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TECHNICAL PAPERS

# Design and Testing of a Transonic Linear Cascade Tunnel With Optimized Slotted Walls

[+] Author and Article Information
Aldo Rona

Department of Engineering, University of Leicester, Leicester LE1 7RH, UKar45@leicester.ac.uk

Renato Paciorri

Department of Mechanics and Aeronautics, University of Rome “La Sapienza,” Via Eudossiana 18, Rome 00184, Italypaciorri@dma.ing.uniroma1.it

Marco Geron

Department of Mechanics and Aeronautics, University of Rome “La Sapienza,” Via Eudossiana 18, Rome 00184, Italy

J. Turbomach 128(1), 23-34 (Jun 23, 2005) (12 pages) doi:10.1115/1.2101856 History: Received October 06, 2004; Revised June 23, 2005

## Abstract

In linear cascade wind tunnel tests, a high level of pitchwise periodicity is desirable to reproduce the azimuthal periodicity in the stage of an axial compressor or turbine. Transonic tests in a cascade wind tunnel with open jet boundaries have been shown to suffer from spurious waves, reflected at the jet boundary, that compromise the flow periodicity in pitch. This problem can be tackled by placing at this boundary a slotted tailboard with a specific wall void ratio $s$ and pitch angle $α$. The optimal value of the $s-α$ pair depends on the test section geometry and on the tunnel running conditions. An inviscid two-dimensional numerical method has been developed to predict transonic linear cascade flows, with and without a tailboard, and quantify the nonperiodicity in the discharge. This method includes a new computational boundary condition to model the effects of the tailboard slots on the cascade interior flow. This method has been applied to a six-blade turbine nozzle cascade, transonically tested at the University of Leicester. The numerical results identified a specific slotted tailboard geometry, able to minimize the spurious reflected waves and regain some pitchwise flow periodicity. The wind tunnel open jet test section was redesigned accordingly. Pressure measurements at the cascade outlet and synchronous spark schlieren visualization of the test section, with and without the optimized slotted tailboard, have confirmed the gain in pitchwise periodicity predicted by the numerical model.

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## Figures

Figure 1

Side view of the wind tunnel test section. All dimensions are in mm.

Figure 2

Double pass schlieren schematics

Figure 3

Blade passage exit geometry in the cascade linear assembly, showing the wall pressure tappings (•). All dimensions are normalized by the blade pitch h.

Figure 4

Experimental schlieren visualization of the cascade flow discharge without a slotted wall

Figure 5

Normalized wall pressure distribution in the discharge from a free jet cascade along the trailing edge line. (–, ◻) passage 1-2, (– –, ▵) passage 2-3, (– ∙ –, ▿) passage 3-4, (◇) passage 4-5

Figure 6

Shock reflecting over a slotted flat plate

Figure 7

Control volume around a slot

Figure 8

Differential pressure across a slotted wall versus normalized slot mass flow rate

Figure 9

Reflection of an impinging oblique shock over a straight boundary. Thick wall model.

Figure 10

Mesh and boundary conditions of the cascade model

Figure 11

Isentropic Mach number distributions along the blades: (a) open jet cascade, (b) tailboard with s=0.5 and α=62 deg, and (c) tailboard with s=0.143 and α=64 deg

Figure 12

Pressure distribution at the blade trailing edge lines: (a) open jet cascade, (b) tailboard with s=0.5 and α=62 deg, and (c) tailboard with s=0.143 and α=64 deg

Figure 13

Tailboards tested in the cascade tunnel: (a) support chassis for slotted board (b) and (b) 15% void ratio board. All dimensions are in mm.

Figure 14

Experimental schlieren visualization of cascade flow discharge with a 15% void ratio tailboard.

Figure 15

Normalized wall pressure distribution in the discharge from a cascade with 15% void ratio tailboard at 64° to the inflow direction. (–, ◻) passage 1-2, (– –, ▵) passage 2-3, (– ∙ –, ▿) passage 3-4, (◇) passage 4-5

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