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									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
Contents


        Introduction

        Calculation Models and Methods

        Results
           – Unsteady Flow Structure

           – Effect of Downstream Stator Row

           – Non-Synchronous Vibration

        Concluding Remarks




                                  1            Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
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.
 1 Mailach, 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
                                                                               2                           Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
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




                                          3              Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
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.
                                                                      4                      Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Calculation Models and Methods (2/4)
 Performance Map                                                                            H s            cx
                                                                                                2
                                                                                                    , 
                                                                                             1U              Ut
   – Averaging 4 stages                                                                      2 t



                                             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

                          [Wisler (1981)]
                                             The calculation underestimated the
                                            pressure rise by 7% at the design point.

                                             The operating range from the
                                            calculation is shorter.



                                    5                  Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
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
    Negative Jet Effect - Mailach et al. (2008)1              - Sirakov et al. (2003)2


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

                                                                        6                        Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
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
         Numerical monitor

                                 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
                                       7                 Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Unsteady Flow Structure (1/5)
 Entropy Distribution                       s  C p ln(
                                                             T
                                                                 )  R ln(
                                                                            p
                                                                                ) , ref : inlet of rotor
                                                            Tref           pref
   – 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




                                      8             Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
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




                                      9        Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
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




                                         10              Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Unsteady Flow Structure (4/5)
 Axial Velocity Distribution
   – Contours at the exit of the rotor row
   – 86% of the design point




                      Unsteady                     Blockage
                                                   • The steady-state calculation predicts
                                                    more blockage near the casing.
                                                   • In the unsteady calculation result, the
                                                    tip leakage vortex blocks less area.


                                                    Predicted blockage may have caused
                     Steady-state
                                                    the pressure rise difference.




                                             11            Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Unsteady Flow Structure (5/5)
 Velocity Distribution
   – Axial & tangential velocities at the exit of the rotor row                    Plateau

   – Circumferentially averaged




                                                                           Pressure rise does not
                                                                           significantly decrease at
   Axial Velocity                Tangential Velocity                     the low flow rates.
   • Higher at 72% than that      • Flow turning at 72% is not
    at 80% near the casing          much smaller than that at
                                    80%.

                                            12              Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
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             Streamlines

         Velocity vectors




                                         13              Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Effect of Downstream Stator Row (2/2)
 Rotating Instability

                                                        Circumferential mode order is nearly half
                                                        the blade number - Mailach et al. (2001)
                                                                                                                              Experiment
                                                        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                                          CFD
        Propagation of Rotating Instabilities
        - Mailach et al. (2001)                         - Drolet et al. (2009)                                                Single blade

                                                        Self-induced unsteadiness
                                                        - Du et al. (2010)




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.

                                                                   14                    Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Non-Synchronous Vibration (1/3)
 Tip Leakage Flow
   – Time-variation at the rotor row     Rotating Instability
   – Design mass flow rate               • 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

                                       15              Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Non-Synchronous Vibration (2/3)
 Tip Leakage Flow
   – Time-variation at the rotor row      Rotating Instability
   – 80% of the design mass flow rate     • 4 discrete vortices
                                          • Rotates at 67% of the rotor speed
                                          • NSV frequency: 285Hz




         Negative Axial Velocity                      Pressure Signal

                                        16             Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Non-Synchronous Vibration (3/3)
 Tip Leakage Flow
   – At the rotor row                     Rotating Instability
   – 72% of the design mass flow rate     • 3 discrete vortices
                                          • Rotates at 72% of the rotor speed
                                          • NSV frequency: 231Hz




           Negative Axial Velocity
                                             Operating point
                                                                        100%             80%            72%
                                             (Mass flow rate)

                                        Circumferential mode
                                                                           80              32              24
                                          order per annulus

                                              Rotating speed              47%            67%             72%


                                              NSV frequency              498Hz          285Hz           231Hz




              Pressure Signal
                                        17                     Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
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.


                                                    18                Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Thank you for your attention.




             19       Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024

								
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