# Design of a Compressor Test Rig for Immobilization of the Stall Cell
## 2 Materials and Methods
### Test Rig Description
The design speed of the first row of rotors in the low-speed counter-rotating compressor test rig is 1100 r/min, while the design speed of the second row of rotors is 1000 r/min. Both rows have equal chord lengths along their radial direction, measuring 54 mm each. To facilitate flow field measurement within rotor clearance, an intentionally enlarged axial clearance between both rows' rotor blades was designed at approximately 46% relative to the blade chord length. Additionally, a tip clearance value for each rotor was set at 1.0 mm according to the design specifications. Following the C4 blade design principles, both rows' rotor blades adopt a radial stacking configuration, with circular arcs forming their middle sections. Table 3 presents geometric angles for three span sections (blade angle relative to axial direction). The approach of directly designing the blade with simple modeling was selected because the original purpose of building the experimental test rig was to measure the rotating stall flow fields. While the performance requirements for a typical compressor are not overly demanding, traditional modeling techniques already achieve good aerodynamic performance for conventional load compressors, such as the counter-rotating compressor. The blade design was carried out using commercial software Autoblade and examined using Fine of Numeca, while photosensitive resin was used to print the blades from a 3D printer. As shown in Figure 8, two rows of blades were installed on the hub of the two rotors with dovetail slots, which facilitates easy and quick redesign and replacement. The test rig is specifically designed for measuring the flow field of a rotating stall. In addition to immobilizing the stall cell, the test rig possesses the following characteristics: The two blade rows are driven by two specially designed wheel hub motors which have the same technical parameters, as shown in Figure 9a,b. The axial position of them is shown in Figure 5. The utilization of these motors enables a shafting-free design, which increases the internal space within the hub and enables measurements from its wall. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Span**} & \multicolumn{2}{c}{**Rotor 1**} & \multicolumn{2}{c}{**Rotor 2**} \\ \cline{2-5}  & **LE** & **TE** & **LE** & **TE** \\ \hline Hub & 55 & 22 & 64.5 & 46 \\ Mid-span & 59 & 36.5 & 65.5 & 51 \\ Tip & 62.5 & 46 & 66.5 & 55 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Blade angles of the blades. Figure 7: Relative position of circumferential adjustment plate and stall cell. Finite element analysis was conducted on the stiffness of support structures, vibration characteristics, and deformation tendencies of the entire rotor component (results depicted in Figure 9). Table 4 provides detailed calculation results from finite element analysis, revealing that the maximum speed frequency of the rotor stands at 33.3 Hz, which is significantly lower than any support mode frequency, indicating no risk of support vibration. Moreover, axial deformation is considerably smaller than the gap between the two rotors (0.5 mm), suggesting no risk of wheel rub on the rotor; similarly, given minimal radial deformation, there is no risk of radial blade scratching. Figure 8: The model and photograph of rotor blades. Figure 9: Finite element analysis on rotor components of wheel hub motor.