This invention relates generally to free-tipped axial-flow fans, and more particularly to free-tipped fans that may be utilized as automotive engine-cooling fans.
Engine-cooling fans are used in automotive vehicles to move air through a set of heat exchangers which typically includes a radiator to cool an internal combustion engine, an air-conditioner condenser, and perhaps additional heat exchangers. These fans are generally enclosed by a shroud which serves to reduce recirculation and to direct air between the heat exchangers and the fan.
The shroud plenum (that portion of the shroud adjacent to the heat exchangers) is generally rectangular and the inflow to the fan is not axisymmetric. The radiator typically has a fin-and-tube structure which contributes additional non-axisymmetric flow structures to the inflow. This lack of symmetry in the inflow causes unsteady blade loading, and the generation of acoustic tones. In addition there are several sources of broadband noise. In order to reduce both tonal and broadband noise, the fan blades are often skewed.
The fans are typically injection-molded in plastic, a material with limited mechanical properties. Plastic fans exhibit creep deflection when subject to rotational and aerodynamic loading at high temperature. This is particularly an issue when the fan is mounted downstream of the heat exchangers, where the fan operates in high-temperature air, and is further subject to radiant heat from various under-hood components. This deflection must be accounted for in the design process.
Although some engine-cooling fans have rotating tip bands, many are free-tipped. These fans are designed to have a tip gap, or running clearance, between the blade tips and the shroud barrel. This tip gap must be sufficient to allow for both manufacturing tolerances and the maximum deflection that may occur over the service life of the fan assembly. Unfortunately, large tip gaps generally result in reduced fan efficiency and increased fan noise.
Many fan assemblies using free-tipped fans are relatively low-power assemblies. These fans do not consume a large amount of electric power, nor do they make much noise. They are often designed with large tip gaps, and minimal blade skew. The resulting decrease in performance and increase in noise may not be as important as would be the case with more powerful fan assemblies.
Other fan assemblies, however, consume considerable electric power, and make objectionable noise. These assemblies must be designed to minimize noise, and maximize efficiency. To accomplish this the tip gap should be as small as possible. There is therefore a need for a fan design which minimizes the deflection of the blade tip. A problem faced by the fan designer is that the blade skew which is desirable for noise reduction often results in increased deflection.
Free-tipped fans are often designed to have a constant-radius tip shape, and to operate in a shroud barrel which is cylindrical in the area of closest clearance with the fan blades. In this case, the radial component of tip deflection is the main component of concern. However, U.S. Pat. No. 6,595,744 describes a free-tipped engine-cooling fan where the blade tips conform to a flared shroud barrel. In this case, both axial and radial tip deflection can change the size of the tip gap. Although U.S. Pat. No. 6,595,744 further describes a fan geometry which minimizes axial deflection of the blade tip for a given skew, it does not prescribe skew distributions which minimize radial deflection.
The invention serves the need for a fan which is skewed to reduce fan noise, but which experiences low radial blade tip deflection. By minimizing radial deflection, the tip gap can be minimized, and performance improved.
In one aspect, the present invention provides a free-tipped axial fan assembly comprising a fan rotatable about an axis and having a radius R and a diameter D. The fan includes a hub having a radius Rhub, and a plurality of blades extending generally radially from the hub. Each of the plurality of blades has a leading edge, a trailing edge, a blade tip, and a span S equal to the difference between the fan radius R and the hub radius Rhub. A shroud of the fan assembly includes a shroud barrel surrounding at least a portion of the blade tips. A tip gap is defined between the shroud barrel and the blade tips. Each of the plurality of blades has a geometry, as viewed in axial projection, which at every radial position has a leading-edge skew angle and a trailing-edge skew angle. The leading-edge skew angle has a maximum value, and the difference between the maximum value of the leading-edge skew angle and the leading-edge skew angle at the fan radius R is at least 10 degrees. The trailing-edge skew angle has a maximum value, and the difference between the maximum value of the leading-edge skew angle and the leading-edge skew angle at the fan radius R is at least 2.5 times the difference between the maximum value of the trailing-edge skew angle and the trailing-edge skew angle at the fan radius R.
In some constructions, the difference between the maximum value of the leading-edge skew angle and the leading-edge skew angle at the fan radius R is at least 3.5 times greater than the difference between the maximum value of the trailing-edge skew angle and the trailing-edge skew angle at the fan radius R.
In some constructions, the difference between the maximum value of the leading-edge skew angle and the leading-edge skew angle at the fan radius R is at least 4.5 times greater than the difference between the maximum value of the trailing-edge skew angle and the trailing-edge skew angle at the fan radius R.
