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Research Papers

The Influence of Blade Lean on Straight and Annular Turbine Cascade Flow Field

[+] Author and Article Information
Gabriele D’Ippolito

Dipartimento di Energia, Politecnico di Milano, I-20133 Milano, Italygabriele.dippolito@polimi.it

Vincenzo Dossena, Alessandro Mora

Dipartimento di Energia, Politecnico di Milano, I-20133 Milano, Italy

J. Turbomach 133(1), 011013 (Sep 21, 2010) (9 pages) doi:10.1115/1.4000536 History: Received February 25, 2009; Revised March 27, 2009; Published September 21, 2010; Online September 21, 2010

The work proposes a detailed description of the flow field throughout leaned turbine nozzles and reports a sensitivity analysis with respect to the lean angle. A phenomenological approach focuses the attention on pressure contours distribution on both inside and outside the passage. The study involves both straight and annular cascades mounting a typical intermediate reaction degree section, designed for steam turbines. Blades are built by stacking the same 2D profile along different linear axes, characterized by different angles with respect to the normal or radial direction: α=0deg for prismatic blade and α=10deg, 15 deg, and 20 deg for the leaned ones are considered. Experimental and numerical tests were performed at the nominal inlet flow angle in order to avoid any effect related to blade sweep. Experimental tests were carried out at the design outlet Mach number of 0.65; measurements were performed at the Laboratorio di Fluidodinamica delle Macchine of Politecnico di Milano. Only linear cascades with prismatic and 20 deg leaned blades were experimentally tested, providing data both downstream and inside the blade passage by means of pressure probe traversing, endwall pressure taps, and oil flow visualization. Experimental results were also used to validate the numerical setup, which provided a detailed computational picture of the flow field throughout the channel. The influence of the pressure contours’ shape on secondary vorticity activity downstream of the passage is highlighted and discussed, focusing the attention on secondary structures and loss distribution in this region. The resulting description of the flow field, based on the representation of pressure contours, supports the sensitivity analysis with respect to the blade lean angle, identifying the mechanism that leads the secondary vorticity to grow in regions where secondary losses and blade loading decrease.

Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Experimental setup: schematic view of the straight cascade with blade profile traces at the hub

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Figure 2

Arrangement of pressure taps at hub endwall inside the passage for the straight cascade

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Figure 3

Picture of the full leaned blade and sketch of its axis arrangement (view from upstream)

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Figure 4

Straight cascade: (a) comparison between numerical and experimental results in terms of pressure distributions at the endwall; (b) computed wall shear stress superimposed to flow visualization

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Figure 5

Straight cascade: numerical pressure contours on the plane at x/B=−0.2 and sketch of plane location

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Figure 6

Straight cascade: numerical pressure contours on the plane at x/B=0.65 and sketch of plane location

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Figure 7

Straight cascade: (a) a schematic of the view plane and (b) prismatic blade, with (c) suction side flow visualization for leaned blade (α=20 deg)

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Figure 8

Straight cascade: (a) superimposition of oil and dye visualization with computed dimensionless pressure contours for the 20 deg leaned blade (Δp/p0=0.015); (b) dimensionless surface pressure distribution at different blade heights for the same blade

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Figure 9

Straight cascade: numerical pressure contours on the plane at x/B=1.4 and a sketch of plane location

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Figure 10

Straight cascade: spanwise velocity as percentage of the 3D velocity (mass-averaged on planes at constant x) and thrust sign on the flow on the considered planes

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Figure 11

Vorticity evolution for straight cascade (α=20): (a) passage vortices and their rotation sense inside the passage; (b) pressure contours with superimposition of vorticity induced by blade lean inside the passage (located as sketched on the left); (c) overall effect just downstream the trailing edge (located as sketched on the right)

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Figure 12

Straight cascade: computed vorticity maps downstream of the cascade at x/B=1.5 for α=0 and α=20

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Figure 13

Straight cascade: experimental loss and vorticity contours at x/B=1.5 for α=0 and α=20

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Figure 14

Straight cascade: nondimensional pressure contours p/p0 inside the passage at x/B=0.65 for leaned blades with α=10 deg (frame a), 15 deg (frame b), and 20 deg (frame c)

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Figure 15

Straight cascade: spanwise distribution of pitchwise mass-averaged yaw and pitch angle at x/B=1.4

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Figure 16

Straight cascade: spanwise velocity as percentage of the 3D velocity (mass-averaged on planes at constant x/B) for the three leaned cascades

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Figure 17

Annular cascade: dimensionless pressure contours inside the prismatic and 20 deg leaned channels

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Figure 18

Annular cascade: dimensionless pressure distribution on blade profile for α=0 and α=20 at hub and tip sections

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Figure 19

Annular cascade: (a) dimensionless pressure contours inside the passage (x/B=0.65); (b) spanwise velocity as percentage of the 3D velocity (mass-averaged on planes at constant x/B) for prismatic and leaned cascades

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Figure 20

Annular cascade: spanwise distribution of pitchwise mass-averaged values of massflow, yaw, and pitch angles downstream of the cascades at x/B=1.4

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Figure 21

Annular cascade: overall mass-averaged results. Values of discharge angle, reported with respect to massflow and loss.

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