WO2001006122A1

WO2001006122A1 - Diffuser

Info

Publication number: WO2001006122A1
Application number: PCT/NZ2000/000128
Authority: WO
Inventors: Miles Peter Fox; Derek Grant Phillips; Serguei Timofeevich Zavtrak
Original assignee: Vortec Energy Limited
Priority date: 1999-07-21
Filing date: 2000-07-18
Publication date: 2001-01-25
Also published as: AU6325000A

Abstract

A diffuser of a Diffuser Augmented Turbine (DAT) adapted to surround the rotor of a turbine for wind or underwater application having an aerofoil like cross sectional shape (10) and wherein there are provided one or more inlets (20) on the nose (12) and/or the exterior surface (13) of the diffuser and a plurality of exits (24) provided on the interior surface (11) of the diffuser.

Description

DIFFUSER

TECHNICAL FIELD OF THE INVENTION

This invention relates to a diffuser for a turbine, and has particular application to the design of a Diffuser Augmented Turbine (abbreviated to "DAT") for wind or undersea application.

BACKGROUND

A DAT has a duct which surrounds the rotor blades and increases in cross-sectional area downstream of the blade plane. The increasing duct area downstream, results in a reduction in the mean velocity of the fluid (air or water) flow through the diffuser. In this specification the "interior surface" of the diffuser is the interior surface of the duct, whilst the '"exterior surface" of the diffuser is the exterior surface of the duct.

OBJECT

It is an object of this invention to provide an improved diffuser for a turbine, or an improved Diffuser Augmented Turbine, or one which will provide the public with a useful choice.

STATEMENT OF INVENTION

In one aspect the invention provides a diffuser adapted to surround the rotor of a turbine, the diffuser having a nose and a tail, and an interior surface and an exterior surface which are spaced apart from one another between the nose and the tail, the diffuser having a generally venturi-like shape at least along the interior surface of the diffuser, wherein one or more inlets are provided on the nose or on the exterior surface of the diffuser, and a plurality of exits are provided on the interior surface of the diffuser.

In another aspect, the invention provides a diffuser adapted to surround the rotor of a turbine, the diffuser having a nose and a tail, and an interior surface and an exterior surface which are spaced apart from one another between the nose and the tail, the diffuser having a generally venturi-like shape at least along the interior surface of the diffuser, wherein one or more inlets are provided on the nose of the diffuser, and a plurality of exits are provided on both the interior surface and the exterior surface of the diffuser.

Preferably the exits are circumferential slots. These slots may be interrupted by structural elements so that they are not fully circumferential openings although it is preferred that they extend as completely as possible around the entire circumference.

Preferably the exit slots are stepped along the surface of the diffuser.

Preferably the inlet or inlets is/are circumferential slot(s).

Preferably the inlet or inlets to the interior of the diffuser at the nose of the diffuser is or are controlled by a diaphragm.

In use, depending on application, wind or water enters the diffuser at the leading edge of the diffuser section, which is preferably of a diameter which is less than the diameter of the trailing edge of the diffuser. By also allowing air or water to enter the inlet or inlets in the diffuser and escape via the multiple exits of the diffuser it is possible to make a more efficient DAT as will be explained below.

In its most preferred form of the invention, the diffuser section is in the shape of an aerofoil (when viewed in cross-section). Though it will be appreciated that the complete primary diffuser section is in the shape of a duct or ring with the interior surface and exterior surface separated from one another because of the substantially aerofoil-like shape of this section.

In its most preferred form of the invention, the diffuser section closely surrounds the turbine blades (ie. a slightly greater radius than the rotor blades), although it is possible to modify the diffuser to include an inner ring between the interior surface of the primary diffuser section and the tips of the rotor blades. This however is a much less preferred arrangement.

In another aspect, the invention provides a Diffuser Augmented Turbine having a nacelle centrally supporting two or more rotor blades, the rotor blades being surrounded by a diffuser as described above. Preferably the nacelle is streamlined to assist in creating a venturi-like shape between the exterior surface of the nacelle and the interior surface of the diffuser, when viewed in cross- section.

Any number of rotor blades can be used, although in the following examples, four such rotor blades are described.

