Patent ID: 7255533

Claim:
A centrifugal fan for use in an electric device comprising: a motor including a static part and a rotational part rotating around a rotational axis; an impeller having a tubular-shaped outline that is mounted on a radially outer circumferential part of the rotational part; and a casing housing the impeller and the motor, wherein the casing includes: an upper plate and a lower plate which are located in parallel and opposing each other in axial direction; a peripheral wall portion formed at a radially outer circumference of the casing, wherein a curvature radius of the peripheral wall portion with respect to the rotational axis is gradually enlarged; and a blowing opening formed by the opposite ends of the peripheral wall portion and ends of the upper plate and lower plate; wherein an air channel is formed within a space enclosed by the upper plate and the lower plate, a radial outer circumference of the impeller and an inner circumferential surface of the peripheral wall portion of the casing, wherein a radial gap is defined as a distance from the radial outer circumference of the impeller to an inner circumferential surface of the peripheral wall portion and the radial gap is gradually enlarged along a rotational direction of the impeller except for at least one specific region provided at the peripheral wall portion of the casing, while the radial gap is kept constant or gradually reduced in the specific region, wherein an air channel enlarged part is located at a vicinity of the specific region, wherein an axial gap between the upper plate and the lower plate at the air channel enlarged portion is enlarged along a rotational direction of the impeller, and wherein a cross sectional area of the air channel cut by a plane passing through the rotational axis is kept constant or is gradually enlarged along the rotational direction of the impeller.