FLUID IMPELLER DIFFUSER AND METHOD OF OPERATION

Abstract: A rotary bladed impeller (1) is disposed at the transition between an entrance passageway (3) to an enlarged diameter diffusion duct (11) surrounding the impeller (1). An annular structure (5) is arranged upstream of the blade tips and projecting inwardly of the duct (11) to define the entrance passageway to guide fluid flowing therethrough within the rotary path of the blade tips. The duct (11) and structure (5) partly define an annular space (7) extending past the blade tips within the duct (11) within which is formed a blade tip driven annular vortex (6) turning so as to diffuse the flow passing through the impeller (1). The annular structure (5) may on its upstream surface be formed to serve as an inlet duct to the impeller without the need for extended inlet ducting, and the diffuser duct may be of other than circular cross-section, and also may be of short length e.g. between 20 and 50 % of the swept diameter of the impeller.

Description: "FLUID IMPELLER DIFFUSERS AND METHOD OF OPERATION" This invention relates to fluid flow diffusers of the type widely employed in association with axial flow impellers to convert kinetic energy in a fluid stream into static pressure. In the case of an axial flow fan, for example, the diffuser commonly has a conical form. The efficiency of such a diffuser form varies with the angle of divergence of the cone, and an included angle of about 15° is regarded as the optimum for many applications. Smaller angles than this increase the length of the diffuser with little further gain in efficiency, while greater angles progressively reduce the efficiency and increase the risk of flow detachment from the wall of the diffuser accompanied by a big loss in diffuser effect- iveness. A diffuser of 15° ' included angle in practice often proves to be a large structure which can be both expensive to manufacture and awkward to accommodate in a given installation. Because of these drawbacks, arising from the large size of conventional diffusers, many attempts have been made to make efficient diffusers of shorter length by finding means for keeping the flow attached to the walls of rapidly diverging passages. Various forms of boundary layer flow control have been shown to achieve this, but the complexity and cost involved in applying such means have prevented their popular adoption. Another means that has been examined has been the use of a standing ring vortex to diverge the flow passing through its linear axis. For example, if at the conjunction of two coaxial circular ducts of different diameters there could be established a standing ring vortex with its outside diameter the same as that of the larger duct and its inner coincident with the flow emerging from the smaller duct and its motion compatible therewith, then the action of the vortex would be expected to cause the flow from the smaller duct to follow the motion of the vortex and diverge to fill the full diameter of the larger duct. In practice a simple arrangement with a pair of coaxial circular ducts fails to produce an adequate vortex for the effect to be realised. One reason for this is that the vortex produced by the flow emerging from the smaller duct is of the forced type with the angular velocity within the vortex proportional to the radius. Since the pressure at the core of such a vortex is higher than at the periphery there is a tendency for the vortex to expand. What is finally produced in practice may only be a large slow recirculation of the outer flow in the larger duct, leaving the core of faster air discharged from the smaller duct with little effective diffusion. Other factors which affect the performance of such an arrangement are the nature of the boundary layer in the smaller duct and any non-uniformities in the flow. Nevertheless special arrangements of the geo¬ metry of the conjunction of two ducts can yield a stable ring vortex, at least in laboratory conditions and the introduction of certain flow control means can achieve the same result. While such constructions have served to demonstrate the effectiveness of a standing ring vortex for diffusing flow they have not produced a solution, widely acceptable in practice, to the problem. "It is an object to provide improved diffusion with a fluid impeller. If a ring vortex were of the free type, in which the angular velocity is inversely proportional to the radius, it would have a natural stability of its own for its core would be at a lower pressure than its periphery. I have found that in certain conditions the direct action of the tips of the blades of an axial flow impeller can be utilised to generate such a vortex which may then act to diffuse the flow discharged from the impeller. Accordingly the present invention consists in the provision of a structure upstream of the tips of the blades of an axial flow