Effects of unsteady interaction on the performance of an ultra-high-pressure-ratio centrifugal compressor

A numerical investigation of an ultra-high-pressure-ratio centrifugal compressor is presented, mainly focusing on the unsteady impeller-diffuser interaction. The compressor is characterized by a high total pressure ratio of about 11.5 and a small radial gap between the impeller and diffuser, whose radius ratio is 1.067. The unsteady effect is stringently assessed by comparing the results of steady and unsteady Reynolds-averaged Navier-Stokes simulations. Within the impeller, the unsteady perturbation, dominated by the blade passing frequency of the diffuser, can only propagate to 20% of the normalized streamwise length from the impeller outlet. The velocity component distribution at the impeller outlet is affected by the unsteadiness, resulting in a variation of the impeller efficiency. In contrast with the impeller, the unsteady perturbation is present inside the whole diffuser passage. The circumferential flow angle variation at the diffuser inlet induces the unsteady formation and shedding of the diffuser leading edge vortex, increasing the flow loss correspondingly. However, the smaller incidence angle at the diffuser inlet makes for better diffusion inside the diffuser passage. The positive and negative effects of the unsteadiness counteract each other. Therefore, both the stage and component performances change slightly, although the flow field changes significantly due to the unsteadiness. (C) 2020 Elsevier Masson SAS. All rights reserved.