US2910268A

US2910268A - Axial flow fluid machines

Info

Publication number: US2910268A
Application number: US313746A
Authority: US
Inventors: Davies David Omri; Jones John Sutcliffe
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1951-10-10
Filing date: 1952-10-08
Publication date: 1959-10-27
Anticipated expiration: 1976-10-27
Also published as: CH313849A; FR1068974A; GB712051A

Description

Oct. 27, 1959 D. o. DAVIES ETAL 2,910,268

AXIAL FLOW FLUID MACHINES m g 4 u n 1 Oct 1959 D. o. DAVIES ETAL- 2,910,268

AXIAL FLOW FLUID MACHINES 5 Sheets-Sheet 2 Filed Oct. 8, 1952 D. a. mums 4- a: s. JONES 5/ WW; 2??

HTT'IS.

0a. 27, 1959 D. O; D VIES EIAL 2,910,268

AXIAL FLOW FLUID MACHINES 5 Sheets-Sheet 3 Filed Oct. 8, 1952 12a DAV/E8, J 3 (IO/V55 ATTORNE 5 AXIAL FLOW FLUlD MACHINES David Omri Davies, Ammanford, Wales, and John Sutclilfe Jones, Wallasey, England, assignors to Rolls- Royce Limited, Derby, England, a British company 7 Application October 8, 1952, Serial No. 313,746

Claims priority, application Great Britain r October 10, 1951' 20 Claims. (Cl. 25339.15)

This invention relates to axial-flow fluid machines having bladed rotors and stators and relates more specifically to tapping means for drawing off through the rotor of the machine part of the pressurised working fluid flowing in the working fluid channel of the machine. An axial-flow fluid machine having such tapping means will, for convenience, be referred to hereinafter as a fluidmachine of the kind referred to.

One prior arrangement of fluid machine of the kind referred to is describedin the specification of U.S. Patent No. 2,636,665 in the name of Adrian Albert Lombard and assigned to the same assignee as the present application, and in tms specification there is described a gasturbine engine having'an axial-flow compressor through the rotor structure of which compressed air is drawn oif from the working fluid channel for use in cooling other engine parts. In U.S. Patent No. 2,636,665 the axialfiow compressor is described as comprising a pluralityof blade-carrying discs mounted in axially-spaced relation to one another on a central shaft and spaced apart adjacent their blade-carrying peripheries by spacer rings, and the tapping means is provided by forming one of the spacer rings in two parts, one part to abut the upstream disc and the other part to abut the downstream disc and the two parts to abut one another. The abutting surfaces of the two parts are so formed as to afford adjacent the working fluid channel circumferentially-extending ports leading from the working fluid channel to a space between radially-inwardly directed annular flanges on the two parts. The compressed air flowing inwardly. in the space between the two flanges passes from the space through ports formed in the shaft and is conveyed through the shaft to its location of use.

It has now been found thatalthough this prior arrange ment enables adequate quantities of air to be tapped 011 for the purposes required, the efliciency with which the arrangement operates is low due to the large pressure.

drop between the working fluid channel of the fluid machine and the location of its use. considered as being made up of three separate losses of which the first may be termed the entry loss, of which the second may be termed the vortex loss, and of which the third is the pressure loss involved in driving the compressed air along the shaft to the location of its use and is only a small proportion of the total pressure loss.

The present invention has for an object to provide a fluid machine of the kind referred to, whereinthe pressure losses involved in tapping off compressed working fluid from the working fluid channel through the rotor are substantially reduced.

According to this invention in one aspect the tapping means in a fluid machine of the kind referred to comprises stator vanes located radially inwards of stator blading in the machine, which vanes provide passages communicating with the working fluid channel in the machine and are arranged to act on the working fluid entering the rotor through the passages so that, it has at the-entry to the rotor a desired whirl in the direction of United States Patent 2,910,268 Patented Och-27,1959

rotation of the rotor. By imparting a suitable whirl to the working fluid at the entry to the rotor, the entry loss can be substantially eliminated. The vanes mayact to increase or reduce the whirl of the working fluid as it passes between them.

Preferably the stator vanes are supported from the radiallyinner ends of the stator blades.

The inventionhas an important application When, as described in U.S. Patent No. 2,636,665 referred to above, the rotor comprises a hollow central shaft supporting a rotor drum assembly, and it is desired to convey the working fluid to the interior of the shaft, and in such an arrangement it is preferred in addition to the stator vanes to provide rotor guide vanes within the rotor drum structure which extend from the point, of entry of working fluid into the rotor drum structure inwards to adjacent the hollow central shaft.

