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

R. Mailach, H.Sauer, K. Vogeler

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. This fact has to be considered for the design of turbomachine bladings 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 periodical fluctuation of the blade tip vortex and it¥blade can be shown, which results in a fluctuation of the blade tip vortices of the whole rotor blade row. This flow pattern moves along the blade row and is today termed as rotating instability. It can be described as a multicell configuration with strongly fluctuating cell number and size. The structure and propagation of the disturbance can be summarized in a model of the periodic tip clearance flow.
Additional experiments were carried out in a plane cascade to improve the understanding of this flow phenomenon. It can be shown by means of time-resolved measurements that the same disturbance exists at comparable inlet flow conditions in the blade tip region of the cascade. Simple flow visualisations show, that the blade tip vortex is very strongly fluctuating and moves sometimes ahead the leading edge of the adjacent blade. The result of it is a short length-scale flow pattern, which is propagating along the blade row. These experiments confirm the model on the periodic tip clearance flow. A Strouhal-number for the estimation of the frequency of the flow fluctuation will be presented, which includes both design and aerodynamic parameters.

reference:

ASME 2001-GT-0299, 46th ASME International Gas Turbine & Aeroengine Technical Congress, Exposition and Users Symposium, New Orleans, USA, June 4-7, 2001.