In some constructions, the difference between the maximum value of the leading-edge skew angle and the leading-edge skew angle at the fan radius R is at least 15 degrees.
In some constructions, the difference between the maximum value of the leading-edge skew angle and the leading-edge skew angle at the fan radius R is at least 20 degrees.
In some constructions, the maximum value of the leading-edge skew angle is at least 2 degrees.
In some constructions, the maximum value of the leading-edge skew angle is at least 5 degrees.
In some constructions, the maximum value of the leading-edge skew angle is at least 9 degrees.
In some constructions, the maximum value of the leading-edge skew angle occurs at a blade spanwise position between about 0.2 times the blade span S and about 0.6 times the blade span S.
In some constructions, the maximum value of the leading-edge skew angle occurs at a blade spanwise position between about 0.3 times the blade span S and about 0.5 times the blade span S.
In some constructions, the shroud barrel is flared, and the blade tip leading edge extends further radially outward than the blade tip trailing edge.
In some constructions, the tip gap is less than 0.02 times the fan diameter D.
In some constructions, the blades are molded of a plastic material.
In some constructions, the fan assembly is a puller-type automotive engine-cooling fan assembly.
In some constructions, each of the plurality of blades has a geometry, as viewed in axial projection, which at every radial position has a leading-edge sweep angle, and the leading-edge sweep angle at the fan radius R is at least 47 degrees in a backward direction.
In some constructions, each of the plurality of blades has a geometry, as viewed in axial projection, which at every radial position has a leading-edge sweep angle, and the leading-edge sweep angle at the fan radius R is at least 55 degrees in a backward direction.
In some constructions, each of the plurality of blades has a geometry, as viewed in axial projection, which at every radial position has a leading-edge sweep angle, and the leading-edge sweep angle at the fan radius R is at least 62 degrees in a backward direction.
a is a schematic of a free-tipped engine-cooling fan assembly, showing a constant-radius blade tip and a cylindrical shroud barrel.
b is a schematic of a free-tipped engine-cooling fan assembly, showing a blade tip which conforms to the shape of a flared shroud barrel.
c is a swept view of a free-tipped fan with a constant-radius blade tip, with definitions of various geometric parameters.
d is a swept view of a free-tipped fan with a blade tip of varying radius, with definitions of various geometric parameters.
a shows an axial projection of a prior-art fan with a constant-radius blade tip and a positive leading-edge sweep angle in a radially outer region.
b shows an axial projection of one blade of the fan shown in
a shows an axial projection of a prior-art fan with a blade tip which conforms to a flared shroud and a negative leading-edge sweep angle in a radially outer region.
b shows an axial projection of one blade of the fan shown in
c is a schematic of the bending forces exerted on the trailing-edge portion of the radially outer region of the blade shown in
a shows an axial projection of a fan according to one construction of the present invention.
b shows an axial projection of one blade of the fan shown in
a shows an axial projection of a fan according to one construction of the invention.
b shows an axial projection of one blade of the fan shown in
a shows a free-tipped axial fan assembly 1 that is configured for use as an engine-cooling fan assembly mounted adjacent to a set of heat exchangers 2. This set of heat exchangers typically includes a radiator 3, which cools an internal combustion engine, but in alternatively-powered vehicles could include heat exchangers to cool batteries, motors, etc. A shroud 4 guides cooling air from the radiator 3 to the fan 5. The fan 5 rotates about an axis 6 and comprises a hub 7 and generally radially-extending blades 8. One of the blades 8 is shown in a swept view, where the axial extent is plotted as a function of radius. The end of the blade 8 adjacent to the hub 7 is the blade root 9, and the outermost end of the blade 8 is blade tip 10a. The blade tips 10a are surrounded by the shroud barrel 11a. A tip gap 12a provides a running clearance between the blade tips 10a and the shroud barrel 11a.
Although most typically the fan is in a “puller” configuration and located downstream of the heat exchangers, in some cases the fan is a “pusher”, and located upstream of the heat exchangers. Although
a shows the blade tip 10a to be at a constant radius, and the shroud barrel 11a to be cylindrical in the region of close proximity to the blade tip 10a. This example shows the entire blade tip 10a in close proximity with the shroud barrel 11a. In other cases, the blade tip 10a is allowed to protrude from the barrel 11a (e.g., extending out to the left in
b shows a free-tipped axial fan assembly that is configured for use as an engine-cooling fan assembly where the shroud barrel 11b is flared, and the blade tip 10b conforms to the shape of the flared shroud barrel 11b. A tip gap 12b provides running clearance between the blade tips 10b and the shroud barrel 11b. As shown by the dashed line in
c is a swept view of a free-tipped fan with a constant-radius blade tip. The radius of the tip is R, and the radius of the hub is Rhub. If the hub has a non-cylindrical shape, Rhub can be defined as the hub radius at the blade trailing edge TE. The span of the blade S is the radial distance between the hub at the blade trailing edge and the blade tip, or (R−Rhub). The blade geometry can be described as a function of radial position r, often non-dimensionalized as r/R, or as a function of the spanwise position s, which is equal to (r−Rhub). The spanwise position can be non-dimensionalized as s/S. Both the radial position r and the spanwise position are defined as increasing in the radially outward direction.
