Abstract:
An air foil includes a leading edge, a trailing edge, a hub contour, and a tip contour, whereby the hub contour and tip contour extend between the leading and trailing edges to define a blade surface therebetween. A localized thick spot is disposed between the leading edge and the trailing edge and extends from a first point on the hub contour to a second point on the tip contour. The localized thick spot includes a generally sheared profile and is operable to provide the air foil with a desired strength while concurrently providing desired aerodynamic properties.

Description:
FIELD OF THE INVENTION 
     The present invention relates to impellers for compressors and pumps and the like, and more particularly, to an improved blade design for an impeller. 
     BACKGROUND OF THE INVENTION 
     Impellers are widely used in a variety of applications to compress a fluid. For example, impellers are often used in air compressor applications for use in generating compressed air to power pneumatic tools and the like. Alternatively, impellers are used to compress a fluid for use in a pressurized system such as in supplying a pressurized fluid stream for use on a fire truck or pumping station. Further yet, such impellers are commonly used in the design and operation of aircraft engines, whereby a compressed fluid stream is provided via an impeller to propel an airplane in a desired direction. In any of the foregoing applications, it is desirable to provide an impeller capable of operating under varying flow conditions to provide a continuous supply of pressurized fluid, regardless of external forces. 
     As can be appreciated from the foregoing discussion, impellers are operable to compress a fluid stream for use in a plurality of applications. As previously discussed, one such application is an air compressor. Conventional compressors typically include an impeller, a diffuser, and a volute, whereby the diffuser is in fluid communication with both the impeller and the volute and is operable to transfer a compressed air stream from the impeller to the volute for use in an external system. The impeller commonly includes a plurality of blades that are operable to receive and compress an external air stream between a hub of the impeller and a stationary shroud. Specifically, the impeller captures the external air stream at an inducer disposed proximate to a leading edge of each blade such that the captured mass air flow is forced between the hub and the stationary shroud through rotation of the impeller. The inducer is generally operable to capture the external air stream and force it between the hub and the stationary shroud as the impeller is rotated due to the generally curved or arcuate shape of the leading edge of each blade. 
     As can be appreciated, as the air stream travels between each of the blades, the shape of the shroud and hub are such that the air stream is compressed prior to reaching the volute. The compressed air stream is received into the diffuser for distribution to the volute prior to being used by an external application such as a pneumatic tool or a vehicle engine or a fuel cell. The diffuser commonly includes a plurality of stationary vanes which are operable to diffuse the air stream from the impeller in an effort to increase the static pressure of the compressed air. Such increases in static pressure generally increase the pressure of the air stream, thereby providing a desired output of pressurized air from the compressor. 
     In compressor design, it is increasingly important to deliver a constant stream of pressurized air to ensure proper operation of an external device. As can be appreciated, interruption of a compressor can cause external devices, such as pneumatic tools, to seize and abruptly stop working. A common occurrence of such compressor failure is impeller blade fracture or blade cracking due to stresses imparted on the impeller blades through compression of an air stream. Such blade facture or cracking impedes the performance of the impeller as the requisite pressurized air cannot be delivered without first replacing the fractured or cracked impeller blade. Conventional air compressors commonly include an impeller disposed within a sealed housing such that replacement and repair of the impeller commonly requires a significant amount of time and accompanying expense when blade fracture or cracking occurs. As can be appreciated, such repairs can be costly both from the standpoint of requiring a replacement impeller and also from the standpoint that the compressor is unusable until the requisite repairs can be completed. 
     To obviate the need for impeller repair, conventional impeller designs have commonly incorporated impeller blades having an increased thickness to stave off blade cracking and fracture. In most cases, such increases in blade thickness come with an aerodynamic penalty. More particularly, by increasing the thickness of each blade in an effort to improve strength characteristics and limit blade fracture and cracking, aerodynamic performance of the blade is sacrificed as thinner blade profiles typically provide for improved aerodynamic performance and efficiency. In this manner, conventional impellers, and impeller blades, suffer from the disadvantage of sacrificing aerodynamic performance to meet requisite strength characteristics. 
     Therefore, an impeller incorporating an airfoil or blade which provides adequate structural support while concurrently providing optimum aerodynamic performance of each airfoil or blade is desirable in the industry. 
