Rotating instabilities (RI) have been observed in axial flow fans, centrifugal compressors as well as in low-speed and high-speed axial compressors. They are responsible for the excitation of high amplitude rotor blade vibrations and noise generation. This flow phenomenon moves relative to the rotor blades and causes periodic vortex separations at the blade tips and an axial reversed flow through the tip clearance of the rotor blades.The paper describes experimental investigations of RI in the Dresden Low-Speed Research Compressor (LSRC). The objective is to show that the fluctuation of the blade tip vortex is responsible for the origination of this flow phenomenon.RI have been found at operating points near the stability limit of the compressor with relatively large tip clearance of the rotor blades. The application of time-resolving sensors in both fixed and rotating frame of reference enables a detailed description of the circumferential structure and the spatial development of this unsteady flow phenomenon, which is limited to the blade tip region.Laser-Doppler-Anemometry (LDA) within the rotor blade passages and within the tip clearance as well as unsteady pressure measurements on the rotor blades show the structure of the blade tip vortex.It will be shown that the periodical interaction of the blade tip vortex of one blade with the flow at the adjacent blade is responsible for the generation of a rotating structure with high mode orders, termed as rotating instability (RI).Copyright © 2000 by ASME

Rotating Instabilities in an Axial Compressor Originating From the Fluctuating Blade Tip Vortex

Work on rotating stall and its related disturbances has been in progress since the Second World War. During this period, certain ‘hot topics’ have come to the fore — mostly in response to pressing problems associated with new engine designs. This paper will take a semi-historical look at some of these fields of study (stall, surge, active control, rotating instabilities etc.) and will examine the ideas which underpin each topic. Good progress can be reported, but the paper will not be an unrestricted celebration of our successes because, after 75 years of research, we are still unable to predict the stalling behaviour of a new compressor or to contribute much to the design a more stall resistant machine. Looking forward from where we are today, it is clear that future developments will come from CFD in the form of better performance predictions, better flow modelling and improved interpretation of experimental results. It is also clear that future experimental work will be most effective when focussed on real compressors with real problems — such as stage matching, large tip clearances, eccentricity and service life degradation. Today’s topics of interest are mostly associated with compressible effects and so further research will require more high speed testing.Copyright © 2015 by ASME

Stall, Surge, and 75 Years of Research

In this paper we describe the structures that produce a spike-type route to rotating stall and explain the physical mechanism for their formation. The descriptions and explanations are based on numerical simulations, complemented and corroborated by experiments. It is found that spikes are caused by a loss of pressure rise capability in the rotor tip region, due to flow separation resulting from high incidence. The separation gives rise to shedding of vorticity from the leading edge and the consequent formation of vortices that span between the suction surface and the casing. As seen in the rotor frame of reference, near the casing the vortex convects toward the pressure surface of the adjacent blade. The approach of the vortex to the adjacent blade triggers a separation on that blade so the structure propagates. The above sequence of events constitutes a spike. The simulations show shed vortices over a range of tip clearances including zero. The implication is that they are not part of the tip clearance vortex, in accord with recent experimental findings. Evidence is presented for the existence of these vortex structures immediately prior to spike-type stall and, more strongly, for their causal connection with spike-type stall inception. Data from several compressors are shown to reproduce the pressure and velocity signature of the spike-type stall inception seen in the simulations.Copyright © 2012 by ASME

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Origins and structure of spike-type rotating stall

In this paper, we describe the structures that produce a spike-type route to rotating stall and explain the physical mechanism for their formation. The descriptions and explanations are based on numerical simulations, complemented and corroborated by experiments. It is found that spikes are caused by a separation at the leading edge due to high incidence. The separation gives rise to shedding of vorticity from the leading edge and the consequent formation of vortices that span between the suction surface and the casing. As seen in the rotor frame of reference, near the casing the vortex convects toward the pressure surface of the adjacent blade. The approach of the vortex to the adjacent blade triggers a separation on that blade so the structure propagates. The above sequence of events constitutes a spike. The computed structure of the spike is shown to be consistent with rotor leading edge pressure measurements from the casing of several compressors: the centre of the vortex is responsible for a pressure drop and the partially blocked passages associated with leading edge separations produce a pressure rise. The simulations show leading edge separation and shed vortices over a range of tip clearances including zero. The implication, in accord with recent experimental findings, is that they are not part of the tip clearance vortex. Although the computations always show high incidence to be the cause of the spike, the conditions that give rise to this incidence (e.g., blockage from a corner separation or the tip leakage jet from the adjacent blade) do depend on the details of the compressor.

Origins and Structure of Spike-Type Rotating Stall

This paper describes the introduction of 3D blade designs into the core compressors for the Rolls-Royce Trent engine with particular emphasis on the use of sweep and dihedral in the rotor designs. It follows the development of the basic ideas in a university research project, through multistage low-speed model testing, to the application to high pressure engine compressors. An essential element of the project was the use of multistage CFD and some of the development of the method to allow the designs to take place is also discussed. The first part of the paper concentrates on the university-based research and the methods development. The second part describes additional low-speed multistage design and testing and the high-speed engine compressor design and test.Copyright © 2002 by ASME

The Use of Sweep and Dihedral in Multistage Axial Flow Compressor Blading—Part I: University Research and Methods Development

