US2494328A - Axial flow elastic fluid turbine - Google Patents

Description

Jan. 10, 1950 D. J. BLOOMBERG AXIAL FLOW ELASTIC FLUID TURBINE Filed March 22, 1946 Ill\ Invent David J. Bloomber by MW His AttOffley,

Patented Jan. 10, 1950 AXIAL FLOW ELASTIC FLUID TURBINE David J. Bloomberg, Newton, General Electric Company,

New York Mass., assignor to a corporation of Application March 22, 1946, Serial No. 656,448 6 Claims. (01.1253-57) This invention relates to an elastic fluid turbine arrangement, particularly to a stationary shroud arrangement for an axial flow turbine bucket-wheel having shroudless buckets. While not limited thereto, it is particularly useful in connection with single stage high speed, high temperature gas turbines, such as those used to drive supercharger compressors in reciprocating internal combustion engine powei'plants.

An object of my invention is to provide a simple, effective shroud arrangement for preventing leakage of motive fluid from the flow paths defined by the shroudless buckets of an axial flow turbine bucket-wheel.

Another object is to provide a turbine bucketwheel arrangement having shroudless buckets and improved efilciency.

A further object is to provide a stationary shroud arrangement for moving shroudless turbine buckets permitting appreciable radial clearances with the buckets to facilitate assembly and maintenance.

Other objects and advantages will be apparent from the following description taken in connection with the accompanying drawings, in which Fig. 1 is a view, partly in section, of a single stage axial flow turbine having a shroud arrangement made in accordancewith my invention; Fig. 2 is an end view of several buckets of the turbine rotor; Fig. 3 is a front view of the buckets of Fig. 2, taken in the direction of the arrows 3--3 in Fig. 1; and Fig. 4 is a sectional view showing a modification of my shroud arrangement.

In conventional steam turbine practice it has usually been the custom to employ axial flow buckets provided with a cover or shroud secured to the buckets and arranged to prevent leak-age of motive fluid radially outward from the bucket flow passages. In high temperature, high speed gas turbines, where it is desired to pass a large quantity of motive fluid of smaller density through a bucket annulus of comparatively small area, it becomes necessary to use buckets of rather large radial length with circumferential spacing between buckets of such magnitude that it is impractical to use the prior art shrouds which are secured to the buckets. However, the arrangements for wheels having shroudless buckets known to the prior art were lneflicient by reason of leakage losses due to motive fluid passing radially outward from the bucket flow passages. The purpose of the present invention is to provide a simple yet effective stationary shroud arrangement for a wheel having shroudless buckets, which will enable such a bucket-wheel to be used with V 2 V efliciencies comparable to those obtained with the shrouded bucket structures known to the prior art.

Referringnow to Fig. 1, my turbine arrangement is represented as comprising a bucket-wheel indicated generally at I and consisting of a disk 2 having welded thereto a circumferential row of shroudless buckets 3. The shape of the buckets may be more clearly seen by reference to Figs. 2 and 3. The bucket-wheel is secured by three or more suitable threaded fastenings 4 to a flange 5 of a shaft supported by suitable bearings (not shown) contained within bearing housing 6. Secured to the bearing housing by means of studs 1 is a nozzle-box support balile 8. The nozzle box 9 has a cast nozzle diaphragm l0 which defines a complete ring of circumferentially spaced nozzles H and is supported on the inner end of support baflle 8 by a plurality of circumferentially spaced lugs l2.

The end cap 6a of bearing housing 6 is provided with a cylindrical axial extension I3 which forms a labyrinth seal it cooperating with the outer circumference of shaft flange 5, in a manner which will be obvious from Fig. 1.

