June 17, 2010

1. ASME Turbo Expo 2010 / GT2010-23024 Numerical Study on Unsteadiness of Tip Clearance Flow Induced by Downstream Stator Row in Axial Compressor Yoojun Hwang* ∙ Shin-Hyoung Kang* ∙ Sungryoung Lee** June 17, 2010 * Mechanical and Aerospace Engineering, Seoul National University, Korea ** Doosan Heavy Industries and Construction Co., Ltd, Korea

2. Contents • Introduction • Calculation Models and Methods • Results • Unsteady Flow Structure • Effect of Downstream Stator Row • Non-Synchronous Vibration • Concluding Remarks

3. Introduction (1/2) • Previous Studies • Unsteady Tip Clearance Flow in Axial Compressors near Stall • Periodically fluctuating tip leakage vortex was investigated by Mailach et al. (2001)1, Marz et al. (2002)2, Kielb et al. (2003)3, Bae et al. (2004)4, Hah et al. (2008)5, etc. • The unsteady flow was referred to as self-induced unsteadiness. • The origin or the role of the flow has been studied by Du et al. (2010)6, Thomassin et al. (2009)7, Drolet et al. (2009)8, etc. 1Mailach, R., Lehmann, I., and Vogeler, K., 2001, “Rotating Instabilities in an Axial Compressor Originating From the Fluctuating Blade Tip Vortex,” Journal of Turbomachinery, Vol. 123, pp. 453-463. 2 März, J., Hah, C., and Neise, W., 2002, “An Experimental and Numerical Investigating Into the Mechanisms of Rotating Instability,” Journal of Turbomachinery, Vol. 124, pp. 367-375. 3 Kielb, R. E., Barter, J. W., Thomas, J. P., and Hall, K. C., 2003, “Blade Excitation by Aerodynamics Instatbilites — A Compressor Blade Study,” ASME Turbo Expo 2003, GT2003-38634. 4 Bae, J., Breuer, K. S., and Tan, C. S., 2004, “Periodic Unsteadiness of Compressor Tip Clearance Vortex,” ASME Turbo Expo 2004, GT2004-53015. 5 Hah, C., Bergner, J., and Schiffer, H.-P., 2008, “Tip Clearance Vortex Oscillation, Vortex Shedding and Rotating Instability in an Axial Transonic Compressor Rotor,” ASME Turbo Expo 2008, GT2008-50105. 6 Du, J., Lin, F., Zhang, H., and Chen, J., 2010, “Numerical Investigation on the Self-Induced Unsteadiness in Tip Leakage Flow for a Transonic Fan Rotor,” Journal of Turbomachinery, Vol. 132, pp. 021017. 7 Thomassin, J., Vo, H. D., and Mureithi, N. W., 2009, “Blade Tip Clearance Flow and Compressor Nonsynchronous Vibrations: The Jet Core Feedback Theory as the Coupling Mechanism,” Journal of Turbomachinery, Vol. 132, pp. 011013. 8 Drolet, M., Thomassin, J., Vo, H. D., and Mureithi, N. W., 2009, “Numerical Investigation into Non-Synchronous Vibrations of Axial Flow Compressors by the Resonant Tip Clearance Flow,” ASME Turbo Expo 2009, GT2009-59074

4. Introduction (2/2) • Motivation • In the previous studies, the unsteady tip leakage flow has been found to be inherent. • Most of the numerical investigations have been done only for a rotor row or for single blade passages. • Objective • Investigate the influence of the downstream stator row on the unsteady flow • Conduct time-accurate numerical calculations for a stage • Performance characteristic, unsteady flow structure, tip leakage flow vibration

5. Calculation Models and Methods (1/4) • Compressor Model • Low speed research axial compressor (LSRC) • 4 stages • Number of blades: IGV(53), Rotor(54), Stator(74) • Hub-to-tip ratio: 0.85 • Tip clearance size to blade height: 2.8% • Experimentally Measured Data • Performance measured by Wisler (1981)1 • 1st stage has a casing treatment with circumferential grooves. • Numerically Calculated Data • Code: ANSYS-CFX 11.0 • Standard k-ε model with the wall function • Structured H-mesh with as a coarse grid as 40,000 cells/blade passage • No casing treatment 1 Wisler, D. C., 1981, “Core Compressor Exit Stage Study Volume IV—Data and Performance Report for the Best Stage Configuration,” NASA CR-165357.

6. Calculation Models and Methods (2/4) • Performance Map • Averaging 4 stages • Calculation Data • Steady-state assumption at the frame interfaces • Single blade passage • Casing Treatment Effect - Wisler (1981) • No change in pressure rise • 8.2% improvement in stall margin • Similar trend around the design point • The calculation underestimated the pressure rise by 7% at the design point. • The operating range from the calculation is shorter. [Wisler (1981)]

7. Calculation Models and Methods (3/4) • Effect of the Wake from the Upstream Blade • Total temperature in the wake is higher than that in the core flow. • Not captured in steady-state calculations • Effect of the Wake • Up to 5.2% of pressure rise  Need Unsteady Calculations Beneficial Effect of Wake - Sirakov et al. (2003)2 Negative Jet Effect - Mailach et al. (2008)1 1 Mailach, R., Lehmann, I. and Vogeler, K., 2008, “Periodic Unsteady Flow Within a Rotor Row of an Axial Compressor— Part II: Wake-Tip Clearance Vortex Interaction,” ASME J. Turbomachinery, Vol. 130, 041005. 2 Sirakov, B. T. and Tan, C. S., 2003, “Effect of Unsteady Stator Wake—Rotor Double-Leakage Tip Clearance Flow Interaction on Time-Average Compressor Performance,” ASME J. Turbomachinery, Vol. 125, pp. 465-474

