Abstract:
A fan has an air conveying conduit ( 16 ) and a fan wheel ( 22 ) arranged therein, which wheel is rotatable about a central axis ( 25 ) and is formed with a central hub ( 20; 120 ) having an outer periphery ( 27 ) on which fan blades ( 26 ) are mounted. These extend with their radially outer rims ( 40 ) as far as a surface ( 17 ) that is substantially coaxial with the central axis ( 25 ) and delimits the air conveying conduit ( 16 ) externally. The blades ( 26 ) have a profile similar to an airfoil profile. A flow element ( 42 ) is provided along the radial outer edge ( 40 ) of a fan blade and serves as an obstacle to a compensating flow proceeding around that radial outer edge ( 40 ) from the delivery side to the intake side, and likewise has, in cross section, an airfoil profile. 
     Adjacent the front edge ( 28 ) and rear edge ( 36 ) of a blade ( 26 ), it has substantially the same outline as the adjacent part of the associated blade ( 26 ), and in a middle region ( 48 ) between the front and back edge is wider, by an approximately constant amount, than the adjacent part of the blade ( 26 ).

Description:
CROSS REFERENCE 
   This application is a section 371 of PCT/EP 2004/003916, filed 14 Apr. 2004, published 4 Nov. 2004 as WO 2004/094835-A1. 
   FIELD OF THE INVENTION 
   The present invention relates to a fan having an air conveying conduit and having a fan wheel arranged rotatably therein, the blades of which wheel are equipped, in the region of their external edges, with flow elements that have low resistance to the conveyed flow and that constitute an obstacle to the compensating flows proceeding around the outer edges of the blades from the delivery side to the intake side. 
   BACKGROUND 
   A fan having such flow elements is known from the commonly assigned DE 30 17 226 A and corresponding GB 2 050 530-A, HARMSEN. This These unexamined applications describes a variety of designs for such flow elements, in combination with fan blades stamped out of sheet metal. These flow elements reduce the leakage flow in a fan equipped therewith. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide a new fan that exhibits a reduced noise level, at least in a predetermined operating range. 
   According to a first aspect of the invention, this object is achieved by a fan in which the fan blades are sickle-shaped and are provided, adjacent their tips, with flow-pattern obstacles which minimize air leakage between the intake side of the fan and the delivery side of the fan. It has been shown that, surprisingly, in such a fan the fan noise decreases, in particular, in the so-called laminar region, i.e. with high conveying volumes and a relatively small pressure rise Δp. A noise reduction occurs with such a fan in the non-laminar region as well, i.e. with higher back pressures and smaller air quantities. A theoretical explanation might be that an air flow occurs along the sickle-shaped front edges of the fan blades, and this air flow flows practically as far as the outer periphery of the hub, where the circumferential velocity is lowest, and consequently little noise is generated by this flow. The degree of sickling is, of course, limited by the fact that with a very pronounced sickle shape, the axial length of such a fan might become too great. 
   The stated object is achieved in another way by providing ends of the fan blades with flow elements which themselves are airfoil-shaped and which, in a middle region between their front and back edges, are wider than an adjacent part of the fan blade. It has been shown that this type of configuration of the profile of the blade and flow element contributes to particularly quiet running of the fan. 

   
     BRIEF FIGURE DESCRIPTION 
     Further details and advantageous refinements of the invention are evident from the exemplifying embodiments, in no way to be understood as a limitation of the invention, that are described below and depicted in the drawings. 
     In the drawings: 
       FIG. 1  is a plan view of an equipment fan, in this case an axial fan, according to a first exemplifying embodiment of the invention; 
       FIG. 2  depicts the fan wheel of the fan of  FIG. 1  in an enlarged depiction; 
       FIG. 3  is a three-dimensional depiction of the fan wheel according to  FIGS. 1 and 2 ; 
       FIG. 4  is a side view of the fan wheel of  FIGS. 1 to 3 ; 
       FIG. 5  is a section viewed along line V-V of  FIG. 2 ; 
       FIG. 6  is a sagittal section through a blade of the fan of  FIGS. 1 to 5 , viewed along line VI-VI of  FIG. 2 ; 
       FIG. 7  is a section viewed along line VII-VII of  FIG. 2 , in an enlarged depiction; 
       FIG. 8  is a section analogous to  FIG. 7 , viewed along line VIII-VIII of  FIG. 2 ; 
       FIG. 9  is a section analogous to  FIG. 7 , viewed along line IX-IX of  FIG. 2 ; 
       FIG. 10  is a depiction of the acoustic pressure level Lp and pressure increase Δp plotted against the slider position of a test stand, for an axial fan whose fan blades have no flow elements on the outer edge; 
       FIG. 11  is a depiction analogous to  FIG. 10 , for a fan of the same construction but in which the fan blades are equipped on their outer edge with special flow elements; 
       FIG. 12  is a depiction comparing the curves in  FIGS. 10 and 11 ; it is apparent that, with this exemplifying embodiment, a reduction in the acoustic pressure level Lp is obtained in particularly pronounced fashion in the laminar region, but also in the turbulent region; 
       FIG. 13  is a plan view, analogous to  FIG. 2 , of a fan wheel  122  according to a second embodiment of the invention; 
       FIG. 14  is a three-dimensional depiction of fan wheel  122  of  FIG. 13  in a depiction analogous to  FIG. 3 ; and 
       FIG. 15  is a comparative depiction showing fan characteristic curves for fan wheel  122  according to  FIGS. 13 and 14 , with and without the special flow elements (winglets). 
