Fluid flow directing device

Abstract

The invention relates to a fluid flow directing device (fig.1, 12) for minimising or preventing stall in a fan (fig.1, 10), and a fan including the fluid flow directing device, wherein the fluid flow

Description

The present invention relates to a fluid flow directing device, in particular a fluid flow directing device for stall management in a fan. A problem with high-pressure fans, such as mixed-flow axial fans, is the occurrence of rotating stall which is understood to include circumferential distortion of fluid before it flows into the blades. A small unitary fan typically has a tile front adjacent the blades which, towards high pressures provides containment for a recirculating flow at the fan's inlet with the result that the fan cannot develop further pressure. This results in a levelling off in the performance of the fan. At the stall point, the relationship between pressure and speed for the fan breaks down. This means that passive constant volume techniques are not possible to implement and so it is necessary to resort detection techniques included in the fan such as external flow and / or pressure sensors for constant volume capability. This results in increased costs. Small unitary fans typically have inlet guide vanes to direct fluid flow that are located within the internal diameter of the fan grille. One such conventional grille 100 with inlet guide vanes is illustrated in Figure 12, which comprises a plurality of vanes 101 extending radially from a central hub 102, strengthened with circumferential rings 103, and axially limited so as not to extend beyond the outside surface profile of the grille 100. An object of the present invention is to reduce the adverse impact of stall on the overall performance and pressure-speed characteristic of an axial flow fan, and thereby enable comparatively inexpensive passive constant volume techniques to be used. SUMMARY OF THE INVENTION According to a first aspect, the present invention provides a fluid flow directing device for directing a flow of a fluid for a fan so as to prevent or at least minimize the occurrence of a stall effect, the fluid flow directing device comprising: a frame having an inner perimeter defining an opening and a front surface; a plurality of first members for guiding fluid flow, disposed in spaced relation on and / or around the inner perimeter of the frame; and at least one second member for generating turbulent fluid flow, disposed on and / or around the inner perimeter of the frame; the first members, and, optionally, the at least one second member, having surfaces projecting a distance into the opening, whereby a central portion of the opening is an open space allowing unobstructed fluid flow. According to a second aspect, the present invention provides a fan including: a casing including an fluid channel having a suction port on one side in an axial direction and a discharge port on the other side in the axial direction, an impeller including a plurality of blades and configured to rotate in the fluid channel, a fluid flow directing device upstream of the impeller, the fluid flow directing device comprising: a frame having an inner perimeter defining an opening and a front surface; a plurality of first members for guiding fluid flow, disposed in spaced relation on and / or around the inner perimeter of the frame; and at least one second member for generating turbulent fluid flow, disposed on and / or around the inner perimeter of the frame; the first members and, optionally, the at least one second member, having surfaces projecting into the opening a distance, whereby a central portion of the opening is an open space allowing unobstructed fluid flow. With this configuration, fluid which is drawn in towards the flow directing device is deflected by the second members and guided by the first members so that fluid flow to the inlet port is uniform or substantially uniform in the axial direction. The present invention also extends to an extraction unit incorporating the above-described fluid flow directing device, optionally the extraction unit is mounted to a ceiling and the flow device is adapted to extract air flow from the ceiling. In one embodiment, each first member has a surface that projects into the opening a distance that is less than 50%, preferably 10-40%, more preferably 20-30% of the width of the opening. In one embodiment, the first members are disposed at circumferentially spaced intervals, preferably equi-spaced intervals. In one embodiment, the first members are each disposed to project radially and axially. In one embodiment, the first members are elongate having a leading or upstream end and a trailing or downstream end, wherein the leading or upstream end is disposed on the front surface of the frame and the trailing or downstream end is disposed on the inner perimeter of the frame. In one embodiment, the first members each have opposed radially projecting and axially extending side surfaces for guiding fluid flow towards and / or through the opening. In this embodiment, channels formed between adjacent first members may communicate with the flow passage at the inlet port. In one embodiment, each first member is in the form of a vane. In one embodiment, the first members each project from the frame to a height that varies along its axial length. In one embodiment, the at least one second member is disposed on the front surface of the frame. In one embodiment, the at least one second member comprises a single, substantially annular member. In one embodiment, the one or more projecting surfaces of the second