Cooling air is supplied so as to flow over the bearing housing 6 and around the nozzle box 9 in a manner indicated by the arrows 35 in Fig. 1. This bearing housing and nozzle box assembly has come to be well known to the art in connection with turbosupercharger construction, being disclosed more fully in a number of prior patents, for instance, No. 2,269,181, issued January 6, 1942, in the name of Edward B. Clarke. In order to prevent the cooling air 35 from coming into direct contact with the rotating bucket-wheel, and from entering the stream of hot turbine motive fluid, a sealing plate assembly indicated generally at I5 is provided. The details of the construction of this sealing plate do not form a part Of the present invention, but are more specifically described and claimed in an application Serial No. 541,244, filed June 20, 1944, in the name of Claude H. Auger, now Patent 2,442,579, issued June 1, 1948. It is sufficient to note here that the sealing plate extends from the nozzle diaphragm H) to the axial extension I3 of the bearing housing.

For cooling the exhaust side of the bucket wheel, a suitable cooling cap, indicated generally at 21, ma be mounted adjacent the downstream side of turbine disk 2. Such cooling caps are more particularly disclosed and claimed in prior patcuts, for instance No. 2,364,037, issued November 28, 1944, in the name of Chester W. Smith.

Secured to the nozzle diaphragm It, as by a circumferential weld at It. is a wall ll deflnim an annular chamber completely surrounding the bucket-wheel. A suitable casing l8 forming the turbine discharge conduit is secured to housing I! by a circumferential row or threaded fastenings I. A rabbeted joint between conduit II and wall ll defines an annular groove 20 arranged to receive the radially extending flange of an annular member 2|, so as to hold ring 2| in a desired position relative to the exit edges of the buckets 2.

There is an appreciable clearance space 22 between the nozzles H and the entrance edges of the buckets 3. This clearance may be made quite large for certain aerodynamic reasons; but

it must at least be sufliciently large, in view 01' the necessary manufacturing tolerances, to prevent any danger oi rubbing of the buckets against the nozzles. There is also a certain axial clearance between the exit edges of the buckets 3 and the adjacent edge of ring 2|. The inner diameter of ring 21 is slightly less than the overall or tip diameter of the bucket-wheel. On the other hand, the inner diameter of the wall H at the rabbet 20 is slightly larger than the tip diameter of the bucket wheel. By removing threaded fastenings 19, the discharge casing l8 and the shroud ring 2| can readily be removed for easy access to the bucket-wheel for inspection, servicing, or removal of the wheel through the opening defined by wall II.

It will be understood by those skilled in the art that, regardless of the size of the nozzle-tobucket clearance space 22, there will inevitably be some leakage of motive fluid radially outward through this clearance space, as represented diagrammatically by the arrow 23 in Fig. 1. It will likewise be understood that with shroudless buckets of the type shown in the drawing, there will inevitably be a certain amount of leakage of motive fluid radially outward from the open ends of the bucket flow paths, as represented by the arrow 24. This leakage of motive fluid represented by arrows 23 and 24 constitutes a thermodynamic loss which may be of appreciable magnitude, unless steps are taken to minimize such leakage.

The leakage losses referred to above are very largely prevented by my shroud arrangement. The invention comprises a closed shroud chamber surrounding th open ends of the shroudless buckets in such a manner that the leakage fluid represented by arrows 23, 24 is collected therein so as to build up an annular "cushion of fluid under pressure which will tend to prevent further leakage.

My invention is applicable to turbine buckets shaped to provide impulse action, as well as to those arranged to operate as reaction buckets and to those which operate partly by impulse and partly by reaction. If the buckets are designed to operate as substantially pure impulse buckets, then there will be littl or no static pressure drop from the clearance space 22 to the bucket exits. If, on the other hand,'the buckets are designed to operate with a substantial amount of reaction, then there will be a certain pressuredrop from clearance 22 across the buckets. Because of the higher pressure in the clearance space 22, the leakage represented by arrow 23 will be increased. This same pressure difierential will also cause the leakage 24 from the inlet portions of'the bucket flow paths to be somewhat greater than it would be otherwise. Furthermore, with a reaction type bucket, static pressures in the bucket flow paths adjacent the exits thereof may drop 4 low enough that there will be some tendency for gas in the shroud chamber to recirculate" back into the bucket flow path, as indicated diagrammatically by dotted arrow 25. In thus reentering the bucket flow path, this fluid delivers some energy to the bucket wheel. It the degree of reaction for which the buckets are designed is small, then there may be no flow as indicated by the arrow 25; but in all cases there will tend to be some small leakage flow through the axial clearance between the buckets and the shroud ring 2|,

in the manner indicated by the arrow 26.

