Cengiz Camci Professor of Aerospace Engineering e-mail cxc11@psu.edu Debashis Dey 1 Research Assistant Levent Kavurmacioglu 2 Visiting Professor Turbomachinery Heat Transfer Laboratory, Department of Aerospace Engineering, The Pennsylvania State University, 223 Hammond Building, University Park, PA 16802 Aerodynamics of Tip Leakage Flows Near Partial Squealer Rims in an Axial Flow Turbine Stage This paper deals with an experimental investigation of aerodynamic characteristics of full and partial-length squealer rims in a turbine stage. Full and partial-length squealer rims are investigated separately on the pressure side and on the suction side in the ‘‘Axial Flow Turbine Research Facility’’(AFTRF) of the Pennsylvania State University. The streamwise length of these ‘‘partial squealer tips’’and their chordwise position are varied to find an optimal aerodynamic tip configuration. The optimal configuration in this cold turbine study is defined as the one that is minimizing the stage exit total pressure defect in the tip vortex dominated zone. A new ‘‘channel arrangement’’diverting some of the leakage flow into the trailing edge zone is also studied. Current results indicate that the use of ‘‘partial squealer rims’’ in axial flow turbines can positively affect the local aerodynamic field by weakening the tip leakage vortex. Results also show that the suction side partial squealers are aerodynamically superior to the pressure side squealers and the channel arrangement. The suction side partial squealers are capable of reducing the stage exit total pressure defect associated with the tip leakage flow to a significant degree. @DOI: 10.1115/1.1791279# Introduction Aerodynamic Character of Tip Clearance Flow: The gap required between the tips of rotating blades and the stationary casing of an axial flow turbine is a significant source of ineffi- ciency. The leakage flow mainly induced by the pressure differen- tial between the pressure side and suction side of a rotor tip usu- ally rolls into a streamwise vortical structure. Total pressure losses of this flow structure measured at the exit of a turbine stage are directly proportional with the tip gap height. The leakage flow mixing with the rotor passage flow causes total pressure loss and significantly reduces turbine stage efficiency. Tip leakage related losses might account for as much as a third of the aerodynamic losses in a stage. Early tip leakage flow visualization experiments in water led to the observation of the typical separation bubble near the entrance section of the gap. These studies concluded that viscous flow con- tributions could probably be ignored because of the magnitude of Reynolds number in the tip gap. Additional visualizations showed that the casing boundary layers changed the pressure field in the gap so much that rotational effects could not be ignored for an accurate determination of leakage losses. The rotation does have a very significant impact on the aerodynamic structure of tip clear- ance flows in turbomachinery systems. The acceleration of the flow into the entrance region of the tip gap near the pressure side results in the relaminarization of an otherwise turbulent tip surface boundary layer. A number of in- vestigators measured heat transfer coefficients on the tip surface by using a moving outer casing. They claimed that different rota- tional speeds imposed significant shear layer variations only near the outer casing. Flow features controlling the local tip heat trans- fer were not influenced from the rotation dependent shear layer near the outer casing. Heyes and Hodson @1# reported an iterative two-dimensional method of solving the mass flow through the tip gap. Their experimentally confirmed results showed that the chordwise pressure gradients had a significant influence on the separation zone and the mass flow rate through the gap region. Another set of experiments from Sjolander and Cao in Ref. @2# supported the Heyes and Hodson tip gap flow model, emphasizing the importance of having realistic chordwise pressure gradients. Their model was characterized by a vena-contracta forming a uni- form isentropic jet along the endwall and a wake-mixing region along the blade tip surface downstream of the separation bubble. The main flow near the vena-contracta suffered very little loss. High losses were generated in the shear flow zone near the surface of the separation bubble. Morphis and Bindon @3# performed an annular cascade experi- ment with a rotating outer casing. They found that the width of the separation bubble was directly controlled by the tip gap height. They experimentally confirmed that motion of the outer casing in the opposite direction to the tip gap flow reduced the leakage mass flow rate and momentum. The size of the separation bubble was reduced when the outer casing motion was imposed. The effect of tip gap size and turbulence intensity on heat trans- fer distribution was investigated in a five bladed linear cascade by Azad, Han, Teng and Boyle @4#. They measured relatively high heat transfer near the entrance section of the gap near the pressure side because of the flow entrance effect. A larger tip gap generally resulted in a higher overall heat transfer coefficient because of the increase in the magnitude of the tip leakage flow. A 15–20% increase in heat transfer level along the leakage flow path resulted when the free stream turbulence intensity level was increased from 6.1% to 9.7%. A new tip desensitization method based on a pressure side tip extension was discussed in Dey and Camci @5#. Phase-averaged total pressure maps downstream of the rotor showed the tip vortex leakage related local flow modifications. The rotating turbine study indicated that the momentum defect in the tip vortex of an untreated turbine blade tip could be effectively reduced by a sug- gested ‘‘pressure side tip platform extension.’’ Squealer Tips: The function of a conventional squealer tip de- sign is threefold. The squealer tip provides an effective reduction in tip gap flow. The specific approach also protects the blade tip from the full impact of high temperature leakage gases. A third function of this design approach is its protective ability against incidental rubs. A conventional full squealer approach forms an 1 Presently at GE Global Research Center, Schenectady, NY. 2 Currently at the Mechanical Engineering Department, Istanbul Technical Univer- sity, Istanbul, Turkey. Contributed by the International Gas Turbine Institute and presented at the Inter- national Gas Turbine and Aeroengine Congress and Exhibition, Atlanta, GA, June 16 –19, 2003. Manuscript received by the IGTI Dec. 2002; final revision Mar. 2003. Paper No. 2003-GT-38979. Review Chair: H. R. Simmons. 14 Õ Vol. 127, JANUARY 2005 Copyright © 2005 by ASME Transactions of the ASME Downloaded From: http://turbomachinery.asmedigitalcollection.asme.org/ on 03/28/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-use