The inventionalso has-important application when the fiuid machine of the kind referred to has an outer loadcarrying stator structure and a central tube extending through the rotor to provide ducting for the working This pressure loss is medium tapped from the working fluid channel; in such an arrangement rotor guide vanes are preferably provided to extend from the point of entry into the rotor drum structure to adjacent the central tube.

Such rotor guide vanes are, in accordance with an im-. portant feature of the present invention, curved in the sense of extending forwardly in the direction of rotation of the rotor from radial lines drawn through their inner ends. By suitably shaping the curved guide vanes, the flow of working fluid between the vanescan be in the nature of a free vortex, i.e. one in which the whirl or tangential velocity is inversely proportional to the radius.

Alternatively, the rotor guide vanes maybe purely radial, and the flow between them will be in the nature of a forced vortex, i.e. one in which the whirl or tangential velocity-is proportional to the radius. However, a curved vanearrangement giving approximately a free vortex flow is preferred.

In constructions having rotor guide vanes as just de--- scribed, the stator guide vanes are designed in such a manner that the shock at entry of the fluid to the rotor from the stator guide vane assembly is eliminated or reduced to a low value, i.e. the whirl velocity imposed by the stator vanes should be such as to give an entry angle to the rotor approximating to the incidence of the rotor guide vanes when the machine is. rotating at a preselected. design speed. Where the vanes are curved, the whirl velocity will preferably be less than the periphery velocity of the rotor by an amount such that the resultant of the difference of the velocities and the radial velocity of the air is at an angle aligned withthe rotor guIde vanes. When the rotor guide vanes are straight and radial, the whirl velocity should preferably be equal entry are substantially reduced, and moreover when rotor guide'vanes are used the entry slots are preferably arranged to form continuations of the passages between therotor guide vanes for the same purpose. I

In accordance with a further feature of the invention, a fluid machine of the kind referred to has a'rotor which comprises a central duct or shaft and which is constructed so that compress ed working fluid flows through apassage therein frorn the working fluid channel to);

the machine to the duct or shaft with a flow which is approximately a free vortex of which the whirl velocity With such an arrangement, the rotor guide 'vanes need.

not be used, and any entry slots in the rotor drum structure are preferably designed to present inlet anglesfat their edges appropriate to the whirl velocity imposed by the stator vanes. In this manner also shock losses at entry may be substantially reduced.

According to another preferred arrangement of this invention, the compressed working fluid passing through the entry slots is arranged to be confined between twodiscs set close together so that a high radial velocity is imposed on the working fluid in flowing inwards from the entry slots. In this way the time taken for the compressed working fluid to flow from the entry slots to collector means at the centre of the rotor, say a hollow shaft, is small so that the effect of the viscous forces tending to produce solid rotation of the working fluid within the rotor is substantially reduced.

In another preferred arrangement according to this feature of the invention, the space into which the compressed working fluid flows on passing through the entry slots is arranged to have a large axial extent compared with the axial extent of entry slots and the entry slots are located in the axial sense substantially symmetrically of said space; it is found that the central core will tend to have a free vortex motion, although the air adjacent the axially-spaced walls will tend to rotate with the walls, and in this arrangement the central core of air in the space will therefore be employed to provide the desired supply of pressure working fluid.

By arranging that a whirl is imparted to the pressurised working fluid entering the entry slots and that the vortex motion of the pressurised working fluid witha in the rotor is either an imposed or induced free vortex,

substantial reductions in the pressure losses liable to occur can be achieved with a consequent improvement in efliciency.

In arrangements according to this invention in which entry slots are provided, these are preferably long in the circumferential sense and are preferably chamfered at their ends.

Some embodiments of this invention will now be described by way of example as applied to a tapping means for conveying pressure air from the working fluid channel of an axial-flow compressor into a hollow shaft of the compressor rotor. The description refers to the accompanying drawings in which:

Figure 1 is an axial section through the intermediate blading stages of the axial-flow compressor adjacent which the tapping-off of compressed air is to be effected,

Figure 2 is a section on the line 22 of Figure I,

Figure 3 is a fragmentary view of part of the rotor of the compressor in the direction of the arrow 3 of Figure 2,

Figure 4 is a view similar to part of Figure l illustrating a modification,

Figure 5 is a view similar to part of Figure 2 illustrating a further modification, and

Figure 6 is a view corresponding to Figure 1 of yet another construction, and Figure 7 is a view corresponding to Figure 2 of yet another construction.

