Patent Publication Number: US-6902377-B2

Title: High performance axial fan

Description:
FIELD OF THE INVENTION 
     This invention relates to cooling fans for use in electronic cooling environments and, more particularly, to a high-performance fan with no intake restriction. 
     BACKGROUND OF THE INVENTION 
     A fan is an air pump, powered by a motor, which produces a volumetric flow of air at a certain pressure. The rotating portion of the fan, known as an impeller, comprises a hub with radiating blades that converts torque from the motor to increase static pressure across the hub. The increased static pressure increases the kinetic energy of the air particles, causing them to move. Fans are thus useful for air movement and ventilation. 
     Fans come in many forms. Axial fans include impellers that rotate to move large amounts of air at low pressure. The air moves in a direction parallel to the fan blade axis. Axial fans can produce a high rate of airflow and are inexpensive to produce, but are useful only in low-pressure environments. Further, axial fans are noisy when the ambient conditions are unfavorable, such as when there is insufficient air or when the airflow is blocked, such as in ductwork. 
     Centrifugal fans, also known,as blowers, also include rotating plates with radially extending blades, but blowers use centrifugal force to move the air. Airflow from the blower tends to be perpendicular to the blade axis, and at a lower flow rate than with axial fans. Centrifugal fans are more expensive to produce than axial fans and can generally operate at about four times the pressure of axial fans. 
     Although fans come in many varieties, higher-quality fans tend to operate more quietly and more efficiently. A good quality fan may include ball bearings for smoother operation of the impeller, and preferably has a snug fit between the blades and the fan housing, to ensure that leakage does not occur during operation. Care in manufacture, such as guaranteeing that each blade matches in size, weight, and configuration, may also improve fan efficiency. 
     The amount of airflow delivered by the fan is related to the fan&#39;s construction and placement. The number and length of the fan blades are important, as well as the distance of the fan from other objects and the speed of the fan motor. Ultimately, though the fan efficiency is determined by the design and arrangement of the fan blades. 
     Processor-based systems, such as desktop computers, generate a substantial amount of heat. These systems often include fans for the power supply, the hard disk drive, and one or more heat sinks placed on the heat-producing microprocessor. Surprisingly, little attention is paid to the design of the fan blades for these uses. The limitation in air intake within the processor-based system, as well as the increasing demand for more effective heat sinks makes the design of a fan in such systems of paramount concern. 
     Thus, there is a need for a fan assembly wherein the volume of air available for intake into the fan as well as the amount expelled from the fan is maximized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of a fan impeller according to some embodiments of the invention; 
         FIG. 2  is an isometric view of the fan impeller of  FIG. 1 ; 
         FIGS. 3A and 3B  are diagrams of airfoils according to the prior art; 
         FIGS. 4A-4C  are diagrams of NACA airfoils according to the prior art; 
         FIG. 5  is a graph of a fan curve according to the prior art; 
         FIG. 6  is a comparison graph of fan curves for both the fan impeller of  FIG. 1 and a  prior art fan; 
         FIG. 7  is an RPM vs. CFM graph superimposed on the prior art fan curve of  FIG. 4  according to the prior art; 
         FIG. 8  is an RPM vs. CFM graph superimposed on the fan curve for the fan impeller of  FIG. 1 ; and 
         FIG. 9  is an isometric view of the fan impeller of  FIG. 1 , including axial and centrifugal airflow lines. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with some embodiments described herein, a fan impeller is disclosed for maximizing both intake and expelled air during use. The impeller utilizes airfoil shapes to efficiently impart momentum to the surrounding air. The air expelled from fans using the disclosed impeller is at a higher pressure than can be delivered by comparably sized prior art fans. 
     The fan impeller employs a distinct airfoil shape for the fan blades to substantially move the ambient air. The use of airfoil-shaped as well as overlapping blades improve the blade lift and consequent mass flow and exit pressure. The blade stall is eliminated, as is evident in a smoother fan curve for the fan impeller relative to prior art fans. The blade sweep angle is optimally arranged to control the radial flow characteristics of the ambient air. Housing sidewalls are removed from the fan assembly to remove parasitic drag and improve the motion of air passing through the fan. 
