Compressor Stability Enhancement Using Discrete Tip Injection

Mass injection upstream of the tip of a high-speed axial compressor rotor is a stabilityenhancement approach known to be effective in suppressing stall in tip-critical rotors.This process is examined in a transonic axial compressor rotor through experiments andtime-averaged Navier-Stokes CFD simulations. Measurements and simulations for dis-crete injection are presented for a range of injection rates and distributions of injectorsaround the annulus. The simulations indicate that tip injection increases stability byunloading the rotor tip and that increasing injection velocity improves the effectiveness oftip injection. For the tested rotor, experimental results demonstrate that at 70 percentspeed the stalling flow coefficient can be reduced by 30 percent using an injected mass-flow equivalent to 1 percent of the annulus flow. At design speed, the stalling flow coef-ficient was reduced by 6 percent using an injected massflow equivalent to 2 percent of theannulus flow. The experiments show that stability enhancement is related to the mass-averaged axial velocity at the tip. For a given injected massflow, the mass-averaged axialvelocity at the tip is increased by injecting flow over discrete portions of the circumfer-ence as opposed to full-annular injection. The implications of these results on the designof recirculating casing treatments and other methods to enhance stability will bediscussed. @DOI: 10.1115/1.1330272#