Physics of Prestall Propagating Disturbances in Axial Compressors and Their Potential as a Stall Warning Indicator

Abstract

Axial compressors in aero engines are prone to suffering a breakdown of orderly flow when operating at the peak of the pressure rise characteristic. The damaging potential of separated flows is why a safe distance has to be left between every possible operating point and an operating point at which stall occurs. During earlier investigations of stall inception mechanisms, a new type of prestall instability has been found. In this study, it could be demonstrated that the prestall instability characterised by discrete flow disturbances can be clearly assigned to the subject of "Rotating Instabilities". Propagating disturbances are responsible for the rise in blade passing irregularity. If the mass flow is reduced successively, the level of irregularity increases until the prestall condition devolves into rotating stall. The primary objective of the current work is to highlight the basic physics behind these prestall disturbances by complementary experimental and numerical investigations. Before reaching the peak of the pressure rise characteristic flow, disturbances appear as small vortex tubes with one end attached to the casing and the other attached to the suction surface of the rotor blade. These vortex structures arise when the entire tip region is affected by blockage and at the same time the critical rotor incidence is not exceeded in this flow regime. Furthermore, a new stall indicator was developed by applying statistical methods to the unsteady pressure signal measured over the rotor blade tips, thus granting a better control of the safety margin.

Evolution Process of Diffuser Stall in a Centrifugal Compressor With Vaned Diffuser

“…Pullan et al demonstrated that the spike-type stall inception in axial compressors is also triggered by the leading edge separation on the blade suction surface [10]. As Eck et al accurately pointed out, the rotating instability cells are induced by the small vortices propagating at the rotor leading edge [11]. We have also reported that the unsteady behavior of the diffuser stall is influenced by the unsteady vortices at the leading edge of the diffuser vane, during off-design operation [12].…”

Non-synchronous vibration in axial compressors: Lock-in mechanism and semi-analytical model

“…Although not linked to rotating stall itself, NSV has been associated to short length-scale stall precursors, such as 'spikes' and the controversially termed 'rotating instabilities' [17]. These precursors have been an active area of research and are known to originate from a variety of aerodynamic features, such as tip clearance vortex oscillations [18,19], leading edge spillage and trailing edge backflow [20,3], leading edge separations [21], shear layer instabilities [22] as well as vortex shedding instabilities [23]. An interaction with blade vibration has been reported in several cases [4,3,7] and recently, Brandstetter et al [16] provided experimental evidence for the lock-in between propagating radial vortices, which are known to be a source of spike-type stall inception through the work of Inoue et al [24] and Young and Pullan [25,21], and blisk vibration patterns [16].…”

Identification and classification of operating flow regimes and prediction of stall in a contra-rotating axial fan using machine learning

“…This network model works for both spike type stall and modal wave, at different rotor speeds if trained with sufficient data. The physics of the pre-stall instability described by discrete flow disturbances, or rotating instability was studied by Eck et al [25], using complementary experimental and numerical methods. The instability is characterised by the development of vortex tubes that are attached to the casing and suction surface of the rotor blades.…”