Numerical investigation into the underlying mechanism connecting the vortex breakdown to the flow unsteadiness in a transonic compressor rotor

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.

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

High flow rate aeroengines typically employ axial flow compressors, where aerodynamic loss is predominantly due to secondary flow features such as tip leakage and corner vortices. In very high altitude missions, turbomachinery operates at low density ambient atmosphere, and the recent trend toward more compact engine core inevitably leads to the reduction of blade size, which in turn increases the relative height of the blade tip clearance. Low Reynolds number flowfield as a result of these two factors amplifies the relative importance of secondary flow effects. This paper focuses on the behavior of tip leakage flow, investigating by use of both experimental and numerical approaches. In order to understand the complex secondary flow behavior, cascade tests are usually conducted using intrusive probes to determine the loss. However relatively few experimental studies are published on tip leakage flows which take into account the interaction between a rotating blade row and its casing wall. Hence a new linear cascade facility has been designed with a moving belt casing in order to reproduce more realistic flowfield as encountered by a rotating compressor row. Numerical simulations were also performed to aid in the understanding of the complex flow features. The experimental results indicate a significant difference in the flowfield when the moving belt casing is present. The numerical simulations reveal that the leakage vortex is pulled by the shearing motion of the endwall toward the pressure side of the adjacent blade. The results highlight the importance of casing wall relative motion in analyzing leakage flow effects.

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Experimental and numerical investigation on compressor cascade flows with tip clearance at a low Reynolds number condition

Experimental and Numerical Investigations of Rotating Instability Phenomenon in An Axial Compressor Stator

This work addresses the phenomenon of rotating instability (RI), which is known to occur in turbomachines of axial and radial types. In axial compressors, RI typically occurs in the vicinity of the stability line. Without occurrence of RI, high efficiencies would be achieved there. The emergence of RI is accompanied by an increase in acoustic emissions, a decrease in efficiency and high amplitude pressure fluctuations, which feature a characteristic structure. Under unfavorable conditions these pressure fluctuations may cause blade vibrations and lead to mechanical failure due to high cycle fatigue. In order to prevent inception of RI and thus to utilise the high efficiencies of the compressor system, detailed knowledge of the flow phenomenon is necessary. A large number of descriptions and interpretations of RI characteristics are found in literature. In order to support understanding of the particular phenomenon, the flow within an annular compressor cascade featuring the RI is modelled by computational fluid dynamics (CFD). Subsequently, data from the CFD is analysed with respect to known characteristics of RI. In this regard, a large-scale turbulence resolving simulation is employed to model the most relevant turbulent fluctuations within the flow. Additionally, a numerical scheme is employed, allowing the accurate prediction of both fluid dynamic and acoustic fluctuations. Further, the full annulus of the compressor stator cascade is modelled in CFD to ensure unaffected reproduction of the characteristic circumferential structure of the RI phenomenon. In a second step, findings from the CFD are transferred to a model configuration. The model derived simulates the velocity fluctuations within the source region related to RI, whereas the dynamics develop freely from a given initial configuration. The model basically employs potential theory methodologies. After a brief introduction to theoretical aspects of axial compressor systems and flow phenomena occurring therein the fundamentals of CFD are described. Details of application of CFD to the flow through the annular compressor cascade are followed by validation of RI occurrence within the flow simulation. A hypothesis on the originating mechanism of the characteristic pressure fluctuations of RI is then formulated based on detailed analyses of simulation data. The identified flow configuration is modelled by fundamental solutions from potential theory. From the temporal integration of the modelled flow states a fluid flow dynamic develops, which is very similar to that related to RI. Pressure and velocity fluctuations calculated by the model fulfil all criteria typically applied to measured or simulated data in order to detect occurrence of RI. In this way, the hypothesis on the causing mechanism of pressure fluctuations characteristic to RI is proved and thus provides another keystone for understanding the complex flow phenomenon of rotating instability.

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Numerical simulation and modelling of sound generated by rotating instabilities in an annular compressor cascade

The paper describes experimental investigations of the tip clearance vortex in a linear compressor cascade with a tip clearance to one side and a compressor with a rotor tip section represented by the cascade. The aim is to show experimentally that breakdown of the tip clearance vortex can take place in subsonic compressors. As a first step, the flow in the linear compressor cascade has been investigated at different flow angles from the design point up to the stability limit of the cascade. In a second step the flow in the tip region of the rotor in a low-speed single-stage compressor has been investigated from the design point up to the stall limit of the compressor. The analysis of PIV measurements reveal a low momentum fluid area in the passage both in the cascade and the compressor when approaching the stall limit. This area is separated from the main flow by an interface that is characterized by high standard deviations of the velocities. The location of this interface correlates well with the positions at which the analysis of the unsteady pressure signals obtained with several flush mounted high-response pressure transducers reveals a bump of increased amplitude at a certain nondimensional frequency. The results in the compressor are for the most part identical to those obtained in the cascade. When approaching stall a distinct bump shows up in the casing wall pressure spectra at a nondimensional frequency comparable to the nondimensional frequency found in the cascade. At the same time an area of very low momentum fluid accumulates in the vicinity of the original vortex axis, as can be shown by PIV measurements in the rotor of the compressor. Additionally it can be shown, that the blockage that is due to the broken down tip clearance vortex leads to a rotating phenomenon, comparable to the phenomenon of ‘rotating instabilities’.© 2008 ASME

Henner Schrapp

Papers

Breakdown of the Tip Clearance Vortex in a Rotor Equivalent Cascade and in a Single-Stage Low-Speed Compressor