   

   DETAILED DESCRIPTION 
   In the figures that follow, the same reference characters are used in each case for identical or identically functioning components, incremented by 100 if applicable (e.g. 122 instead of 22), and these components are usually described only once. 
     FIG. 1  shows an equipment fan  10  of ordinary design. The present invention can be realized implemented in an axial fan and, alternatively, in a diagonal fan. Fan  10 , depicted in  FIG. 1 , has an external housing  12 , at the four corners of which respective mounting openings  14  are provided and which defines in its interior an air conveying conduit  16 , which conduit is limited toward the outside by a rotation surface  17  and in which conduit is rotatably mounted, via struts  18 , the central hub  20  of a fan wheel  22  that, in operation, is rotated about a central axis  25  ( FIGS. 4 and 5 ) by an electric motor arranged inside this hub  20 . In  FIG. 1 , hub  20  rotates counterclockwise in the direction of an arrow  24 . The air flow is such that the air is blown out over struts  18 , i.e. through the back or “delivery” side of fan  10  with reference to  FIG. 1 . 
   As  FIGS. 1 to 5  show, five fan blades  26 , labeled  26 A to  26 E, are mounted on outer periphery  27  of hub  20 . In this exemplifying embodiment, the angular distance beta ( FIG. 2 ) from front edge  28 A of fan blade  26 A to front edge  28 B of blade  26 B is 74°. Blades  26  are distributed irregularly over the periphery of the hub in order to obtain a more pleasant frequency spectrum. The type of distribution depicted represents, of course, only a preferred embodiment. 
   As  FIGS. 1 to 3  show, front edges  28 A to  28 E of blades  26  are embodied in concave and sickle-shaped fashion. The rear edges of blades  26  are labeled  36 A to  36 E, and are convex. They are implemented in such a way that their intersection with struts  18  occurs in “grazing” fashion, i.e. “with a grazing intersection.” This means that, in most or all rotational positions and when viewed in plan, the imaginary intersection between a strut  18  and a rear edge  36  (which of course do not touch another) occurs at an angle as clearly shown, for example, in  FIG. 1 . This feature contributes to noise damping. 
   The radially outer edges of blades  26  are labeled  40 A to  40 E. As depicted in  FIG. 5 , these edges  40  are at a radial distance d from inner side  17  of external housing  12 . This “air gap” d should be as small as possible. If it is large, a considerable leakage flow flows through it from the delivery side to the intake side of fan  10 . 
   To reduce this air flow, the individual blades  26  are equipped in the region of their radially outer edges  40  with flow elements  42 A to  42 E, specifically with enlargements of outer blade edges  40 , which enlargements preferably extend in the axial direction toward the intake side and the delivery side. (With diagonal fans, it is preferable to use blades on which such flow elements are present only on the intake side.) As is evident from the sagittal sections of  FIGS. 6 to 9 , blades  26  have approximately the cross-sectional shape of an aircraft airfoil, i.e. front edge  28 C is round and relatively blunt. From there, the thickness D ( FIG. 6 ) of a blade  26  first increases and then decreases again toward rear edge  36 , and blade  26  tapers to a sharp rear edge  36 , in order to reduce or prevent the creation of eddies there, and consequently the creation of noise. 
   Flow elements  42  have an outline analogous to that of the associated blades (cf.  FIG. 6 ), i.e. they likewise taper to a sharp rear edge  36  and are rounded at front edge  28 ; and in intermediate region  48  between the region of front edge  28  and the region of rear edge  36 , they protrude beyond blade  26  by a substantially constant amount in the axial direction, as clearly shown by  FIGS. 5 and 6 . A smooth transition is provided at both ends, i.e. the constant amount smoothly decreases there to zero. 
