The Periodical Interaction of the Tip Clearance Flow in the Blade Rows of Axial Compressors

Abstract

Current models on the tip clearance flow in turbomachines only describe the time-averaged behaviour of the flow structures. However, the real tip clearance flow is periodically fluctuating in time. This fact has to be considered for the design of turbomachine bladings especially with regard to blade vibrations and tip clearance noise. Detailed experimental investigations on the time-resolved behaviour of the flow in the rotor blade tip region were carried out in a four-stage low-speed research compressor. A strong time-periodic interaction of the blade tip vortices of adjacent blades can be shown for relatively large tip clearance of the rotor blades for operating points near the stability limit of the compressor. The resulting flow pattern, which frequency is not related to the rotor frequency, moves along the blade row. It can be described as a multicell configuration with strongly fluctuating cell number and size. The structure and propagation of the flow instability can be summarized in a model of the periodic fluctuating tip clearance flow (Mailach et al., 2000). Additional experiments were carried out in a straight cascade to improve the understanding of this flow phenomenon. It can be shown by means of time-resolved measurements that the same disturbance exists for comparable inlet flow conditions in the blade tip region of the cascade. Flow visualizations show that the blade tip vortex is strongly fluctuating and moves sometimes ahead of the leading edge of the adjacent blade. The result of this is a short-lengthscale flow pattern, which is propagating along the blade row. These experiments confirm the model of the time-periodic tip clearance flow proposed for compressors. A Strouhal-number for the estimation of the frequency of the flow fluctuation will be presented, which includes both design and aerodynamic parameters.

Numerical simulation of rotating stall in a centrifugal compressor with vaned diffuser

“…As already found in axial compressors [9][10][11][12][13][14], when the mass flow rate is below a certain value the tip leakage flow begins to fluctuate at some frequencies. Especially within a narrow operating range near the stability limit when the compressor has a relatively large tip clearance, a unique phenomenon namely rotating instability occurs, which has been observed in axial and centrifugal compressors.…”

“…This paper will concentrate on the development of rotating stall from normal operating conditions to stalled condition in a centrifugal compressor with vaned diffuser and try to explain the reason for the formation of rotating stall. In the process some interesting phenomena similar to the rotating instability found and investigated by Mailach [9,10] and Marz [11] et al in axial compressors are presented here. Whether this phenomenon is a precursor of rotating stall is still a problem under research.…”

“…The discovery of unsteady characteristics of tip leakage flow furnishes the research about stall initiation. Mailach [9,10] and Marz [11] observed the rotating instability formed by the unsteady tip leakage flow near compressor stability limit with relatively large tip clearance. Furukawa [12] studied the unsteady tip leakage flow due to the breakdown of tip leakage vortex at near stall point by numerical simulation.…”

Non-Synchronous Vibration of Turbomachinery Airfoils

“…Numerous other researchers have studied the NSV problem in turbomachinery and one particular area that has received considerable interest is tip flow instability. Examples of work in this area can be found in Mailach (1999), Mailach et al (2000Mailach et al ( & 2001, Marz et al (1999), Inoue et al (1999, Lenglin &Tan (2002), andVo (2001). Mailach et al (2001) Camp (1999) reports that this vexing problem has been observed in a high speed compressor.…”

“…Examples of work in this area can be found in Mailach (1999), Mailach et al (2000Mailach et al ( & 2001, Marz et al (1999), Inoue et al (1999, Lenglin &Tan (2002), andVo (2001). Mailach et al (2001) Camp (1999) reports that this vexing problem has been observed in a high speed compressor. As a result, an experimental study was performed in a low speed compressor facility.…”