A CFD MODEL FOR PREDICTION OF LEAKAGE AND DYNAMIC FORCE COEFFICIENTS IN POCKET DAMPER SEALS: FROM GAS TO A WET GAS

Transcription

1 May 2018 A CFD MODEL FOR PREDICTION OF LEAKAGE AND DYNAMIC FORCE COEFFICIENTS IN POCKET DAMPER SEALS: FROM GAS TO A WET GAS TRC-SEAL Dr. Luis San Andrés Mast-Childs Chair Professor Dr. Jing Yang Post-Doctoral Research Associate yangjing@tamu.edu Xueliang Lu Graduate Research Assistant PROJECT: CFD ANALYSIS OF WET GAS POCKET DAMPER SEAL AND VALIDATION AGAINST TEST DATA 1

2 Justification In 1991, pocket damper seal (PDS) Invented by Vance at Turbo Lab [1]. In 2016, a wet gas PDS shows superior stability than a labyrinth seal [2] Time average LVF contour. Seal inlet LVF = 30%, P s = 12.3 bar, P a = 10.2 bar. PDSs are gas seals with high damping coefficient, and + leakage than a labyrinth seal of same dimensions. Labyrinth seal Few published research on wet gas PDS, in particular for rotordynamic coefficients! PDS: Less trapped liquid! 2

3 Objective To present a comprehensive analysis, numerical and experimental, for leakage and force coefficients of a wet gas PDS. Content 1. CFD model & predictions Verification case 2. CFD model & predictions vs. tests Gas PDS at Turbo Lab 3. CFD model & predictions vs. tests Wet gas PDS at Turbo Lab 3

4 CFD procedure 3D flow solver A multi-frequency, elliptic orbit method [3] Displacements d x(t), d y(t) ) 3D unsteady state flow solution: Force (F x(t), F y(t) ) steady state performance: Seal leakage (m); Static pressure (P); Velocities (W and U θ ); Drag torque, etc. d a cos( t); d b sin( t) t N Quantify how seal works! t x i y i i1 i1 Seal force coefficients: Direct dynamic stiffness H R ; Cross-coupled dynamic stiffness h R ; Direct damping C; N H j Discrete Fourier Transform (DFT) Frequency domain: (d x(ω), d y(ω) ) and (F x(ω), F y(ω) ) af ib F Xj Y af ibf j j Yj Xj ; h 2 2 j a b a 2 b 2 4

5 1. CFD model & predictions Verification case 5

6 An eight-ribs pocket damper seal (PDS) Test case to validate accuracy of CFD method against test data [4]. Figure. Cross-section view of eight-rib pocket damper seal (not to scale). Eight ribs (axial) Seven pocket sections Seal length, L Rotor diameter, D r Clearance height, C r Number of ribs Number of ridges mm 170 mm 0.3 mm 8 (axial) 8 (circumferential) Supply pressure, P S Supply temperature, T S Rotor speed, Ω Surface speed, ΩR Air (ideal gas) 6.9 bar (a) K 15,000 rpm m/s [4] Ertas, B. H., Delgado, A., and Vannini, G., 2012, Rotordynamic Force Coefficients for Three Types of Annular Gas Seals with Inlet Pre-swirl and High Differential Pressure Ratio, ASME J. Eng. Gas Turbines Power, 134(4). 6

7 3D mesh for CFD analysis Medium mesh node count = Finer mesh with node count (not shown). 7

8 CFD vs tests: force coefficients Leakage: CFD : g/s; BFM : g/s. BFM: Bulk-Flow Model PDSeal Direct stiffness H R Cross stiffness h R frequency CFD H R and h R agree well with test data. CFD C is smaller than experimental one. BFM is poor but still captures trends as frequency increases. Damping C Effective damping C eff frequency 8

9 Predicted force coefficients per pocket Direct stiffness H R /L C Direct damping C/L C frequency frequency The force coefficients for each pocket show similar trends vs. frequency. #1 pocket shows the largest positive direct stiffness and smallest damping. #7 pocket shows the lowest direct stiffness. 9

10 2. CFD model & predictions vs. tests Gas PDS at Turbo Lab 10

11 TRC wet seal test rig LVF: liquid volume fraction Oil Inlet (ISO VG 10) Valve Air Inlet Valve Sparger (mixing) element Test seal section P s : pressure at seal inlet plane P a : ambient pressure= 1 bar(a) LVF at inlet: LVF Q g : gas flow rate at P s Q l : liquid flow rate Q l Q +Q P / P l g a s Supply pressure (P s ) 1.0~3.5 bar Oil ISO VG 10 density(ρ l ) 830 kg/m 3 journal speed: 3.5 krpm (23.3 m/s) viscosity (μ l ) at 34 ºC 10.6 cp Air density (ρ ga ) at 1bar 1.1 kg/m 3 viscosity(μ ga ) at 20 ºC 0.02 cp 11

