Inducer/Exducer Matching Characteristics inside Tandem Impellers of a Highly Loaded Centrifugal Compressor

Abstract

A centrifugal compressor usually operates with low isentropic efficiency and a terrible stable operating range, resulting from the complex impeller flow structure companied with the intense interaction among the impeller and the diffuser downstream. In many studies, the potential of centrifugal compressor tandem-impeller configurations for improving the compressor has been demonstrated. Whereas, compared with the convincing results on the tandem-designed axial compressors, the results on tandem impellers are limited and contradictory. Very little insight has been provided into the flow mechanisms inside tandem impellers, which is considered to be the primary reason for the confusion in tandem impeller design and application. Tandem impellers are expected to exhibit a totally different behavior due to the intense aerodynamic interaction between the inducer and the exducer, which substantially contributes to the flow structure and the compressor performance change. In the present study, a numerical study of a highly-loaded centrifugal compressor with various tandem designs was conducted to explore the inducer/exducer matching characteristics and the underlying flow mechanism inside tandem impellers. Two tandem impeller design parameters, namely, the inducer/exducer clocking fraction and the axial gap (overlap), were considered in the tandem impeller design process. The tandem impeller was also compared to the existing conventional impeller which the tandem impeller was redesigned for. The results demonstrated that the tandem-designed impeller can improve the centrifugal compressor stage performance and intense inducer/exducer interaction can be observed with changes in the clocking fraction and the axial gap (overlap). The tandem impeller performance is sensitive to changes in axial gap (overlap) when the suction side of the exducer blade is circumferentially close to the inducer blade. The fundamental reason for the performance variation in the inducer and the exducer lies in the inducer pressure change in the blade trailing edge that is determined by the Kutta condition. Additionally, the correlation between the tandem impeller slip effect and the discharge flow quality should be emphasized in the inducer/exducer gap jet analysis, in which the jet injection angle and the Coanda effect of the exducer suction surface critically affect the discharge flow characteristics.