c shows the axial position of a blade leading edge LE and a blade trailing edge TE plotted as a function of radial position r. The midchord line at a radial position r is shown to be axially midway between the leading and trailing edges at that radial position r. The midchord rake of the blade XMID at a radial position r is defined to be the axial distance of the midchord line at that radial position r from the position of the midchord line at the hub radius Rhub. The midchord rake angle ΘMID at a radial position r is the angle formed between a radial line and a line tangent to the midchord line at that radial position r. The rake XMID and the angle ΘMID are both shown to be positive at the arbitrary radial position r illustrated in
d is a swept view of a free-tipped fan with a blade tip that is flared to conform to a flared shroud barrel, as shown in
The conventions for defining radial position r and spanwise position s of any position along the blade are shown in
The diameter D of the fan is taken to be two times the fan radius, that is two times the blade tip radius R as shown in
a is an axial projection of a prior-art free-tipped fan, where the fan geometry is projected onto a plane normal to the fan's rotation axis. The fan has a constant-radius blade tip 10a. The rotation is clockwise, and the fan leading edge LE and trailing edge TE are as shown.
b is an axial projection of a single blade of the fan shown in
The sweep angle of an edge at a radial position r is the angle in an axial projection formed by a radial line to the edge at that radial position r and a line tangent to the edge at that radial position r. The sweep angle of the leading edge is shown in
The skew angle of an edge at a radial position r is the angle in an axial projection formed by a radial line to the edge at that radial position r and a radial line to the same edge at the blade root. The skew angle of the leading edge is shown in
a is an axial projection of a prior-art free-tipped fan with a blade tip that conforms to a flared shroud, as shown in
Although sweep angles are not labeled on the fan of
b is an axial projection of a single blade of the fan shown in
b shows the leading-edge sweep angle at the fan radius, ΛLE(R), to be approximately −62 degrees. Leading-edge sweep can reduce both tones and broadband noise, particularly turbulence-ingestion noise.
b also shows the radial position of the maximum skew angle of the leading edge, rΦLEmax, which is equal to about 0.625 times the fan radius R. The spanwise position of the maximum leading-edge skew angle, sΦLEmax, is about 0.375 times the blade span S.
c is a simple schematic of the forces due to rotation which act on the trailing-edge region of the tip of the blade shown in
a is an axial projection of a fan according to one construction of the present invention. It has a blade tip that conforms to a flared shroud. The rotation is clockwise. As in
b is an axial projection of a single blade of the fan shown in
The blade tip of
a is an axial projection of a fan according to one construction of the present invention. It has a blade tip that conforms to a flared shroud. The rotation is clockwise. As in
b is an axial projection of a single blade of the fan shown in
The blade tip of
A measure of the potential for noise reduction is the value of the leading-edge skew differential ΔΦLE. Although the fans of
U.S. Pat. No. 6,595,744 describes a rake distribution which minimizes the axial deflection of the blade tip. For a blade which is forward-swept at the root and back-swept at the tip, it prescribes a forward rake angle at the root, and a rearward rake angle at the tip. In order to maintain an axially compact fan geometry, the amount of forward sweep in the radially inner region should balance the amount of back sweep in the radially outer region. A measure of the amount of forward sweep in the radially inner region is the value of maximum skew angle of the leading edge, ΦLE(max). Although
The fans of
Although the fans of
Although the intersection between the leading edge and the blade tip is not shown to be locally rounded in
Although the fans of
Furthermore, the radial position of the maximum value of trailing-edge skew ΦTE(max) is not limited to that shown in the drawings, and can occur at any radial position r from the hub radius Rhub to the fan radius R, including those extremes.
Although the fans of
Although the benefits of the invention are generally greater when the fan assembly is in a puller configuration, fan assemblies according to the present invention can be in either a pusher or puller configuration, except where explicitly claimed otherwise.
This application claims priority to U.S. Provisional Patent Application No. 61/312,487 filed Mar. 10, 2010, the entire contents of which are hereby incorporated by reference.
Number | Date | Country | |
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61312487 | Mar 2010 | US |