impeller so as partly to define a space in which a blade tip driven ring vortex turning so as to diffuse the flow which has passed through the said impeller can be established and be maintained in a stable condition. A number of other constructions involving fluid impellers with associated structures locating vortices at or in the vicinity of the blade tips are known, for example U.S. Patents 3 Oil 762, 3 872 916, 3 934 410, 4 061 188, 4 050 845, French Patents 2 034 406, 2 096 639 and British Patents 1 502 000, 2 034 435. In these examples there is either no significant diffusion process or because there is an enclosing duct in the plane of and close to the tips of the impeller blades or because the core of the vortex is filled with a structure or because the passage immediately upstream of the impeller, as defined by a duct or other boundary structure, is of greater diameter than the tips of the blades of the impeller or of the bounding streamline which will pass directly through the impeller close to the blade tips in a manner which would not generate a vortex as described in this invention. Further reference may be made to patents concerning a diffusion process, such as U.S. Patent 3 447 741, which involves the action of the tips of the blades of an impeller but does not involve a standing ring vortex, and British Patents 970 047 and 1 314 819 which concern diffusion by ring vortices but not in direct association with the tips of an impeller. The invention will now be described, by way of example, with reference to the accompanying partly diagrammatic drawings, in which:- Fig. 1 is a sectional elevation of a bladed impeller mounted coaxially at the junction of two circular ducts of different diameters and showing a ring vortex developed at the tips of the blades of the impeller; Fig. 2 is a sectional elevation of an embodiment of the invention applied to an axial flow fan; Figs. 3a, b, c, d are fragmentary sectional elevations of various structures which may be used to partly define a space in which the ring vortex develops; - _>- Fig. 4 is a sectional elevation of an embodiment of the invention applied to a two-stage fan; Fig. 5 is a sectional elevation of an embodiment of the invention applied to a fan with upstream guide vanes; Figs. 6a, b are sectional elevations of embodi¬ ments of the invention applied to fans with downstream guide vanes; Fig. 7 is a sectional elevation of a further example of the invention applied to a fan with down¬ stream guide vanes; Fig. 8 is a fragmentary partly sectional elevation of an embodiment of the invention applied to a fan for cooling an internal combustion engine; Fig. 9 is a sectional elevation of an embodiment of the invention applied to a ducted propeller for use in air, Fig. 10 is a fragmentary partly sectional elevation of an embodiment of the invention applied to a marine propeller; Figs. 11a, b, c and d are fragmentary sectional views of alternative duct arrangements, and Figs. 12a, b and c are fragmentary sectional elevations showing alternative impeller blade tip shapes. In Figure 1 a bladed impeller 1 driven by a motor 2 is located coaxially at the junction between two ducts 3,4 of which the duct 4 is of larger diameter than the duct 3 and which are joined by an annular C-section member 5 concave towards the duct 4. The diameter of the impeller 1 is slightly greater than that of the smaller duct 3 and the impeller 1 is arranged so that the tips of the blades 6 of the impeller 1 project into a space 7 within duct 4 which on its upstream face, as defined by the direction of the arrow 8, is bounded by the member 5. When the impeller is rotated so as to induce a flow in the direction of the arrow 8 the initial effect is for the tips of blades 6 to induce the development of a forced vortex in the aforesaid space 7. The tips of the blades 6 of the impeller 1 will shed free vortices of the usual type during this process and the absorption of these into the forced vortex will convert the latter into a stable free ring vortex 9 with a low pressure core 10. .The action of this vortex 9 imparts to the flow which is drawn through the smaller diameter duct 3 a motion which tends to deflect it radially outwards so that a streamline close to the walls of the ducts 3 and 4 will have an indicated path 11. As a result effective diffusion of fluid flow through the impeller is obtained. In the embodiment of Figure 2 the invention is applied to an axial flow fan which consists of a bladed impeller 12 driven by a motor 13 the two being coaxially mounted, by means of stays or other structure (not shown) in a circular duct 14 which is of substantially greater diameter than that of the impeller 12. An annular member 15 having a C- shaped cross-section, mounted inside the duct 14 immediately upstream of the impeller, has an outside diameter equal to that of the internal diameter of the duct 14 and