211 Wind tunnel

The wind tunnel used in the present study is a multi-purpose blowing type wind tunnel built by Engineering Laboratory in 1991 in the Heat Transfer Laboratory at the University of Minnesota.

At normal operating conditions, air at room temperature is drawn through a filter by a 22.4kW blower, flowing through a long diffuser and a heat exchanger before entering a couple of settling chambers with screens inside. After passing through a second screen layer, the air flow is guided through a squared contraction nozzle with an area ratio of 6.25 and an exit area of 45.7 x 45.7cm2 into the test section, which will be described in the next section. Exiting the test section, the air is discharged into the room and then outside the building through windows. As the exit of the contraction, the air flow can attain a velocity of about 40m/s with 0.2% turbulence intensity as designed by the manufacturer.

The mainstream velocity in the wind tunnel is controlled by a variable torque system and can be tuned by adjusting the rotating frequency of the blower.

2.1.2 Test section

The test section as shown in fig. 2.3.1 is connected to the exit of the contraction of wind tunnel and is composed of the straight and triangular sections, the linear cascade with different tip clearances, and the tailboard section. All the walls of the test section except the top and bottom wall of the cascade section are made of 1.9cm thick Plexiglas. The straight section with a cross-section of 45.7 x 45.7cm2 and a length of 61.0cm, has slots reserved for generation of mainstream turbulence by inserting various grid turbulence generators, and is followed by the triangular section connecting to the linear cascade. In present study, a bar grid turbulence generator is inserted into one of the slots to obtain a high free stream turbulence level.

To obtain a fully developed turbulent boundary layer on the endwall, a 1 mm diameter trip wire is placed at the exit of the contraction section, which is 82.5cm ahead of the stagnation point of the central blade. At the incoming flow measurement plane, which is at Xi = 67.0cm, ahead of of the leading edge of the center blade in the direction of the incoming flow, the incoming flow velocity and turbulence intensity are measured using a hot wire anemometry and a traverse system. The incoming flow dynamic pressure (velocity), static pressure and temperature are also observed at this position, using a pitot tube and a T type thermocouple respectively. Measurement planel is at X = — 20.8cm, ahead and parallel to the leading edges of the blades, where the boundary layer on the tip-endwall is measured at three different locations at half pitch distance. Velocity and turbulence intensity are also measured at measurement plane2 (X = 28.2), which is downstream of the linear cascade and parallel to the trailing edge of the blades.

The tailboard section has two tailboards attached to the trailing edges of two outside blades. It has two endwalls connected to endwalls of the cascade section.

2.1.3 Linear turbine cascade with tip clearance

The linear turbine cascade consists of five 45.7cm long high pressure turbine rotor blades made of aluminum with central blade configuration (blade 3 in fig. 2.3.1). The blade profile data and other cascade geometries are listed in table 2.2 and shown in fig. 2.2. Basically, the blade has a profile of the high performance turbine blade in high pressure stage and the cascade has a blade chord of 18.4cm with a solidity of 0.75 and a high turning angle of 107.5°.

The two outside blades (blade 1 and 5) fixed by screws from the bottom and the top walls, together with two bleeds and two downstream tailboards, form the sidewalls for the mainstream. The space between the bleeds and the outside blades as well as the orientation of the tailboards are adjustable to obtain periodic flows in blade passages. Three middle blades (blade 2, 3 and 4) are fixed by screws at the bottom wall. The central blade is composed of three parts, one of which is the test blade and can be either pressure blade or naphthalene blade placed near the tip-endwall. The other two are solid aluminum blades with equal height of 13.0cm (5.125 inch) and can be placed between the bottom endwall and the test blade during the measurement. However, the test blade can also be placed between the other two solid part for two-dimensional mid-span measurement.

The bottom and the top wall of the linear cascade are 1.27cm thick aluminum plates,

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