GB2285485A - Housing for axial flow fan - Google Patents

Abstract

A cylindrical housing for an axial-flow fan, formed with an annular recess to provide an alternative path for air flow in the reverse direction when it occurs near the outer diameter of the fan blading 1. The recess contains vanes whose curvature about the housing axis 12 changes progressively from the inlet end of the recess to the outlet end, and which at least at the inlet end have a curvature about an axis 23 generally parallel to the housing axis, so that air which is travelling generally circumferentially as it enters the recess is initially deflected towards a radially-outward direction by encounter with the vanes. The recess may also contain a ring-shaped 7 fairing which physically separates the recess inlet from its outlet, and which may serve to diminish turbulence in the air travelling through the recess. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO HOUSINGS FOR AXIAL FLOW FANS This invention relates to the design of the housing of an axial flow fan, with the object of improving the stable range of volume or mass flow obtainable from that fan.

Figure 1 of the accompanying drawings is a graph indicating the relationship of pressure rise to volume flow at constant speed for a typical axial flow fan. From this Figure it is apparent that over part A ofthe graph pressure increases as volume falls, until a maximum pressure is reached at point B. Over the negative gradient of part A of the graph, the pressure/volume characteristic of the fan is stable. However when the volume flow falls below the maximum value B, and the graph enters the positive-gradient part C, flow instability usually results and normal operation is not possible. It is an objective of this invention to extend the stable operating range.When the point of instability or stall is reached it is common experience that a reverse flow - that is to say, a flow in the reverse direction relative to the main flow of incoming air to the fan - appears near the outer diameter of the blading of the fan. This is often confined to one or more cells which rotate in the same direction as the blades of the fan rotor, but at a lower speed. The cells may occupy only a part of the blade span or the total blade span, the latter being more common when the ratio of hub/tip diameter of the blades is high, say greater than 0.75, and the former when that ratio is lower.

The reverse flow proceeds upstream away from the rotor with a circumferential velocity of the same order as that of the moving blades, and a comparatively small axial velocity component.

The main incoming flow is necessarily diverted away from the cells of reverse flow, either towards the hub in the case of partial span cells, or to sectors between the cells in the case of full span cells. Where the reverse flow is mixed with the incoming flow to re-enter the fan it will produce a component of circumferential velocity in the direction of blade rotation. This will reduce the fan's ability to produce its normal pressure rise or volume flow.

There have been several prior proposals to overcome this harmful effect by forming an annular recess or chamber in the wall of the fan housing. In describing these prior proposals, and henceforth generally in this specification, the words "upstream" and "downstream" will relate always to the direction of the main flow of air through the fan.

Typically, in the prior proposals just referred to, one axial end - the inlet end - of that recess has been located close to the tips of the rotor blades and the opposite, outlet, end further upstream. This has allowed reverse flow, when generated near the blade tips, to escape into the recess instead of remaining closely upstream ofthe fan and so obstructing the incoming air. Once inside the recess, the reverse flow has tended both to continue moving in a reverse direction, and to lose some of its unwanted circumferential velocity. This air has then emerged back into main bore of the housing from the outlet end of the recess and rejoined the main flow of incoming air. Such prior proposals have also sometimes included designs of vanes to fit within such recesses, to guide the reverse flow air as it moves through them from inlet to outlet.Patent Specification GB-A-2124303 is one such proposal, and one of its teachings is apparently that the vanes, at least at the inlet end of the recess, should be curved about a radial axis - that is to say, a line intersecting the rotor axis at right angles - so that they have the effect of turning the direction of the incoming air from circumferential to axially-upstream. Patent Specification US-A4602410 and 4630993 show further proposals, in each of which it is clear that the vanes have a uniform profile which does not substantially change from one end of.the recess to the other..

The present invention arises from appreciating the advantages of "turning" the air within the recess in a different manner from that proposed in GB-A-2124303, and more progressively than would result from the constant-profile vanes of US-A-4602410 and 4630993. The invention is defined by the claims, the contents of which are to be read as included within the disclosure ofthe specification, and includes housings and fans as shown in the following further figures of drawings.The invention will now be described by way of example with reference to those further figures in which: Figure 2 is a cross-section through an axial flow fant Figure 3 is a section on the line A-A in Figure 2; Figure 4 is a cross-section on the line B-B in Figure 2; Figure 5 shows an alternative embodiment of the invention, in a view similar to that of Figure 3; Figure 6 shows a further embodiment of the invention, in a view similar to that of Figure 4, and Figure 7 shows yet a further embodiment of the invention, in a view similar to that of part of Figure 2.