   Flow elements  42 , in combination with the narrow air gap d ( FIG. 5 ), present an elevated resistance to the leakage flow that proceeds, during operation, around outer rim  40  of blades  26  from the delivery side to the intake side. 
   As is apparent in particular from  FIGS. 3 and 4 , the individual blades  26  are twisted, i.e. the location from which a blade  26 , so to speak, “grows” out of hub  20  has approximately the shape of a screw-thread segment, and outer edges  40  of blades are likewise shaped in the manner of a screw-thread segment, although, as depicted shown, the pitch of the screw-thread segments is greater in the region of hub  20  than in the region of the radially outer edges  40 . 
     FIG. 10  shows the pressure rise Δp 1  and acoustic pressure level Lp 1  for a fan whose blades  26  are not equipped with flow elements  42 . The curves were measured on an ordinary fan test stand in which an adjustable throttle (not shown) is arranged on the delivery side of fan  10 . The opening ODR of this throttle is indicated on the horizontal axis with values between 0 and 2500, “0” meaning that the throttle is closed. 
   It is apparent that for a throttle opening below 1000, fan  10  is working in the turbulent flow region, with the pressure Δp 1  and acoustic pressure level Lp 1  rising toward the left. 
   For values to the right of the value of 1000 for the throttle opening, i.e. as the throttle is opened further, the pressure Δp 1  decreases and the volume of air conveyed rises correspondingly, this being associated with a higher Lp 1 . 
     FIG. 11  shows curves for the exemplifying embodiment described here, i.e. the fan is the same as in  FIG. 10  but fan wheel  22  is equipped with the above-described flow elements  42 . 
   The profile of the pressure curve (Δp 2 ) is the same as in  FIG. 10 , but the acoustic pressure level Lp 2  is reduced by approximately 1.5 to 2 dB(A), especially in the region of larger throttle openings (approximately 1100 and up). 
   Curves Lp 1  and Lp 2  are largely coincident in the region around a throttle opening of 1000, but a drop in the acoustic pressure level is once again observable in the region below a throttle opening of 600. 
   The above-described flow elements  42  thus yield, without any additional effort, a reduction in acoustic pressure level Lp which is acoustically perceptible and whose magnitude depends on the working point at which the relevant fan  10  is operated. The sickling of front edges  28  likewise contributes to a diminution in noise. 
     FIGS. 13 and 14  show a fan wheel  122  according to a second, particularly preferred exemplifying embodiment of the invention, having a central hub  120 . The external housing of this fan wheel has the same shape as external housing  12  of  FIG. 1 , and is therefore not depicted again. The rotation direction is labeled  124 , i.e. fan wheel  122  rotates clockwise.  FIG. 14  is a view toward the intake side of fan wheel  122 . 
   As  FIGS. 13 and 14  show, five fan blades  126  labeled  126 A to  126 E are mounted on outer periphery  127  of hub  120 . Just as in the first exemplifying embodiment, these blades are distributed unevenly around periphery  127  of hub  120  in order to obtain a pleasant frequency spectrum for the fan noise. 
   As  FIGS. 13 and 14  show, front edges  128 A to  128 E of blades  126  are concave and strongly sickle-shaped in configuration. In this exemplifying embodiment outer end  130 A to  130 E of sickles  128  is preferably located, when viewed in rotation direction  124 , in front of transition point  132 A to  132 E of sickles  128  into hub  120 ; in particularly preferred fashion these transition points  132 A to  132 E are located all the way at the back with reference to rotation direction  124 , i.e. the entire sickle  128  extends, as depicted, from this transition point  132  forward in the rotation direction. This results, for example at transition point  132 A, in a value of approximately 78° for the angle alpha at which sickle edge  128 A emerges from hub  120 . This angle alpha is, for example, greater than 90° in FIGS.  1  to  12 ,. It should preferably be less than 90° and has preferred values between 70 and 90°, in particular between 75 and 85°. 
   As explained below with reference to measurement curves, this configuration yields a considerable additional noise reduction, but usually requires a larger axial extension of the fan than with the version according to  FIGS. 1 to 12 . 
   For comparison, it should be noted that in the case of fan wheel  22  according to  FIGS. 1 to 12 , outer end  30 A to  30 E of sickles  28  is located in each case on the same radius vector as inner end  32 A to  32 E, which yields an axially shorter construction but is less favorable for noise reduction than the version according to  FIGS. 13 to 15 , as is evident from a comparison of the measurement curves according to  FIG. 12  and  FIG. 15 . 
