###### Abstract  
At present, two-stage centrifugal compressors based on air bearing and high-speed motor technology are widely used in automotive hydrogen fuel cells. The low-pressure stage and high-pressure stage of the compressor are directly connected through an interstage pipeline, thus the structure of the interstage pipeline has an important influence on the aerodynamic performance of the compressor. In this work, a two-stage compressor with three different interstage pipelines were investigated experimentally and numerically. Results show that affected by the interstage pipeline bend section, the flow distortion will be induced at the impeller inlet of the high-pressure stage, and the distortion intensity changes with the pipeline structure. Among the three models, the EPC (elbow pipe change) model induces the most intense total pressure distortion at the condition of 80 kr/min and 130% mass flow rate, resulting in an efficiency reduction of 2 and 1.5 percentage points compared with the SPC (straight pipe change) and the TPC (total pipe change) model, respectively. Further research indicates that the upstream distortion has obvious influence on the downstream motor blades. As the blade height decreases, the load becomes more uniform on the main blades, while the load extremum migrates to the trailing edge on the splitter blades. Finally, three models are tested, and their performance is compared at three typical rotational speeds., It is recommended that interstage pipelines similar to SPC models should be chosen to improve the two-stage compressor efficiency in the design. type=multistage, inner-outer  
through recycling the engine exhaust energy [11; 12; 13; 14]. The main reason for the difference between the two driving modes is that the stack exhaust energy is not enough to drive the compressor, so the compressor must consume the electric energy generated by the stack, and with the increase of the mass flow rate (MFR) and pressure ratio, the energy consumed by the compressor also increases. Studies have shown that in HFC systems, compressor power consumption is close to 20% of the system power [15]. In the case of a 120 kW HFC system, the maximum compressor speed is 100 kr/min due to the influence of rotor weight and shafting stability, and the required MFR and pressure ratio of the stack are generally 0.15 kg/s and 2.8, respectively. If the compressor uses a single-stage compression form, this allows the impeller to have a lower specific speed and isentropic efficiency [16; 17; 18; 19]; while the compressor with two-stage compression technology can effectively solve the problem caused by the specific speed, it is therefore widely used in HFC systems at present. However, the layout of the two-stage centrifugal compressor needs to meet the requirement of compact overall structure, which forces the interstage pipeline connecting the low-pressure stage (LPS) and the high-pressure stage (HPS) to be bent and twisted, leading to the pressure distortion and efficiency penalty of the HPS. Therefore, it has important engineering application value to study the influence of interstage pipeline on the performance and internal flow of two-stage compressors used in HFCs. However, to date, no related research literature has been reported, since most is still focused on the single-stage centrifugal compressors [20; 21; 22; 23; 24]. The experimental study performed by [PERSON] [25] in a single-stage centrifugal compressor revealed that by changing the installation form of honeycomb mesh angles, radial and circumferential pressure distortion were generated at the impeller inlet. The intake distortion changes the inlet attack angle of the impeller, which significantly reduces the impeller efficiency, and the surge line of the compressor deviates to the direction of low flow.