Referring to Figures 1 to 3 of the accompanying drawings,-the axial-flow compressor illustrated is of the kind such as is described in the specification of U.S. Patent No. 2,636,665 and comprises a compressor casing 10 supporting a plurality of axially-spaced rows of stator blad- 2,910,268 I e I g a ing 11 and comprises also a rotor afforded by a rotor drum structure carrying at its periphery a plurality of axially-spaced rows of rotor blading 12, the rotor being supported within the compressor casing with the rows of stator blading 11 alternating with the rows of rotor blading 12.

In this construction the rotor drum structure comprises a central hollow shaft 13, a plurality of blade-carrying discs 14, one for each row of rotor blading 12, each of which discs is torsionally engaged with the shaft 13 through co-operating spline formations 15 at its centre and on the adjacent portion of the shaft 13, and each of which is axially thickened at its periphery 16 and provided with radial flanges '17 to which the blades forming the associated row of rotor blading are connected by pivot pins 18, and spacer rings 19 extending between the disc peripheries to hold them in spaced relation.

The rows of stator blades 11 carry at their inner ends shrouding 20 which forms with the root portions 21 of the rotor blades 12 the inner wall of the working fluid channel of the compressor, the outer wall of the channel being afforded by the compressor casing 10.

Means is provided to tap ofl'f compressed air from the compressor and convey it away from the working fluid channel of the compressor through the hollow central shaft 13, and for this purpose compressed air from the working fluid channel is allowed to flow between one pair of the discs 14 inwardly to adjacent the shaft 13 and then to flow into the shaft through ports 22 cut in the shaft.

In accordance with this invention, means is provided to reduce pressure losses heretofore experienced in eifecting such a tapping of compressed air.

In the construction illustrated in Figures 1 to 3, the means comprises the provision of stator guide vanes 23 radially inwards of the stator blades 11 of the row of stator blades adjacent which the tapping is effected, the provision of entry slots 24 into the rotor by which the entry shock is reduced, and the provision of rotating guide vanes 25 which extend from adjacent the periphery of the rotor drum structure to adjacent the ports 22.

The stator vanes 23 are located adjacent the trailing edges of the stator blades 11 carrying them and are formed by curved metal strips extending between a radial flange 26 on the inner shrouding 20 of the row of stator blading 11 and a radially-extending annular ring 27, the flange 26 and ring 27 forming axially-spaced shrouds for the stator guide vanes 23. Conveniently the vanes 23 are brazed to the flange 26 and ring 27 and the assembly is made in sections each comprising say three or four vanes 23.

The guide vanes are arranged to impart to the air flowing between them towards the rotor drum structure a whirl in the direction of rotation of the rotor drum structure and for instance the vanes may make an angle of, say, 80 to the adjacent tangent at their inlet ends and an angle of, say, 48 to the adjacent tangent at their outlet ends and they are thus outwardly concave and inwardly convex.

The entry slots 24 into the rotor drum structure are conveniently formed in the spacer ring 19 separating the pair of discs 14 between which the tapped-off air flows towards the shaft 13 and the slots 24 are preferably circumferentially-elongated, the ends being chamfered so that the narrow walls 24a of the spacer ring 19 which separate pairs of slots 24 are inclined forwardly in the direction of rotation from their radially inner ends to their radially outer ends.

The rotary guide vanes 25 are in the construction illustrated formed from channel-section strip and are curved so that they are nearly radial at their inner ends and so that they curve outwardly and forwardly from their inner ends to adjacent the narrow walls 24a of the spacer ring 19, the walls 24a conveniently forming smooth continuations of the adjacent guide vanes 25. The

- stant circumferential flow area.

number of walls 24a willbe an integral multiple of the number of'rotary guide vanes 25. The guide vanes 25 are formed so that the flow of fluid between them is a free vortex, that is its circumferential velocity is inversely proportional to the radius, while the circumferential velocity adjacent the shaft 13 is the same as that of the shaft. These relations determine the circumferential velocity of the air at the periphery of

vanes

25 and 24a, and thus the desired whirl at outlet from vanes 23.

