Mechanism of Strong Effects of Stall Suppression by an Air Separator Investigated by Internal Flow Measurements in an Axial Flow Fan

Abstract

Radial-vaned air separators produce strong stall improvement effect in axial flow fans. Changes in the fan internal flow patterns achieved by the device are confirmed by Pitot traverse measurements. For the air separator condition, with a decrease in the fan flow rate the meridional streamline inclinations through the rotor blades have become increasingly steep and the circumferential velocities increase toward the tip region, in contrast to relatively little changes in the behaviors of the streamlines and flow patterns in the solid wall condition. It is suggested that the stall suppression is achieved by a combination of the following effects of the air separator; (1) suction of casing boundary layers and elimination of embryos of stall, (2) separation of reversed flow from the main flow, (3) induction of the flow toward the casing wall and strengthening of meridional streamline inclination, thus keeping the Euler head increasing, and (4) reinforcement of axi-symmetric flow structure. Appearance of these effects in the descending order is considered to achieve the great stall suppression effect that would be difficult by other types of stall prevention devices.