In Figure 2 the fan consists of a row of rotating blades 1 followed downstream relative to direction 18 of the main flow of incoming air - by a typical row of stationary blades 2. The rotating blades are mounted on a disc 3 which is driven by a shaft 10 connected to a driving motor, turbine or other power source 11. Reference 12 indicates the common axis of the fan and of its surrounding casing or housing 5, in which an annular recess is formed. The downstream wall 13 of this recess lies in a radial plane relative to axis 12, and typically lies close to the mid-chord 14 of the blades 1. The recess extends upstream ofthe leading edge 15 ofthe blades by a distance substantially of the same order as the blade chord 16.The upstream wall 6 ofthe recess may be shaped as shown in Figure 2, if it is desired to impart an axially-downstream component of velocity to the air as it leaves the recess. Alternatively the upstream wall ofthe recess may be radial, as shown at 17 in Figure 7.

The radial depth of the recess is preferably related to the radial height of the rotor blades. The ratio h/H may for instance be between 0.1 and 0.3, where h is the radial depth of the recess and H is the radial height of the rotor blade, larger values being preferred for the ratio where space and other considerations permit.

The recess, and any guiding means associated with it, should be chosen so that at the inlet end of the recess, close to the wall 13, circulating air that would otherwise have exerted a hail reverse flow effect within the main bore of the housing enters the recess instead. The shaping of the recess, and of any guiding vanes within it, should be chosen to guide the air received from the blade tips in both an outwards and an upstream direction, and then to discharge the recirculated air back into the main flow at the upstream end of the recess with a radially-inward and possibly also some component of downstream velocity.

It is important that the cross-section of the flow as defined by the total structure of the recess should everywhere be adequate to pass the recirculating flow without throttling.

Reference 7 indicates a profiled ring mounted in the mouth ofthe recess to contribute to the guiding of the air as it recirculates within the recess, and also to provide a positive division between the inlet opening to the recess at its downstream end and the outlet opening at its upstream end. The inner radius ofthe ring may, as shown, substantially coincide with that ofthe housing 5. Reference 20 generally indicates one of several guiding vanes, mounted according to the invention at substantially equal angular intervals around the recess to help improve the flow of air recirculating within it.

Figures 3 to 6 show different configurations of guiding vanes 20 in more detail.

Each such vane is held in place within the recess by being attached, by welding or otherwise, along its lengthwise outer edge, to the outer wall 21 of the recess, the line of such attachment being generally parallel to the axis 12. Each vane may also be similarly attached to the recess end walls 13 and 6. The vanes are also similarly attached to the ring 7, and thus provide the means of support and positioning for that ring. In the embodiment of Figures 3 and 4, Figure 3 shows that at their inlet ends the vanes 20 show a curvature (radius 22) about an axis 23 lying perpendicular to the paper and therefore parallel to axis 12. With the fan blades 1 rotating as shown in direction 24, the effect of this curvature is clearly to start to turn the air entering the recess.That air, as it enters the recess at its inlet end 4a from the vicinity of the blade tips 8 (where it will be tra'-elling circumferentially, parallel to arrow 24) will start to turn to an outward radial direction 25. The tips 29 of the inlet ends ofthe vanes may typically lie at an angle of about 600 to the radius ofthe housing which passes through those tips. As the air then travels on in the upstream direction within the recess, towards the outlet end 4b, during which passage it is guided by the surfaces of the vanes 20, the outer wall 21 and ring 7 by which it is bounded, the angular setting ofthe outer part 26 of each vane relative to the wall 21 does not change.However the curvature of the inner part 27 changes progressively so that the outlet end tip (28, Figure 4) points substantially radially relative to the fan. Thus at the inlet end of the recess the inner parts of the vanes 20 start to "unwind" the entering air from a circumferential to a radial direction, and by the outlet end the vanes have finished the process so that the air enters the main incoming stream with little if any circumferential velocity. As Figure 2 shows best, the inner end 30 of recess end wall 6 and the confronting face 31 of ring 7 mav if desired be shaped so as to add a component of downstream motion to the air leaving the recess.In the alternative embodiment of Figures 5 and 6 the outer parts 32 of the vanes lie radially relative to the wall 21 throughout their length, instead of at an incline as at 26 in Figures 3 and 4, and at the inlet end ofthe recess the inner parts 33 ofthe vanes turn the air from circumferential to radial more sharply than the corresponding parts adjacent the tips 29 in Figure 3. At the outlet end ofthe recess the inner parts part 34 are straight and radial over their full radial depth, instead of reaching the radial only at their tips as in Figure 4 at 28.In broken lines at 35, Figure 5 also shows the option of slightly angling the tips of the inner parts of the vanes, at the recess outlet, to ensure a deliberate circumferential component of velocity, in either rotary direction, to the air as it leaves the recess. Such a component could be desirable, for instance where there is an inlet guide vane or other means to produce a similar whirl component of velocity in the main flow upstream of the rotor. Even without such whirl component some variation of the reverse flow discharge may be advantageous to optimise the performance improvement of the fan.

