# Design of a Compressor Test Rig for Immobilization of the Stall Cell
## 2 Materials and Methods
### Test Rig Description  
The physical representation and schematic layout of the low-speed counter-rotating axial flow compressor test rig are depicted in Figures 4 and 5, respectively. Table 2 presents the key parameters of the low-speed counter-rotating axial flow compressor test rig. The outer diameter of the compressor is measured at 0.5 m, with a hub ratio of 0.75. Comprising two rows of counter-rotating rotor blades, the low-speed counter-rotating compressor test rig consists of a first row with 38 blades and a second row with 41 blades. In order to assess the primary characteristics of the stall cell, appropriate extensions were made to the axial dimensions of both the upstream and downstream regions of the test rig. Consequently, the total axial length of the test rig reached approximately 2.6 m. The distance from the bell mouth to the leading edge of the first row's rotor blades is approximately double its outer  
Figure 3: Design of a counter-rotating compressor for immobilization of stall cell. Figure 2: Wall pressure signal on the leading edge of the first rotor during stall. When a rotating stall is observed, one or more stall cells will be present in the flow field, resulting in an uneven flow field within the compressor as opposed to a circumferentially uniform flow field. Subsequently, a circumferential distortion adjustment plate is positioned downstream of the rotor for the adjustment of the position of the stall cell. The configuration and appearance of the circumferential adjustment plate are depicted in Figure 6, which comprises a main plate for circumferential adjustment along with two extension plates. The axial positioning of the circumferential distortion plate on the test rig can be seen in Figure 5. The original intention behind the design of the distortion plate was to create six holes in it, aiming to adjust the position of the stall cell by varying the size of the metal wire mesh installed on these holes. However, experiments revealed that this scheme was unfeasible due to the low sensitivity of the stall cell towards the distortion plate. Therefore, an extender plate concept was adopted with different shapes attached to the main plate. To evaluate its effectiveness, three types of extension plates were designed for experimentation, as illustrated in Figure 6. The results indicate that the No.2 extension plate yields optimal adjustments. The circumferential distortion plate serves two primary functions: firstly, it ensures the stabilization of the stall cell at a specific circumferential position within the compressor stall flow field; secondly, it enables adjustment of both the circumferential position and phase of the stall cell. The relative position of the circumferential distortion plate and the stall cell is shown in Figure 7, which also demonstrates how circumferential distortion plates work. As the circumferential distortion plate rotates along its circumference, it functions like a \"knob\", enabling the adjustment of the stall cell's circumferential position. When the circumferential distortion plate is in position, the stall cell aligns with its position. Upon rotation of the circumferential distortion plate to position, the stall cell shifts to position. The relative phase and circumferential positions between the stall cell and the circumferential distortion plate remain essentially unchanged throughout this process. In the measurement, the stall cell is adjusted to the ideal position through the circumferential distortion plate, with the distortion plate and stall cell keeping static during the experiment. The IAI motor drives movement in the circumferential distortion plate with an angle adjustment accuracy of 0.1\({}^{\circ}\), satisfying requirements for the precise regulation of the stall cell's circumferential position. Figure 6: Schematic diagram and photograph of circumferential adjustment plate.