Patent ID: 8348593

Claim:
A serial axial fan comprising: a first axial fan; and a second axial fan connected to the first axial fan along a central axis of the first axial fan; wherein the first axial fan includes: a first motor portion; a first impeller having a plurality of first blades extending radially outward, centered about the central axis and arranged at regular intervals in a circumferential direction and to rotate about the central axis in response to action of the first motor portion to produce an air flow along the central axis and toward the second axial fan; and a first housing surrounding an outer circumference of the first impeller; the second axial fan includes: a second motor portion; a second impeller having a plurality of second blades extending radially outward, centered about the central axis and arranged at regular intervals in the circumferential direction and to rotate about the central axis in response to action of the second motor portion in an opposite direction to that in which the first impeller rotates, to produce an air flow in the same direction as that of the air flow produced by the first impeller; and a second housing surrounding an outer circumference of the second impeller; in the first impeller, an angle defined by a rotational plane, perpendicular or substantially perpendicular to the central axis, of the first impeller with a chord of each first blade on an imaginary cylindrical surface centered about the central axis increases as a radius of the imaginary cylindrical surface increases; a first solidity, which is defined as a ratio of a chord length of the first blades of the first impeller to a pitch of the first blades, is greater than a second solidity, which is defined as a ratio of a chord length of the second blades of the second impeller to a pitch of the second blades, when the first solidity and the second solidity correspond to an additional imaginary cylindrical surface taken at an outer radial position of each of the first blades and the second blades; and the first solidity is greater than the second solidity along most of the radius of the imaginary cylindrical surface.