The paper outlines the results of a numerical analysis of a low-speed centrifugal compressor operating at design and off-design conditions. This included the analysis of surge predecessors. Surge is known to be a reason for the severe damage of compressing units and systems working with them. This phenomenon is also known to have a very quick and sudden onset. Therefore, it is important to understand the nature of flow structures that appear locally prior to the surge onset. It is generally believed that these could include: impeller rotating stall (abrupt or progressive), diffuser rotating stall, and inlet or outlet recirculation. This classification is, however, not sharp and observed flows could represent only selected features of the different phenomena. It is also not clear which structure is most likely to appear in a given system. The paper presents an exhaustive examination of flow structures observed in the case of a low-speed centrifugal compressor. Transient numerical simulations were conducted for both design and off-design conditions. The results were related to the experimental data presented in another paper. The numerical test domain consisted of blower, volute, inlet nozzle and elements of inlet and outlet channels assuring no influence of the boundary conditions on the local flow fluctuations. The analysis of the results included many factors such as: flow in the volute, flow in the impeller, flow in the diffuser and stagnation zones. The most significant flow structures appearing at pre-surge were identified as the inlet recirculation and the phenomenon representing some features of the rotating stall. The simulation, confirmed by experimental data, allowed for a better understanding of pre-surge flow structures in the case of a low-speed compressor, which is very important for the identification of early surge indicators.
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