   The rear edges of blades  126 A to  126 E are labeled  136 A to  136 E and likewise have a more pronounced sickle-shaped curvature than in the version according to  FIGS. 1 to 12 . Their intersection with struts  18  of housing  12  once again occurs “with a grazing intersection,” as described in detail with reference to  FIGS. 1 to 12 . 
   It should be noted, in this context, that for the version according to  FIGS. 13 to 15 , a shape was used for the external housing such that struts  18  extend in mirror-image fashion with respect to  FIG. 1 . For example, in  FIG. 1  strut  18  extends from an outer point that would correspond to approximately 6 o&#39;clock on a clock face to an inner point that corresponds to approximately 8 o&#39;clock. In the version according to  FIGS. 13 to 15 , this strut would extend from an outer point corresponding to approximately 6 o&#39;clock to an inner point that corresponds to approximately 4 o&#39;clock. This results in the aforementioned “grazing intersection” for the fan wheels of  FIGS. 13 and 14 . 
   The outer radial edges of blades  126  are labeled  140 A to  140 E. Analogously to  FIG. 5 , these edges  140  are at a small radial distance d from the inner side of fan housing  12 . Through the gap thereby formed, a leakage flow flows from the delivery side to the intake side of the fan. 
   To reduce this air flow, the individual blades  126  are equipped in the region of their radially outer edges  140  with flow elements  142 A to  142 E that extend in the axial direction between the intake side and delivery side. 
   The shape of flow elements  142  may be very easily gathered from the depiction of  FIG. 14 , which very clearly shows, in particular, flow element  142 D and a portion of flow element  142 C. The contour of flow elements  142  is the same as described in detail with reference to  FIG. 6  for flow element  42 C, and the same applies to the profile of blades  126 , so that for this portion the reader may be referred to the description of  FIGS. 1 to 12 . In combination with the narrow air gap d ( FIG. 5 ), flow elements  142  present an increased resistance to the leakage flow that proceeds, during operation, around outer rim  140  of blades  126  from the delivery side to the intake side. 
   As is clearly evident from  FIG. 14 , the individual blades  126  are twisted, i.e. the location from which a blade  126 , so to speak, “grows” out of hub  120  has approximately the shape of a screw-thread segment, and outer edges  140  of blades  126  are likewise shaped in the manner of a screw-thread segment although, as depicted, the pitch is greater in the region of hub  120  than in the region of the radially outer edges  140 . 
     FIG. 15  shows, in comparative fashion, fan characteristic curves for fan wheel  122  without flow elements and for fan wheel  122  with flow elements  142 , with the same air gap d (as in the depictions of  FIGS. 1 to 12 ). The pressure rise for a fan wheel without flow elements  142  is labeled Δp 3 , and the pressure rise for the same fan wheel  122  with flow elements  142  is labeled Δp 4 . It is apparent that a slightly greater pressure rise Δp is obtained without flow elements  142 . 
   The acoustic pressure level for a fan wheel without flow elements is labeled Lp 3 , and the acoustic pressure level for the same fan wheel  122  with elements  142  is labeled Lp 4 . For this measurement, just as for  FIGS. 1 to 12 , the measurement microphone was located in front of the intake side of the fan at the axial height of the fan. 
   Comparing  FIG. 15  with  FIG. 12 , it is evident that the greater sickling of front edges  128 , in combination with flow elements  142 , has resulted here in a reduction in the acoustic pressure level Lp over the entire measurement range, that reduction being very pronounced especially in the laminar region. For practical use, the noise reduction depends on the region of the relevant fan&#39;s characteristic curve in which it is operated, as is common knowledge among those skilled in the art of fans. A physical reason for the noise reduction might be that an air flow can form in the region of the sickle-shaped front edges  128  and flow along an entire front edge  128  from outside to inside, and thus to a region with a low circumferential velocity, flow elements  142  having a positive influence on the beginning of this air flow. 
   A measurement of the acoustic power LWA for the version according to  FIGS. 13 to 15  has revealed that, particularly in the range of the middle-third frequencies from 5 to 20 kHz, it was possible to achieve a reduction in acoustic power as a result of the flow elements. In the region from 160 to 4000 Hz, on the other hand, the acoustic power values differ only slightly, i.e. it is rushing noise in particular that is reduced by flow elements  42  and  142 . 
   Many variants and modifications are, of course, possible within the scope of the present invention.