     SUMMARY OF THE INVENTION 
     Accordingly, an air foil is provided and includes a leading edge, a trailing edge, a hub contour, and a tip contour, whereby the hub contour and tip contour extend between the leading and trailing edges to define a blade surface therebetween. A localized thick spot is disposed between the leading edge and the trailing edge and extends from a first point on the hub contour to a second point on the tip contour. The localized thick spot includes a generally sheared profile and is operable to provide the air foil with a desired strength while concurrently providing desired aerodynamic properties. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a sectional view of an air foil in accordance with the principals of the present invention incorporated into an impeller arrangement; 
         FIG. 2  is a graphical representation of the air foil of  FIG. 1 ; and 
         FIG. 3  is a perspective view of an impeller incorporating a plurality of air foils in accordance with the principals of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     With reference to the figures, an airfoil  10  of an impeller is provided and includes a leading edge  12  operable to capture airflow and a trailing edge  14  formed on an opposite end from the leading edge  12  and operable to receive the airflow from the leading edge  12 . In addition, the airfoil  10  includes hub contour  16  and a tip contour  18  extending between the leading edge  12  and trailing edge  14 . 
     The leading edge  12  and trailing edge  14  serve to define an overall length of the airfoil  10  while the hub contour  16  and tip contour  18  serve to define an overall height of the airfoil  10 , as best shown in  FIG. 1 . In this regard, the airfoil  10  includes a blade surface  20  extending along the length of the airfoil  10  between the leading edge  12  and the trailing edge  14  and between the hub contour  16  and tip contour  18 . The blade surface  20  is operable to receive airflow from the leading edge  12  and transmit the flow to the trailing edge  14 , as will be discussed further below. 
     The blade surface  20  defines the general shape of the airfoil  10 . In one embodiment, the blade surface  20  is a generally sweeping, arcuate surface, as best shown in  FIGS. 1 and 3 . In this regard, the blade surface  20  is operable to direct the airflow from the leading edge  12  to the trailing edge  14  and to transmit a force accompanying the airflow along the blade surface  20 . In this regard, the blade surface  20  is concurrently responsible for transmitting the airflow between the leading edge  12  and the trailing edge  14  and withstanding the accompanying forces associated with the flow of air. 
     The blade surface  20  further includes a localized thick spot  22  and a tapered surface  24 , as best shown in  FIGS. 1 and 3 . The localized thick spot  22  is formed integrally with the blade surface  20  and extends between the hub contour  16  and the tip contour  18 . More particularly, the localized thick spot  22  includes a hub junction  26  adjacent to, and abutting, the hub contour  16  and a tip junction  28  adjacent to the tip contour  18 . The hub junction  26  is formed a distance “X” away from the leading edge  12 , whereby the distance X is generally equivalent to 8–12% of a total length of the airfoil  10 . The tip junction  28  is formed a distance “Y” away from the leading edge  12 , whereby the distance Y is generally equivalent to 28–32% of the total length of the airfoil  10  as measured between the leading edge  12  and trailing edge  14 . 
     As described, the localized thick spot  22  includes a generally sheared or angular relationship relative to a mean axis  30  of the airfoil  10 , as shown in  FIGS. 1 and 2 . In this regard, the localized thick spot  22  crosses the mean  30  at a distance “Z” away from the leading edge  12 , whereby the distance Z is generally equivalent to 18–22% of the total length of the airfoil  10 . In other words, the distance Z is disposed generally between the X and Y positions, as best shown in  FIGS. 1 and 2 . In this manner, the localized thick spot  22  is formed at a sheared or angled profile relative to the central axis  30 . 
     The localized thick spot  22  is tapered between the hub contour  16  and tip contour  18 , as graphically illustrated in  FIG. 2 . In this regard, the thickness of the localized thick spot  22  is greatest at the hub contour  16  and tapers as the localized thick spot  22  approaches the tip contour  18 . Generally speaking, the thick spot  22  is reduced by a ratio of 2:1 moving from the hub contour  16  to the mean  30  and further reduced by a ratio of 2:1 moving from the mean  30  to the tip contour  18 . In other words, the hub contour  16  to mean  30  ratio is substantially 2.0 having an acceptable range of 1.75–2.25 while the mean  30  to tip contour  18  ratio is similarly 2.0 having an acceptable range of 1.75–2.25. 
