Computational Fluid Dynamics (CFD) Analysis for Wet Gas and Bubbly Liquid Seals

* Wet gas: has up to 5% liquid volume fraction.

On how millions of nodes and equations can be useful to learn more about the operation of mechanical elements. CFD supported by extensive test data.

Who said more is not better?

 

The work: more number crunching to crunch more learning!

Publications

1.    Yang, J., and San Andrés, L., 2022, “Making Better Swirl Brakes Using Computational Fluid Dynamics: Performance Enhancement From Geometry Variation,” ASME J. Eng. Gas Turbines Power, Vol. 144(2): 021027, https://doi.org/10.1115/1.4051962  (ASME GT2021-58956).

 

2.    San Andrés, L., and Yang, J., 2021,”An Analytical Two-Phase Flow Model for Prediction of Leakage in Wet Gas Labyrinth Seals and Pocket Damper Seals. Is Simplicity Still Desired?” ASME J. Eng. Gas Turbines Power, Vol. 143(12): 121016, https://doi.org/10.1115/1.4051916 (ASME GT2021-58958).

 

3.    Yang, J, San Andrés, L., and, Lu, X., 2019, “Leakage and Dynamic Force Coefficients of a Pocket Damper Seal Operating Under a Wet Gas condition: Tests vs. Predictions,” ASME J. Eng. Gas Turbines Power, Vol. 141(11):111001, DOI: 10.1115/1.4044307 (ASME GT2019-90331)..

 

4.    Yang, J., and San Andrés, L., 2019, “On the Influence of the Entrance Section on the Rotordynamic Performance of a Pump Seal with Uniform Clearance: a Sharp Edge VS. a Round Inlet,” ASME J. Eng. Gas Turbines Power, 141(3), pp. 031029. DOI: 10.1115/1.4040742 (ASME GT20018-75414)

 

5.    San Andrés, L., Yang, J., and Lu, X., 2019, “On the Leakage, Torque and Dynamic Force Coefficients of an Air in Oil (Wet) Annular Seal: a CFD Analysis Anchored to Test Data,” ASME J. Eng. Gas Turbines Power, 141(2), pp. 021008. DOI: 10.1115/1.4040766 (ASME GT20018-77140)

 

6.    Yang, J., and San Andrés, L., 2019, “On the Influence of the Entrance Section on the Rotordynamic Performance of a Pump Seal with Uniform Clearance: a Sharp Edge vs. a Round Inlet,” ASME J. Eng. Gas Turbines Power, Vol. 141(3), 032109, DOI: 10.1115/1.4040742 (ASME GT20018-75414)

 

7.    San Andrés, L., Yang, J., and Xu, L., 2019, “On the Leakage, Torque and Dynamic Force Coefficients of an Air in Oil (Wet) Annular Seal: a CFD Analysis Anchored to Test Data,” ASME J. Eng. Gas Turbines Power, Vol. 141(2), 021008, DOI: 10.1115/1.4040766 (ASME GT20018-77140).

 

8.       San Andrés, L., and Yang, J., 2018, “The Influence of Corner Shape on the Static and Dynamic Performance on an Annular Pressure Seal,” Proc. Global Power and Propulsion Society Forum 18, Zurich, Switzerland, 10-12 January 2018, Paper GPPS-2018-55 (www.gpps.global )

 

 

Sponsors

1. Texas A&M University (TAMU) Turbomachinery Laboratory (2016-2018).

2. TAMU Turbomachinery Research Consortium (TRC) (2017-2021).

Computational Resources TAMU High-Performance Computing Center & TAMU Turbomachinery Laboratory Clusters

 

Research Background

è Cost efficient subsea factories must rely on multiple-phase flow compression and pump systems that reduce tieback systems and perform full flow separation on the sea floor.

è Operation requirements of subsea turbomachinery: ~ 5% liquid volume fraction (LVF) in wet compressors and up to ~ 90% gas volume fraction (GVF) in pumps.

è It is already known that seals operating under a wet gas or bubbly flow conditions do affect system rotordynamic stability.

 

Objective Sound Engineering to Quantify the Leakage and Dynamic Forced Performance of Annular Seals under Wet Gas Conditions!