an internal diameter is less than that of the impeller 16, so that tips of the blades of the impeller overlap a radially inner portion of the member 15. The annular member 15 is arranged with its concave side facing the blade tips and the downstream portion of duct -14 as illustrated in the Figure. When the impeller is rotated to drive the working fluid in the direction of the arrow 17 a flow pattern with a stable free ring vortex develops in the manner previously described in connection with Figure 1. Such a construction presents an efficient diffusion and is simpler than that of a conventional arrangement which requires a bell-mouth entry, a casing fitting closely over the blade tips and a conical diffuser. The annular member behind which the ring vortex forms may have a cross-section other than C-shaped. In the arrangement of Figure 3a the annular member has an S-shaped or sinuous cross-section 18 extending inwardly from the duct initially in arcuate convex manner in a downstream facing, rightward, direction, inwardly to a reversely arcuately curved inner portion concave towards the downstream direction and terminating radially inwards of the impeller blade tips 16. In the arrangement of Figure 3b, the annular member is formed with a linear cross- section 19 at an inclination 24 rearwardly and inwardly of the duct wall towards the blade tips 16. In the arrangement of Figure 3c the annular member has a cross-section 20 defined by two linear portions of which the outer extends inwardly at a steep angle and the inner at a less steep angle towards the blade tips 16. Whilst as shown in Figures 3b and 3c the blade tips 16 terminate slightly inwardly of the innermost portions of the annular members 19,20 it will be appreciated that in operation the flow of fluid past the annular member will be such as to cause a well developed vena contracta to form immediat- ely downstream of the annular member. The blade tips 16 are arranged to project outwardly of the boundary of the vena contracta and they generate a free ring vortex as previously described. Such an arrangement may be advantageous in installations where access to the downstream side is limited since it is possible to pass the complete impeller through the aperture provided by the annular member for purposes of assembly or removal. In the example of Figure 3b the angle 24 may be a right -angle and in that of Figure 3c the outer section may be at right angles to the duct wall and to the inner section of the member 20. In the arrangement of Figure 3d the annular member has a section 21 of generally J-form with a radially outer section extending radially through the duct wall and being secured between flanges 23 to facilitate assembly and disassembly. The radially inner portion extends rearwardly in arcuate manner, concave towards the downstream direction. The invention may be applied in the manner here- tofore described at each stage of a multistage fan -9- with contraction between stages, or at some stages and not others or at the final stage only. An applic¬ ation to the final stage of a two-stage fan is illus¬ trated in Figure 4. It will often be desirable for the impellers at each stage to be of the same diameter. Two such impellers 26,27 are shown mounted at the opposite ends of a pair of back-to-back motors 28,29. The upstream impeller 26 is provided with a suitably shaped inlet 30 and an annular casing 31 encloses this impeller and extends rearwards over the motors. Upstream of the second impeller 27 the casing 31 is provided with a short inward tapering conical section 32 which deflects the approaching stream inwards so that the tips of the second impeller 27 can produce a free ring vortex as in the previous examples. A C-section annular member 33 is provided to locate this vortex as previously described. Guidevanes are commonly employed in connection with axial flow fans and these may be placed upstream or downstream of the impeller. The example shown in Figure 5 has vanes 34 upstream of the impeller, which are enclosed within a casing 35. The vortex may be allowed to form outside this casing and behind a C-sectioned annular member 36. In order to reduce the size of this vortex a further C-sectioned annular member 37 (shown dashed) may be introduced into the space between the outside duct 38 and the inner casing 35 for the guidevanes 34. In this, as in other examples the shape of the members 36 and 37 is not limited to C-section. When the guidevanes are downstream of the impeller various means may be employed to permit their effective operation. In Figures 6a and 6b two possibilities are illustrated and a third is shown in Figure 7. In Figure 6a the guidevanes are enclosed in a short annular duct 39 which is of smaller diameter than the impeller blades the tips 40 which generate a vortex outside the said short duct 39. In Figure 6b the short duct is omitted, the flow