# Design of a Compressor Test Rig for Immobilization of the Stall Cell
## 4 Example Measurements of the Near-Wall Flow Field in the Stall Cell  
As the circumferential position of the stall cell can be adjusted using a distortion mesh at the outlet of the compressor, the whole flow structure can be achieved using this partitioning idea. The compressor characteristics indicate that the pressure rise coefficient and flow coefficient of the stall flow field generated after installing the distortion plate are identical to those without installation, thus suggesting a preliminary inference that simulates an equivalent stall cell. The oil flow visualization measurement technique was employed in this study to illustrate the near-wall flow field within the stall cell. The schematic diagram of the oil flow visualization measurement setup is presented in Figure 17. To minimize the influence of gravity, the measuring plane for oil flow visualization on the casing consisted of two plates positioned directly above it (as depicted in Figure 17). Additionally, matte wallpaper was applied to the corresponding hub wall to visualize the oil flow in the hub region. Following each measurement, adjustments were made to the circumferential position of the stall cell using a distortion mesh until all circumferential positions were covered. Figure 17: The scheme of oil flow visualization measurement. The oil flow coating formula ratio (volume ratio) is shown in Table 5, and the preparation of oil flow visualization is shown in Figure 18. The criterion for choosing the oil ratio is to achieve a ratio that prevents easy detachment of the oil flow coating by fluid and maintains sensitivity to slow airflow. The prepared coating was evenly applied on the inner wall of the casing; the interval of oil flow coating points was approximately 1.5 cm, and the oil flow coating points were approximately equal in size. Therefore, a set of oil flow coating plates was designed to achieve rapid coating of the casing. The design scheme and optimization process of the coating plate and scraper are shown in Figure 19. The coating plate was made of 1 mm thick plexiglass plates, and five kinds of apertures were designed. The experiment revealed that the coating plate with an aperture size of 1 mm exhibited optimal efficacy. The measured image photos are stitched into a complete oil flow map, as shown in Figure 20. The test procedures for the oil flow visualization measurement of the hub are basically the same as those for the casing. The difference is that oil flow coating points are not painted on the inner wall of the measurement plates, but they are painted on the black matte self-adhesive wallpaper, which is then pasted onto the hub of the test rig. After the test, the wallpaper was removed, and a photo was taken, as shown in Figure 20.  
Since the measurement of oil flow visualization of the hub is carried out by the black matte self-adhesive wallpaper, it is not restricted by the circumferential size of the window or affected by the squirrel-cage structure casing beam, and a complete oil flow map can be obtained after four experiments. In the traditional oil flow visualization measurement technique, obtaining a good-quality, complete image photo indicates the test's success. Building upon this foundation, the present study conducted further research. In this paper, a processing method was developed to obtain the two-dimensional velocity vector and limit the streamline of the flow field from the image measured using the oil flow visualization measurement technique. Firstly, the images obtained by step-by-step measurement were stitched into a complete oil flow map with images meticulously arranged and aligned in sequential order using drawing software. Then, the oil flow coating points were manually transcribed into vector line segments accompanied by arrows (wherein circles represent the starting point and sharp points indicate the end point of approximately drop-shaped measuring points). And then the velocity vectors of each oil flow coating point were obtained according to the oil flow map.