 

Project I

Wet Gas in a Smooth Annular Seal

 

To complement experimental work by revealing flow field structures in multiple-phase flow seals through Computational Fluid Dynamics (CFD) and to validate/update engineering (BFM) predictive tools.

 

Operating conditions

Supply pressure: 1.0 ~ 3.5 bar(a); Rotor speed: 3500 rpm (surface speed 23 m/s);

Inlet gas volume fraction (GVF): 0, 0.2, 0.4, 0.6, 0.8, 0.9, 1.

Oil density: 830 kg/m3 & viscosity: 8.2 cP

 

A Wet Smooth Surface Annular Seal

 

 

2D CFD Predicted Gas Volume Fraction (GVF) vs axial length

 

 

 

 

 

2D CFD Predicted Axial Velocity (W) vs axial length



 

 

CFD Predicted Seal Direct Dynamic Stiffness against Test Data

 

 

 

CFD Predicted Seal Cross-coupled Dynamic Stiffness against Test Data

 

 

 

CFD Predicted Seal Direct damping against Test Data

 

Stiffness Hardening Effect

 

 

 

Conclusions

 

1. CFD predictions (leakage, power loss) agree with test data, and also produce high fidelity flow field variables, including pressure, speeds, and GVF.

2. Operation with a low GVF (< 0.4) produces a significant hardening effect which makes positive the direct stiffness. Test data shows same rapid stiffness increase as GVFà 0.2.

3. Stiffness hardening effect is due to the dramatic reduction in sound speed brought by a small amount of gas (fluid becomes more compressible).

4. The combination of test results and CFD and BFM analyses furthers the engineering of seals for wet gas compressors and bubbly liquids in multiple phase pumps.

 

 

Project II

Wet Gas in a Pocket Damper Seal

 

Aim of the work

1. Experimentally and numerically investigate the leakage and dynamic force coefficients of a fully partitioned pocket damper seal, operating with just air (dry condition) and oil in air (wet condition).

2.  Apply CFD to showcase the flow fields in pocket damper seal under a wet gas condition.

Operating conditions

Supply pressure: 1.0 ~ 3.2 bar(a); Rotor speed: 5250 rpm (surface speed 35 m/s);

Inlet liquid volume fraction (LVF): 0, 0.4%.  Oil density: 830 kg/m3 & viscosity 8.2 cP

 

A Four-Rib, Eight-Pocket Damper Seal

 

 

3D Mesh for the Pocket Damper Seal

 

 

Test Measured, CFD Predicted and BFM Predicted Leakage for the Pocket Damper Seal Operating with Air

 

CFD Predicted Liquid Volume Fraction (LVF) Contours on Rotor and Stator Surfaces (Wet Gas Pocket Damper Seal, inlet LVF = 0.4%)

 

CFD Predicted Dynamic Force Coefficients against Test Data for Gas Pocket Damper Seal and Wet Gas Pocket Damper Seal (inlet LVF = 0.4%)

 

 

 

Conclusions

1. For dry gas condition, the measured mass flow rate nonlinearly increases with the inlet pressure. For operation with a wet gas, the measured leakage increases rapidly as the liquid density is much larger than that of the gas. CFD predictions match the recorded leakage.


2. Under a dry gas condition, the experimental direct dynamic stiffness (HR) increases with excitation frequency, whereas the cross-coupled dynamic stiffness (hR) is a minute fraction of HR and showing a high variability as frequency increases. The test effective damping (Ceff) is relatively constant as the excitation frequency (w) increases.

 

3. For a wet gas condition are with inlet LVF=0.4%, the test derived force coefficients show a large variation along the two orthogonal directions of forced excitation. The large liquid mass fraction (57%) makes the dynamic direct stiffness HR negative though decreasing in magnitude as the excitation frequency increases.

 

4.  The CFD model for wet gas operation delivers high fidelity fields, such as pressure, liquid fraction and velocities, which demonstrate the ridges in a pocket limit the development of the circumferential flow speed while reducing the liquid content in the middle of a pocket.

 

Last update: August 11, 2022