Controllable-pitch moving blade type axial fan

In an axial fan of the controllable-pitch moving blade type for providing an air flow ranging from a large air flow to a small air flow, comprising a fan rotor hub rotatable about an axis, a plurality of movable blades extending radially outwardly from said hub, and each having a radially outer end and a variable setting angle inclined relatiive to said rotor hub axis, a controllable-pitch mechanism mounted in said hub for changing the setting angle of said blades during operation, each of said blades having a tip at the radially outer end thereof more remote from said hub with the tips of said blades traveling in a tip circle, an imperforate axially extending casing laterally enclosing said blades and including a wall having an inner wall surface spaced closely outwardly from the tips of said blades and said wall extending radially outwardly from said inner surface, wherein the improvement comprises an annular circumferentially extending recess formed in said inner wall surface with the base of the recess spaced radially outwardly from said inner wall surface and located within said wall, said recess aligned opposite and closely outwardly from said tip circle, a casing treatment located in said recess and extending radially outwardly from said inner wall surface of said casing into said recess and extending annularly around the circumference of said recess, said casing treatment having a radially inner surface generally flush with said inner wall surface of said casing, said casing treatment extending in the axial direction of said hub for at least a portion of the axial dimension of said blades and the axial extent of said casing treatment remains constant while the axial extent of the tips of said blades vary in accordance with the air flow with the axial extent determined by said controllable-pitch mechanism and the axial extent of the tips of said blades ranging from an axial dimension greater than the axial extent of said casing for the larger air flow to an axial dimension less than the axial extent of said casing for the small air flow, whereby said casing treatment affords operating conditions providing high pressure in the low volume range and also permitting improvement of the stall characteristics toward the low flow side.