An Experimental Investigation on the Unsteady Excitation Effect of Casing Treatment on a Low Speed Axial Compressor

Abstract

The performances of compressor should be remarkably improved if the interaction between the flow within treatment slots and the separated flow field around the rotor blades could be properly organized. An experiment was performed on a single stage low speed axial compressor to investigate the unsteady excitation effect of three arc curve skewed slot casing treatments which differ only in the number of treatment slots under uniform and distorted inlet flow conditions. The separated flow field around the rotor blade was investigated with a dynamic pressure probe to obtain the information on unsteady vortex shedding, and the frequency spectrum of the dynamic information was analyzed and utilized to guide the design of the casing treatment in confirming the number of slots. The performances of the compressor were measured for smooth casing and arc curve skewed slot casing treatments under uniform and distorted inlet flow conditions respectively. The stable operating range of the compressor can be improved with all these arc curve skewed slot casing treatments compared to smooth casing. However, only the casing treatment representing the same frequency as that of separated flow around the rotor blades could achieve prominent improvement in stability and total pressure rise coefficient of the compressor without efficiency penalty, where the unsteady separated flow around the rotor blades could couple with that injected from the treatment slots. Consequently, the flow structure was remarkably improved and the time-averaged aerodynamic performances especially the stability of the compressor were significantly enhanced. The number of treatment slots would effectively influence the unsteady interaction, and hence influence the effectiveness of the casing treatment. The frequency of unsteady interaction should be tuned close to the vortex shedding frequency or within the range of lock-in frequency to couple with the separated flow field around the rotor blades, which might be utilized to guide the design of casing treatment in confirming the number of treatment slots.