Patent Application
20110280719
The present invention relates to an axial flow fan and shroud assembly.
Rotating engine mounted cooling fans are used to push air through the radiator of an engine cooling system. Normally, a fan shroud is mounted around such a fan. Typical pushing fan assemblies include a flat plate box shroud which is simple to manufacture and is common in low volume applications. Such a shroud design does not allow the design to match a shroud to a fan for a given application or maximize fan efficiency. It has been determined that for a pushing fan application with high static pressure, the relationship between the inlet shroud geometry and the fan is critical to fan performance. It desired to maximize the fan efficiency and air flow for a given fan speed and system restriction in such a fan/shroud assembly. In agriculture applications where fan diameters range from 550 millimeters to 1100 millimeters, ideal inlet conditions are not achievable in the space available. By designing a complex three dimensional fan shroud with specific inlet and outlet shapes, the efficiency of the fan/shroud/cooling system can be enhanced.
Accordingly, an object of this invention is to provide an axial flow fan assembly with improved efficiency.
This and other objects are achieved by the present invention, wherein an axial flow fan assembly has a plurality of fan blades surrounded by an annular shroud. The shroud includes an annular first wall which extends radially outwardly at an upstream end of the assembly, an annular second wall which extends axially and radially outwardly at a downstream end of the assembly, and an annular third wall which extends between the first wall and the second wall. The third wall is outwardly convex and has a curved cross sectional shape and a minimum diameter part which is aligned with a radial plane which is positioned between the upstream and downstream edges of the fan blades. The third wall has a radius of curvature which is preferably less than 0.05 times the diameter of the fan blades. The first wall has a height in the radial direction which is preferably greater than 0.05 times the diameter of the fan blades. The second annular wall extends at an angle of preferably approximately 27 degrees with respect an axis of rotation of the fan. The fan blades have an upstream edge which is upstream with respect to the first wall.
An axial flow fan assembly 10 includes a fan 11 surrounded by a non-rotating annular shroud 16. The fan 11 has a plurality of fan blades 12 which extend outwardly from a central hub 14. Each fan blade 12 has an upstream edge 18 and a downstream edge 20 which define a projected axial width W when projected onto a plane (not shown) which contains the central rotation axis Ar. Each fan blade has a diameter D which is equal to twice the fan radius Rf measured from axis Ar to its outer edge 30.
The shroud 16 includes a first annular wall 22 which extends radially outwardly from an inner edge 21 to an outer edge 23 near an upstream end of the assembly. Wall 22 is fixed relative to the fan 11 and controls concentricity and immersion of the fan 11. The shroud 16 also includes a second annular wall 24 which extends axially and radially outwardly from an inner edge 25 to an outer edge 27 towards a downstream end of the assembly. The second wall 24 is downstream with respect to the first wall 22 in the air flow direction and includes a flexible seal joint 29. Wall 24 is supported by an annular housing 31 which is fixed to a vehicle chassis (not shown).
The shroud 16 also includes an annular third wall 26 which has a curved cross sectional shape, and which extends between the first wall 22 and the second wall 24. The third wall 26 is outwardly concave in cross-section, and has a minimum diameter part 28 which is aligned with a radial plane P which is positioned approximately half-way between the upstream and downstream edges 18, 20. The third wall 26 has a radius of curvature R which is preferably less than 0.05 times the blade diameter D. The first wall 22 preferably has a height H in the radial direction which is greater than 0.05 times the blade diameter D. The second wall 24 extends from the minimum diameter part 28 of the third wall 26. The second wall 24 preferably extends at an angle A with respect to a cylindrical surface S which is defined by the rotating our edges 30 of the blades 12, or with respect to a rotation axis of the fan. The angle A should be maximized until air flow separation occurs between the air exiting the fan blade 12 and the shroud 16.
Thus, starting at its upstream end, the shroud 16 forms a radially extending portion or first wall 22, a trumpet shaped portion or third wall 26, and an axially and radially outwardly extending frustoconical portion or second wall 24. Preferably, the shroud 16 has smooth transitions between the first wall 22 and the third wall 26, and between the third wall 26 and the second wall 24. The upstream edges 18 of the blades 12 are slightly upstream of the first wall 22.
Preferably, the height H of wall 22 should be greater than 0.05×D and the radius R should be less than 0.05×D. The fan immersion is preferably 0.5×the fan projected width W, but similar results were achieved in a range of 0.4×W to 0.6×W. Fan immersion is the axial amount of the fan relative to actual fan depth that the fan is surrounded by the shroud at the closest point radially.
The shape of the outlet 24 of the shroud 16 also affects the uniformity of the air and a quick expansion, referred to as the angle A of the shroud. It was also determined that for further optimal performance, the fan diameter must be smaller than the height and width of the heat exchanger cores (not shown) to allow room for the downstream end of the shroud to expand at angle A.
While the present invention has been described in conjunction with a specific embodiment, it is understood that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, this invention is intended to embrace all such alternatives, modifications and variations which fall within the spirit and scope of the appended claims.