       FIG. 2  is a graphical representation of the hub  12  to mean  30  ratio and mean  30  to tip  18  ratio and provides an example of each ratio. For example, if the hub contour  16  is assigned a normalized thickness value of 1.0, the mean  30  would then have a normalized thickness value substantially equal to 0.5 due to the 2:1 ratio, previously discussed. In addition, if the normalized thickness value of the mean  30  is 0.5, the normalized thickness value of the tip contour  18  is substantially equal to 0.25, as graphically demonstrated in  FIG. 2 . In this regard, the localized thick spot  22  extends from the hub contour  16  at its thickest point to the tip contour  18  at its thinnest point. 
     The tapered surface  24  of the airfoil is disposed adjacent to the thick spot  22  and extends along the length of the airfoil  10 , as shown in  FIG. 3  and graphically represented in  FIG. 2 . The tapered surface  24  similarly includes a hub  16  to mean  30  ratio of 2:1 and a mean  30  to tip  18  ratio of 2:1. In this manner, the hub contour  16  to mean  30  ratio is substantially 2.0 having an acceptable range of 1.75–2.25 while the mean  30  to tip contour  18  ratio is similarly 2.0 having an acceptable range of 1.75–2.25. 
     As previously discussed,  FIG. 2  is a graphical representation of the hub  16  to mean  30  ratio and mean  30  to tip  18  ratio. For example, if the hub contour  16  is assigned a normalized thickness value of 0.4 (as illustrated), the mean  30  would then have a normalized thickness value substantially equal to 0.2. In addition, if the normalized thickness value of the mean  30  is 0.2, the normalized thickness value of the tip contour  18  is substantially equal to 0.1, as graphically demonstrated in  FIG. 2 . In this regard, the tapered surface  24  extends from the hub contour  16  at its thickest point to the tip contour  18  at its thinnest point. As can be seen from  FIG. 2 , the localized thick spot  22  is approximately 2.5 times the thickness of the blade along the hub contour, mean axis and tip contour respectively. 
     With reference to  FIGS. 1 and 3 , the airfoil  10  is shown incorporated into an impeller  100 . The impeller  100  includes a hub  102 , a central axis of rotation  104 , and a plurality of airfoils  10  disposed radially around the hub  102 . The airfoils  10  are positioned around the hub  102  such that rotation of the impeller  100  around axis  104  causes the airfoils  10  to capture an air flow and transmit the air flow between the leading edge  12  and the trailing edge  14 . As can be appreciated, such movement of the air flow between the leading edge  12  and trailing edge  14  compresses the air to a predetermined pressure. 
     The pressurized air flow is commonly received by a collecting assembly  106  having a diffuser  110  and a volute  108 . The diffuser  110  and volute  108  cooperate to receive the pressurized air flow from the impeller  100  and deliver the pressurized stream to an external source. In this regard, the air flow is captured by the leading edge  12  of each airfoil  10  and is caused to travel along each airfoil  10  along the blade surface  20 . Such travel along the blade surface  20  imparts a force on the airfoil  10  as the air travels between the leading edge  12  and the trailing edge  14 . 
     Such forces are received by the blade surface  20  and are transmitted to the localized thick spot  22  to prevent fracture or cracking of the airfoil  10 . In this manner, the localized thick spot  22  strengthens the airfoil  10  at a predetermined location along the blade surface  20  to account for the air pressure forces. As the localized thick spot  22  is formed at a predetermined position along the blade surface  20 , the remainder of the blade surface  20  can be formed such that the aerodynamic performance of the airfoil  10  is optimized. In other words, the remainder of the airfoil  10  can be relatively thin without concern for fracture or cracking due to the position and thickness of the localized thick spot  22 . 
     While the airfoil  10  has been described in an impeller application, it should be understood that such an airfoil design is applicable to other forms of turbo machinery such as, but not limited to, turbines, pumps, fans, and blowers. In each of the foregoing applications, strength and aerodynamic performance of a blade or airfoil can be concurrently optimized due to the placement and nature of the localized thick spot  22 . 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.