12 A four-ribs pocket damper seal Tested at TRC wet seal rig facility Cross-section (not to scale) Seal length, L 48 mm Rotor diameter, D r 127 mm Stator diameter, D s 68.5 mm Clearance height, C r mm Number of ribs 4 (axial) Number of ridges 8 (45º) PDS View (180º cut). 12

13 3D mesh for CFD analysis Medium mesh node count Finer mesh with node count (not shown). 13

14 Seal supplied with air (LVF=0) Air (ideal gas) Supply pressure, P S 1.6, 2.3, 3.2 bar (a) Exit pressure, P a 1 bar (a) Supply temperature, T S 315 K Rotor speed, Ω 5,250 rpm Surface speed, ΩR 35 m/s Leakage Pressure field P/P a Seal leakage vs. pressure ratio (P S /P a ). Pressure drop as air leaves a pocket and decreases on film below a rib. P ~ constant within a pocket. 14

15 CFD vs tests: force coefficients Air only Direct stiffness Cross stiffness H R h R Test cross stiffness h R is very small & with great variability. CFD stiffnesses, H R & h R, match well test data. CFD C is ~ 30% lower than test C. BFM C ~ 0 C eff < 0 (poor model). frequency Damping C Effective damping frequency C eff 15

16 3. CFD model & predictions vs. tests Wet Gas PDS at Turbo Lab 16

17 Operation with wet air Oil in air mixture Supply pressure, P S 2.3 bar (a) Exit pressure, P a 1 bar (a) Supply temperature, T S 315 K Oil density, ρ l 830 kg/m 3 Viscosity at (P S, T S ), μ l 8.2 cp Rotor speed, Ω 5,250 rpm Surface speed, ΩR 35 m/s Liquid volume fraction (LVF) 0, 0.4%, 2.2% Seal Leakage conditions Test data CFD Case Inlet Inlet ṁ ṁ P No. S /P l ṁ ṁ l a LVF LMF [g/s] [g/s] [g/s] [g/s] % 57% 27.2± % 84% 68.7± LVF at exit plane= 0.17% and 0.7% A small LVF (<1%) amounts to a large liquid mass CFD predictions match measured flows Reynolds number For oil in air mixture (LVF = 0.4%) Dry gas Wet gas WC m D m m r Rea m Re c m( RC ) r 2 Re a Re c Re Re a /Re c 2, , m m 17

18 LVF contours on stator and rotor LVF =0.4% at inlet Stator Rotor Liquid leaves as a jet; Air expands 18

19 CFD vs tests: force coefficients Wet gas H R Direct stiffness Cross stiffness hr Under wet gas operation, test data shows ++ variability. Test H R <0 but CFD H R >0. CFD h R & C show same trends as test data. frequency Damping C Effective damping C eff frequency 19

20 Conclusions CFD model (3D flow solver + multi-frequency, elliptic orbit method + DFT) predicts force coefficients for two PDS. Comparisons to test data for GE-Ertas et al. PDS validates accuracy of CFD model. PDS at Turbo Lab ** For air only, the CFD predictions match well test data. BFM under estimates damping C. ** For wet gas, test direct stiffness (H R ) turns negative, whereas CFD predicted H R > 0. ** Both test and predicted cross-coupled stiffness and direct damping increases compared with those for gas PDS. 20

21 Acknowledgements Thanks to TRC and High Performance Research Center (HPRC) in TAMU. Questions (?) 21

22 References [1] Vance, J. M., and Shultz, R. R, 1993, A New Damper Seal for Turbomachinery, Proc. of the 14th Vibration and Noise Conference, September 19-22, Albuquerque, NM, ASME DE-60, pp [2] Vannini, G., Bertoneri, M., Nielsen, K. K, Ludiciani, P., and Stronach, R., 2016, Experimental Results and Computational Fluid Dynamics Simulations of Labyrinth and Pocket Damper Seals for Wet Gas Compression, ASME J. Eng. Gas Turbines Power, 138(5), pp [3] Li, J., Li, Z., and Feng, Z., 2012, Investigations on the Rotordynamic Coefficients of Pocket Damper Seals Using the Multi-frequency, One-Dimensional, Whirling Orbit Model and RANS Solutions, ASME J. Eng. Gas Turbines Power, 134(10), pp [4] Ertas, B. H., Delgado, A., and Vannini, G., 2012, Rotordynamic Force Coefficients for Three Types of Annular Gas Seals with Inlet Pre-swirl and High Differential Pressure Ratio, ASME J. Eng. Gas Turbines Power, 134(4), p [5] San Andrés, L., 2012, Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals, ASME J. Eng. Gas Turbine Power, 134(2), pp [6] Benckert, H., and Wachter, J., 1980, Flow Induced Spring Constants of Labyrinth Seals for Applications in Rotor Dynamics, Proc. of the 1st Workshop on Rotordynamic Instability Problems in High-Performance Turbomachinery, May 12-14, College Station, Texas, Texas A&M University, pp

23 Pressure & velocities along seal P W LVF α At exit, air slows down and expands. Oil adheres to rotor surface. CFD homogeneous flow! 23