Improving near-stall performance of axial flow compressors using variable rotor tandem stage. Unsteady analysis

This paper investigates the instability inception phenomenon in a variable rotor tandem (VRT) stage compressor by developing a numerical model based on unsteady Reynolds-averaged Navier–Stokes (URANS) equations. The main objective is to assess the idea of replacing a combination of one inlet guide vane row and one conventional axial-flow stage with a VRT stage to reduce the weight and size of axial-flow compressors. Steady performance analysis of a tandem stage axial-flow compressor, designed and constructed in the Dana laboratory of the Amirkabir University of Technology, was previously reported. In that study, it was discovered that at both design and off-design speeds, a VRT stage is more effective than a variable stator tandem stage in postponing the surge point and extending the safe operation range. However, Reynolds-averaged Navier–Stokes (RANS) modeling did not show the origin of instability inception and particulars of such highly complex flow phenomena. To explore the physics of flow more accurately, a full-annulus three-dimensional URANS model is developed to study near-stall performance. Transient simulations at mass flow rates less than the stall point show that instability originates from hub corner stall formation on the suction surface of the front stator tandem blade at about 0.97 revolution time. In 4.62 revolutions, the stall zone propagates radially toward the stator mid blade height and then affects the upstream tandem rotor row. This study explains how a VRT blade mechanism with 8° of positive rotation of the aft rotor controls and suppresses the instability well before the stall zone affects the whole stage.