Evolution and structure of multiple stall cells with short-length-scale in an axial compressor rotor have been investigated experimentally. In a low-speed research compressor rotor tested, a short-length-scale stall cell appeared at first, but did not grow rapidly in size, unlike a so-called spike-type stall inception observed in many multistage compressors. Alternatively, the number of cells increased to a certain stable state (a mild stall state) under a fixed throttle condition. In the mild stall state the multiple stall cells, the size of which was on the same order of the inception cell (a few blade spacings), were rotating at 72 percent of rotor speed and at intervals of 4.8 blade spacings. With further throttling, a long-length-scale wave appeared overlapping the multiple short-length-scale waves, then developed to a deep stall state with a large cell. In order to capture the short-length-scale cells in the mild stall state, a so-called double phase-locked averaging technique has been developed, by which the flow field can be measured phase locked to both the rotor and the stall cell rotation. Then, time-dependent ensemble averages of the three-dimensional velocity components upstream and downstream of the rotor have been obtained with a slanted hot-wire, and the pressure distributions on the casing wall with high-response pressure transducers. By a physically plausible explanation for the experimental results, a model for the flow mechanism of the short-length-scale stall cell has been presented. The distinctive feature of the stall cell structure is on the separation vortex bubble with a leg traveling ahead of the rotor, with changing the blade in turn on which the vortex leg stands.

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Propagation of Multiple Short-Length-Scale Stall Cells in an Axial Compressor Rotor

In a low-speed compressor test rig at Kyushu University, multiple short length-scale stall cells appeared under a mild stall condition and turned into a long length-scale cell under a deep stall condition. Then, for the both types of stall cell, the pressure distribution on the casing wall and the velocity distributions upstream and downstream of the rotor have been measured by high response pressure transducers and a slanted hot-wire, respectively. The time-dependent ensemble averages of these distributions have been obtained phase-locking to both of the rotor and the stall cell rotation by using a so-called ‘double phase-locked averaging technique’ developed by the authors.Structure of the two stall cells are compared with each other: The short length-scale stall cell is characterized by a concentrated vortex spanning from the casing wall ahead of the rotor to the blade suction surface. In the long length-scale stall cell, the separation vortices go upstream irregularly when blade separation develops in the front half of the cell, and reenter the rotor on the hub side in the rear half of it.The unsteady aerodynamic force and torsional moment acting on the blade tip section have been evaluated from the time-dependent ensemble averages of the casing wall pressure distribution. The force fluctuation due to the short length-scale cells is somewhat smaller than that for the long length-scale cell. The blade suffers two peaks of the force during a period of the short length-scale cells passing through it. The moment fluctuation for the short length-scale cells is considerably larger than that for the long length-scale cell.© 2000 ASME

Comparative studies on short and long length-scale stall cell propagating in an axial compressor rotor

This research aims to develop an advanced technology of highly loaded axial compressor stages with high efficiency and sufficient surge margin. To improve endwall boundary layer flows which lead to energy loss and instability at an operation of low flow rate, the Controlled-Endwall-Flow (CEF) rotor blades were designed and tested in the low speed rotating cascade facility of Kyushu University. The CEF rotor blades have three distinctive features: the leading-edge sweep near hub and casing wall, the leading-edge bend near the casing, and the same exit metal angle of blade evaluated by a conventional design method. Mechanical strength of the blade was verified by a numerical simulation at a high speed condition. The baseline rotor blades were designed under the same design condition and tested to compare with the CEF rotor. The results showed that the maximum stage efficiency of the CEF rotor was higher by 0.7 percent and the increase in surge margin was more than 20 percent in comparison with the baseline rotor. The results of both internal flow survey and 3D Navier-Stokes analysis showed that improvement of the overall stage performance resulted from activation of the endwall boundary layers, and suggested that further improvement might be expected by combination of end-bend stator blades and a highly loaded axial compressor stage could be developed by use of the CEF rotor.Copyright © 1997 by ASME

Controlled-endwall-flow blading for multistage axial compressor rotor

Controlled-Endwall-Flow Blading for Multistage Axial Compressor Rotor

We investigated the effective use of cross-flow wind turbines for small-scale wind power generation to increase the output power by using a casing, which is a kind of wind-collecting device, composed of three flow deflector plates having the shape of a circular-arc airfoil. Drag-type vertical-axis wind turbines have an undesirable part of about half of the swept area where the inflow of wind results in low output performance. To solve this problem, we devised a casing consisting of three flow deflector plates, two of which were to block the unwanted inflow of wind and the remaining flow deflector plate having an angle of attack with respect to the wind direction to increase the flow toward the rotor. In this study, output performance experiments using a wind tunnel and numerical fluid analysis were conducted on a cross-flow wind turbine with three flow deflector plates to evaluate the effectiveness of the casing on output performance improvement. As a result, it was confirmed that the casing could improve the output performance of the cross-flow wind turbine by approximately 60% at the maximum performance point and could also maintain the output performance about 50% higher compared to the bare cross-flow wind turbine without the casing within a deviation angle of ±10 degrees, even when the casing direction was inclined against the wind direction due to changes in wind direction.

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T. Tanino

Papers

Propagation of Multiple Short-Length-Scale Stall Cells in an Axial Compressor Rotor

Comparative studies on short and long length-scale stall cell propagating in an axial compressor rotor

Controlled-endwall-flow blading for multistage axial compressor rotor

Controlled-Endwall-Flow Blading for Multistage Axial Compressor Rotor

A Study on a Casing Consisting of Three Flow Deflectors for Performance Improvement of Cross-Flow Wind Turbine