In wind generated applications, the invention lends itself to both small and large diameter DAT's, the smaller DAT's typically ranging from 1 metre to 5 metres with a typical size being a 3 metre diameter DAT for use in stand alone power generation, with a larger sizes from 5 metres diameter upwards, typically about 50 metres diameter, being designed for use in providing power to the national grid. The larger sizes can provide significant quantities of power, for example, with this invention a 54 metre diameter DAT should provide 3.5 MW of power, whilst a 91 metre diameter DAT should provide approximately 10 MW of power. For environmental reasons, a DAT of up to about 54 metres diameter could be land-based, with larger diameters being used on floating off-shore islands.

These and other aspects of this invention which shall be considered in all its novel aspects, will become apparent from the following description, which is given by way of example only, with reference to the accompanying drawings, in which:

DRAWINGS

Figure 1 is a first example of the geometry of a preferred DAT according to Example 1 showing the relationship of the inlet in the diffuser and the exits on the interior of the diffuser.

Figure 2 is an expanded detail of one of the exit slots of Figure 1 showing the diffuser made up from ribbed "long-run" rolled metal cladding.

Figure 3 is an aerodynamic plot of a prior art diffuser without slots.

Figure 4 is an aerodynamic plot of a preferred diffuser with multiple exit slots.

Figure 5 shows a section through a preferred diffuser to show the internal structure but omitting the slots. Figure 6 is a radial section through a preferred DAT according to Example 2 showing the relationship of the inlet to the interior of the diffuser and the exits on the interior and exterior surfaces of the diffuser.

Figures 7-11 relate to various theoretical aspects of the invention as described according to Example 3.

PREFERRED EMBODIMENTS

Example 1

Figure 1 of Example 1 shows a section through a portion of the diffuser body, showing that it is of an aerofoil shape 10, having an interior surface 1 1 corresponding to the shape of a venturi (this will be appreciated if the section is rotated through 360° and recognised as surrounding the blades of a turbine (not shown)). It has a relatively bull shaped (i.e. blunt) nose 12, and an exterior surface 13, both the interior and exterior surfaces 1 1 and 13 tapering towards the tail 14 which is of greater diameter than the diameter of the nose 12.

Preferably a pressure inlet 20 is provided on the nose 12 adjacent the high pressure zone 21 (i.e. adjacent the exterior surface 13). Alternatively one or more inlets may be provided along the high pressure zone, e.g. along or on the exterior surface 13.

Preferably a plurality of exit slots 24 are provided along the interior surface 1 1 of the diffuser 10. between the nose and the tail. More preferably this plurality of slots 24, commences approximately at a point near the downstream position of the shortest radius along the exterior surface 1 1 , and extends almost to the tip of the tail 14.

Preferably the slots 24 are stepped tangential to the diffuser wall surface as shown in Figure 1.

It is particularly preferred that these exit slots 24 are circumferential slots, or approximate circumferential slots. Any structure linking the portions of the cladding making up the slots is preferably of small size relative to the slot width in order to provide minimal interference with the air flow from the interior of the diffuser body through the exit slots. Figure 5 shows the type of structure that might be used on the inside of the diffuser body showing a number of struts 50 and wires 51. making up a "space-frame" structure. In Figure 5, the slots have been omitted to simplify the illustration, to show the type of structure that might be used within the diffuser body.

Figure 2 shows a detail of one of the stepped slots 24, and the type of cladding material used to make up the diffuser. In this case, assume that the diffuser body, is a diffuser of 54 metres diameter (this is the interior diameter), and its structure is provided by a space-frame structure, then it is preferred that the cladding of the diffuser is made up a number of sheets of ribbed metal cladding 30, with the ribs 31, 32 positioned on the interior of the diffuser body 10. By using rolled metal cladding of the long-run roofing type, it is possible to use a readily available material, but by positioning it with the ribs 31, 32 on the inside of the body, it then provides a relatively smooth external surface both on the interior surface 1 1 and the external surface 13.

Ribs 31, 32. allow for the connection between adjacent long-run sections 30. To provide for the exit 24, it is preferred that the rib shown at 33 is rolled over, flattened, or physically removed, so as not to impede the fluid (air or water) flow shown by arrows 35. By way of example, a 54 metre diameter DAT would have about ten slots each of about 70 mm in width (arrow 39 in Figure 2).

