Roles and mechanisms of casing treatment on different scales of flow instability in high pressure ratio centrifugal compressors

At present, off-design centrifugal compressor suffers from great instabilities, such as rotating stall and surge, which lead to work failure, strong pressure fluctuations, and violent vibrations of the entire system. One cause of these instabilities is the rotating stall occurring in vaneless diffuser. In this review, recent researches on rotating stall of vaneless diffuser in centrifugal compressor are reviewed, and its inductive mechanism and characteristic description are emphatically introduced. In vaneless diffuser, the rotating stall starts up and fully develops under non-design conditions with the characteristic of increasing pressure fluctuation amplitude. The negative effects of rotating stall include reverse flow and vortex, both of which have significant effects on the instability of centrifugal compressors. Some improvement methods of rotating stall are also introduced, and a lot of examples are given. Finally, some suggestions and discussions are put forward for the future work.

A Review of Rotating Stall in Vaneless Diffuser of Centrifugal Compressor

Flow instability evolution in high pressure ratio centrifugal compressor with vaned diffuser

Development and validation of a 1D model for turbocharger compressors under deep-surge operation

Two stall stages in a centrifugal compressor with a vaneless diffuser

The present paper gives the experimental results obtained in a centrifugal compressor stage designed and built by SAFRAN Helicopter Engines. The compressor is composed of inlet guide vanes, a backswept splittered unshrouded impeller, a splittered vaned radial diffuser and axial outlet guide vanes. Previous numerical simulations revealed a particular S-shape pressure rise characteristic at partial rotation speed and predicted an alternate flow pattern in the vaned radial diffuser at low mass flow rate. This alternate flow pattern involves two adjacent vane passages. One passage exhibits very low momentum and a low pressure recovery, whereas the adjacent passage has very high momentum in the passage inlet and diffuses efficiently. Experimental measurements confirm the S-shape of the pressure rise characteristic even if the stability limit experimentally occurs at higher mass flow than numerically predicted. At low mass flow the alternate stall pattern is confirmed thanks to the data obtained by high-frequency pressure sensors. As the compressor is throttled the path to instability has been registered and a first scenario of the surge inception is given. The compressor first experiences a steady alternate stall in the diffuser. As the mass flow decreases, the alternate stall amplifies and triggers the mild surge in the vaned diffuser. An unsteady behavior results from the interaction of the alternate stall and the mild surge. Finally, when the pressure gradient becomes too strong, the alternate stall blows away and the compressor enters into deep surge.

An experimental description of the flow in a centrifugal compressor from alternate stall to surge

A boundary layer suction strategy is tested in a centrifugal compressor stage designed and built by Safran Helicopter Engines. According to previous numerical results, suction slots are disposed in the radial diffuser at the hub and the shroud near the vane suction side. Experimental results performed in the compressor without any boundary layer suction at partial speed showed a specific feature consisting in a two-channel pattern in the radial diffuser characterized by one stalled passage over two. If throttled the compressor experiences mild surge during which both rotating instabilities and alternate stall co-exist, the latter being then highly unsteady. When the boundary layer suction is activated no surge margin improvement is registered. However, the boundary layer suction allows to revert the pattern in the radial diffuser and to improve the compressor performance both at design and off-design operating points depending on the location of the boundary layer strategy.

Pierre Laucher

Papers

An experimental description of the flow in a centrifugal compressor from alternate stall to surge

Experimental Application of a Boundary Layer Control Strategy in a Radial Vaned Diffuser