     In the following detailed description, reference is made to the accompanying drawings, which show by way of illustration specific embodiments in which the invention may be practiced. However, it is to be understood that other embodiments will become apparent to those of ordinary skill in the art upon reading this disclosure. The following detailed description is, therefore, not to be construed in a limiting sense, as the scope of the present invention is defined by the claims. 
     In  FIGS. 1 and 2 , top and isometric views, respectively, of a fan impeller  100  are shown. The impeller  100  includes a plurality of blades  10  arranged around a hub  14 . Otherwise hidden edges of the blades  10  in the image of  FIG. 1  are made visible, for a more precise understanding of the blade arrangement. 
     The hub  14  of the impeller  100  is a cylindrical body to which the blades  10  are connected. The part of the blade that is closest to the hub, known as the blade root  58 , extends across the cylindrical walls of the hub  14 . (The part of the blade farthest from the hub is known as the blade tip  68 .) As shown in  FIG. 2 , the blade root  58  overlaps the bottom of the hub  14 . 
     The hub  14  is closed off at one end by a cover  30 , a flat, circular plate, that connects transverse to the top of the hub. A blade axle  12 , disposed at the center of the cover  30 , may be a rigid rod positioned orthogonal to the cover  30 . Upon turning the blade axle  12 , the fan impeller  100  rotates. Typically, the blade axle is powered by a motor (not shown). 
     The blades  10  have a leading edge  22 , a trailing edge  24 , an overlapping portion  18 , and a blade sweep angle  16 . The leading edge  22  is the portion of the blade that first makes contact with the ambient air, at a front intake area  78 . The trailing edge  24  is the portion that last makes contact with the ambient air, at a rear discharge area  88 . 
     Blade Geometry 
     The fan impeller  100  is designed for more efficient operation than typical fan impellers. The blade geometry is optimized to perform at a predetermined speed, or revolutions per minute (RPM) range. The blade sweep angle is optimally arranged to control radial flow characteristics of the ambient air. The airfoil design and the angle of the blades  10 , or blade angle, are designed for optimal performance of the fan impeller  100  at a specified operating condition. 
     Varying Cross-sectional Thickness 
     In contrast to typical fan impellers, in which the blades are of uniform thickness throughout, the blades  10  of the fan impeller  100  have varying cross-sectional thickness. In particular, a cross-section of the blades  10  reveals that the blades  10  are airfoil-shaped. An airfoil is a surface designed so that air flowing around it produces useful motion. Usually describing a cross-section of an airplane wing, airfoils are generally designed to produce lift. More broadly, airfoils are useful for efficiently controlling the flow of air around them. The shape of the airfoil affects the speed of air flowing both over and under the airfoil. Airfoil-shaped blades minimize airflow turbulence, maximize useful angles of attack, and reduce sound level problems. Airfoil properties are discussed in more detail, below. 
     Smooth Leading Edges 
     In addition to their airfoil shape, the blades  10  have rounded or smooth leading edges  22 . The smooth leading edges reduce blade drag, which improves the efficiency of the impeller  100 . Further, impeller blades with smooth leading edges tend to produce less noise than those without such a feature. 
     Concave Blades 
     The blades  10  of the fan impeller  100  are concave, when viewed from the leading edge  22 , to draw air toward the inside of the fan impeller. The cup shape of the blades provides a scooping effect, for improving the intake volume of air, which is pulled in radially as well as axially. The greater volume from which air can be drawn results in a relatively greater expelled volume by the impeller  100 , as compared to typical fan impellers. 
     Looking at  FIG. 1 , the intake air is described as axial where the air is received into the fan impeller  100  from behind. The intake air is described as radial where the air is received into the fan impeller from the sides. The fan impeller  100  utilizes both axial and radial intake air during operation. 