'The rotary guide vanes are mounted between a pair of

thin discs

28 and 29 and are secured to the discs, for instance by spot welding their lateral flanges to the discs. The

discs

28 and 29 have turned-over outer peripheries and are unconnected at their outer peripheries with the adjacent spacer ring 19. The assembly comprising the two

discs

28 and 29 and the guide'vanes 25 is however torsionally connected with the shaft 13 by securing to the inner periphery of the disc 29 an internallysplined ring 30, the splines of which co-operate with the spline formations 15 provided on the shaft for the adjacent blade-carrying disc 14. The disc 23 is provided at its centre with an axially-directed flange 28a which extends to adjacent the other of the pair of blade-carrying discs 14.

With the arrangement just described, the stator guide vanes 23 impart a whirl to the tapped air, the whirl being in a direction of rotation of the rotor drum structure, but it will be appreciated that the whirl component to be imparted to the air in flowing between the stator guide vanes 23 will depend upon the whirl velocity of the air as it enters the passages between the guide vanes 23 and the desired whirl velocity at the entry slots 24.

Thus if the Whirl of the air entering the passages between the stator guide vanes 23 is greater than the desired Whirl velocity at the entry slots 24, the stator guide vanes 23 will be required to impart a component of whirl to the air in a directionopposite to the direction of rotation of the rotor. ranged as indicated in Figure '5 with their convex surfaces outwardly facing and their concave surfaces inwardly facing. In all cases, however, the'air leaving the guide vanes 23 Will have a resultant whirl component in the direction of rotation of the rotor.

In one construction of compressor having a tapping means as just set forth, a pressure loss of about 3 lbs. per sq. inch or less occurred in tapping-off the air' as compared with a'pressure loss of about 16.8 lbs. per square inch which occurred with a construction as described in the specification of US. Patent No. 2,63 6,665. In an alternative arrangement (Figure 7), the rotary guide vanes 125 are'straight'andradial, and in this case the flow between them will approximate to the'forced vortex condition, that is such that the circumferential velocity at any point is directly proportional to the radius. In this case, in order to ensure shock-free entry conditions, the whirl at outlet from vanes 23 will be arranged to be equal to the peripheral velocity at the tip of the'rotary' vanes. Constructionally these rotary guide vanes 125 will be similar to the vanes 25 above described.

Referring now to Figure 4, there is illustrated another alternative arrangement in which the rotary guide vanes 25 are omitted and the air flows from the entry slots 24 between a' pair of narrowly-spaced discs 31 which may be constructed to

besimilar tothe discs

28 and 29 above described and to be located in the rotary drum structure in a similar manner. The discs 31 are held apart by spacers 32 through which extend retaining bolts 33, and strengthening rings 34 may be secured to the outwardlyfacing surfaces of the discs 31. The discs 31 are preferably arranged so that the path between them has a con- In this construction the air entering the rotor drum structure flows towards the shaft 13 with a high radial velocity between the discs 31 In this case the guide vanes will be ar-' 6 so that thereby the time taken for the. air to fl'ow from the entry slots 24 to the ports 22 is small and the effect of the viscous forces tending to produce solid rotation of the working fluid within the rotor drum structure is substantially reduced.

In yet another arrangement (Figure 6), the rotary structure is arranged so that the axial spacing between a pair of adjacent discs 131 is relatively large and so that the circumferentially-elongated slots 24 in the spacer ring are, in the axial sense, substantially symmetrical with respect to the two discs 131. Similarly the ports 22 providing communication between the space between the discs 131 and the interior of the hollow shaft 13 are disposed symmetrically with respect to the two discs 131. In this way only the central layer of air in the space between the two discs 131 is drawn off into the hollow shaft and the motion of the air in this layer will tend to be a free vortex motion since it is remote from the boundary layers on the two adjacent discs 131.

The air led to the space within shaft 13 will be arranged to flow along the interior of the shaft and may be arranged for example to flow out through ports spaced axially from ports 22 for the purpose cfcooling an associated turbine, or of cooling a shaft bearing, or for any j other suitable purpose.