Figures 3 and 4 show a comparison between a diffuser body without slots (a prior art diffuser), and Figure 4 in which the diffuser has the plurality of exit slots of the type shown in Figure 1. These slots are shown in order to provide an indication of pressures, and the boundary layer effect. In the case of Figure 3, the diffuser without slots shows the boundary layer breaking away approximately one third of the way along the interior surface 11 of the diffuser. It also shows that there is no pressure within the diffuser, as it is fully closed.

Figure 4 on the other hand shows the effect of the pressure inlet 20, and the escape of this high pressure fluid within the diffuser body space 29 via the plurality of exit slots 24 along the interior surface 1 1 , with the result that the boundary layer more closely follows the contour of the interior surface 1 1 of the diffuser. resulting in a more slippery aerodynamic diffuser. The air pressure within space 29 will be greater than the air pressure at low pressure zone 28. Example 2 (Figure 6)

Figure 6 shows a section through a portion of the diffuser body, showing that it is of an aerofoil shape 1 10 according to Example 2, having an interior surface 1 1 1 corresponding to the shape of a venturi (this will be appreciated if the section is rotated through 360° and recognised as surrounding the blades of a turbine (not shown)). It has a relatively bull shaped (i.e. blunt) nose 1 12, and an exterior surface 1 13, both the interior and exterior surfaces 1 11 and 113 tapering towards the tail 114 which is of greater diameter than the diameter of the nose 112.

Preferably a pressure inlet 120 is provided on the nose 112 adjacent the high pressure zone 121 (i.e. adjacent the exterior surface 1 13). Alternatively one or more inlets may be provided along the high pressure zone. e.g. along or on the exterior surface 1 13.

Preferably a plurality of exit slots 124 are provided along the interior surface 111 of the diffuser 1 10, between the nose and the tail. More preferably this plurality of slots 124, commences approximately at a point near the downstream position of the shortest radius along the exterior surface 1 1 1. and extends almost to the tip of the tail 1 14.

Preferably the slots 124 are stepped tangential to the diffuser wall surface as shown in Figure 1.

It is particularly preferred that these exit slots 124 are circumferential slots, or approximate circumferential slots. Any structure linking the portions of the cladding making up the slots is preferably of small size relative to the slot width in order to provide minimal interference with the fluid flow from the interior of the diffuser body through the exit slots.

Figure 5 shows the type of structure that might be used on the inside of the diffuser body showing a number of struts 150 and wires 151, making up a "space-frame" structure. In Figure 5, the slots have been omitted to simplify the illustration, to show the type of structure that might be used within the diffuser body.

Figure 2 shows a detail of one of the stepped slots 124. and the type of cladding material used to make up the diffuser. In this case, assume that the diffuser body, is a diffuser of 54 metres diameter (this is the interior diameter), and its structure is provided by a space-frame structure, then it is preferred that the cladding of the diffuser is made up a number of sheets of ribbed metal cladding 130, with the ribs 131, 132 positioned on the interior of the diffuser body 110. By using rolled metal cladding of the long-run roofing type, it is possible to use a readily available material, but by positioning it with the ribs 131. 132 on the inside of the body, it then provides a relatively smooth external surface both on the interior surface 11 1 and the external surface 1 13.

Ribs 131 , 132, allow for the connection between adjacent long-run sections 130. To provide for the exit 124. it is preferred that the rib shown at 133 is rolled over, flattened, or physically removed, so as not to impede the fluid flow shown by arrows 135. By way of example, a 54 metre diameter DAT would have about five (5) slots each of about 70 mm in width (arrow 139 in Figure 2).

Figures 3 and 4 show a comparison between a diffuser body without slots (a prior art diffuser), and Figure 4 in which the diffuser has the plurality of exit slots of the type shown in Figure 1. These slots are shown in order to provide an indication of pressures, and the boundary layer effect. In the case of Figure 4, the diffuser without slots shows the boundary layer breaking away approximately one third of the way along the interior surface 11 1 of the diffuser. It also shows that there is no pressure within the diffuser, as it is fully closed.