Figure 7 shows an alternative section of ring 7a, more rectangular than that of the ring 7 of Figure 2. The section 7a could be better suited to the vane shape of Figures 5 and 6 because it provides a less rapidly expanding cross-sectional area to the recirculating flow in the initial part of the vaned passage.

Although it is contemplated that the tips of the inlet ends of the vanes (as shown for instance at 29a in Figure 3) could lie at the same radius from axis 12 as the tips ofthe outlet ends (28, Figure 4), the alternative arrangement shown in full lines in Figures 3 and 4 may be preferable. In this arrangement the outlet end tips 28 lie at a radius substantially equal to or, as shown in Figures 2, 4 and 6, only slightly greater than that of the inner wall of housing 5. At the inlet end of the recess, however, the vane tips (as shown in Figure 5 and in full lines at 29 in Figure 3) may lie at a slightly greater radius so as to allow a greater clearance between them and the fan blade tips 8 and so help to avoid undesirable effects that could possibly result from too small a clearance between these two components moving at high relative velocity. The optimum radius for the tips of the vanes at their inlet end could, as indicated in Figures 2 and 7, substantially coincide with the mid-radius of the ring 7 or 7a.

Claims (13)

1. A cylindrical housing for an axial-flow fan adapted to generate a unidirectional main flow of air and formed with an annular recess to provide an alternative path for flow in the reverse direction when it occurs near the outer diameter of the fan blading (1), in which the recess extends from an inlet end (4a) of the alternative path to an outlet end (4b), and in which the recess contains a plurality of air-guidance vanes (20) circumferentiallyspaced from each other and operable to diminish circumferential velocity of the air as it travels within the recess from inlet to outlet, characterised in that at least at the inlet end of the recess the vanes have a curvature about an axis (23) generally parallel to the housing axis (12), whereby air entering the recess inlet after travelling generally circumferentially in the region of the blading tips is deflected towards a radially-outward direction by encounter with the said curvature of the vanes, and that the curvature of the vanes about the said axis changes progressively from the inlet (4a) to the outlet (4b) end.

2. A housing according to Claim 1 in which, at the outlet end of the recess, the radially-innermost end of each vane (34, Figure 6) lies in a substantially radial plane of the housing.

3. A housing according to Claim 1 or Claim 2 in which the radially-outermost part (32) of each vane, throughout its axial length, contacts and lies substantially at the same angle to the outer wall (21) of the recess.

4. A housing according to Claim 1 in which the axial end wall (6) of the recess, at the outlet end of that recess, is shaped so as to guide air leaving the recess with a component of velocity in the direction of the main flow of air.

5. A housing according to Claim 1 in which, between its inlet and outlet, the recess contains a ring-shaped fairing (7) coaxial with the housing and operative to diminish turbulence in the air as it travels through the recess, the fairing having substantial area when viewed in section in a plane including the fan axis

6. A housing according to Claim 1 in which the inlet end of the recess axially overlaps the rotor blading tips by a distance equal to substantially about 50% of the axial chord of the blading.

7. A housing according to Claim 1 in which the tip (29, Figure 3) of a vane, atthe inlet end of the recess, makes an angle of substantially 60O with a radius passing through both that tip and the fan axis.

8. A housing according to Claim 1 in which the tip of a vane, at the outlet end of the recess, is angled (35, Figure 6) so as to impart a limited circumferential component of velocity to the air leaving the recess.

9. A housing according to Claim 1 in which the tip of each vane at the inlet end of the recess lies at a greater radius than the corresponding tip at the outlet end.

10. A housing according to Claim 5 in which the tip of each vane, at the inlet end of the recess, lies at a radius substantially coinciding with the mid-radius of the fairing.

11. An axial flow fan including a bladed rotor, and a housing according to any of the preceding claims.

12. A housing according to Claim 1, substantially as described with reference to the accompanying drawings -

13. An axial flow fan according to Claim 11, substantially as described with reference to the accompanying drawings.