being free to pass radially outward between the vanes 41 according to the arrows 42. In Figure 7 the guidevanes 43 are placed further downstream after the process of diffusion has been largely completed. Combinations of these arrangements may be employed. In the embodiment of Figure 7 the inlet ducting on the upstream side of the annular member 36 is omitted and it is to be understood that the outer, upstream surface of the annular member 36 which serves on its inner, downstream, side to define the vortex chamber, can serve as an inlet duct to the impeller, as in Figure 7, without the need for extended inlet ducting. An advantageous application of the invention is shown in Figure 8 where a fan used to draw air through a radiator for cooling an internal combustion engine. This is particularly useful in a vehicle where commonly such a fan is mounted in a confined space between the engine and the radiator and there is insufficient room to install a diffuser for pressure recovery. When the fan is mounted directly on the engine it may not be practical to install a close fitting cowl or duct in the plane of the fan blades to prevent recirculation of air because of the differ¬ ential movement between the freely mounted engine and the rigidly mounted radiator. Due to these features such fans tend to be inefficient and wasteful of power. The present invention can be applied to these fans due to the very short length of diffuser necessary and the elimination of wasteful recirculation through the main part of the fan disc. Accordingly, as shown in Figure 8 a shaped member 44 is fitted behind the radiator 45 to provide an entry nozzle directing the flow into the fan 46 mounted on the engine 47. A C-sectioned annular member 48 is mounted on the end of the shaped member 44 such that the inner lip 49 of the C-section directs the flow into the fan inside the path swept by the blade tips which generate a free ring vortex within the C-section. The outer lip of the C-section is continued rearwards in the form of a short parallel duct 50 past the blades. It will be understood that the exact form the invention takes in this application will depend on the space available for its installation and the performance required from the fan. The invention may further be applied to provide a diffusing shroud for a propeller used in air or other fluid. In Figure 9 an example of an installation suitable for a hovercraft is shown in which a propeller 51 mounted on a pylon 52 above a deck 53 is surrouned by a shroud 54. The internal surface of the shroud is provided with an annular curved concave recess 55 into which the tips of the propeller blades 56 project and in which a blade-tip generated free ring vortex forms as previously described when the propeller rotates in a direction which drives the ambient air in the direction of the arrows 57. The passage from the curved recess to the trailing edge of the shroud 58 may be parallel to or diverge from the common axis 59 of the propeller and shroud or it may converge, as shown at 60 in the drawing. The lower part of the shroud may be attached to the pylon by means of a bracket 61 or otherwise and by radial stays (not shown) . If the propeller is of the variable pitch type sufficient clearance 62 must be provided between the blade tips 56 and the inner lip 63 of the recess to accommodate the movement and any deflect- tion the blade may suffer during operation. In Figure 10 a marine installation is shown. The shroud 63 mounted between the underside of the stern S4 and the skeg 65 has an internal cut-out similar to that of the - previous example of Figure 8 with the tips 66 of the propeller blades projecting into it. Shroud forms of this type allow a considerably greater rate of diffusion to be achieved over a given axial length than is possible with a conventional form and the substantial radial clearance between the blade tips and the inner surface of the shroud make installation and removal of the propeller with the shroud in place a good deal easier. In both this and the previous example the shroud may be of considerably shorter axial length than a corresponding conventional shroud. In this application the invention not only provides improved diffusion but by protecting the tips of the blades provides a safety feature. Such an arrangement finds advantageous application in, for example, high power outboard motors where both performance and safety are important . The shape of the cross-section of the shroud is not confined to those of Figures 9 and 10 but may be adapted to the requirements of a particular installation for reasons of performance, strength or simplicity of construction. Some further examples of shroud cross-sections are shown in Figures 11a, b, c. in the shroud of Figure 11a a faired leading outer end 68 is provided from which an outer duct surface 67 extends rearwardly and inwardly at a