We claim:

I. A multi-stage axial-flow fluid machine comprising a stator casing, a rotor drum structure, said casing and drum structure defining therebetween an annular working fluid channel, a plurality of rows of stator blades mounted on said casing to extend into said working fluid channel, a plurality of rows of rotor blades mounted on the periphery of said drum structure and alternating with said rows of stator blades, and means for tapping off compressed working fluid from said annular channel comprising means defining a hollow collector space centrally Within said rotor, a pair of annular walls extending between said collector-space-defining means and the periphery of said rotor drum structure and thereby defining an annular space, means placing said annular space in communication with said hollow collector space, means defining entry ports to said annular space in said rotor drum structure, and a plurality of stator guide vanes supported.

at the inner periphery of a row of stator blades in alignment with and radially outside said entry ports and adapted and arranged so that the working fluid at outlet from said guide vanes has a circumferential component of velocity in the direction of rotation of said rotor before entry of the rotor.

2. A multi-stage axial-flow fluid machine as claimed in tation of the rotor towards the periphery of the rotor drum structure.

3. A multi-stage axial-flow fluid machine as claimed in claim 1 comprising also a plurality of rotary guide vanes connected at their axially-spaced ends to said annular walls and extending substantially radially from adjacent said collector-space-defining means to adjacent the periplicry of the rotor drum structure.

4. A fluid machine as claimed in claim 1, wherein said pair of annular walls are constituted by two discs having a small axial spacing, the axial spacing at each radius from the axis of rotation being such that the circumferential area of the flow path between the discs is constant.

5. A fluid machine as claimed in claim 1,-wherein the annular space has an axial extent which is large cornpared with the axial extent of the entry slots.

6. A fluid machine as claimed in claim 1, wherein the entry ports are circumferentially-spaced slots and the axially-extending end surfaces of the slots are chamfered to be inclined forwardly from their radially inner ends to their outer ends. I I

7; A multi-stage axial-flow fluid machine comprising a stator casing, a rotor drum structure, said casing and drum structure defining therebetween an annular working fluid space, a plurality of rows of stator blades mounted on said casing to extend into said working fluid channel, a plurality of rows of rotor blades mounted on the periphery of said drum structure and alternating with said rows of stator blades, and means for tapping oif compressed working fluid from said annular space comprising means defining a hollow collector space centrally within said rotor, a pair of annular walls extending between said collector-space-defining means and the periphery of said rotor drum structure and thereby defining an annular space, a plurality of straight radial rotary guide vanes in said annular space and connected at their axially-spaced ends to said annular walls, means placing said annular space in communication with said hollow collector space, means defining entry ports to said annular space in'said rotor drum structure, said entry ports being aligned with the spaces between said rotary guide vanes, a shroud ring supported at the inner periphery of a row of stator blades and having a radially-inwardly-directed flange, an annular member spaced axially from said flange, and a plurality of stator guide vanes extending between said flange and said annular member in alignment with said entry ports and adapted and arranged so that the work ing fluid at outlet from said guide vanes has a circumferential component of velocity in the direction of rotation of said rotor before entry to the rotor and substantially equal to the peripheral velocity of the radially outer ends of said rotary guide vanes. r

8. in a multi-stage axial-flow fluid machine comprising a stator casing, a rotor drum structure, said stator casing and said drum structure defining therebetween an annular working fluid channel, a plurality of rows of stator blades mounted on said casing to extend into said working fluid channel, and a plurality of rows of rotor blades mounted on said rotor drum to extend into said working fluid channel and alternating with therows of stator blades; means for tapping oif compressed working fluid from the working fluid channel comprising means defining a hollow collector space centrally within said drum structure, a pair of axially-spaced annular walls extending outwardly from said collector space to adjacent the periphery of the rotor drum structure and defining therebetween a transfer space, said transfer space communicating with said collector space, port-affording means in the periphery of the rotor drum structure providing entry ports to said transfer space, said port-affording means being spaced radially inwards from the ends of the stator blades of one of said rows of stator blades, and a plurality of stator guide vanes carried at the inner ends of the said one row of stator blades, said stator guide vanes extending inwards from the stator blades to adclaim 13, wherein the stator guide vanes have each a jacent said entry ports and being inclined to the radial so that their inner ends are displaced with respect to their outer ends in the direction of rotation of the rotor drum structure, and said stator guide vanes aifording between them passages for the flow of working fluid from the working fluid channel to the entry ports.

9. A multi-stage axial-flow machine as claimed in claim 8 comprising also rotary guide vanes in said transfer space between said annular walls, said rotary guide vanes extending outwardly from adjacent the collector space to adjacent the entry ports.

10. A multi-stage axial-flow machine as claimed in claim 9, having said rotary guide vanes curved to be substantially radial at their inner ends and to extend outwardly and forwardly in the direction of rotation at'their outer ends.

