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Research Papers

Influence of Stator-Rotor Interaction on the Aerothermal Performance of Recess Blade Tips

[+] Author and Article Information
Bob Mischo, Reza S. Abhari

Department of Mechanical and Process Engineering, LSM, Turbomachinery Laboratory, Laboratory for Energy Conversion, IET ETH Zürich, Sonneggstrasse 3, CH-8092 Zürich, Switzerland

André Burdet

Gas Turbine Business, Alstom (Switzerland) Ltd., CH-5401 Baden, Switzerland

J. Turbomach 133(1), 011023 (Sep 24, 2010) (11 pages) doi:10.1115/1.4001134 History: Received June 28, 2008; Revised September 19, 2009; Published September 24, 2010; Online September 24, 2010

This paper investigates the influence of stator-rotor interaction on the stage performance of three blade tip geometries. A reference flat tip is used to assess two different recess blade geometries. The study is made in the context of the realistic turbine stage configuration provided by the ETHZ 1.5-stage LISA turbine research facility. This numerical investigation describes the details of unsteady recess cavity flow structure and confirms the beneficial effects of the improved recess geometry over the flat tip and the nominal recess design both in terms of stage efficiency and tip heat load. The tip flow field obtained from the improved recess design combines the advantages of a nominal recess design (aerodynamic sealing) and the flat tip configuration. The turbine stage capacity is almost unchanged between the flat tip and the improved recess tip cases, which simplifies the design procedure when using the improved recess design. The overall heat load in the improved recess case is reduced by 26% compared with the flat tip and by 14% compared with the nominal recess. A key finding of this study is the difference in effects of the upstream stator wake on the recess cavity flow. Where cavity flow in the nominal design is only moderately influenced, the improved recess cavity flow shows enhanced flow unsteadiness. The tip Nusselt number from a purely steady-state prediction in the nominal recess case is nearly identical to the time-average prediction. The improved design shows a 6% difference between steady-state and time-average tip Nusselt number. This is due to the strong influence of the wake passing on the recess cavity flow. In fact, the wake enhances a small flow difference at the leading edge of the recess cavity between the nominal and improved recess cavities, which results in a completely different flow field further downstream in the recess cavity.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Blade tip geometries

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Figure 2

Expanded view of the computational domain and grid

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Figure 3

Steady-state recess flow structure for the nominal recess (left) and improved recess (right)

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Figure 4

Time-averaged blade tip loading at 95% span

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Figure 5

Predicted and measured time- and pitch-averaged rotor outlet pressure coefficients (left) and predicted entropy (right)

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Figure 6

Time-averaged contours of absolute momentum of tip leakage flow for flat tip (top), nominal recess (middle), and improved recess (bottom)

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Figure 7

Axial distribution of tip leakage mass flow at tip midheight

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Figure 8

Velocity triangles for freestream and wake fluid at blade tip

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Figure 9

Normalized (by minimal cavity entropy) entropy contours of nominal recess case, superimposed with flow streamlines. Four snapshots at t∗=[0.5;0.6;0.7;0.8] are represented. Radial cut at 99% span (top) and axial cut (22% cax) downstream of rotor leading edge (bottom).

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Figure 10

Normalized (by minimal cavity entropy) entropy contours of improved recess case, superimposed with flow streamlines. Four snapshots at t∗=[0.5;0.6;0.7;0.8] are represented. Radial cut at 99% span (top) and axial cut (22% cax) downstream of rotor leading edge (bottom).

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Figure 11

Time space diagram of entropy along a line traversing the recess cavity. Nominal recess case (left) and improved recess case (right).

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Figure 12

Predicted steady-state and time-averaged heat load for the investigated test cases

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Figure 13

Time-accurate (t∗=0.0, top row), time-averaged (second row), line time-averaged profiles (third row), and steady-state (bottom row) prediction of Nusselt number on the blade tip wetted surface for the flat tip (first column), nominal recess (second column), and improved recess (third column) configurations.

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