Unconventional blade design for compressors

Abstract

One of the factors causing the low efficiency of axial flow compressors operating at decreased flow rates is the flow separation at blade extremities. To postpone this tendency to separate to lower flow rates, compressor blades were designed with increased chord at the blade extremities, which are immersed in the hub and casing boundary layers. The experimental results have shown that the stage static pressure rise is not significantly altered when the modification is used, but the rotor and stage efficiencies are significantly higher at low flow rates. A lifting vortex, similar to that encountered on delta wings at moderate angles of attack, has been observed near the hub region of the modified rotor. The observed differences in the turning through the rotor have been explained satisfactorily using a model incorporating the effects of the lifting vortex. The model also demonstrates that the total drag of the modified rotor can be expected to be smaller than that of the conventional rotor at low flow rates. In the Appendix, some theories based on the lifting line approach are described, which predict the behavior of a stationary cascade facing an inlet flow with spanwise variations in the angle of attack. The comparison of the theoretical results with the very limited amount of experimental data available is considered satisfactory.