The effect of injector size on compressor performance in a transonic axial compressor with discrete tip injection

The present study investigates the effects of injector size, which covers both circumferential extent and radial extent on compressor stability and radial work redistribution using time-accurate three-dimensional numerical simulations for multi-passages in a transonic compressor with discrete tip injection. Injector width ranges from 1% to 14.5% of casing perimeter and injector throat height varies from 2 to 6 times the height of rotor tip clearance. Six identical injectors are uniformly mounted around the annulus. Results indicate that, with sufficient covering of injection, tip injection can significantly improve compressor stability. Stability margin is improved by 3.5% employing 0.65% of the annulus mass flow. With injector width increased, stability improves sharply when the width is less than 4.5% of casing perimeter, but increases flatly for the rest coverage. Expanding injector throat height to an intermediate level can further enhance compressor stability, but excessive throat height is detrimental on account of the deterioration of the aerodynamic characteristic of Coanda injector. The average decrement of the blockage in injection region, which is defined as injection efficiency, is a key factor that takes primary responsibility for the capacity of tip injection in stability improvement. Injection efficiency derived from the same operating condition for all the configurations with tip injection is well-qualified to estimate the trend of stability improvement. The unsteady influence on radial work redistribution is much more significant than the time-averaged one.