Centrifugal fan structure

1. A centrifugal fan structure, comprising:

a top case comprising an air inlet;

a body comprising a sidewall, a tongue portion, and a bottom case, wherein the sidewall and the tongue portion are disposed on the bottom case, and the sidewall comprises an opening;

a fan disposed in the body, wherein the fan comprises a shaft and a plurality of blades connected to the shaft; and

a muffler pipe disposed on the sidewall of the body, and comprising an opening corresponding to the opening of the sidewall of the body, a first side surface disposed along the sidewall of the body, a second side surface opposite to the first side surface, a top surface and a bottom surface disposed at a top side and a bottom side of the muffler pipe, and an end surface disposed at one end of the muffler pipe distal to the opening of the muffler pipe along a long axis of the muffler pipe, wherein the end surface is formed as a wall and extends between the first side surface and the second side surface, and between the top surface and the bottom surface to fully close said one end of the muffler pipe distal to the opening of the muffler pipe along the long axis of the muffler pipe, wherein the fan has a steady state rotation speed under operation, a length of the muffler pipe is substantially equal to a quarter of a wavelength of a blade passing frequency of the fan, and wherein the blade passing frequency is a product of the steady state rotation speed of the fan and a number of the blades, and the wavelength is equal to a sound speed divided by the blade passing frequency.

2. The centrifugal fan structure of claim 1 , wherein the opening of the muffler pipe and the corresponding opening of the sidewall are substantially of the same size.

3. The centrifugal fan structure of claim 1 , wherein the opening of the muffler pipe is disposed at an edge of the short side surface.

4. The centrifugal fan structure of claim 1 , wherein a profile of the short side surface fits a profile of the sidewall.

5. The centrifugal fan structure of claim 1 , wherein the long side surface and the short side surface are intersected at another end of the muffler pipe along the long axis, and the opening of the muffler pipe is disposed at the short side surface.

6. The centrifugal fan structure of claim 5 , wherein the opening of the muffler pipe and the corresponding opening of the sidewall are substantially of the same size.

7. The centrifugal fan structure of claim 5 , wherein the opening of the muffler pipe is disposed at an edge of the short side surface.

8. The centrifugal fan structure of claim 5 , wherein a profile of the short side surface is conformal with a profile of the sidewall.

9. The centrifugal fan structure of claim 5 , wherein the muffler pipe and the body are formed integrally in one piece.

10. The centrifugal fan structure of claim 5 , wherein the muffler pipe is detachably disposed on the body.

11. The centrifugal fan structure of claim 5 , wherein the muffler pipe is made of plastic.

12. The centrifugal fan structure of claim 5 , wherein the muffler pipe is made of rubber or soft plastic.

13. The centrifugal fan structure of claim 5 , wherein:

a first direction is parallel to a bottom edge of the bottom case and passes through a center of the shaft,

a second direction is extended from the center of the shaft toward the opening of the sidewall, and

the first direction and the second direction form a 120-degree angle between the first direction and the second direction.

14. The centrifugal fan structure of claim 5 , wherein an intersection of the long side surface and the short side surface contacts the body.

15. The centrifugal fan structure of claim 5 , wherein the muffler pipe has five surfaces.

16. The centrifugal fan structure of claim 5 , wherein a profile of the muffler pipe is conformal to the sidewall of the body.

17. The centrifugal fan structure of claim 1 , wherein the muffler pipe is free of additional openings in addition to the opening of the muffler pipe.

18. The centrifugal fan structure of claim 1 , wherein the opening of the muffler pipe is the only entrance and exit for air.