Figure 5 on the other hand shows the effect of the pressure inlet 120, and the escape of this high pressure fluid within the diffuser body space 129 via the plurality of exit slots 124 along the interior surface 1 1 1. with the result that the boundary layer more closely follows the contour of the interior surface 1 1 1 of the diffuser, resulting in a more slippery aerodynamic diffuser. The fluid pressure within space 129 will be greater than the fluid pressure at low pressure zone 128.

Example 3

The inventor has done mathematical calculations to prepare an article (for publication) and a patent on the diffuser and centre body (CB) configuration for application to wind powered turbines.

Looking through Ref. [4] of the appended bibliography, the inventor has identified that the best airfoils have been made with slots. The basic idea to make slots on the internal surfaces of the diffuser appears to be very logical. The difference is a simple inlet instead of many entrance holes (shown by Figure 7, A and B). The inventor has developed a pure analytical solution for a 2-dimensional diffuser (see Fig. 8, Configuration A). According to the inventor's calculation, the wind speed inside the diffuser at the narrowest area (swept by the blades) may be represented as a sum

" 1 ' " ' geometry ' Or circulation^ T ) Where "1 " is chosen as a reference wind speed, hV_geometry is a contribution caused by the geometry of the diffuser and δV_cιrcuι_{a o}„ is a contribution given by the circulation (see Fig. 9).

In 2-dimensional case shown on Fig. 3 there are two separate circulations T+ and T. around to wings. The total sum Y_fuu = T+ + T. is obviously equal to zero because of the symmetry. In a real 3-dimensional case there are local toroidal circulations. Geometry contribution bV_geometry is in practice always positive. As shown in Ref. [1], it is determined by the extension ratio.

The most interesting thing is the second contribution. It has never been discussed in the literature on wind turbines before (probably, because the technology is still not widespread). The following question presented itself immediately. Imagine, having two diffusers with the same extension ratios (see Fig. 8, configurations A and B). The difference is in the circulations only. As can be seen from this figure, the circulations are opposite. Which diffuser does work better? The answer is not obvious. According to the inventor's calculations, contribution δV ci culation is positive at configuration A and negative at configuration B. So, configuration A is the best.

It is well known that the lift force acting at the airfoil is proportional to the full circulation (no circulation = no lift) around the airfoil (see Fig. 7) and may be represented by Jukovsky's formula

F,_lfl = pUJ u (2)

Where p is the density of air, U_∞ is the wind speed and T_juii is the total circulation taken around the whole airfoil [2,3]

where ϋ is the local wind speed (vector taken in each point along the curve of integration and dϊ is the element of length. In its turn, this integral may be written as the sum

of contributions taken in each point of the curve.

The slots on the airfoil solve two basic problems:

• the slots pump air slightly through the holes to prevent flow separation

• increasing the wind speed at the bottom area (Fig. 7) and decreasing it at the top one that results in an increase of the circulation i.e. the lift force.

Very roughly, the circulation for the airfoil can be written as

r - (f _Aono(II - f/_/Λp ) - c , (5)

where M _bottom is the wind speed at the bottom area (under the airfoil), \ _top is the wind speed at the top area (over the airfoil) and c is the chord. In pure mathematical terms, by making the slots (see Fig. 7, B), total circulation Yf_uu is the sum Yf_uu = Ti + T + T₃ + T + ... The same takes place when considering the diffuser's blade (see Fig. 7, B). The difference is that the entrance holes (airfoil) are replaced by one common entrance gap (diffuser).

If equation (5) is examined, it will be understood what must be done to increase the diffuser's performance. The total circulation, i.e. the useful positive contribution hV_cιrcuι_{a on} in equation (1) must be increased. Then, the diffuser will work like a sucking pump.

So. with the same geometry there may be a significant gain in wind power due to increase in the circulation. Virtually, it looks like an increase in geometry but, in reality the gain is produced by the sucking action (it involves bigger wind flow area into the diffuser). The circulation may be increased in two ways:

^• by increasing U _bottom (this way is already found when making slots on the diffuser's surface) • by decreasing (drag) U,_op.

