Patent Application
20180223861
The present invention relates to fans, and more particularly to flexible fan blades that operate over a large range of speed and pressure.
In conventional fan assemblies, a highly pitched, fixed-wing fan blade is efficient at low differential pressure with high output flow. However, the same highly pitched, fixed-wing fan blade stalls as the output flow approaches zero. At the point of stall, as the output flow decreases, the power input increases while the pressure increases very little or may decrease. This is equivalent to the stall of an airplane wing. When the angle of attack increases beyond a critical point, airflow across the top of the wing separates from the wing and continues without being deflected downward with the wing. Thus, because the airflow on the upper surface of the wing is not pulled downward by the wind, the wing is not pulled upward by the airflow above the wing. Thus, the plane loses lift, though the airflow on the lower surface of the wing continues to provide some lift as it is deflected downward.
For other fan assemblies, a low pitched, fixed-wing fan blade is efficient at high differential pressure with low output flow. No stall occurs. However, at low differential pressure, the same fan is inefficient and the output flow is low. The fan speed may be increased to increase the output flow, but the additional fan blade drag keeps the efficiency low and the power input high.
One design is to allow for variable pitch in the fan blade and hub assembly. This design provides for rotation of the fan blade along its longitude, thereby controlling the pitch. However, additional mechanisms must be provided to control the pitch according to differential pressure and/or fan speed. One disadvantage of this design is that the solid blade has a fixed helical twist (high pitch angle near the fan hub and lower pitch angle near the blade wingtip). The predetermined, helical twist is optimized for a particular angular position of the blade. As the solid blade is rotated to reduce the pitch under high differential pressure conditions, the pitch angle is reduced by the same amount along the length of the blade. Therefore, the pitch at the wingtip is overcompensated relative to the blade's pitch near the fan hub. Another disadvantage is the cost and maintenance of the mechanism to rotate each of the fan's blades, as well as the systems to control the rotation. Also, failure of these mechanisms and systems can cause great loss in critical, high-value applications.
Another design is to allow for flexibility in the wing of the fan blade itself. Some fans combine a rigid leading edge element with a curved, flexible wing element. The curved (cambered), flexible wing element trails the rigid leading edge and is sandwiched between and upper and lower portion of the rigid leading edge. The rigid leading edge is set at a fixed pitch. As the fan speed increases, thereby increasing the differential pressure (given the fixed system resistance coefficient), the flexible wing element is deflected away from the higher pressure side (the “lower” side as viewed as an airplane wing). The greatest degree of bending in the flexible wing element occurs where this flexible wing element connects to the rigid leading edge. Preloading (biasing) elements and/or limiters are provided to reduce localized stress and vibration, both of which could lead to failure.
One disadvantage of the above design is that the overall camber of the wing is more significantly reduced by the high differential pressure than the overall pitch of the wing. Thus, the lift that creates the differential pressure, generated by the angle of attach of the wing, is much greater than the lift generated by the camber of the wing under high differential pressure. Thus, this flexible fan blade can stall occur under high differential pressure, low flow conditions. Another disadvantage of this design is that the flexible wing element rubs against the preloading elements and/or limiters as it bends under high and low differential pressure or vibrates. Additionally, the preloading elements and/or limiters, located on the upper wing surface, affect the airflow over the airfoil and can contribute to the separation (stall) of airflow over the upper wing surface.
Yet another conventional design is a flexible fan blade that attaches directly to the fan hub, thus fixing both the camber and pitch of the wing near the fan hub. Between the fan hub and the wingtip, the leading edge is relatively rigid, while the curved, flexible trailing wing portion is deflected by the differential pressure. The fan wing is typically of one piece construction. While this design solves the problem of localized stress, rubbing and perturbed airflow as in the other designs described above, the wing pitch near the fan hub is fixed and can stall in this area. Also, the wingtip is subject to deflecting and vibrating about the blade's longitude, therefore limiting the safe speed and pressure differential of the fan.
Still yet another design includes a fan blade of flexible material attached to a rigid leading edge and includes materials of differing thermal expansion coefficients, whereby the blade curvature is increased by higher temperature and decreased by lower temperatures and aerodynamic lift on the blade. This type of fan is directed toward cooling of internal combustion engines. However, as with the other prior art designs, the overall camber of the wing is more significantly reduced by the high differential pressure than the overall pitch of the wing.
This document describes a fan blade with a flexible airfoil wing. The fan blades maintain high efficiency over a wide range of pressure differentials and output flow.
In one aspect, an apparatus includes a flexible fan blade including a main spar and a curved, flexible wing, the lower surface of the main spar connecting to a lower portion of the curved, flexible wing. The lower portion of the curved, flexible wing extends to a leading edge of the curved, flexible wing. The leading edge of the curved, flexible wing extends to an upper surface of the curved, flexible wing, thereby creating a flexible airfoil of the flexible fan blade.
In another aspect, a fan includes a plurality of flexible fan blades connected at the root end of each of a plurality of multiple main spars that are connected to a common fan hub. Each of the flexible fan blades includes a main spar and a curved, flexible wing, the lower surface of the main spar connecting to a lower portion of the curved, flexible wing, as described above.
