Abstract
This paper presents the analysis of the instabilities inception in a transonic centrifugal compressor for different rotation speeds. The analysis was conducted from experimental results obtained with unsteady pressure sensors implanted in the inducer, vaneless diffuser and vaned diffuser. Beyond the stability limit the compressor enters into a deep surge without any precursor, whatever the speed. The surge process is initiated in the vaned diffuser by a massive boundary layer separation. For low speeds, together with the surge which remains triggered in the diffuser, aerodynamic instabilities are detected in the inducer. These instabilities can be understood as “tip clearance rotating disturbances” because they are generated at the leading edge of the impeller main blades and move along the tip clearance trajectory.
Highlights
Because instabilities can be catastrophic for performance and even damaging for the machine, predicting the onset of instability is an essential part of the description of the compressor performance map
Short Length Scale Disturbances (SLSD) come from a local perturbation leading to disturbances which rotate typically between 50 and 80% of the rotor speed and evolving in Article published by EDP Sciences
Few revolutions to rotating stall. Both SLSD and Long Length Scale Disturbances (LLSD) can be present at the same time and it is possible to switch between SLSD toward LLSD setting up flow control process [6]
Summary
Because instabilities can be catastrophic for performance and even damaging for the machine, predicting the onset of instability is an essential part of the description of the compressor performance map. The instability (rotating stall or surge) may be triggered by Long Length Scale Disturbances (LLSD) or Short Length Scale Disturbances (SLSD); the reference length scale being the blade-to-blade passage [4, 5]. The sensors placed in the diffuser are mounted on the shroud They are distributed over the whole circumference of the machine but, for clarity of presentation, are brought together in Figure 2 as if they were located in a single diffuser passage. Few revolutions to rotating stall (or surge) Both SLSD and LLSD can be present at the same time and it is possible to switch between SLSD toward LLSD setting up flow control process [6]. The present contribution aims at describing the mechanisms involved in the surge onset in a transonic centrifugal compressor from 60% to 100% of the nominal rotation speed. The aerodynamic instabilities which have been experienced in both the diffuser and inducer are discussed
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