Of course, there is a third way to increase the geometry, i.e. the chord c but, this is expensive.

From the practical point of view, the internal surface of the diffuser works properly due to the slots but. the front surface does not.

To improve the situation there has to be other slots on the front surface as shown on Fig. 1 1.

The inventor has evaluated the performance of the diffuser with additional slots on the front surface and provides a brief evaluation.

Coefficient C_p is proportional to cube of the wind speed, i.e.

C P. ~ U:

where U_∞ is the wind speed of the incident flow (far from the diffuser). What does the diffuser do? It increases the wind speed, i.e. wind speed in the diffuser increases and may be represented as

t/ = « <l + Δ),

where dimension-less parameter Δ > 0 shows the relative increase.

We know exactly that slots help and increase the C_p twice. It means we can write

C_p(with slots) = (1 + Δ)³ • C_p(without slots)

Herewith.

,(wifhout slots) = 0.9 ,(with slots) = 1.2 Because the ratio

C (with slots)

— i ₌ 2 ₅

C (without slots) parameter Δ can be found caused by the slots (denoted as Δ+)

Δ₊ - 2^1/3 - 1 = 0.25992

As shown previously, increase in wind speed due to the circulation is the sum of two contributions. The first is Δ, which is determined by the slots on the exterior surface (we have to speed the flow there to increase the circulation). The second one Δ. is determined by new slots on the front surface (we have to drag the flow there).

Total contribution is the sum

Δ = Δ₊ + Δ.

In the case of the slots we have

Δ = Δ₊,

Δ. = 0

In case of new additional slots we have (we suppose new slots to produce the same effect as existing ones)

Δ. = Δ₊

and

Δ = 2Δ+

So, in case of new additional slots we will have

Cp(wifh new additional slots) = (1 + 2Δ₊) • C^ without existing slots)

Calculation gives us

C_p (with new additional slots) = 0.9 (2 ^2 - l)³ = 1.56

An additional note

A very simple explanation of the general idea. Imagine two rotating cylinders (rotating in opposite directions) in water as shown in Fig. 9. These two cylinders are very similar to the diffuser. The circulation is caused by the rotation. Will it help? The answer is obvious in the case shown in Fig. 10 (T > 0). It will suck water.

In the opposite case (T < 0) shown in Fig. 10 it will not help because the circulation will produce an opposite flow (opposite to the incident flow).

Note, the slots 25 on the front surface 1 have to be opposite to the slots 24 on the back surface 2 to drag in the wind flow. The slots on the back surface increase the wind flow. Generally, the total circulation increases.

Preferably, the front slots have to be built on the same principles as the back ones (the same distance between them and the same width) because it is already checked by the experiments in the wind tunnel. The tests might be carried out on the 0.5m model because the wood form for the front surface is already built.

Of course, the front diffuser's gap have to be left at this stage. Only improvement we can make is to put a diaphragm 21 (see Fig. 1 1) to regulate the flow through the slots. It might be done, if we put a long light rubber tube inside the diffuser. The regulation might be achieved by pumping the tube or decompressing it like children blowing up rubber toys.

References

1 ) G.M. Lilley and W.J. Rainbird. A Preliminary Report on the Design and Performance of Ducted Windmills. The College of Aeronautics. Granfield. Report No. 102. 1956.

2) B.R. Munson, D.F. Young and T.H. Okiishi. Fundamentals of Fluid Mechanics. John Wiley & Sons. NY. 1990.

3) L.D. Landau and E.M. Lifshitz. Fluid Mechanics. Pergamon Press. Oxford. 1966.

4) I.H. Abbott & A.E. Von Doenhoff. Theory of Wing Sections. Dover Publications, INC. New York. 1949.

ADVANTAGES

Advantages of the DAT and diffuser design for this invention include: A) Preventing or minimising separation on the interior diffuser wall.

B) An improved power to weight ratio over a conical diffuser.

C) The ability to scale this diffuser design for both large and small diameter DAT's.

D) A more slippery aerodynamic body which reduces the frontal loads on the diffuser, which in turn reduces loads on the supports and foundation.