11. A multi-stage axial-flow machine as claimed in claim 10, wherein said port-affording means comprises a plurality of circumferentially-spaced radially-extending convex surface and a concave surface and are supported with their convex surfaces inwardly facing.

15. A multi-stage axial-flow machine as claimed in claim 8, having said stator guide vanes formed as curved sheet metal strips, each of said guide vanes having a convex surface and a concave surface, and the stator guide vanes being supported with their convex surfaces outwardly facing.

16. A multi-stage axial-flow machine as claimed in claim 8, comprising also radially-arranged rotary guide vanes in said transfer space between said annular walls and having said stator guide vanes adapted so that the working fluid at their inner ends has a whirl equal to the peripheral velocity of the outer ends of the rotary guide vanes.

17. A multi-stage axial-flow machine as claimed in claim 8, having the axial spacing of said axially-spaced annular walls substantially greater than the axial dimension of said entry ports, and having said entry ports substantially symmetrically disposed with respect to said annular walls.

18. In a multi-stage axial-flow fluid machine comprising a stator casing, a rotor drum structure, said casing and drum structure defining therebetween an annular working fluid channel, a plurality of rows of stator blades mounted on said casing to extend into said Working fluid channel, and a plurality of rows of rotor blades mounted on the periphery of said drum structure and alternating with said rows of stator blades; means for tapping ofi compressed working fluid from said annular working fluid channel comprising means defining a hollow collector space centrally within said rotor, a pair of annular walls extending between said collector-space-defining means and the periphery of said rotor drum structure and thereby defining an annular space, means placing said annular space in communication with said hollow collector space, means defining entry ports to said annular space in said rotor drum structure, and a plurality of stator guide vanes supported at the inner periphery of a row of stator blades in alignment with said entry ports and affording passages between them opening at one end to the working fluid channel at a point in which the air has a low circumferential component of velocity compared with the peripheral velocity of the rotor and opening at their opposite ends to deliver air to said entry ports, said stator guide vanes guiding the air flowing in said passages to have a desired circumferential component of velocity adjacent said entry ports. e

19. In a multi-stage axial-flow fluid machine comprising a stator casing, a rotor drum structure, said stator casing and said drum structure defining therebetween an annular working fluid channel, a plurality of rows of stator blades mounted on said casing to extend into said working fluid channel, and a plurality of rows of rotor blades mounted on said rotor drum to extend into said working fluid channel and alternating with the rows of stator blades; means for tapping olf compressed working fluid from the working fluid channel comprising a hollow shaft centrally within said drum structure, a pair of axiallyspaced annular walls extending outwardly from said collector space to adjacent the periphery of the rotor drum of the rotor drum structure providing entry ports to said a transfer space, said port-affording means being spaced' radially inwards from the ends of the stator blades of one of said rows of stator blades, and a plurality of stator guide vanes carried at the inner ends of the said one row of stator blades, said stator guide vanes extending inwards from the stator blades to adjacent said entry ports and being inclined to the radial so that their inner ends are displaced with respect to their outer ends in the direction of rotation of the rotor drum structure, and said stator guide vanes affording between them passages for the inward flow of working fluid from the working fluid channel to the entry ports, the inner ends of said stationary guide vanes being inclined to impart to the air passing through the passages formed between them a whirl velocity in the direction of rotation of the rotor which is less than the periphery velocity of the central shaft and is in the ratio to it which the radius of the central shaft bears to the radius of the periphery of the rotor drum structure.

20. A multi-stage axial-flow machine as claimed in claim 19, comprising also rotary guide vanes in said transfer space between said annular walls, said rotary guide vanes extending outwardly from adjacent the central shaft to adjacent the entry ports and being inclined at any radial location to produce a whirl velocity at said location which is less than the peripheral velocity of the central shaft and is in the ratio to it which the radius of the central shaft bears to the radius at said location.

References Cited in the file of this patent UNITED STATES PATENTS 2,177,159 Stalker Oct. 24, 1939 2,294,586 Troller Sept. 1, 1942 2,419,669 Birmann Apr. 29, 1947 2,618,433 Loos et a1 Nov. 18, 1952 2,636,665 Lombard Apr. 28, 1953 FOREIGN PATENTS 479,427 Great Britain I an. 31, 1938 838,188 Germany May 5, 1952