Counter-rotating axial flow fan

摘要

A counter-rotating axial flow fan (1) with reduced noise at the target operating point achieved without modifying a front impeller (15), a rear impeller (27), or a middle stationary portion (19) is provided. An annular rib including a projecting surface (29) for generating turbulent flow is formed on an inner wall portion (4) of a casing (3) at a position off from the middle stationary portion (19) to a side of the rear impeller (27), the projecting surface extending radially inwardly of the inner wall portion (4) and extending continuously in the circumferential direction of the inner wall portion (4). A fluid striking the projecting surface (29) for generating turbulent flow is partially disturbed to form a turbulent flow before entering an area in which the rear impeller (27) is provided. The turbulent flow suppresses flow separation of a fluid flowing along the surfaces of rear blades (23) of the rear impeller (27) from the surfaces of the rear blades.