Active Flow Control to Improve the Aerodynamic Performance of Axial Turbomachines

Abstract

Axial turbomachines have a radial gap between the casing and the rotor blades. The static pressure difference between the suction and the pressure side of the impeller blades produces a secondary flow over the tip of the rotor blades. This tip clearance flow is important for the aerodynamic performance of the fan. Fan pressure and efficiency drop, and the usable range of the performance characteristics is diminished as the rotor flow is stalled at low flow rates. Previous investigations have shown that one method for increasing the aerodynamic performance is to control the flow in the tip clearance gap via air injection into the gap. The goal of this paper is to compare the different effects of steady and unsteady air injection on the aerodynamic performance curves and to implement various closed-loop extremum-seeking control algorithms. The main purpose of these active flow control methods is to stabilize the flow at operating points, where it is stalled otherwise. To compare the effect of steady and unsteady air injection, the aerodynamic performance curves (fan pressure rise and efficiency) were measured for different sets of frequencies with the air injection rate held constant. To control the air injection rate automatically and to find optimal actuation parameters, a SISO-extremum-seeking control algorithm was applied. For the improvement of the control performance, the controller was extended by a slope-seeker. Moreover, an extended Kalman filter was used to speed up the control via a faster slope detection to accelerate the estimation of the local gradient of the static input-output map of the process. This new approach led to an almost fivefold increase in closed-loop control speed.