Abstract:
An extraction fan rotor including a hub and a plurality of vanes extending radially therefrom, wherein said vanes have an aerofoil profile in cross-section.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the field of design and manufacture of extraction fans for domestic or commercial applications. In particular, the invention relates to an improved rotor blade configuration. 
       BACKGROUND OF THE INVENTION 
       [0002]    Air extraction fans are used in a number of domestic and commercial applications, such as in extracting poisonous gases from fume hoods, or, most commonly, extracting odours and vapour from kitchens, bathrooms and the like. 
         [0003]    Typically, these fans are mounted inside a duct and include a rotor, which is designed to impart mechanical energy to air inside the duct to impel air flow through the duct away from the room; and a stator downstream of the rotor which is designed to aid smooth airflow through the duct. 
         [0004]    However, many existing designs are not particularly efficient in performing this task. Testing of many prior art designs reveals that domestic kitchen extraction fans are particularly inefficient in moving air through the duct. Very few designs actually maintain significant operational downstream pressure, or suction at the duct inlet, often doing little more than internally agitate the air. Often the fan does little more than suck in some air, agitate it, actually blow it back out of the inlet and then re-extract it again—effectively setting up a recycling stream wherein only a small portion of the agitated air is actually impelled through the duct. 
         [0005]    Accordingly, it is an object of the invention to provide an extraction fan that efficiently meets the objective of such devices, by causing a substantially greater proportion of the air to be extracted from the source room and impelled away from the room to its target destination. 
       SUMMARY OF THE INVENTION 
       [0006]    According to a first aspect of the invention, there is provided an extraction fan rotor, said rotor including a hub and a plurality of vanes extending radially therefrom, wherein said vanes have an aerofoil profile in cross-section. 
         [0007]    The new and inventive use of an aerofoil profile for the vanes of an extraction fan provides a number of functional advantages. These include a significantly greater transfer of mechanical energy from the rotor to the air mass, thereby generating greater pressure and less noise. This is due mainly to the ability of a true aerofoil to maintain linear flow of air at the boundary layer along the vast majority of its surface. 
         [0008]    Preferably, the profile of said aerofoil corresponds to a Selig SG6043 aerofoil profile. 
         [0009]    Advantageously, the vanes are configured such that the angle of attack of the vanes increases with distance from the centre of the rotor. This appears to resemble a ‘twisting’ of the vane when viewed in a direction from the end of the vane toward the hub. This accounts for a different linear speed at each point of the vane and significantly improves the efficiency of the rotor. 
         [0010]    The extraction fan rotor of any preceding claim, wherein the hub has a substantially hemispherical profile. This profile provides particularly good performance, and in particular, where the leading edges of the vanes are coplanar with the apex of the hemispherical profile of the hub. 
         [0011]    According to another aspect of the invention, there is provided an extraction fan, said fan including a rotor according with that described above. It is advantageous if the rotor is couples with a downstream stator, wherein said stator includes a core and a plurality of vanes extending radially therefrom, and which make operative connection with the inner wall of a duct, in which the rotor and stator are housed. 
         [0012]    Preferably, the cross-sectional profile of said hub is approximately parabolic, and is operably arranged such that the diameter of said hub decreases in the direction of air flow. This profile has been found by the inventors to enhance the ability of the extraction fan to impart energy to the movement of air through the duct. It is thought that the parabolic curvature of the stator helps to reduce turbulence in the air flowing through said duct. 
         [0013]    Now will be described, with by way of a specific, non-limiting example, a preferred embodiment of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0014]      FIG. 1   a  is an end-on view of a rotor for an air extraction fan according to the invention. 
           [0015]      FIG. 1   b  is an end-on view of a rotor for an air extraction fan typical of the prior art. 
           [0016]      FIG. 2   a  is a side view of a rotor for an air extraction fan according to the invention. 
           [0017]      FIG. 2   b  is a side view of a rotor for an air extraction fan typical of the prior art. 
           [0018]      FIG. 3   a  is a detail view of a single vane of the rotor of  FIG. 2   a.    
           [0019]      FIG. 3   b  is a scale representation of the cross-sectional profile of the vane of  FIG. 3   a.    
           [0020]      FIG. 4  is a computer generated fluid dynamic model of turbulence generated by simple inclined plane rotor blade, typical of the prior art. 
           [0021]      FIG. 5  is a computer generated representation of modelled flow path lines for a conventional inclined plane rotor vane, typical of the prior art. 
           [0022]      FIG. 6  is a computer generated representation of modelled flow path lines for a rotor for an air extraction fan according to the invention. 
           [0023]      FIG. 7   a  is an isometric view cut-away view of a stator according to the invention mounted in a duct. 
           [0024]      FIG. 7   b  is an isometric view cut-away view of a stator typical of the prior art mounted in a duct. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0025]    The invention provides an improved air extraction fan rotor which alleviates the problems of the prior art principally via a superior vane profile design, and which also features an improved hub and vane configuration. 