members extend continuously or at intervals in a circumferential direction. In one embodiment, two second members are provided and disposed coaxially. In one embodiment, two second members are provided, wherein one second member has a different axial height from the frame outer surface to the other second member. In one embodiment, the projecting surface of the at least one second member extends substantially perpendicularly from the inner perimeter and / or front surface of the frame. In order to form one or more projecting surfaces for generating turbulent flow, one or more ribs may preferably be formed on the front surface of the frame at a position off from the inner perimeter away from the impeller, the ribs projecting upwardly from the front surface of the frame and extending continuously or at intervals in the circumferential direction. In one embodiment, each first member has a trailing end section, a leading end section, and a middle section between the trailing and leading end sections, wherein the trailing end section includes a champfered edge portion and an axial edge portion, and the leading end section includes a tapered edge portion. In this embodiment, the tapered edge portion may define an acute angle 0v,le to a line perpendicular to the axial direction X. Preferably, the angle 0v,le is less than 45°, more preferably 20° to 40°. The champfered edge portion may define an acute angle 0v,te to a line perpendicular to the axial direction X. Preferably, the angle 0v,te is greater than 45°, more preferably 50° to 60°. In this embodiment, the middle section may have an outer edge in the form of an arc. In one embodiment, the first members and the at least one second member intersect on the frame outer surface. The first and second members may intersect at substantially right angles. The frame is preferably formed of molded plastic. The first and second members may be integral with the frame. The frame may curve outwardly upstream from the inner perimeter defining the opening and then curve downwardly and / or extend substantially perpendicularly to the frame outermost perimeter in relation to the axial direction. In this way, the front of the frame may form a curved or curved / flat outer surface profile. The present invention alleviates or eliminates developing or developed rotating stalls and allows for fan designs with high operating efficiency and performance. Further, the present invention provides a relatively inexpensive replacement for stall control devices currently used in fans, such as mixed flow fans, and the associated actuators and control algorithms. Since such a fluid flow directing device can easily replace a conventional grille, anti-stall measures can be implemented at a low cost. Preferred embodiments of the present invention will now be described hereinbelow by way of example only with reference to the accompanying drawings, in which: BRIEF DESCRIPTION OF THE FIGURES Figure 1 illustrates a front view of a fan, incorporating a fluid flow directing device in accordance with a preferred embodiment of the present invention; Figure 2 illustrates a sectional view in perspective of a fan incorporating a fluid flow directing device in accordance with a preferred embodiment of the present invention; Figure 3 illustrates a perspective view of the front side of a fluid flow directing device in accordance with a preferred embodiment of the present invention; Figure 4 illustrates the flow of fluid into and through the fluid flow directing device in accordance with a preferred embodiment of the present invention. This Figure shows how the fluid flow directing device of the present invention makes flow in the passage of the fan uniform, thereby preventing the occurrence of rotating stall; Figures 5a and 5b illustrate a sectional view (along section II-II in Figure 1) of a guide vane of the fluid flow directing device in accordance with a preferred embodiment of the present invention; Figure 6 illustrates a sectional view (along section III-III in Figure 1) of a turbulator of the fluid flow directing device in accordance with a preferred embodiment of the present invention; Figure 7 illustrates the stall region A on a plot of pressure vs flow rate (performance curve) for an existing axial flow fan; Figure 8 illustrates the stall region A on a plot of pressure vs speed for an existing axial flow fan; Figures 7 and 8 show the breakdown of the functional relationship between pressure and speed / flow at the stall region. This relationship is important for passive constant volume schemes to work effectively. The stall region is detrimental to both the performance and predictability of the fan; Figure 9 illustrates plots of pressure vs flow rate (performance curve) for a fan having a fluid flow directing device according to the present invention as compared to an existing axial flow fan; Figure 10 illustrates plots of pressure vs speed for a fan having the fluid flow directing device according to the present invention as compared to an existing axial flow fan; Figures 9 and 10 show that the fluid flow directing device according to the present invention increases the performance of a fan at / near the stall point and importantly recovers the functional relationship between pressure and flow rate / speed; Figure 11 illustrates plots of pressure vs speed for a fan having the fluid flow directing device according to the present invention as compared to both an existing axial flow fan and to a fan having a fluid flow directing device including guide vanes but no turbulator. This Figure shows the importance of the turbulator(s) alongside