One key aspect of centrifugal compressor performance is the stability range, in particular at lower than optimal mass flows. This paper focuses on the stability improvement of a turbocharger-relevant compressor stage equipped with a trimmed vaneless diffuser, incorporated into its interior volute. Evidence in the literature suggests that such a diffuser might increase the stall margin, especially for the high speed lines. As is the case with all supercharged internal combustion engines, this would lead to improved overall efficiency, operational flexibility and lower emissions. Using advanced RANS turbulence modeling, a batch of test cases was setup in order to assess the feasibility of the concept as well as its potential drawbacks. Spalart-Allmaras and k-ω SST models, both with rotation and curvature corrections, were benchmarked against the baseline experimental data. The new diffuser was compared with conventional diffuser pinching and shelving and the fully trimmed embodiment was found to be superior to all others. Stability was assessed through a number of user defined parameters, tailored to reveal the signs of flow instability even under steady state assumptions. In addition to monitoring the mass flow imbalance, individual loading of blade passages and radial force magnitude and orientation were also monitored. Stability was found to be increased with the degree of diffuser trimming, alas this came at an ever increasing pressure loss. Most entropy was found to be generated across the volute as the flow velocity was significantly higher than the baseline. Another aspect was the static pressure distortion across the outlet of the vaneless diffuser, which paradoxically is linked with higher stability. Hence, in order to maximize both stability and mitigate pressure losses, further optimization on the scroll area distribution will have to be carried out after considering the new scroll inlet angle and speed circumferential distributions.

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