The tandem-bladed impeller centrifugal compressor, which may offer potential aerodynamic benefits over conventional designs, is rarely employed in production gas-turbine applications. Conventional impeller designs are often favored because of concerns for both significant performance losses and increased manufacturing costs associated with tandem configurations. In addition, much of the available literature concerning the characteristics of tandem-impellers is inconclusive, and at times contradictory. Because of the scarcity of tandem-impellers, rules for their design are nearly non-existent. Also, the effects of inducer/exducer clocking upon tandem-impeller performance and exit flow characteristics are not fully understood. In the present study, a numerical investigation was performed to investigate the aerodynamic characteristics of a tandem-impeller design for the rear-stage of a gas-turbine compressor (target impeller PRt-t≈3.0). A parametric study of tandem-impeller inducer/exducer clocking was performed in order to explore its effect upon performance and exit flow quality. Because the tandem impeller was designed to be retrofittable for an existing conventional impeller, it was also compared to predictions for the baseline conventional design.
Results of the study indicated that the tandem-impeller was less efficient than the conventional design for all clocking configurations studied. Tandem-impeller blade clocking was found to have a significant effect upon predicted pressure-ratio, temperature-ratio, efficiency, and slip-factor; the maximum values for these parameters were predicted for the in-line tandem configuration. The minimum predicted tandem-impeller isentropic efficiency occurred at a clocking fraction of 50%, falling 3.8 points below that of the in-line case. Although the tandem-impeller performance was predicted to diminish as blade clocking was increased, significant improvements in the uniformity of impeller exit velocity profiles were observed. Profiles of both total pressure and swirl at impeller exit indicated that the tandem-impeller design may offer both improved diffuser recovery and stalling margin over the conventional design.