the guide vanes in order for the functional relationship between speed and pressure to be preserved across the entirety of the curve; and Figure 12 illustrates a conventional grille for a fan. The figures show that the fluid flow directing device of the present invention prevents a rotating stall from taking place during increasing or decreasing speed of the fan. Definitions For the purposes of this specification, the term "axial" refers to a direction parallel to the longitudinal axis of the fan casing (shown by the line X in Figure 2), the term "cross-sectional" refers to a direction perpendicular to the longitudinal axis of the fan casing, and the term "radial" refers to a direction extending radially from or towards the longitudinal axis of the fan casing. Within the context of the present application, the terms "comprises" and "comprising" are interpreted to mean "includes, among other things". These terms are not intended to be construed as "consists of only". Unless otherwise stated or indicated, the terms such as "front", "rear", "upper", "lower", "side", "horizontal", "vertical", and the like, are used as words of convenience to provide reference points and are not to be construed as limiting terms. The terms "about" and "substantially" means plus or minus 20%, more preferably plus or minus 10%, even more preferably plus or minus 5%, most preferably plus or minus 2%. DESCRIPTION OF THE PREFERRED EMBODIMENTS An axial flow fan according to an embodiment of the present invention will be described below with reference to the drawings. Figure 1 illustrates a front view of a fan or extraction unit 10 including a fluid flow directing device 12 according to one embodiment of the present invention. In this embodiment, the extraction unit 10 is mounted to a ceiling, and the flow directing device 12 is adapted to extract air from the ceiling area. As shown in Figure 2, the fan has a cylindrical casing 14. The casing 14 includes an air channel 16 having an inlet port 18 on one side in an axial direction of an axial line X and a discharge port (not shown) on the other side in the axial direction. The inner wall of the air channel 16 extends axially to the inlet port 18. The casing 14 houses an impeller 20, which comprises a hub 22 and a plurality of blades 24 fixed to the hub 22. The impeller 20 is disposed inside the air channel 16 at the side of the inlet port 18 and is configured to rotate in the air channel 16. The plurality of blades 24 are disposed at equidistant intervals in the circumferential direction of the hub 22. A motor (not shown) serving as a drive source for the impeller 20 is fixed inside the hub 22. The rotor blades 24 are located immediately or adjacently downstream, i.e., physically distanced but next to or close to, the air flow directing device 12. The air flow directing device 12 includes a frame 26 having a front surface 28, an inner perimeter 30 and an outer perimeter 32. The inner perimeter 30 of the frame 26 defines an opening 34 which lies adjacent to the inlet port 18. In this embodiment, the frame 26 curves outwardly initially from the inner perimeter in a direction away from the impeller 20 and then curves gently rearwardly to the outer perimeter 32 so that the front surface 28 of the frame 26 has a curved profile (see figure 2). The air flow directing device 12 comprises a plurality of first members 36, in this embodiment in the form of vanes, which are disposed at equi-distant intervals in circumferentially spaced relation around the axial line X, and two second members, 38a and 38b, in this embodiment in the form of annular ribs, which are disposed in axially and radially spaced relationship. In this embodiment, twelve guide vanes 36 upstream of the blades 24 are provided. In figure 2, the air flow directing device 12 is shown to have a protective cover 40 in place, extending over the front surface 28 and opening 34 of the frame 26 of the fan. The vanes 36 each have an upper edge 42, two opposed sides 44 that are exposed to air flow, and a lower edge 45 by which the vane 36 is attached to the outer surface 28 of the frame 26 (figure 1). As seen most clearly in Figure 5a, each vane 36 has a leading or upstream end section 46 disposed on the outer surface 28 of the frame 26 and a trailing or downstream end section 48 disposed at or within the inner perimeter 30 in the axial direction of the fan. The vane 36 has a middle section 50 between the leading and trailing end sections, 46, 48, of generally curved arc profile in longitudinal cross-section. The trailing end section 48 includes a chamfered edge 52 and an axial edge 54. The leading end section 46 includes a tapering / sloping edge 56. In this embodiment, two annular ribs 38a, 38b, including respective projecting surfaces for generating turbulent flow are formed on the frame 26, the projecting surfaces of the ribs 38a, 38b, extending upwardly from the front surface 28 of the frame 26 by, respectively, 4mm and 5 mm, and extending continuously in the circumferential direction of the inner perimeter 30. In this embodiment, the vanes 36 are attached to the frame 26 over the ribs 38a, 38b. In this embodiment, the trailing end section 48 of each vane 36 projects radially by 13 mm into the opening 34 defined by the inner perimeter 30. Each vane 36 projects into the opening 34 a limited distance from the inner perimeter 30 such that a central open space 58 is defined between upper edges of opposed radially extending vanes 36 at their trailing end sections 48. In this embodiment, each vane 36 has a maximum projection into the opening of around a fifth of the diameter