     Constant Blade Angle 
     The blades  10  have a constant or nearly constant blade angle. The blade angle is measured by connecting a line between the leading edge and the trailing edge of the blade (known as the chord), where that line then intersects with a horizontal plane when the hub  14  is disposed horizontally. (Blade angle  36  is shown in FIG.  2 ). In some prior art fan impellers, the blade angle varies in the radial direction, from root to tip, possibly to simplify manufacture and/or to produce uniform axial flow. The blade may twist, from root to tip, such that the blade angle at the tip is different from the blade angle at the root. In contrast, the blade angle of the fan impeller  100  at the blade root  58  and at the blade tip  68  are substantially similar to one another, or substantially constant. Put another way, the blades  10  of the fan impeller  100  do not twist from the root  58  to the tip  68 . 
     Trailing Edge 50% Longer than Leading Edge 
     The constancy of the blade angle  36 , from root to tip, results in a trailing edge  24  that is approximately fifty percent longer than the leading edge  22 . This substantially increases the blade area, which allows the fan impeller  100  to operate with increased lift, higher mass flow, and higher exit pressure. 
     Low Blade Angle 
     Furthermore, the blade angle  52  is low, relative to prior art fan impellers. The blade angle  52  may fall between 20 and 50 degrees, preferably between 30 and 40 degrees. In some embodiments, the blade angle  52  is 40 degrees. In some other embodiments, the blade angle  52  is 30 degrees. 
     Overlapping Blades 
     In the fan impeller  100 , the blade surfaces are overlapping, when viewing the fan impeller in the direction of the blade axle  12 , such as in FIG.  1 . Prior art fan impellers are generally designed such that the blades do not overlap when viewed from the blade axis  12 . This allows the impeller  100  to be pulled axially during manufacture (typically by plastic injection molding), simplifying the injection mold tool. The presence of blade overlap in the impeller  100  allows for constant blade angles and increases the blade surface area, at the cost of a slightly more complex plastic injection tool. 
     Blade Sweep Angle 
     In addition to having an overlapping portion  18 , in which the leading edge  22  of one blade overlaps the trailing edge  24  of an adjacent blade, the blade sweep angle  16  of the blades  10  may vary. 
     In the top view of  FIG. 1 , for a given blade  10 , the blade tip  68  leads, or precedes, the blade root  58 , going in the direction of rotation  50 . Thus, the blade  10  is “forward swept.” The blade sweep angle  16  is greater than 90°, but less than 180°. The triangular shape of the forward sweep emphasizes the blade tip  68 , resulting in a more even overall intake of air volume, and thus, less turbulent operation of the fan impeller  100 . 
     Alternatively, the blades  10  may be positioned such that there is no forward sweep. In other words, the blade tip  68  does not precede the blade root  58 , going in the direction of rotation  50 . Rather, the leading edge  22  extends substantially perpendicular from the hub  14 , such that the blade sweep angle  16  is approximately 90°. In such a configuration, the blade  10  is said to have “no sweep.” 
     As a further alternative, the blades  10  may be positioned such that the blade root  58  precedes the blade tip  68 , going in the direction of rotation  50 . The blade sweep angle  16  is greater than 180°, but less than 135°. The blade  10  is thus “backward swept.” The fan impeller  100  blades may be forward swept, backward swept, or may include no sweep, as indicated by the blade sweep angle  16 . 
     Airfoil Properties 
     As previously indicated, the blades  10  of the fan impeller  100  are airfoils. Airfoils  20 A and  20 B are depicted in  FIGS. 3A and 3B , respectively. Several features useful for discussing airfoils are illustrated: the leading edge  22  and the trailing edge  24 , already shown in the fan impeller  100 , a camber line  26 , a chord  28 , and a blade angle  36 . The leading edge  22  of the airfoil  20  is the portion that first makes contact with the surrounding air. The trailing edge  24  is the point at which airflow passing over the upper surface  32  meets with airflow passing over the lower surface  34  of the airfoil  20 . The chord  28  is an imaginary straight line drawn through the airfoil between the leading edge  22  and the trailing edge  24 . The camber line  26  follows the midpoint between the upper surface  32  and the lower surface  34 . As shown in  FIG. 3B , the blade angle  36  is formed by the intersection of the chord  28  and an imaginary horizontal plane  38 . 
