14th European Conference on Turbomachinery Fluid dynamics & Thermodynamics
In many centrifugal compressors, especially compressors for automotive applications, areas of reversed flow frequently develop near the endwalls at partial rotation speed. The present contribution details such a recirculated flow zone in a research centrifugal compressor stage designed and built by Safran Helicopter Engines for an aeronautical application. The compressor stage includes 4 blade rows: axial inlet guide vanes (IGV), a backswept splittered impeller, a splittered vaned radial diffuser and axial outlet guide vanes. The effects of the change in the IGV stagger angle on the occurrence and development of the recirculated area is analyzed thanks to both experimental and numerical results. Three IGV stagger angles are investigated: 0, 20 and 40° (positive pre-swirl) at a partial rotation speed. The numerical results come from Reynolds-Averaged Navier-Stokes (RANS) simulations. The experimental ones come from steady and unsteady pressure sensors embedded in the shroud and hub throughout the compressor. The numerical simulations show that the recirculated flow zone arises at the leading edge of the impeller splitter blades near the shroud. This recirculated zone extends towards the hub and upstream as the compressor is throttled for the three IGV stagger angles. Even if the experimental detection of reversed flow is rather tricky with only pressure sensors, it is shown that a sudden change of the circumferential velocity of decaying rotating disturbances emerging at the impeller leading edge matches well with the critical mass flow rate related to the occurrence of inducer recirculation predicted in numerical simulations. A particular behavior at the IGV stagger angle of 40° is observed: the recirculated zone stretches downstream until it reaches the impeller trailing edge at an operating point close to deep surge. As a result, the diffuser incidence is altered due to the increase of radial velocity outside the recirculation zone. This alteration can be linked with a change of the flow topology in the radial diffuser. Moreover, a second sudden change of circumferential velocity of decaying rotating disturbances is also observed as the recirculation reaches the impeller trailing edge. Finally, the onset of the inducer recirculation leads to a sudden increase of the slip factor. This sudden increase is investigated with numerical flow fields.