I have discovered that with a shroud arrangement as in Fig. 1, the leakage gases 23, 24 build up a slight positive pressure in the shroud chamber, which pressure has a tendency to resist further leakage. It is also found that the gas in the shroud chamber sets up a vortex whirl about the axis of the bucket-wheel. This vortex may be strong enough to create a definite radial pressure gradient in the fluid in the shroud chamber, in a manner which will be readily understood by those familiar with the mechanics of compressible fluid flow. Thus the static pressure in the shroud chamber may be somewhat higher at the radially outer portion of the shroud chamber than it is immediately adjacent the bucket tips. This vortex whirl set up in the shroud chamber appears to have a further beneficial action in preventing recirculation of the fluid from the shroud chamber back into the motive fluid stream, in the manner of arrows 25, 26.

The axial clearance between the left-hand edge of shroud ring 2| and the exit edges of the buckets 3 is made as small as possible consistent with manufacturing tolerances and the requirements of ease of assembly and other mechanical consideration's, for instance the tendency of ring 2| or other parts of the assembly to warp slightly under the influence of the extreme temperature gradients encountered in the operation of high temperature gas turbines. Reducing this axial clearance to the smallest practical value has two important effects: (1') Itwill be obvious that the smaller this axial clearance, the smaller will be the effective area of the clearance space and therefore the smaller the leakage flow (per arrow 25) for a given pressure diiferential between the shroud chamber and the discharge conduit. (2) If this clearance space, bucket-to-shroud, is too wide, then there will be a tendency for the main stream of motive fluid discharged at high velocity from the bucket flow paths to entrain the comparatively static gas molecules in the clearance space, with the result that there will be a tendency for fluid to be sucked from the shroud chamber by a sort of ejector action. This ejector action can be reducedto a very low value, or prevented entirely, by reducing the axial clearance to a practical minimum.

In small, high performance gas turbines operating at very high speeds and high temperatures, such as those in turbosuperchargers used in connection with reciprocating in ernal combustion engines for aircraft, the normal working stresses may rather closely approach the maximum allowable safe limits for the materials used, so that it sometimes happens that a bucket or a portion thereof breaks and is thrown radially outward from the wheel. Such an accident may have serious consequences, resulting in the complete destruction of the turbosupercharger and perhaps loss of the aircraft, if the broken bucket fragment does not leave the bucket-wheel freely 1'6 but jams against adjacent structure in such a manner as to foul other buckets. It is a purpose of the arrangement shown in Fig. 1 to provide a shroud chamber of such size that any bucket fragments thrown from the wheel can be freely received in the shroud chamber, where they can rattle around" without interfering with adjacent buckets and causing further damage to the bucket-wheel. In this connection it will be noted that a bucketthrown from the wheel will follow a path in space having an appreciable tangential component, which will cause it to contact the inner surface of the housing I! at an acute angle, so that the fragment is readily deflected without rupture of the wall II. To take care of those cases where a whole bucket leaves the wheel, it is desirable that the radial width of the shroud chamber be of the same order of magnitude as the length of the buckets. Of course it is also necessary that the total axial clearance from the nozzle diaphragm III to the adjacent edge of shroud ring 2| be sufflcient to permit the widest part of the bucket to pass through freely.