The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims.
These and other aspects will now be described in detail with reference to the following drawings.
Like reference symbols in the various drawings indicate like elements.
This document describes a fan assembly including one or more fan blades having a flexible airfoil wing. In particular, a curved, flexible wing is connected to a main spar element located between the upper and lower portions of the curved, flexible wing element. The curved, flexible wing forms the entire upper surface of the wing, the entire leading edge of the wing, and a portion of the lower surface of the wing. As used herein, the terms “upper” and “lower” refer to the direction of the low pressure side and high pressure side of the fan, respectively.
The main spar is connected to the upper surface of the lower portion of the wing element at substantially the lower surface of the main spar (shown in
A main spar may be solid or hollow. The material composition, dimensions and wall thickness of the main spar are sufficient to resist aerodynamic forces of lift, drag and torsion. In some implementations, the main spar and flexible wing may be molded from a single mold so as to form one unit. The main spar may be cable-stayed or the like, by one or more cables connecting a point or points on the spar near the wingtip to the fan axis, such as the fan shaft, in order to increase the differential pressure capacity of the fan, and/or to otherwise decrease the axial load in the main spar itself.
The main spar may preload the wing's leading edge with internal torque to delay the deflection (bending) of the leading edge. This is accomplished with a main spar that is rounded near the leading edge of the wing with a radius of curvature greater than the relaxed radius of curvature of the leading edge of the wing. The main spar can be forced tight against the wing's leading edge, and then fastened to the upper surface of the lower portion of the wing element. The result of this implementation is to allow for a greater reduction in camber lift relative to angle of attack lift as the fan's differential pressure increases. Without the preloading, the camber lift remains relatively high compared to the angle of attack lift as the fan's differential pressure increases.
The flexible wing may be a composite of a thin, flexible material and an energy absorbing, vibration damping material. The energy absorbing, vibration damping material is preferably positioned inside the curve of the thin, flexible material, which would protect the energy damping material, especially at the leading edge of the wing.
The flexible wing may be of constant or varying thickness. If the wing thickness is greater in the area of the lower portion and the leading edge relative to the upper portion of the wing element, then the wing will exhibit a greater reduction in camber lift relative to angle of attack lift as the fan's differential pressure increases. If the thickness of the wing is less in the area of the lower portion and the leading edge relative to the upper portion of the wing, then the wing will exhibit a lesser reduction in camber lift relative to angle of attack lift as the fan's differential pressure increases.
Additionally, wing element thickness may vary from the wing root to the wingtip. If the wing thickness is less in the area of the wing root relative to the wingtip, then the wing root area will exhibit greater deflection as compared to a wing root of uniform thickness to the wingtip as the fan's differential pressure increases.
The flexible wing may be of constant or varying cord length. The aerodynamic lift of a section of wing is proportional to the cord length of that section for a given angle of attach and shape (i.e., camber as a percentage of cord length). The preferred implementation of a fan blade incorporates a wing with a greater cord length near the wing root than the wingtip in order to produce the fan differential pressure with a relatively low airspeed near the wing root.
The elasticity of a section of wing increases with an increased cord length of that section for a given shade. An exemplary preferred implementation of a flexible fan blade incorporates a wing with a greater cord length near the wing root than the wingtip in order to produce the greater wing deflection necessary near the wing root, thereby maintaining an ideal helical twist over the operating range of fan differential pressures.
A fan shroud with an expansion cone can be aligned axially with the fan blades so that the main spar is located at the bottom of the fan shroud, just above the expansion cone.
Furthermore, as the differential pressure increases, the wingtip near the trailing edge is deflected upward into the region of the fan shroud, which allows for the production of maximum differential pressure. Under these conditions, the expansion cone serves little purpose as the air velocity through the expansion cone is minimal.
The flexible wing may be a composite of flexible ribs and a flexible membrane. Each rib forms an airfoil cross-section of the wing, from the cross-section at the wing root to the cross-section at the wingtip. The upper surface of the lower portion of the ribs is connected to the main spar. Referring to
Attached ribs at the trailing edge of the wing reduce the deflection of the ribs toward the middle of the fan blade by the resultant tension, induced by the aerodynamic forces, in the flexible membrane. In contrast, floating ribs at the trailing edge of the wing allow for more independent deflection of the ribs, thereby allowing for a greater independence in wing deflection from wing root to wingtip.
Although a few embodiments have been described in detail above, other modifications are possible. Other embodiments may be within the scope of the following claims.
This application is a Continuation Application of U.S. patent application Ser. No. 14/233,371 filed Jan. 16, 2014, entitled FAN BLADE WITH FLEXIBLE AIRFOIL WING, which is a 371 National Stage Application of International Application No. PCT/US2012/047477 filed Jul. 19, 2012, entitled FAN BLADE WITH FLEXIBLE AIRFOIL WING, which claims benefit of Provisional Application No. 61/509,294 filed Jul. 19, 2011, entitled FAN BLADE WITH FLEXIBLE AIRFOIL WING, the disclosures of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61509294 | Jul 2011 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14233371 | Jan 2014 | US |
| Child | 15804946 | US |
