The apparatus currently has a top speed of 29,800 rpm and can accommodate seal diameters up to 114.3 mm. The air-supply system can provide up to 13.79 MPa (2,000 psi) of pressure at the seal inlet. Test seals are configured in a back-to-back arrangement inside the stator and air enters a central inlet annulus at two opposed radial positions. Labyrinth seals and bleed ports located outboard of each test seal are used to control the pressure drop across the test seals. Two orthogonal, external hydraulic shakers are used to excite the test stator at frequencies up to 400 Hz. At a given operating condition, the apparatus can measure the rotordynamic impedance of a pair of identical seals over a broad frequency range using a single pseudo-random excitation waveform. Measurements are also made of seal leakage rates and upstream and downstream temperatures and pressures.

Sign in / Sign up

Export Citation Format

Share Document