In other applications, as for instance in turbosuperchargers for Diesel engines where the gas turbine is not operating under such extreme conditions, the working stresses may be sufliciently low that there is little chance of bucket breakage. For such applications the enlarged shroud chamber shown in Fig. 1 is not necessary, and

the shroud arrangement shown in Fig. 4 may be employed. In this arrangement, the shroud chamber is formed by a circumferential wall 28 extending axially from the nozzle diaphragm l and formed integral therewith. Wall "is provided with an inwardly extending annular portion 29 which defines a small axial clearance with the exit edges of the buckets 3. The discharge conduit It may have welded thereto a radially extending flange 30 which is detachably secured to a similar flange 3| on wall 28 by means of a known type of "band clamp. This may consist of a channel-shaped ring 32 split at one or more points and having the ends drawn together circumferentially by a suitable tension 'device, such as a bolt 33 projecting through lugs 34 fastened to the respective ends of the split ring 32. Such quick-detachable clamp devices are well known to the art, and the details of the clamp shown in Fig. 4 are not necessary to an understanding of the present invention.

The operation of the shroud arrangement shown in Fig. 4 is, from an aerodynamic stand-- point, very similar to that of the shroud arrangement of Fig. 1. The leakage gases 23, 24 build up a. pressure cushion" in the circumferential chamber defined by wall 28, 29 which cushion acts to prevent further leakage. Because the shroud wall 28, 29 is formed integral with nozzle diaphragm I0, it is obvious that the inner diameter of the portion 29 must be at least as large as the tip diameter of the bucketwheel, in order to permit removal of the wheel from the shroud. While having the inner diameter of wall 29 slightly greater than the tip diameter of the bucket-wheel results in a shroud arrangement not quite so effective aerodynamically (as compared with the results obtainable when the inner diameter of wall 29 is slightly less than the tip diameter of the bucket-wheel, as in Fig. 1), the shroud arrangement of Fig. 4 has been found to be very effective. It has some obvious advantages over the structure of Fig. 1

from the standpoint of smaller weight and mechanical simplicity.

In gas turbines of the type described, it is necessary to have frequent access to the bucketwheel to inspect the condition of the bucket surfaces, check the clearance between the buckets and the nozzle diaphragm and other parts. Both the shroud arrangements of Fig. 1 and Fig. 4 provide simple mechanical constructions which provide ready access to the bucket-wheel and related partsfor such inspection and servicing.

My invention provides a simple yet effective stationary shroud arrangement which permits the use of shroudless bucket-wheels, while obtaining efliciencies which compare favorably with those obtained with the more complex shrouded bucket arrangements known to the prior art.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an elastic fluid turbine having a bucketwheel with a circumferential row of shroudless buckets and nozzle means for supplying motive fluid to the buckets, the combination of stationary bucket shroud means for minimizing leakage comprising walls defining a circumferential chamber surrounding the tips of the buckets and communicating with the open ends of the bucket fiow paths and with the clearance space between the nozzle means and the leading edges of the buckets, said walls including a circumferential portion having substantially no radial clearance relative to the bucket tips and a small axial clearance with the exit edges of the buckets, said last-mentioned clearance being the smallest which will avoid mechanical interference between said circumferential portion and the buckets, and being smaller than the nozzle-tobucket clearance space.

2. In an elastic fluid turbine having a bucketwheel with a circumferential row of shroudless buckets and a nozzle ring for supplying motive fluid to the buckets, the combination of stationary shroud means for minimizing leakage of motive fluid radially outward from the nozzle ring and from the bucket flow paths, the shroud means comprising walls defining a circumferential chamber around the buckets and communieating with the open ends of the bucket flow paths and with the nozzle-to-bucket clearance space, said walls including an annular portion having an inner diameter substantially equal to the tip diameter of the buckets and a minimum clearance in an axial direction with the exit edges of the buckets, said last-mentioned clearance being the smallest which will avoid mechanical interference between said annular portion and the buckets, and being smaller than the nozzleto-bucket clearance space.

