Abstract
The present paper aims at providing an experimental analysis of the path to surge of a centrifugal compressor stage designed and built by Safran Helicopter Engines. Depending on the rotation speed of the compressor, two distinct flow patterns are observed in the radial diffuser at stabilized operating points near the surge, an asymmetric and a symmetric pattern. At medium rotation speed, the alternate pattern consisting of a two-channel pattern in the radial diffuser develops. One passage over two is stalled, the adjacent passage is free and this pattern replicates over the whole circumference while pulsing at a frequency of roughly 12 Hz which is close to the Helmholtz frequency of the test rig. By lowering the rotation speed, the two-channel pattern fades away and gives way to a periodical behavior of the radial diffuser passages called symmetric mode. The flow in each channel is identical presenting a stalled behavior pulsating in phase at a higher frequency of roughly 42 Hz. The two 12 Hz and 42 Hz modes are described and their existences are imputed to a lock-in of the natural frequencies of the instabilities with the acoustic modes of the test rig.
Highlights
Compressor operating range is limited at low mass flow by severe aerodynamic instabilities which occur as the flow inertia becomes too weak to counter the adverse pressure gradient
The compressor is composed of inlet guide vanes (IGV), a backswept splittered unshrouded impeller (IMP), a splittered vaned radial diffuser (RD) with an even number of vanes and axial outlet guide vanes (OGV)
It is likely that the 42 Hz spatially periodic mode results from a coupling between the natural frequency of the instability and the test rig acoustics
Summary
Compressor operating range is limited at low mass flow by severe aerodynamic instabilities which occur as the flow inertia becomes too weak to counter the adverse pressure gradient. The most feared instability is called surge. It consists of a global reversal of the flow which may result in the destruction of the entire engine. As the mass flow is reduced, a typical path to surge involves rotating stall which triggers surge. Centrifugal compressors show much more complex path to surge as stall of one of the subcomponent does not systematically trigger the instability of the whole compressor [1]. General trends may have been established from previous studies, they are flawed in many cases where centrifugal compressors show peculiar behaviors near their stability limits especially at partial rotation speeds [2,3,4]. Most of the time, no clear explanation on the mechanism of surge onset is found and many questions remain about the cause of these complex paths to surge
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