Mechanism analysis of stability improvement with tip injection in a transonic axial-flow compressor rotor

Multichannel, full three-dimensional, steady/unsteady numerical simulation with tip injection was carried out for NASA Rotor 35. The predicted overall performance of solid casing agreed very well with the experimental data. Predicted result showed that the stall margin could be improved by 21.4% when the jet amount was equal to 3.6% of choke mass flow of the rotor. The steady calculations showed that nearly 10% outerspan of flow annulus was influenced by tip injection. The high axial momentum introduced by injection decreased the flow incidence angle and diffusion factor, thus reducing near-tip blade loading, and accordingly the blockage region arising from the interaction between passage shock and tip leakage vortex. Unsteady simulation showed that stall margin improvement attribute to two aspects: one is reducing the loading on a certain blade, and the other is reinjection axial momentum into the low energy region formed by the interaction between shock/leakage vortex. The latter improved the passage flow to larger extent than that of the former. The higher jet frequency can make the tip injection restrain further development of low-speed area in each passage so as to achieve the stability improvement of compressor.