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Research Papers

Effect of the Leakage Flows and the Upstream Platform Geometry on the Endwall Flows of a Turbine Cascade

[+] Author and Article Information
E. de la Rosa Blanco, H. P. Hodson

Whittle Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0DY, UK

R. Vazquez

ITP, Industria de Turbo Propulsores S.A., Avda. Castilla 2. Parque Empresarial San Fernando-Edificio Japon, San Fernando de Henares, Madrid, 28830, Spain

J. Turbomach 131(1), 011004 (Oct 02, 2008) (9 pages) doi:10.1115/1.2950052 History: Received February 11, 2007; Revised December 03, 2007; Published October 02, 2008

This work describes the effect that the injection of leakage flow from a cavity into the mainstream has on the endwall flows and their interaction with a large pressure surface separation bubble in a low-pressure turbine. The effect of a step in hub diameter ahead of the blade row is also simulated. The blade profile under consideration is a typical design of modern low-pressure turbines. The tests are conducted in a low speed linear cascade. These are complemented by numerical simulations. Two different step geometries are investigated, i.e., a backward-facing step and a forward-facing step. The leakage tangential velocity and the leakage mass flow rate are also modified. It was found that the injection of leakage mass flow gives rise to a strengthening of the endwall flows independently of the leakage mass flow rate and the leakage tangential velocity. The experimental results have shown that below a critical value of the leakage tangential velocity, the net mixed-out endwall losses are not significantly altered by a change in the leakage tangential velocity. For these cases, the effect of the leakage mass flow is confined to the wall, as the inlet endwall boundary layer is pushed further away from the wall by the leakage flow. However, for values of the leakage tangential velocity around 100% of the wheel speed, there is a large increase in losses due to a stronger interaction between the endwall flows and the leakage mass flow. This gives rise to a change in the endwall flows’ structure. In all cases, the presence of a forward-facing step produces a strengthening of the endwall flows and an increase of the net mixed-out endwall losses when compared with a backward-facing step. This is because of a strong interaction with the pressure surface separation bubble.

Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 11

Calculated particle path lines around the slot. ṁleak=0.9%ṁ1. (a) Vleak=101%U, backward step. (b) Vleak=44%U, backward step. (c) Vleak=101%U, forward step. (d) Vleak=37%U, forward step.

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Figure 12

Calculated path lines of the inlet endwall boundary layer. Forward step, Vleak: 101%U. ṁleak=0.9%ṁ1.

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Figure 13

Measured pitchwise mass-averaged gross stagnation pressure loss and deviation at 50% Cax downstream of the blade trailing edge. ṁleak=0.7%ṁ1.

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Figure 14

Measured loss contours, streamwise vorticity contours, and secondary velocity vectors at 50% Cax downstream of the blade trailing edge. Backward facing step. ṁleak=0.7%ṁ1. (a) and (b) Leakage tangential velocity: 56%U. (c) and (d) Leakage tangential velocity: 101%U.

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Figure 15

Measured loss contours, streamwise vorticity contours, and secondary velocity vectors at 50% Cax downstream of the blade trailing edge. Forward facing step, ṁleak=0.7%ṁ1. (a) and (b) Leakage tangential velocity: 56%U. (c) and (d) Leakage tangential velocity: 92.5%U.

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Figure 1

Oil flow visualization experiments on the endwall. Turbulent inlet boundary layer. (a) Backward-facing step. (b) Forward-facing step.

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Figure 2

Profile sections and static pressure distributions

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Figure 3

Schematic representation of the cascade and cavity layout

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Figure 4

Cross-sectional view of the cavity. (a) Backward-facing step. (b) Forward-facing step.

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Figure 5

Oil flow visualization experiments on the blade pressure surface with cavity but with no net leakage flow. (a) Backward facing step. (b) Forward facing step.

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Figure 6

Measured pitchwise mass-averaged gross stagnation pressure loss and deviation at 50% Cax downstream of the blade trailing edge

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Figure 7

Measured loss contours, streamwise vorticity contours, and secondary velocity vectors at 50% Cax downstream of the blade trailing edge. Backward facing step. (a) and (b) No cavity. (c) and (d) Cavity. No net leakage flow.

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Figure 8

Measured loss contours, streamwise vorticity contours, and secondary velocity vectors at 50% Cax downstream of the blade trailing edge. Forward facing step. (a) and (b) No cavity. (c) and (d) Cavity. No net leakage flow.

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Figure 9

Oil flow visualization experiments on the endwall. ṁleak=0.7%ṁ1⋅Vleak=56%U. (a) Backward-facing step. (b) Forward-facing step.

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Figure 10

Measured net mixed-out endwall losses

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