     Lift  54  by the blade  10  is generated normal to the blade chord  28 . The lift force is an airfoil characteristic that is preferably increased for efficient impeller design. Lift  54  and drag  56  characteristics are largely dependent upon the airfoil shape and the blade angle  36 . The fan impeller  100  balances against an increase in backpressure or impedance by increasing the blade angle  36 . An increase in the blade angle  36  increases the lift force  54 , up to the point of blade stall, where the lift force decreases. In some embodiments, an optimal blade angle is achieved with the fan impeller  100 , such that stall (from too steep a blade angle) and ineffective lift (from too small a blade angle) are avoided. 
     The National Advisory Committee for Aeronautics (NACA) once maintained as classified a collection of airfoil geometries to be used for aeronautical development and other engineering analysis. (Created in 1915, the National Advisory Committee for Aeronautics operated as an agency of the United Stated Department of Defense until 1958.) Each NACA airfoil is generated by polynomials that represent the shape of the camber line and the thickness of the airfoil. 
     In  FIGS. 4A-4C , three airfoils, NACA 5404, NACA 6404, and NACA 7404, respectively, are depicted. A numbering system is used to classify each airfoil. In a four-digit airfoil, the first (left-most digit) number indicates the amount of bow in the camber line (as a percentage of the airfoil chord). The second number, adjacent to the first, indicates the location of the highest point in the bow as a percentage of the chord. The rightmost two digits indicate the amount of thickness to be added to the camber line as a percentage of the airfoil chord. 
     For the fan impeller  100 , the airfoil geometry, coefficients of lift, coefficients of drag, and pressure distribution of the blades are based on infinite length straight wings. Using one of the NACA geometries described, such as in  FIGS. 4A-4C , the blades  10  of the fan impeller  100  maintain stream-wise airflow relationships that ensure predictable airfoil performance for a radial configuration, according to some embodiments. 
     Elimination of Blade Stall 
     The blade features described above are designed for efficient operation of the fan impeller  100 . Additionally, a condition known as blade stall is minimized or eliminated in the fan impeller  100 . As backpressure or impedance is increased, the impeller balances against the impedance by increasing the angle of attack and, hence, increasing the lift force. At some impedance, however, the airfoil is unable to increase the lift, leading to flow separation. 
     To counter this effect, the blade angle is kept small in the impeller  100 , such that flow separation (or blade stall) is minimized or eliminated. Flow separation is a phenomenon that occurs when the airflow no longer follows the contour of the blade surface. The small blade angle allows the entire blade area to be utilized for lift, resulting in a substantially higher performing impeller and reduced noise generation, in some embodiments. 
     A “knee” in the fan curve of most fan impellers is the flow separation (or blade stall) point. As will be shown, below, the fan impeller  100  has no knee in its fan curve. Instead, the impeller  100  transitions smoothly from operating primarily from its airfoil lift characteristics to a simpler swirl scheme, for more efficient operation. 
     Fan Curve 
       FIG. 5  is a graph of a fan curve  40  for a typical prior art fan impeller. The fan curve  40  depicts airflow versus static pressure. A fan can deliver one quantity of airflow and one pressure in a given environment. Accordingly, at a relatively higher pressure, the prior art impeller delivers a relatively lower airflow, as shown in FIG.  5 . This is depicted as the swirl-dominant region  42  of the fan curve  40 . When the fan impeller operates in the swirl-dominant region  42 , the axial airflow is reduced by the back pressure while the rotational velocity of the fan is essentially unchanged. This results in air exiting the fan with a relatively higher swirl velocity and lower axial velocity. 
     The fan curve graph  40  also includes an airfoil-dominant region  44 . The airfoil-dominant region is the part of the fan curve  40  where the pressure is relatively low and the airflow is relatively high. When the impeller operates in the airfoil-dominant region  44 , the airflow is governed by the airfoil characteristics at that particular velocity. Typically, the impeller will operate somewhere between the swirl-dominant region  42  and the airfoil-dominant region  44 , shown in  FIG. 5  as the transition region  48 . 
