Centrifugal fan

Abstract

A centrifugal fan includes a motor including a rotor, an impeller fixed to the rotor to rotate together with the rotor, a circuit board electrically connected to the motor, and a casing to house the motor, the impeller, and the circuit board. The impeller includes blade portions spaced apart from one another in a circumferential direction and extending radially outward, an annular upper shroud to join axially upper portions of the blade portions, and an annular lower shroud to join axially lower portions of the blade portions. The casing includes an upper casing to cover an axially upper side of the impeller. The upper casing includes a tubular portion radially opposite to the upper shroud with a first gap therebetween, and an annular disk-shaped portion extending radially inward from the tubular portion and axially opposite to an axially upper end portion of the upper shroud with a second gap therebetween. The disk-shaped portion, including an air inlet defined radially inside thereof, includes an axially recessed portion recessed axially upward in a lower surface thereof.

What is claimed is: 
     
         1 . A centrifugal fan comprising:
 a motor including a rotor to rotate about a central axis extending in a vertical direction;   an impeller fixed to the rotor to rotate together with the rotor;   a circuit board electrically connected to the motor; and   a casing to house the motor, the impeller, and the circuit board; wherein   the impeller includes:
 a plurality of blade portions spaced apart from one another in a circumferential direction and extending radially outward; 
 an annular upper shroud to join at least portions of axially upper portions of the blade portions; and 
 an annular lower shroud to join at least portions of axially lower portions of the blade portions; 
   the casing includes an upper casing to cover an axially upper side of the impeller;   the upper casing includes:
 a tubular portion radially opposite to the upper shroud with a first gap therebetween; and 
 an annular disk-shaped portion extending radially inward from the tubular portion and axially opposite to an axially upper end portion of the upper shroud with a second gap therebetween; 
   the disk-shaped portion includes an air inlet defined radially inside thereof; and   the disk-shaped portion includes at least one axially recessed portion recessed axially upward in a lower surface of the disk-shaped portion.   
     
     
         2 . The centrifugal fan according to  claim 1 , wherein the at least one axially recessed portion includes a plurality of axially recessed portions arranged in the circumferential direction. 
     
     
         3 . The centrifugal fan according to  claim 1 , wherein the at least one axially recessed portion is annular when viewed in an axial direction. 
     
     
         4 . The centrifugal fan according to  claim 1 , wherein
 the tubular portion includes a plurality of radially protruding portions each of which projects radially inward and extends in an axial direction at a radially inner surface of the tubular portion; and   an axially upper end portion of each radially protruding portion is connected to the lower surface of the disk-shaped portion.   
     
     
         5 . The centrifugal fan according to  claim 4 , wherein, when viewed in the axial direction, a backward rotation-direction side surface of each radially protruding portion is an inclined surface extending in a forward rotation direction while extending radially inward, or a curved surface that is convex radially inward and in a backward rotation direction, the backward rotation-direction side surface, the forward rotation direction, and the backward rotation direction being defined on a basis of a rotation direction of the impeller. 
     
     
         6 . The centrifugal fan according to  claim 4 , wherein the radially protruding portions are arranged at regular intervals in the circumferential direction. 
     
     
         7 . The centrifugal fan according to  claim 4 , wherein the at least one axially recessed portion circumferentially overlaps with the radially protruding portions. 
     
     
         8 . The centrifugal fan according to  claim 4 , wherein the axially upper end portion of each radially protruding portion is housed in the at least one axially recessed portion. 
     
     
         9 . The centrifugal fan according to  claim 1 , wherein the first gap has a radial width smaller than an axial width of the second gap. 
     
     
         10 . The centrifugal fan according to  claim 1 , wherein
 the impeller further includes an annular wall portion extending from the upper shroud toward the upper casing;   a lower surface of the upper casing includes an annular wall housing portion to house at least a portion of the wall portion, and recessed axially upward; and   each of the wall portion and the wall housing portion is located radially outward of the disk-shaped portion.   
     
     
         11 . The centrifugal fan according to  claim 10 , wherein each of the wall portion and the wall housing portion is located radially inward of a radially outer end of the upper shroud.