Single-channel unsteady numerical simulation was carried out on NASA Rotor 35 to study the influence of radial skewed angle of reversed blade-angle slot casing treatment (RBSCT) on the aerodynamic performance and stall margin of a transonic compressor. Moreover, the influence mechanism is explained by detailed flow field analysis. The radial skewed angles were set to +0°, +30°, +60°, and +75° in the research. The calculated result shows that stall margin improvement (SMI) generated by the slots is increased when the radial skewed angle is gradually increased. The SMI of 17.47% for the slots with +75° radial skewed angle is the biggest among the four RBSCTs. As for the design efficiency improvement (DEI), it is increased first and then decreased with the radial skewed angle increasing. RBSCT with +60°radial skewed angle achieves the greatest DEI of 1.11%. The flow field analysis shows that the radial momentum of the injected and sucked flows is improved with the increasing of radial skewed angle. The improvement can reduce the relative airflow angle of mainstream near the stall condition. Furthermore, the excessive radial skewed angle will increase the flow losses in the slots. Consequently, the design efficiency is reduced at +75°.

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The main objective of this study is to understand the mechanisms of affecting the performance and flow field of a low speed centrifugal fan with blade surface slots. In the investigation, a spanwise slot, which is placed within the range of about 60% to 100% of the blade span, is applied to each blade surface of the impeller, and the partial blade pressure and suction surfaces for each blade are connected via the blade surface slot. The full-passage unsteady numerical results indicate that the performance of fan including the volute is improved among the overall mass flow rate by applying the blade surface slot, and the blade surface slots generate about an enhancement of 3.6% for the total pressure improvement, an improvement of 2.6% for the efficiency respectively at the design mass flow rate. The flow field analyses show that the injecting flows discharging from the blade surface slots can encourage the low energy flows near the blade suction surface in some blade passages. Hence, the boundary layer separations of the blade suction surface are restrained by the injecting flows, and the flow condition in the passag is considerately improved. Moreover, the adverse effect caused by the low energy flows is very serious in the partial blade passages near the shroud, while the flows in the blade surface slots have few positive effect on the low energy flows. So, the improvement by the slots on the flow fields is very small near the shroud region. Because the boundary layer separations on the blade suction surface are absent in most of the blade passages at about 60% of the blade span, the flows emerging from the slots have passive effects on the passage mainstreams. Further, in the case of blade surface slots, there is an obvious vortex near the blade trailing edge at about 60% of the blade span, and the vortex is close to the blade suction surface at some instants. As a result, the wake flow losses in the passage become large with the application of the slots. Briefly, within the slotted span range, the improvement of the flow field in the blade passages becomes large and then gets small, as the blade span decreases. Furthermore, the flow condition in the volute is improved thanks to the amelioration of the flows in some blade passages, and the degree of the second flows in the volute is weakened with the application of blade surface slots. Consequently, the flow losses in the volute are reduced effectively.