Axial flow fan

1. An axial flow fan comprising:

an impeller, a hub of which is attached to a rotation shaft of a rotation driving device; and

a plurality of blades integrally attached to the hub,

a plurality of step portions on a positive pressure surface of each of the plurality of blades, each of the plurality of step portions being curved so as to comply with a curved shape of a front edge portion in a rotation direction of the blades;

one of the plurality of step portions on each of the blades is centrally located between a front edge and a rear edge of the blade and extends in an arcuate pathway from a root of the blade to a tip of the blade.

2. The axial flow fan according to claim 1 ,

wherein the respective step portions are extended and curved from an inner circumferential portion of each blade to an outer circumferential portion thereof, in the positive pressure surface of each blade.

3. The axial flow fan according to claim 1 ,

wherein the step portion located on a rear edge portion side of the blade is curved so that a radius of curvature thereof is smaller than the radius of curvature of the step portion located on a front edge portion side.

4. The axial flow fan according to claim 1 ,

wherein the cross-sectional shape of the blades includes a shape in which streamlined blades are arranged in series from the front edge portion side toward the rear edge portion side.

5. The axial flow fan according to claim 1 , wherein one of the plurality of step portions on each of the blades is centrally located between the front edge and the rear edge of the blade and extends in an arcuate pathway from a root of the blade to a tip of the blade.

6. The axial flow fan according to claim 1 , wherein the one of the plurality of step portions begins at the root of the blade and extends to a tip of the blade.

7. An axial flow fan comprising:

an impeller, a hub of which is attached to a rotation shaft of a rotation driving device; and

a venturi casing that surrounds an outer circumference in a radial direction of the impeller, and includes a suction port and a discharge port facing each other in an axial direction of the rotation shaft,

wherein a positive pressure surface of the plurality of blades integrally attached to the hub includes a plurality of step portions that is curved so as to comply with a curved shape of a front edge portion in a rotation direction of the blades;

wherein one of the plurality of step portions on each of the blades is centrally located between the front edge and a rear edge of the blade and extends in an arcuate pathway from a root of the blade to a tip of the blade.

8. The axial flow fan according to claim 7 , wherein the one of the plurality of step portions begins at the root of the blade and extends to a tip of the blade.

9. An axial flow fan, comprising:

an impeller, a hub of which is attached to a rotation shaft of a rotation driving device; and

a plurality of blades integrally attached to the hub,

wherein a positive pressure surface of the plurality of blades includes a plurality of step portions that is curved so as to comply with a curved shape of a front edge portion in a rotation direction of the blades;

wherein a cross-sectional shape of the positive pressure surface of the blade progressively repeats a shape in which a thickness smoothly increases from the front edge portion side of the blade and rapidly decreases at an edge of the step portion;

wherein one of the plurality of step portions on each of the blades is centrally located between the front edge and a rear edge of the blade and extends in an arcuate pathway from a root of the blade to a tip of the blade.

10. The axial flow fan according to claim 9 , wherein the one of the plurality of step portions begins at the root of the blade and extends to a tip of the blade.