3. In an elastic fiuid turbine having a bucketwheel with a circumferential row of shroudless buckets and a, nozzle ring for supplying motive fluid to the buckets, the combination of stationary shroud means for reducing leakage of motive fluid radially outward from the nozzle ring and from the bucket flow paths, the shroud means comprising a circumferential wall secured in fixed relation to the nozzle ring and surrounding the bucket tips with an appreciable radial clearance, said clearance space constituting a shroud chamber communicating with the nozzle-tobucket clearance space and with the open ends of the bucket flow paths, the inner diameter of the circumferential wall being suflici'ently large to permit removal of the blllelrnt..whml it..-

surface forming at least part of the outer boundary of the bucket-wheel discharge passage, the diameter of said surface being no greater than the tip diameter of the bucket-wheel, said annular member having a circumferential edge portion forming a minimum axial clearance with the exit edges of the buckets.

4. In an elastic fluid turbine having a bucketwheel with a circumferential row of shroudless buckets and a nozzle ring for supplying motive fluid to the buckets, and stationary shroud means for minimizing leakage of motive fluid radially outward from the nozzle ring and from the bucket flow paths, the shroud means comprising a circumferential wall surrounding the bucket tips with a radial clearance therefrom substantially equal to the bucket height, said clearance space constituting a shroud chamber communicating with the nozzle-to-bucket clearance space and with the open ends of the bucket flow paths and adapted to receive broken buckets and fragments thereof thrown radially outward from the bucket-wheel, said circumferential wall.

having a radially inwardly extending annular portion with an inner diameter substantially equal to the tip diameter of the bucket-wheel, and said annular portion having a circumferential edge portion defining a minimum axial clearance with the exit edges of the buckets.

, 5. In an elastic fluid turbine having a bucketwheel with a circumferential row of shroudless buckets and a nozzle ring for supplying motive fluid to the buckets, the combination of stationary shroud means for reducing leakage of motive fluid outward from the nozzle ring and from the bucket flow paths, the shroud means comprising a circumferential wall secured to the nozzle ring and projecting axially therefrom and having an appreciable radial clearance with the bucket tips, said clearance space constituting a shroud chamber communicating with the nozzle-to-bucket clearance space and with the open ends of the bucket fiow paths, said wall having a radially inwardly extending annular portion with an inner diameter not less than the tip diameter of the bucket wheel. and defining a minimum radial clearance and a small axial clearance with the exit edges of the buckets, said small axial clearance being the minimum space which will avoid mechanical interference between said inwardly extending annular wall portion and the buckets. and being smaller than the nozzle-to-bucket clearance space.

6. In an elastic fluid turbine having a bucketwheel with a circumferential row of shroudiess buckets and a nozzle ring for supplying motive fluid to the buckets, the combination of stationary shroud means for reducing leakage of motive fluid outward from the nozzle ring and the bucket flow paths, theshroud means comprising a circumferential wall secured to the nozzle ring and projecting axially therefrom and having an appreciable radial clearance with the bucket tips, said clearance space constituting a shroud chamber communicating with the nozzle-to-bucket clearance space and with the open ends of the bucket flow paths, said wall having an inwardly extending annular portion with an inner diameter substantially equal to the tip diameter of the buckets and defining a minimum axial clearance with the exit edges of the buckets, said last-mentioned axial clearance being the smallest which will avoid mechanical interference between the inwardly extending annular portion of said circumferential wall and the buckets, and being smaller than the nozzle-to-bucket clearance space.

DAVID J. BLOOMBERG.

REFERENCES CITED The followingreferences are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,469,045 MacMurchy Sept. 25, 1923 1,771,023 Allen July 22, 1930 2,232,611 Keller Feb. 18, 1941 2,269,181 Clarke Jan. 6, 1942 2,319,995 Keller May 25, 1943 2,364,037 Smith Nov. 28, 1944 2,402,418 Kroon June 18, 1946 2,442,579 Auger June 1, 1948 2,459,850 Stine Jan. 25, 1949 FOREIGN PATENTS Number Country Date 500,713 Great Britain Feb. 14, 1939- OTHER REFERENCES Steam and Gas Turbine, by Stodola, 1927 edition, vol. 1, pages 697, 699, Fig. 703.

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