         [0026]    The invention further provides an improved air extraction fan which incorporates said rotor and which advantageously further includes an improved stator design. 
         [0027]    Turning to  FIGS. 1   a  and  1   b,  there are contrasted a rotor  1  according to the invention and a rotor  1 ′ typical of the prior art. Both rotors feature a central hub ( 5 ,  5 ′), from which extend radially a plurality of vanes ( 10 ,  10 ′). It will be noted from these views that the hub  5  of the inventive rotor  1  is hemispherical in profile, and that the vanes  10  of the inventive rotor  1  are arranged such that their leading edges  15  are substantially coplanar with the centre of the hub  5 , which represents the apex of said hemisphere. 
         [0028]      FIGS. 2   a  and  2   b  show a side elevation of the rotors of  FIGS. 1   a  and  1   b,  respectively. Most notable in  FIG. 2   b  is the profile of the vanes  10 ′: they are curved and have a substantially consistent thickness. Other typical prior art vanes have a flat profile and are also of substantially consistent thickness. This is one of the main causes of aerodynamic inefficiency amongst prior art vane designs. 
         [0029]    This is illustrated in  FIG. 4 , which is a computer-modelled representation of the turbulence that results from rapid air flow over an ‘inclined plane’ vane. It will be noted from the figure that very substantial turbulent air flow is produced. The principal disadvantages of conventional inclined plane rotor vanes using is turbulence-induced cavitation and substantial form drag. Turbulence prevents the rotor vane from effectively transferring air across its entire span, and form drag increases the energy required to move a parcel of air from one side of the rotor vane to the other. 
         [0030]    This effect is further illustrated in  FIG. 5 . This is a representation of computer-modelled flow path lines  20  for air moving across a conventional inclined plane rotor vane  25  showing that boundary layer separation commences close to the leading edge  30  of the vane  25 . Boundary-layer separation is where the layer of air flowing at the boundary between air and the vane surface  35  (known as the boundary layer) actually separates from the vane surface, creating a region of vacuum  140 . This vacuum region  140  then tends to distort the flow of the layers of air nearest to it, which initiates turbulent air flow. 
         [0031]    By contrast, it will be noted from  FIG. 2   a  that the inventive rotor  1  has vanes  10  that have a profile that corresponds to an aerofoil, in this illustration a preferred Selig SG6043 aerofoil. This profile is illustrated in greater detail in  FIG. 3   b.  The profile is asymmetrical between the upper surface  40  and lower surface  45 , and is thicker toward the leading edge  50 , whilst thinning toward the trailing edge  55 . 
         [0032]    The inventive rotor vane has been designed to minimise turbulence. As illustrated in the representation of computer-modelled flow path lines in  FIG. 6 , air is drawn along the profile of the vane  60  and the varying cross section of the aerofoil reduces and/or delays separation of the boundary layer  65 . Delaying separation of the boundary layer  65  tends to significantly reduce, or eliminate, the onset of turbulent airflow, which makes more efficient use of the energy delivered by the motor to the rotor. 
         [0033]    As illustrated in  FIG. 2   a  and shown in greater detail in  FIG. 3   a,  the inventive vane  10  also appears to incorporate a ‘twist’, in that the angle of attack A (i.e. the angle at which the leading edge  50  of the vane strikes the air, relative to the direction of air flow) of the vane  10  becomes greater with distance from the hub  5 . This is in recognition of the fact that the relative instantaneous linear air speed at any point along the leading edge of the vane will be greater with distance from the centre of the hub. The change in angle of attack at different speeds assists in reducing the likelihood of creating turbulent flow across the vane surface. 
         [0034]    Turning to  FIGS. 7   a  and  7   b,  there is illustrated a cutaway view of an extraction fan ( 100 ,  100 ′) according to the invention and according to the prior art respectively. Both views are shown with the rotor removed. Both fans feature a duct ( 105 ,  105 ′), which houses a stator assembly ( 110 ,  110 ′). The stator assembly in each case is made up of a central core ( 115 ,  115 ′) which is attached to the inner surface of the duct ( 105 ,  105 ′) by a plurality of vanes ( 120 ,  120 ′). Air flows through the duct in the direction of arrows B and B′. 
         [0035]    However, the crucial difference is the particular profile of the core  115  in  FIG. 7   a.  The core  115  is substantially elongated by comparison with the prior art  115 ′. The cross-sectional profile of the core  115  is approximately parabolic. This is illustrated in greater detail in  FIG. 8 . This design has been determined to provide the least turbulent air flow through the duct  105 , especially by comparison with the approximately hemispherical prior art core  115 ′. 
         [0036]    It will be appreciated by those skilled in the art that the foregoing is merely one way in which the invention may be embodied. It will be understood by those skilled in the art that other embodiments may be conceived of which, while differing in some aspects, nevertheless fall within the scope of the invention and the claims appended hereto.