###### Abstract
The research of [PERSON] [26] and [PERSON] [27] showed that when the inlet pressure distortion is generated, the blade load fluctuation amplitude was the largest at the blade leading edge, while with the MFR decreases, the amplitude of the load fluctuation also decreases. [PERSON] [28] studied the effects of straight and bend pipes on compressor performance through experiments and numerical methods, showing that when the 90\({}^{\circ}\)-bend pipe is connected to the inlet of the compressor, the flow distortion will occur and significantly reduce the compressor performance; when the internal structure of the bend pipe is changed and the guide plate is added at the appropriate position, the influence of the secondary flow brought by the bend can be reduced. Without the guide plate, the performance of the compressor and diffuser can be improved. In view of the form of bend inlet diversion in literature [28], the research of [PERSON] [29] showed that the inlet bend with guide vane could increase the compressor stage efficiency by 3%, and the uneven flow at the compressor inlet inevitably causes the distortion of the flow inside the compressor, leading to the forced response of the blades. [PERSON] [30] experimentally studied the unsteady pressure fluctuation of compressor blades under forced response when the inlet flow is distorted and measured the resonance and non-resonance conditions by installing pressure sensors on the blade surface. [PERSON] [31; 32] investigated the compressor blade excitation under the condition of inlet distortion caused by bend pipes and showed that the blade amplitude varied with the MFR. Furthermore, the circumferential asymmetrical static pressure distribution is used to simulate the volute at the diffuser outlet. It is proved that the non-axis symmetrical property of the volute and the inlet pipe have great influence on the vibration of the blade. [PERSON] [33] and [PERSON] [34] experimentally studied the influence of radial distortion of inlet total pressure on the performance of a centrifugal compressor. By installing a radial distortion net in front of the inlet guide vane, a ring radial combined total pressure distortion is created at both tip and root of the blades, and its comprehensive effect is beneficial to the operation of the centrifugal compressor, and the measured pressure ratio and efficiency are higher than those without distortion, besides, its surge margin is also widened. The above literature systematically studies the influence of compressor inlet distortion on its performance and internal flow. For the two-stage centrifugal compressors used in HFCs, there must be a distortion at the impeller inlet for HPS due to the existence of the interstage pipelines. Since, there is limited relevant research on the bend intake of two-stage centrifugal compressors, it is of great significance to study the effects of interstage pipelines on the performance and internal flow of two-stage centrifugal compressors for HFCs. In this work, a two-stage centrifugal compressors applicated in a 120 kW HFC system with different interstage pipelines were studied numerically and experimentally; the influence of the interstage pipelines structure on their internal flow and loss patterns were analyzed. The inlet distortion of downstream HPS compressors is compared in three models, and the performance and flow loss characteristics of the HPS compressor is emphasized. Finally, three kinds of interstage pipes are manufactured, and their performance is tested and compared.