Serial axial fan

1. A serial axial fan comprising:

a first motor portion including a first rotating portion;

a first impeller fixed to the first rotating portion of the first motor portion;

a second motor portion arranged along a central axis of the first motor portion, and including a second rotating portion;

a second impeller fixed to the second rotating portion of the second motor portion;

a tubular wind channel portion arranged to surround outer circumferences of the first and second impellers; and

a plurality of support ribs arranged to join the wind channel portion to both the first and second motor portions; wherein

the first impeller includes a plurality of first blades arranged in a circumferential direction about the central axis, and the second impeller includes a plurality of second blades arranged in the circumferential direction;

each of the first and second impellers is configured to be rotatable in both directions, and a rotation direction of the second impeller is always opposite to a rotation direction of the first impeller;

a portion of a surface of each of the plurality of first blades, the surface facing the second impeller, is concave, and a portion of a surface of each of the plurality of second blades, the surface facing the first impeller, is concave;

the plurality of support ribs are located between the first and second impellers;

an edge of each of the plurality of first blades on an opposite side with respect to the second impeller includes an inclined portion configured to become progressively more distant from the second impeller with increasing distance from the central axis; and

an edge of each of the plurality of the second blades on an opposite side with respect to the first impeller includes an inclined portion configured to become progressively more distant from the first impeller with increasing distance from the central axis.

2. The serial axial fan according to claim 1 , wherein a rotation speed of one of the first and second impellers which is located on an upstream side is higher than a rotation speed of the other of the first and second impellers located on a downstream side.

3. The serial axial fan according to claim 1 , wherein the plurality of first blades and the plurality of second blades are equal in number.

4. The serial axial fan according to claim 1 , wherein

the wind channel portion has a square or rectangular external shape when viewed along the central axis; and

an inner surface of each of four corners of the wind channel portion includes, at each of both opening ends of the wind channel portion, an inclined surface configured to become progressively more distant from the central axis with decreasing distance from the opening end.

5. The serial axial fan according to claim 1 , wherein

a width of a section of each of the plurality of support ribs taken on a plane perpendicular to a direction of extension of the support rib, the width being measured in a direction perpendicular to the central axis, is configured to first gradually increase in a direction away from the first impeller toward the second impeller and then gradually decrease; and

a shape of the plurality of support ribs as viewed from a direction of the first impeller along the central axis is configured to be identical to a shape of the plurality of support ribs as viewed from a direction of the second impeller along the central axis.

6. The serial axial fan according to claim 5 , wherein each of the plurality of support ribs extends straight in a radial direction.

7. The serial axial fan according to claim 1 , wherein a leading edge of each of the plurality of first blades with respect to the rotation direction of the first impeller and a trailing edge of each of the plurality of second blades with respect to the rotation direction of the second impeller are warped in different directions with respect to the rotation directions of the first and second impellers, respectively.

8. The serial axial fan according to claim 1 , wherein each of the plurality of first blades has an axial dimension equal to that of each of the plurality of second blades.

9. The serial axial fan according to claim 1 , wherein

a region in a surface of each of the plurality of first blades on an opposite side with respect to the second impeller, the region being located forward with respect to the rotation direction of the first impeller when the first impeller is configured to send a fluid out of the wind channel portion, is concave; and

a region in a surface of each of the plurality of second blades on an opposite side with respect to the first impeller, the region being located forward with respect to the rotation direction of the second impeller when the second impeller is configured to send the fluid out of the wind channel portion, is concave.