AXIAL FLOW FANS

Abstract: In a reversible flow fan each impeller blade has two aerofoil surfaces reversed with respect to each other, with the leading edge of each aerofoil surface arranged so that when the impeller is rotated in the direction in which that edge provides the leading edge of the blade, the surface lies on the suction side of the blade.

Description: - I - This invention relates o axial flow fans and to impellers therefor. There are a number of applications, for example in.road and railway tunnels, where it is required to drive the air in one or other direction depending upon the conditions that exist at the time and for such applications it is common to utilise reversible fans, ' that is to say fans in which the impellers can be driven selectively in either direction of rotation. Ideally the flow rate of the air in both directions should be equal for a given speed of rotation of the impeller. In order to achieve this blades with conventional aerofoil sections have been disposed alternately at 180° to each other but this results in a loss of efficiency. It is well known that a very high proportion of the work done by a conventional aerofoil section blade takes place due to the shaping of what is termed the upper or suction surface, The opposite or lower surface generally plays a minor role in that it only provides a contribution by way of a small positive pressure. -An object of the present invention is to provide a form of impeller blade for a reversible axial flow fan which can operate in either direction to provide a substantially equal but opposite flow of air with a higher efficiency and consequently a lower power consumption than can be obtained with existing forms of fan having conventional blade configurations. .According to the invention in a reversible axial flow fan each impeller blade has two aerofoil shaped surfaces reversed with respect to each other so that their leading edges are at opposite edges of the blades, with the leading edge of each aerofoil surface arranged so that when the impeller is rotated in the direction in which said edge provides the leading edge of the blade, the surface lies on the suction side of the blade. Preferably in each transverse section of the blade, the opposite surfaces of the blade have equal but oppositely disposed profiles, although the blade p ofil^_^j j ?~ξ- ay vary along the length of the blade. The pitch of the blades may be adjustable for providing a degree of control over the rate at which air is driven by the impeller at its operating speed. Although the blade pitch could be constant along the length of the blades it will, in general, decrease towards the tips of the blades to provide a generally uniform airflow over the area swept by the blades. One reversible axial flow fan in accordance with the invention will now be described by way of example with reference to Figures 1 to 3 of the accompanying schematic drawing, in which Figure 1 represents diagra matically a side view of the fan, and Figures 2 and 3 shows sections through a blade of the fan impeller at different distances from the axis of rotation of the impeller. Referring first to .Figure 1, the fan comprises an impeller consisting of a hub 1 carrying a plurality of radially extending blades 2 uniformly spaced around it, the hub being fixed to the shaft 3 of a reversible electric driving motor and the assembly being mounted coaxially within a short hollow cylindrical casing 5 carrying means (not shown) for securing the fan to a supporting structure such as the roof of a road or railway tunnel, with the axis of rotation of the impeller generall parallel to the tunnel axis. Depending upon conditions existing in the tunnel air may be required to be driven along the tunnel in either direction and this is achieved by causing the motor to rotate the impeller in one direction or the other. In accordance with the invention each blade has two identical aerofoil shaped surfaces reversed with respect to each other so that their leading edges are at opposite edges of the blade, each aerofoil surface being arranged so that-when the impeller is rotated in the direction in which a leading edge of the surface corresponds to the leading edge of the blade, the surface provides the suction surface of the blade as indicated in Figures 2 OR O and 3 ι which represent sections through a typical blade at two positions disposed nearer and further respectively from the inner end of the blade. The pitch of the blade reduces towards its outer end in known manner. For one blade in accordance with the invention approximately 150 mm in length the distances of the blade surface from the chord- line at various positions across the blade at two sections 75 and 150 mm respectively from the blade centre (impeller axis) are as shown below :- Blade Section Figure 2 Blade Station (Distance from Datum A) 0.0 0.33 2.29 8.0 17.15 Distance of Surface .a from Chord Line 0.0 1.04 2.82 5.82 7.10 Blade Station (Distance from Datum A) 28.640.0 1.43 62.87 7 .30 Distance of Surface a from Chord Line 8.308.94 9.00 8.50 7.57 Blade Station (Distance from Datum A) 85.7397-16108.60114.30 Distance of Surface a , from Chord Line 6.18 4.32 1.95 0.635 Blade Section Figure 3 Blade Station (Distance from Datum A)- 0.0 0.813 3-302 8.2516.50 Distance of Surface a , " from Chord in ^ e • 0.0. 1.422 2.90 .4.40 5-64 Blade Station (Distance from Datum A) 24.7733-0241.28 49.53 57-79 Distance of Surface _a from Chord Line 6.28 6.53 6.375 5.84 5.01 Blade Station (Distance from Datum A) 66.0474.3080.49 82.55 Distance of Surface a_ from Chord Line . 3-835 2.31 0.94 0.46 In each case the surface b_ has the same profile as the corresponding surface a. but the blade station distances OMPI t o IPO .-« - *r - are measured from Datum B, all dimensions being in millimetres. It will be see that the blade profile is not constant along the length of the blade, the chord length and the blade width becoming smaller towards the blade tip. However other blade profiles can alternatively be employed. It has been found that an axial flow impeller having blades of the form described operates at high efficiency with low power consumption when driven in either direction, compared with an impeller equipped with blades of conventional aerofoil sections but arranged alternately 180° to each other, or with symmetrically streamlined non-aerofoil shaped blades. The blades may be adjustable about respective radial axes relative to the hub to enable the blade pitch to be varied and provide a degree of control over the flow rate at the operating speed of the impeller. Any convenient means of adjustment may be provided. The invention includes within its scope not only reversible axial flow fans and impellers therefor constructed as described, but also individual blades having both surfaces of aerofoil shape, but reversed with respect to each other, for use in such fans and impellers. W V/

Claims: CLAIMS . . A reversible axial low f n characterised in that each impeller blade has two aerofoil shaped surfaces reversed with respect to each other so that their leading edges are at opposite edges of the blades, with the leading edge of each aerofoil surface arranged so that when the impeller is rotated in the direction • in which said edge provides the leading edge of the blade, the surface lies on the suction side of the blade. 2. A reversible axial flow fan according to Claim 1 characterised in that in each transverse section of the blade, the opposite surfaces of the blade have equal . but oppositely disposed profiles. ■ 3» A reversible axial flow fan according to Claim 2 characterised in that the profile of each blade varies along the length of the blade. 4. A reversible axial flow fan according to any preceding claim characterised in that the pitch of each blade is adjustable ' for providing a degree of control over the rate at which air is driven by the impeller at its operating speed. 5« blade for a reversible axial flow fan having two aerofoil shaped surfaces reversed with respect to each other so that their leading edges are at opposite edges of the blades. 6. An impeller for a reversible axial flow fan having a plurality of blades each having two aerofoil shaped surfaces reversed with respect to each other so that their leading edges are at opposite edges of the blades, with the leading edge of each aerofoil surface arranged so that when the impeller is rotated in the direct!on in which said edge provides the leading edge of the blade, the surface lies on the suction side of the blade. 7. A reversible flow axial flow ' fan substantially - jRE OMPI as shown in and as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawing, 8. A blade for an axial flow fan having profiles at 75 and 50to_ι from the blade centre substantially as shown in and as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawing. -<£ fREA OMPI

Inventors: SMITH T
Applicants: COLCHESTER WOODS, SMITH T, WOODS OF COLCHESTER LTD
Application year: 1979
Main classification: F04D29
IPC codes: F04D19, F04D29