E) The possibility of a steeper inlet/exit angle. This allows for an increase in power output by increasing the area ratio with reliable advantage of maintaining boundary layer flow attachment.

F) A single diffuser body, instead of using a primary and secondary diffuser, as the slots achieve the same purpose as a secondary diffuser body, albeit a multiple of exit slots re- energising the boundary layer continuously along the interior diffuser surface.

G) Simplicity in design, allowing for the use of "long-run" ribbed metal cladding, which can be adapted to provide the covering for the diffuser body.

VARIATIONS

This invention is particularly suited to a space-frame structure, having ribbed metal cladding as the surface of the structure, although it will be appreciated that it could also be used in the design of smaller DATs, where the space-frame structure could be replaced by a specially shaped monocoque construction.

The inlet slot (or slots) and the exit slots may be single continuous circumferential slots or may be a plurality of smaller apertures (e.g. cut, punched or drilled in the cladding).

Although only one inlet slot has been shown, more than one inlet slot could be provided.

Although this invention is suited to the use of a single diffuser body, it will be appreciated that a secondary or tertiary diffuser body could be provided if required.

DAT^'s in accordance with this invention may be made in a variety of sizes or shapes. A particular advantage of this design of the diffuser being that the diffuser length is relatively short allowing for compact and therefore more economical and light weight design than would be the case with a long conical diffuser, or in the case of diffusers having closed primary and secondary sections with a slot in between. The design of the diffuser of this invention lends itself to both small and large DAT's in both land-based and water-based environments.

Where the DAT is used in an underwater or subsea application, the means of tethering or fixing the diffuser will depend on whether the location is in a tidal or a constant current situation. Where tidal currents are involved, the DAT can be mounted on a rotating base to change direction. Where constant ocean or river currents are involved, the DAT could be fixed on permanent foundations.

Finally, various other alterations or modifications may be made to the foregoing without departing from the spirit or the scope of this invention as set forth in the claims.

Claims

CLAIMS:

1. A diffuser adapted to surround the rotor of a turbine, the diffuser having a nose and a tail, and an interior surface and an exterior surface which are spaced apart from one another between the nose and the tail, the diffuser having a generally venturi-like shape at least along the interior surface of the diffuser, wherein one or more inlets are provided on the nose and/or on the exterior surface of the diffuser, and a plurality of exits are provided on the interior surface of the diffuser.

2. A diffuser adapted to surround the rotor of a turbine, the diffuser having a nose and a tail, and an interior surface and an exterior surface which are spaced apart from one another between the nose and the tail, the diffuser having a generally venturi-like shape at least along the interior surface of the diffuser, wherein one or more inlets are provided on the nose of the diffuser, and a plurality of exits are provided on both the interior surface and the exterior surface of the diffuser.

3. A diffuser as claimed in claim 1 or claim 2 wherein the exits are circumferential slots stepped along the surface of the diffuser.

4. A diffuser as claimed in claim 1 or claim 2 wherein the exit slots are a plurality of smaller apertures which are cut, punched or drilled in the surface of the diffuser.

5. A diffuser as claimed or disclosed in any one of the preceding claims wherein the one or more inlets is or are circumferential slots.

6. A diffuser as claimed in claim 5 wherein the one or more inlets are a plurality of smaller apertures cut, punched or drilled in the exterior surface of the diffuser.

7. A diffuser as claimed in any one of the preceding claims wherein the one or more inlets at the nose of the diffuser is or are controlled by a diaphragm.

8. A diffuser as claimed in any one of the preceding claims wherein the diffuser section is in the shape of an aerofoil when viewed in cross section.

9. A diffuser as claimed or disclosed in any one of the preceding claims wherein the diffuser further includes an inner ring between the interior surface of a primary diffuser section and the tips of the rotor blades.

10. A Diffuser Augmented Turbine (DAT) having a diffuser as claimed in any one of the preceding claims and having a nacelle of streamlined shape to assist in creating a venturi- like shape between the exterior surface of the nacelle and interior surface of the diffuser, the nacelle centrally supporting two or more rotor blades surrounded by the diffuser.

1 1. A Diffuser Augmented Turbine (DAT) as claimed in claim 2 having a secondary or a tertiary diffuser body.