# Design of a Compressor Test Rig for Immobilization of the Stall Cell
## 3 Characteristics of the Compressor and Immobilization of the Stall Cell
### Immobilization of the Stall Cell  
Due to strong three-dimensional characteristics and unsteadiness in stall flow field analysis, quantitatively analyzing the relative influences on rotational speeds between rotors and stators while immobilizing cells in specific circumferential positions is challenging; therefore, obtaining a stall cell without a designated location under a specified rotation rate such as 5:3 is common practice for researchers seeking greater accuracy when designing second rotor blades based on revised specifications for both rotors' respective speeds. A revised design of the second rotor blades was carried out with rotating speeds for the first and second rotors of 1100 r/min and 1000 r/min, respectively. Notably, for designing the second row of rotor blades, their shape was determined based on calculations involving velocity triangles after determining rotation speed. After the second round of revised designs, the design speed of the two rows of rotors was adjusted, and the modified blades were installed and tested, successfully immobilizing the stall cell in a circumferential position. As a result, both designs effectively stabilize the stall cell when operating at comparable speed ratios. However, due to its closer proximity to the design value, the second design enables more precise functioning of the compressor resembling that of a typical compressor. It is worth noting that immobilization of the stall cell at a speed ratio of 1.2 was achieved, which did not exhibit any significant deviation from the original design speed ratio of 1.1. As depicted in Figure 15, no substantial disparities were observed in the characteristic curves of the compressor between the speed ratios of 1.1 and 1.2. Therefore, fine-tuning solely reduces the rotational velocity to zero for stall cells without altering their form or affecting the structural aspects within stalled flow fields; hence, further optimization designs were not pursued. As shown in Figure 15, the new test rig and compressor shown in Figure 1 present different characteristic curves [17] for the stable flow regime (mainly for the pressure coefficient). However, in this case, it is  
Figure 14: Cell rotating speed versus rotating speed ratio of the two rotors. important to consider the region of unstable operation of the machine. Even if the two compressors operate in the same flow rate range and the onset into the stall is always sudden, the point of recovery from the stall is substantially different, and this indicates a different evolution of the stall cell in the two machines. To determine the location of the stationary stall cell, pressures were measured at eight evenly distributed pressure ports around the casing in front of the first rotor using Model 3051S pressure transducers. Figure 16 illustrates the distribution of these eight pressures along the circumference. It is known that higher pressure exists inside the stall cell towards the front of the rotor. The analysis revealed that the stall cell was situated at an approximately 40% circumferential scale from its starting point on the top-left side of the compressor's circle. The circumferential position of this stall cell can be adjusted by implementing a distortion mesh at the compressor outlet (as depicted in Figure 6). Consequently, oil flow visualization technology and stationary probes mounted within the casing enable detailed measurements and the examination of flow structures within this stall cell. The implementation of counter-rotation does yield comparable speeds; nevertheless, there are discrepancies in flow conditions due to differing blade designs between the  
Figure 16: Pressures of the eight pressure ports evenly distributed around the casing. Figure 15: Characteristic curves of the compressor at the speed ratio of 1.1 and 1.2.  
counter-rotating compressor and its prototype counterpart. However, prior research [7; 9; 15] has shown that blade-induced fluctuations are considerably milder than those observed within un-stalled regions of a stall cell.