Experimental Off-Design Investigation of Unsteady Secondary Flow Phenomena in a Three-Stage Axial Compressor at 68% Nominal Speed

This paper deals with unsteady measurements in a highspeed three-stage axial compressor with inlet guide vanes (IGV) and controlled diffusion airfoils (CDA) at off-design conditions. The compressor under consideration exhibits design features of real industrial compressors. The main emphasis is put on the experimental investigation of two operating points at 68% nominal speed where a significant mismatching of the stages occurs. The first operating point is the last stable one near the surge margin whereas the second one represents choke. Probe traverses with a high resolution both in space and time show the significant potential upstream influence of the rotor blades. This effect and the disturbances caused by the convected wakes do strongly influence the unsteady boundary layer behaviour of the stator blades which are detected by glue-on hot-film sensors at different spanwise positions. Dynamic pressure transducers on the casing show that the structure of the rotor tip clearance flow strongly depends on the operating conditions of the compressor. Conclusions can be drawn concerning the consideration of the discussed unsteady effects within the design process of multistage axial compressors with respect to the presented results.