8. Calculation Models and Methods (4/4) • Unsteady Calculation Method • Modified 1/8 annulus • Number of Blades for Each Row • Rotor: 54  56 • Stator: 74  72 • IGV + Additional Domain • Single blade + Mixing-Plane • Provide circumferentially uniform flow to the inlet of the rotor row • Boundary Conditions • Inlet: Atmospheric conditions (Pt, Tt) • Exit: Mass flow rate  Adjust operating conditions • Calculation Process • Reducing mass flow rate from the design point • One rotor revolution at each point • Numerical Monitor • Between rotor and stator in the stationary frame • Static pressure Numerical monitor

9. Unsteady Flow Structure (1/5) • Entropy Distribution • Contours at 50% and 90% span height • Design point • 50% Span • Wakes from the rotor blades • The structures are identical at every blade passage. • 90% Span • The tip leakage flow interacts with the wakes. • The structures are not identical at every blade passage. • The tip leakage flow varies with time. 50% span 90% span

10. Unsteady Flow Structure (2/5) • Entropy Distribution • Contours at 50% and 90% span height • Design point • Without stator row • Without Stator • The tip leakage flow structures are identical at every blade passage. • Tip leakage flow varies with time.  Potential effect of the downstream stator row on the rotor tip leakage flow behavior 50% span 90% span

11. Unsteady Flow Structure (3/5) • Performance Characteristics • Unsteady and Steady-state calculations • Modified 1/8 annulus model • Pressure Rise • The unsteady calculation improves the underestimation of the steady-state calculation. • The difference increases as the flow rate decreases. • Operating Range • The steady-state calculation predicts the limit earlier. • Low Flow Rate • Plateau on the curve

12. Unsteady Flow Structure (4/5) • Axial Velocity Distribution • Contours at the exit of the rotor row • 86% of the design point • Blockage • The steady-state calculation predicts more blockage near the casing. • In the unsteady calculation result, the tip leakage vortex blocks less area. Unsteady  Predicted blockage may have caused the pressure rise difference. Steady-state

13. Unsteady Flow Structure (5/5) • Velocity Distribution • Axial & tangential velocities at the exit of the rotor row • Circumferentially averaged Plateau  Pressure rise does not significantly decrease at the low flow rates. • Axial Velocity • Higher at 72% than that at 80% near the casing • Tangential Velocity • Flow turning at 72% is not much smaller than that at 80%.

14. Effect of Downstream Stator Row (1/2) • Formation of Tip Leakage Vortex • Contours of pressure and streamlines at 90% span • At 80% of the design mass flow rate • Role of Pressure Field • Pressure gradient between the rotor and the stator pushes the tip leakage flow. • Pressure difference variation across the blade tip makes the leakage flows different for the two adjacent blade passages.  The leakage flow forms Rotating instability Static pressure Velocity vectors Streamlines

15. Effect of Downstream Stator Row (2/2) • Rotating Instability • Circumferential mode order is nearly half the blade number - Mailach et al. (2001) • Interactions of the upstream wake and the tip leakage flow - Mailach et al. (2008) • Affected by the axial gap between blade rows - Deng et al. (2005)1 • Resonant tip clearance flow • - Drolet et al. (2009) • Self-induced unsteadiness • - Du et al. (2010) Experiment CFD Single blade Propagation of Rotating Instabilities - Mailach et al. (2001) 1 Deng, X., Zhang, H., Chen, J. and Huang, W., 2005, “Unsteady Tip Clearance Flow in a Low-Speed Axial Compressor Rotor with Upstream and Downstream Stators,” ASME paper GT2005-68571.

16. Non-Synchronous Vibration (1/3) • Tip Leakage Flow • Time-variation at the rotor row • Design mass flow rate • Rotating Instability • Not correspond to the blade periodicity • Rotates at 47% of the rotor speed • NSV frequency: 498Hz • Blade passing frequency: 747Hz Negative Axial Velocity Pressure Signal

17. Non-Synchronous Vibration (2/3) • Tip Leakage Flow • Time-variation at the rotor row • 80% of the design mass flow rate • Rotating Instability • 4 discrete vortices • Rotates at 67% of the rotor speed • NSV frequency: 285Hz Negative Axial Velocity Pressure Signal

18. Non-Synchronous Vibration (3/3) • Tip Leakage Flow • At the rotor row • 72% of the design mass flow rate • Rotating Instability • 3 discrete vortices • Rotates at 72% of the rotor speed • NSV frequency: 231Hz Negative Axial Velocity Pressure Signal

19. Concluding Remarks • Unsteady tip leakage flow was influenced by the potential effect of the downstream stator row. • Rotating instability developed as the flow rate was reduced. • The speed and the circumferential mode order of the rotating instability varied with the flow rate, which corresponded to unsteady tip leakage flow frequency. • In future work, further calculations towards or beyond stall is needed to investigate the behavior of the unsteadiness. • Acknowledgement • Supported by R&D Research Fund from the Korea Institute of Energy Technology Evaluation and • Planning in the Ministry of Knowledge Economy, Korea.

20. Thank you for your attention.