shallow inclination, and a forward, upstream facing surface 69 extends inwardly and rearwardly at a steeper general inclination in arcuately convex manner. Rearwardly of the surface 69 is formed the curved recess 55 defining the vortex chamber and merging in faired manner at 63 with the inner end of surface 69. The upper surface of the recess 55 extends rearwardly and inwardly at a slight inclination as an inner duct surface 60 converging with the outer duct surface in faired manner. In the shroud of Figure lib the outer duct surface 67 extends rearwardly and inwardly in steeper manner than in Figure 11a to define a shorter duct length. In the shroud of Figure lie the duct 60 is of uniform diameter and at its forward end is formed with an inwardly projecting annular portion 69 of C-section, concave rearwardly. In the shroud of Figure lid the duct 60 extends rearwardly from a C-section annular member 69 inwardly inclined manner, and at a rear portion is formed with an internal step 73 from which an inner wall surface 74 extends rearwardly as a uniform internal diameter duct section. In some installations and in particular with marine propellers the flow into the propeller may be far from uniform and it may be desirable to vary the cross-sectional shape of the shroud around its periphery. Both the outside and inside profile may be varied, the former chiefly in relation to the local approach velocity of the fluid and the latter to control the motion of the core of the ring vortex. In certain circumstances when the working fluid is a liquid such as water the core of the vortex may cavitate and it may be desirable to vary the inside profile and in particular the cross-sectional shape or area of the recess to control the cavitation, for example to prevent it collapsing at any point within the shroud : It may also be noted that the ring vortex in addition to having the circulatory motion as illustrated by the ellipse 9 in Figure 1 will also have a further rotational motion in the plane of the impeller disc whereby all the fluid in the vortex rotates as an annulus in the same direction as the tips of the impeller blades but at a lower speed. One of the results of this motion as an annulus will be to tend to maintain a vortex of uniform strength right round the periphery, which otherwise might not be the case if the impeller was working in a non-uniform fluid stream. Various modifications may be made to the tips of the impeller blades for operation in conjunction with the various examples given, and some are shown in Figures 12a, b, c. In Figure 12a a portion of the leading part 71 of the blade tip is cut away leaving only the rear part 70 of the tip to generate the vortex. Such an arrangement may be useful with variable pitch blades and if the axis of rotation of the blades is along the radial line shown by the chain dotted line at the leading edge of the tip portion 70 the clearance between the moving and fixed parts will remain substantially constant. In Figure 12b the reverse arrangement is adopted with the rear part 70 of the tip cut away leaving only the leading part 71 to energise the vortex. This arrangement may help to reduce the axial distance occupied by the vortex, but it may allow some of the vorticity shed from the rear of the cut-away portion to pass into the diffusing stream. An arrangement with a sloping tip 72 is shown in Figure 12c. It will be appreciated that the invention is of wide application and is not confined to the examples given here. Other applications include ducts fans for aircraft propulsion, shrouded propellers for slow moving airborne vehicles such as airships and axial flow compressors. Whilst the invention has been described in the above embodiments has generally been disclosed in operation within ducts of circular cross-section, application of the invention is not so limited and it can with advantage be applied in ducting of non- circular cross-section, for example of square or rectangular cross-section as is commonly used in ventilation and air conditioning systems. This is particularly so since it is a feature of the invention that the swept path of the impeller is substantially smaller than the duct cross-section. In an arrangement employing non-circular section ducting the annular member defining the ring-vortex forming space needs to be adapted at its outer part to conform to the ducting cross-section and at its inner portion either to overlap the impeller blade tips or formed to develop a vena contracta outside which the blade tips extend. It will be appreciated from the embodiments described in relation to Figures 8-12 that the fluid impeller diffuser of the invention can with advantage be used with relatively short diffuser duct length surrounding and extending downstream from the impeller, for example a duct length of between 20 and 50% of the swept diameter of the impeller may be used.