     The fan curve  40  includes a knee  46  in the transition region  48 , at which point the relative airflow begins to drop, despite a drop in pressure. The knee  46  is the point at which many prior art fans become inefficient, as the fan speed (RPM) increases with little or no increase in pressure and a substantial loss in airflow. 
     The fan impeller  100  is designed with the inefficiencies of prior art fans in mind. The use of high-lift airfoil shapes in a curved and overlapping blade profile, the smooth leading edges  22 , and the blade position along the hub contribute to the success of the fan impeller  100 , as illustrated in the fan curve  60  of FIG.  6 . 
     In contrast to the prior art fan impeller curve  40 , the fan curve  60  for the impeller  100  provides a consistently higher airflow rate all along the curve. Further, the fan curve  60  has no visible knee, or increase in airflow without a corresponding decrease in static pressure, in the transition area between the swirl-dominant  42  and airfoil-dominant  44  regimes. In contrast, the knee  46  in the prior art fan curve  40  is evident. A significant improvement in impeller performance can be observed in the transition region  48  of the fan curve  60 , which is where fan impellers typically operate. 
     In  FIG. 7 , the flow separation of a typical prior art fan is illustrated. The graph depicts revolutions per minute versus cubic feet per minute (RPM vs. CFM), overlaid on the fan curve  40 . At the knee  46  of the fan curve  40 , the speed (RPM) increases significantly with little increase in pressure and a great loss in airflow. 
     The opposite effect can be seen with the fan impeller  100 , as illustrated in FIG.  8 . At the point in the graph where the transition occurs, the speed (RPM) rises less significantly. The speed then decreases as the fan impeller  100  continues to work against increasing impedance. The fan impeller  100  is able to work against the further increasing impedance by transitioning from an airfoil-dominant operation to a swirl-dominant operation. 
     No Housing Sidewall 
     The fan impeller  100  includes no housing sidewall. Prior art fan impellers typically have a housing that surrounds the blades and provides mechanical structure to the fan. The elimination of the fan housing sidewall ensures that the radial inlet flow path is available in addition to the axial inlet flow path. The availability of both axial inlet flow and radial inlet flow allows a smoother transition from airfoil-dominant to swirl-dominant behavior. 
     The radial inlet air travels a greater distance across the blades  10  than is typical for an axial inlet fan impeller. In the fan impeller  100 , the inlet air crosses the blades  10  along a diagonal. This reduces the pressure gradient (i.e., the same change in airflow momentum from inlet to exit occurs, but is applied across an increased length), which delays flow separation. 
     Further, eliminating the housing sidewalls removes any potential parasitic drag that the fan blades may encounter, due to the boundary layer on the sidewalls. This boundary layer will also impede the motion of air passing through the fan. 
     In  FIG. 9 , an isometric view of the fan impeller  100  shows the mid-plane of the impeller gap. Solid arrows show the swirl-dominant behavior of the impeller  100  while dashed arrows show the airfoil-dominant behavior. 
     Operating Environment 
     In some embodiments, the fan impeller  100  is used in conjunction with a heat sink assembly to transfer heat from a microprocessor or other heat-producing semiconductor device in a processor-based system. Heat sinks often employ fans to increase ambient airflow around the heat sink and the microprocessor. The fan replaces air recently heated by the heat sink assembly with cooler ambient air. The fan, therefore, generally improves the efficiency of the heat sink. 
     Typically, fans used in computing environments, such as those used with heat sinks, power supplies, and hard disk drives, are designed without considering the airfoil properties of the fan blades. This ignorance leads to fan designs that are highly inefficient and noisy. Instead, considerations such as simplifying the manufacture and minimizing the number of moving parts generally influence fan design in such systems. The lack of blade design consideration leads to highly inefficient fan operation. Where the inefficiently designed fan is coupled with a heat sink, the rating of the heat sink design is ultimately limited. 
     The attention to the blade geometry, as well as airfoil principles, makes the fan impeller  100  a preferred choice for use in conjunction with heat sinks. The fan impeller  100  may also be used in other electronic cooling environments, such as with power supplies or other heat-producing electronic equipment. The fan impeller  100  can also be part of an industrial environment, such as a factory or manufacturing facility. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.