Research Papers

Predicting Separation and Transitional Flow in Turbine Blades at Low Reynolds Numbers—Part I: Development of Prediction Methodology

[+] Author and Article Information
Darius D. Sanders, Walter F. O’Brien

Department of Mechanical Engineering, Virginia Tech, Blacksburg, VA 24061

Rolf Sondergaard, Marc D. Polanka, Douglas C. Rabe

Air Force Research Lab, Propulsion Directorate, Wright Patterson AFB, OH 45433

J. Turbomach 133(3), 031011 (Nov 15, 2010) (10 pages) doi:10.1115/1.4001230 History: Received August 13, 2009; Revised October 19, 2009; Published November 15, 2010; Online November 15, 2010

There is an increasing interest in design methods and performance prediction for aircraft engine turbines operating at low Reynolds numbers. In this regime, boundary layer separation may be more likely to occur in the turbine flow passages. For accurate computational fluid dynamics (CFD) predictions of the flow, correct modeling of laminar-turbulent boundary layer transition is essential to capture the details of the flow. To investigate possible improvements in model fidelity, CFD models were created for the flow over two low pressure turbine blade designs. A new three-equation eddy-viscosity type turbulent transitional flow model, originally developed by Walters and Leylek (2004, “A New Model for Boundary Layer Transition Using a Single Point RANS Approach,” ASME J. Turbomach., 126(1), pp. 193–202), was employed for the current Reynolds averaged Navier–Stokes (RANS) CFD calculations. Previous studies demonstrated the ability of this model to accurately predict separation and boundary layer transition characteristics of low Reynolds number flows. The present research tested the capability of CFD with the Walters and Leylek turbulent transitional flow model to predict the boundary layer behavior and performance of two different turbine cascade configurations. Flows over low pressure turbine (LPT) blade airfoils with different blade loading characteristics were simulated over a Reynolds number range of 15,000–100,000 and predictions were compared with experimental cascade results. Part I of this paper discusses the prediction methodology that was developed and its validation using a lightly loaded LPT blade airfoil design. The turbulent transitional flow model sensitivity to turbulent flow parameters was investigated and showed a strong dependence on freestream turbulence intensity with a second-order effect of turbulent length scale. Focusing on the calculation of the total pressure loss coefficients to judge performance, the CFD simulation incorporating Walters and Leylek’s turbulent transitional flow model produced adequate prediction of the Reynolds number performance for the lightly loaded LPT blade cascade geometry. Significant improvements in performance were shown over predictions of conventional RANS turbulence models. Historically, these models cannot adequately predict boundary layer transition.

Copyright © 2011 by American Society of Mechanical Engineers
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Figure 5

Comparison of total pressure loss coefficient for turbulence models at Re=100,000, Tu=1% for the lightly loaded blade

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Figure 7

Wake region total pressure contours for the lightly loaded blade at (a) Lm=8 mm, (b) Lm=14 mm, and (c) Lm=20 mm

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Figure 8

Blade surface turbulent kinetic energy at different inlet turbulent intensities for the lightly loaded blade

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Figure 9

Comparison of boundary layer integral parameters at different inlet freestream turbulent intensities at Re=50,000 for the lightly loaded blade airfoil

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Figure 10

Comparison of the CFD and experimental (11) maximum loss coefficient at a range of inlet Reynolds numbers for the lightly loaded blade airfoil

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Figure 1

Hybrid O-H grid of lightly loaded blade airfoil

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Figure 2

Comparison of the total pressure loss coefficient for the lightly loaded blade airfoil at different grids spatial resolutions

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Figure 3

Air Force Research Lab Low Speed Wind Tunnel view of test section (11)

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Figure 4

Comparison of transition location at Re=100,000, Tu=1% for the lightly loaded blade airfoil

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Figure 6

Maximum wake loss coefficient verses inlet turbulent length scale for the lightly loaded blade




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