Claims: CLAIMS 1. A method of inducing a tip driven annular vortex at the transition between an entry passageway to an enlarged diffuser duct (11) surrounding a bladed impeller (1) which is characterised by arranging an annular structure (5) upstream of the tips of the blades (6) and projecting inwardly of the duct (11) to define the entry passageway and to guide fluid therethrough within the rotary path of the tips of the blades (6) and arranging the duct (11) in radially outwardly spaced relation to the path of the blade tips partly to define with the annular structure (5) an annular space (7) extending from the annular member (5) between the tips of the blades (6) and the duct (11), and rotating the impeller so that the blade tip driven annular vortex (9) is formed within the annular space (7) turning so as to diffuse the flow passing through the impeller (1) . 2. A fluid impeller diffuser comprising a rotary bladed impeller (1) disposed within a surrounding diffused discharge duct (11), characterised in that an annular structure (5) is provided upstream of the tips of the blades (6) of the impeller (1) and projecting inwardly of the duct (11) so as partly to define a space (7) in which a blade tip driven annular vortex (9) turning so as to diffuse the flow (8) which has passed through the impeller (1) can be established and maintained in a stable condition, the space (7) extending between tips of the blades (6) and the duct (11) and the annular structure (5) defining a constricted entrance passageway to the duct (11) . 3. A fluid impeller diffuser as claimed in claim 2, characterised in that the annular member (5,15,18, 19,20,21,33,36,49,63) is formed on its upstream side to define a path for fluid flow passing through the constricted entrance which lies within the path swept by the tips of the impeller blades (6,16,46,51,66,68). 4. A fluid impeller diffuser as claimed in claim 2 or claim 3, characterised in that the diameter of the constricted entrance passageway is other than that of the impeller. 5. A fluid impeller diffuser as claimed in any one of claims 2 to 4, characterised in that the annular structure (5) presents an arcuate concavity facing in the downstream direction. 6. A fluid impeller diffuser as claimed in claim 5, characterised in that the annular structure (5) is of C-shaped radial cross-section. 7. A fluid impeller diffuser as claimed in any one of claims 2 to 6, characterised in that the internal duct diameter (60, Figure 9, Figure 11a, lib, lid) is axially varied. 8. A fluid impeller diffuser as claimed in any one of claims 2 to 7, characterised in that the tips (70,71,72) of the impeller blades are so formed that the impeller diameter at an upstream end differs from that at a downstream end of the impeller. 9. A fluid impeller diffuser as claimed in any one of claims 2 to 7, characterised by a plurality of serially arranged bladed impeller stages (26,27) in which a stage (26) is followed by a convergent guide (32) leading to a constricted entry to the impeller (27) of a following stage having an annular structure (33) provided upstream of the tips of the impeller blades thereof which are surrounded by a diffused discharge duct, the annular structure (33) projecting inwardly of the diffused discharge duct to define the constricted entry and partly defining an annular space in which a blade tip driven annular vortex turning so as to diffuse the flow which passes through the impeller (27) can be established and maintained in stable condition, the annular space extending between tips of the blades and the diffuser duct. 10. A fluid impeller diffuser as claimed in any of claims 2 to 9, characterised in that the upstream side of the impeller diffuser (46,50) is coupled to the downstream side of a vehicle engine radiator (45) by a flow duct cowling (44) directing coolant air drawn through the radiator (45) to the restricted entry to the impeller diffuser (46,50), the impeller diffuser being disposed between the radiator (45) and the engine (47).

Inventors: GERRY ULRICH KEITH, GERRY, ULRICH, KEITH
Applicants: GERRY U K, GERRY, ULRICH, KEITH
Application year: 1984
Main classification: F04D29
IPC codes: F01P5, F04D29