51 Description of the moving wall apparatus

The moving wall apparatus used for simulating rotation of the blades against the stationary casing in a turbine consists of a 2kW 3-phase induction motor, a rubber belt, three rollers on which the moving wall rotates, and two pitch pulleys which transmit power from the motor to the belt. The whole apparatus rests on an aluminium base that fits on to the top wall of the linear cascade section.

The moving wall uses a belt made of N-8 Sampla synthetic rubber that is placed flush with the inner top wall of the test section. The belt moves from the direction of the suction side of the stationary blades to the pressure side. This simulates the relative motion in a turbine where it is the rotation of the blades that causes the relative motion. The belt rotates on 3 aluminium rollers of diameter 75mm supported on bearings housed in an aluminium frame. There are two primary rollers and a tertiary roller; one of the two primary rollers is driven by a 2kW 3-phase induction motor (also bolted to the frame) through a system of pitch pulleys. A tertiary roller with an adjustable axis of rotation is used to keep the belt tight and flat when moving in the windtunnel. The vertical position of the bearings supporting the tertiary roller can be changed by using screws to move each bearing housing independent of the other in a slot provided on the frame. This mechanism is used to prevent 'tracking' of the belt from one end of the roller to another. It was experimentally verified that the 'flapping' effect of the belt due to vibrations during operation resulted in a variation of tip clearance of not more than 0.1% of chord. A schematic diagram of the moving wall apparatus is shown in fig. 5.1.

The motor speed (and hence the belt 'rotation' speed) is controlled by means of an Altivar ATV 18 speed drive controller. The apparatus is designed so that air leakage can occur only from the gap between the roller and the frame, and the very low tolerances used

Bearing housing

Slot for axis movement Roller

Power from motor

Bearing housing

Slot for axis movement Roller

Power from motor

Inner wall of test section

Turbine blades in test section

Figure 5.1: Moving wall apparatus used to simulate rotation

Inner wall of test section

Turbine blades in test section

Figure 5.1: Moving wall apparatus used to simulate rotation during fabrication ensure that there is virtually no such leakage. With the motor operating at its rated speed of 3250 rpm, the reduction in speed during transmission by the pulleys ensures that the maximum belt speed that can be achieved is 25 m/s. In order to simulate engine conditions, where the blade tangential velocity is twice the axial throughflow velocity, the wind tunnel is operated at an inlet speed of about 12.5 m/s.

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