of the opening 34 or inner perimeter 30. In operation, air sucked in by rotation of the blades, striking the projecting surfaces for generating turbulent flow and the vanes is partially disturbed and guided, with the result that flow is directed uniformly into the air channel 16, as shown in figure 4. EXAMPLE An example of a flow directing device according to the present invention is provided below, where the frame is referred to as a grille, the first member is referred to as a guide vane, and the second member is referred to as a turbulator. A guide vane is illustrated in Figure 5b with various parameters; definitions of the parameters are given in the table below. Tv / Tg Where rG is the outer radius of the grille and rv is the radial extent of the guide vane. h v,ax Axial extent of the guide vane measured from the grilles internal and furthest downstream surface. 0 v,LE Angle of guide vanes leading edge to the vertical. 0 v,TE Angle of guide vanes trailing edge to the vertical. R Radius of curvature of inlet guide vane profile (simply a circular arc profile). d v,i Inbound radial distance of vane upstream of rearward chamfer. d v,2 Inbound radial distance of vane downstream of rearward chamfer. d V,3 Axial distance upstream of rearward chamfer. nv Number of equispaced (in the circular pattern sense) guide vanes. tv Guide vane wall thickness. Values of the parameters for a particular embodiment are given in the table below. Guide Vanes Fg = 94.950mm: Parameter Units Value rv pg ul 0.579252 hv,ax mm 32.4 0v,LE Deg 45 0v,TE Deg 45.5 R mm 36.8 dv,i mm 13 dv,2 mm 8 dv,3 mm 2.5 nv ul 12 tv mm 1.3 A turbulator is illustrated in Figure 6 with various parameters; definitions of the parameters are given in the table below. Parameter Description rt,n Fg Where Fg is the outer radius of the grille and Ft is the inside radius of the turbulator, Ft,n is used here to indicate that a unique value for Ft is chosen for each turbulator i.e. flexible spacing. t Turbulator wall thickness. hrt Axial height of the turbulator measured from the outer grille to the top of the turbulator at Ft. nt Number of turbulators. Values of the parameters for a particular embodiment are given in the table below. 10 Turbulators Fg = 94.950mm: No. 1 - Parameter Units Value p / V__ 1 rG ul 0.393892 mm 1.4 hrt, / mm 5 nt ul 2 No. 2 “ Ft,2 Fg ul 0.498157 t2 mm 1.4 hrt..? mm 4 An optimum value of the size and shape of the projecting surfaces for reducing stall is determined according to the structure of the axial flow fan to be provided, such as blade numbers, shapes and sizes, etc. and may be obtained through simulation in the design stage. Accordingly, the projecting surfaces for generating turbulent flow and reducing swirl may arbitrarily be shaped and sized as long as occurrence of rotation of fluid flow at the surfaces of the blades can be prevented or restrained at the target operating point. The projecting surface for generating turbulent flow is not necessarily continuous in the circumferential direction as in the above described embodiment. Rather, one or more ribs may be formed on the outer surface of the frame that extend at intervals in the circumferential direction of the inner perimeter, thereby forming one or more projecting surfaces for generating turbulent flow at intervals in the circumferential direction. In this case, the interval between the projecting surfaces for generating turbulent flow may be appropriately determined according to the structure of the axial flow fan to be provided. While the one or more projecting surfaces for generating turbulent flow extend in the direction parallel to the axial line X in the above embodiment, the one or more projecting surfaces for generating turbulent flow do not necessarily extend in the direction parallel to the axial line X, and may be inclined, curved, or stepped. Thus, the projecting surfaces may arbitrarily be shaped as long as a required turbulent flow can be generated. It has been verified that an axial flow fan having a fluid flow directing device according to the present invention reduces stall, compared to an axial flow fan that is provided with the planar arrangement of the conventional grille, as represented in Figure 12, instead of the fluid flow directing device of the present invention when the two axial flow fans are operated at the same operating point. Although the reason for such reduction has not been sufficiently clarified yet, the inventors infer that a fluid drawn into the inlet port by the impeller is prevented by the guide vanes from forming large scale circulation regions, and the one or more projecting surfaces for generating turbulent flow provide sufficient mixing of fluid, such that the angle of incidence of fluid flow onto the impeller blades is sufficiently favourable to maintain efficient operation of the fan. It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the scope of the present invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications are covered by the appended claims. For example, the frame provided at the opening of the fluid flow channel is described as being round. In fact, any other shape could be made such as rectangular, square, oval or any sided polygon. In addition, although the present invention has been described in relation to the directing of an air flow, the present invention has equal application to other fluids, including liquids. Further, although the present invention has been described in relation to an infeed grille, the present invention has application to a discharge grille, and, indeed, flow directing elements within a flow channel. 26 02 25

