At low mass flow rates the steady axisymmetric flow through an axial compressor breaks down in a process referred to as rotating stall. This blade row instability severely limits the performance of the compressor and can even cause mechanical failure. Under certain conditions, rotating stall can lead to another instability called surge. Surge is characterized by large oscillations in the axisymmetric flow rate through the machine.
These phenomena must be avoided in gas-turbine engines to ensure safe and efficient operation. Compressors are therefore often operated at flow rates much higher than desired to ensure that no transient in flow rate can drive the machine into stall. This ‘stall-margin’ is wasteful as it forces the compressor to operate at lower than peak efficiency and pressure rise.
If the mechanisms that lead to rotating stall can be better understood it is hoped that the wasteful stall margin can be reduced. This can be accomplished by more accurate stall prediction or through active control of the stall inception. The global characteristics of the stall inception process have been extensively studied in both low and high-speed compressors. The detailed nature of the generation of an embryonic stall cell in the blade row is considerably less understood. This is especially true at high-speeds where shock systems interact with the blade passage flow and stall can happen very quickly.
The objective of this research program is to link the blade passage flow structure to the generation of disturbances that can grow into fully developed stall cells. The investigations will be conducted at low and high-speeds. Emphasis will be placed on the influence of the tip clearance flow on the stall inception process. The experiments are being conducted at the Notre Dame Transonic Axial Compressor (ND TAC) Facility using ND Stage 01.
In order to quantify the behavior of the blade tip flows during stall inception, digital Particle Image Velocimetry (PIV) will be used. An optical window in the casing over the rotor of ND Stage 01 provides optical access to rotor blade tips and the rotor blade passage. The PIV data will be used to make both instantaneous and phase averaged measurements throughout inception. The first step in the successful application of this technique is accurate detection and localization of the incipient stall cell. This will be accomplished a posteriori using wavelet analysis. PIV images will be taken along with unsteady casing pressure traces during many stall events. The wavelet analysis will then be used to sort the PIV images into bins according to the stage of stall cell development.