Claims

1. A fluid flow directing device for minimising or preventing stall in a fan, the fluid flow directing device comprising:5 a frame having an inner perimeter, an outer perimeter and a frontsurface between the inner and outer perimeters, the inner perimeter defining an opening;a plurality of first members for guiding fluid flow, disposed in spaced relation on the frame; andio at least one second member for generating turbulent fluid flow,disposed on the front surface of the frame;the first members being disposed radially inward of the inner perimeter in the opening of the frame, whereby a central portion of the opening is an open space allowing unobstructed fluid flow.

152. A fluid flow directing device according to claim 1, wherein each first member has a surface that projects into the opening a distance that is less than 40%, or 10-30%, or 20%, of the width of the opening.20 3. A fluid flow directing device according to claim 1 or claim 2, whereinthe first members are disposed at circumferentially equal intervals on the frame.

4. A fluid flow directing device according to any one of the preceding 25 claims, wherein the at least one second member comprises a single, substantially annular member.

5. A fluid flow directing device according to any one of the preceding claims, wherein two or more second members are arranged co-axially to 30 each other.26 02 256. A fluid flow directing device according to any one of the preceding claims, wherein each first member is in the form of a vane.

7. A fluid flow directing device according to any one of the preceding5 claims, wherein the first members are elongate having a leading or upstream portion and a trailing ordownstream portion, wherein the leading or upstream portion is disposed on the front surface of the frame and the trailing or downstream portion is disposed on the inner perimeter of the frame.io8. A fluid flow directing device according to any one of the preceding claims, wherein adjacent first members define channels therebetween to direct fluid flow in the axial direction or along the longitudinal axis of the fan.

159. A fluid flow directing device according to any one of the preceding claims, wherein the projecting surface of the at least one second member extends substantially perpendicularly to the inner perimeter or front surface of the frame.2010. A fluid flow directing device according to any one of the preceding claims, wherein each first member has a trailing end section, a leading end section, and a middle section between the trailing and leading end sections, wherein the trailing end section includes a chamfered edge portion and an 25 axial edge portion, the leading end section includes a tapered edge portion, and the middle section includes an edge in the form of an arc.

11. A fluid flow directing device according to any one of the preceding claims, wherein the first member includes a leading end section having a 30 tapering portion that defines an acute angle 0 v, le to a line perpendicular to the axis X.

12. A fluid flow directing device according to any one of the preceding claims, wherein the first member includes a trailing end section having a chamfered edge that defines an acute angle 0 v,te to a line perpendicular to the axis X.

513. A fan including:a casing including a fluid channel having a suction port on one side in an axial direction and a discharge port on the other side in the axial direction, an impeller including a plurality of blades and configured to rotate io in the fluid channel,a fluid flow directing device according to any one of claims 1 to 12, upstream of the impeller.26 02 25

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