US2830751A - Gas turbine engines - Google Patents

Description

APril 15, 1958 N. L. QUINN ETAL "2,'83-0,751

GAS TURBINE ENGINES Filed March 21, 1955 fly/fl/f/f/aa f f4 /fff fhg/6 f/J ff ff a f 719 ggz 4W/Jaffa /f/f K ya f f 'aff/fafa /aff NRLQUINNJ United States Patent GAS TURBnsn ENGINES Neville Raymond Lloyd Quinn and Alexander Douglas Carmichael, Bristol, England, assignors, by mesne assignments, to Bristol Aero-Engines Limited, Bristol, England, a British company Application March 21, 1955, Serial No. 495,502 Claims priority, application Great Britain April 1, 1954 8 Claims. (Cl. 230114) This invention relates to gas turbine engines of the kind (hereinafter referred to as the kind described) comprising a compressor system for compressing working uid, said compressor system having more than one compression stage, combustion equipment for heating working iiuid compressed by the compressor system, and a turbine system for driving the compressor system, said turbine system being arranged for operation by the working fluid yheated by the combustion equipment. The turbine system may also drive an external load and may comprise two or more independently rotating rotors some or each of which is coupled to a compressor rotor.

By a compression stage is meant a passage of the working huid past a power-driven axial-flow, centrifugal flow or mixed-flow blade-row and may include a cooperating stationary blade-row whereby velocity head is converted into pressure head.

ln engines of this kind, more particularly those in which all the moving compressor blades are driven at the same speed, diliculty is frequently encountered at low running speeds in that the liow of working medium through the lower pressure compression stages is insuicient to avoid surging. To overcome this diiculty it has been the practice to blow off working fluid from an intermediate compression stage into the atmosphere until the speed or the compression ratio of the compressor exceeds a value atwhich such blowing off may be stopped without danger of surging occurring.

Hitherto, the blowing off of working uid from the compressor has been controlled by a speedor pressureratio sensitive device acting upon a servo system to open or close blow-off valves, but this involves considerable complication, additional weight, and risk of defective operation, and the object of the present invention is to provide a valve-less system which permits working fluid to blow-ofi from an intermediate compression stage only until running conditions are reached at which such bl'owi ing off is no longer necessary. To this end the present invention broadly provides a gas turbine engine of the kind described and further comprising means defining a ow path communicating an intermediate pressure zone of said compressor system with a low pressure zone in or in communication with a main workingliuid ow path through the engine in which low pressure zone there exists at all times during running of the engine a static pressure less than the static pressure in said intermediate zone, said low pressure zone being positioned downstream of a restriction in said main working fluid ow path, which restriction is such that it becomes or tends to become choked as the speed of the compressor system increases, said flow path defining means including a chamber having an inlet from said intermediate zone and an outlet to saidlow pressure zone which` outlet is at a lesser radial distance from the axis of said chamber than said inlet, said chamber being arranged to rotate` about its axis duringoperation of the engine, at aspeed related to the speed of a compressor of the compressor system, the arrangement being such that below a predetermined speed ICC of said compressor a free vortex of working fluid is generated in the chamber, and above said predetermined speed a partially degenerated or totally degenerated vortex is generated in the chamber.

The compressor system may comprise a bladedcompressor rotor having a number of blade rows, in which case the intermediate pressure zone is a zone between an adjacent pair of rows of blades.

In another case, however, the compressor system may comprise a low pressure compressor rotor anda high pressure compressor rotor, in which case, the intermediate pressure zone may be at the exhaust end of the low pressure compressor, or it may be at an intermediate pressure stage of the low pressure or high pressure compresysor in the case where the low pressure compressor or the high pressure compressor comprises a bladed rotor having a number of blade rows.

As will hereinafter be more fully explained, when a free vortex exists in the chamber a ow of working tiuid takes place from the intermediate zone through the chamber to the low pressure zone. When, however, a degenerated or partially degenerated vortex exists in the chamber blow-off of working fluid from the intermediate zone does not take place. Up to said predetermined speed of said compressor, therefore, working `fluid is blown off from the intermediate pressure zone but above the predetermined speed the blow-off ceases, the predetermined speed is chosen to be that speed of said 'corn-` pressor at which running conditions are such that blowoff is no longer necessary, and no valve means is provided in the flow path defined between the interme diate pressure zone and the low pressure zone.` The invention, therefore, provides a valve-less system which permits working fluid to blow off from an inter-` mediate compression stage only until running conditions are reached at which said blowing-0E is no longer necessary. In one form the present invention may consist in a gas turbine engine of the kind described, including aring of turbine nozzle guide blades, a driven rotor having an interior chamber bounded in each axial direction bywalls which are surfaces of revolution about the axes of rotation of the Vrotor and which are substantially freefrom features likely to hinder free-vortex` movement of a gaseous medium in the chamber, means defining a first opening to the interior of the chamber, means defining a second opening to the interior of the chamber,fsaid second opening being positioned at a lesser radial distance from the axis of rotation of said rotor than said first opening, means defining an opening (hereinafter referred to as the third opening) to the main working uid flow path through the engine at a position in the compressor system corresponding to partial compression' of the working fluid during running of the engine, first conduit means connecting the third opening with said first opening, and second conduit means connecting said second opening with a low pressure zone in or in communication with a main working fluid flow path through ithe engine vdownstream of said ring of turbine nozzle guide blades in which low pressure zone the static pressure during running of the engine is lower than the static pressure at the third opening, the radial distances between said first opening and the third kopening and between said first opening and said second opening being so related to the changing pressure differential set up between the pressure at the third opening and the pressure in said low pressure zone during variable speed running of the engine that a significant ow of working fluid takes place from said intermediate zone to the low pressure Azone through the third opening and the chamber and .said conduit means only at speeds below a predetermined speed.

jlln,- ano therv fnorm, the present invention may consist inhagas turbine engine of the kind described including a ring of turbine nozzle guide blades and a driven compressor rotor having an intermediate compression stage, said rotor comprising axial-flow blading carried vupon the outer periphery'of the rotor, the rotor having an interior,,chambergbunded by the outer peripheral wall of the rotor and walls spaced apart axiallyof the rotor, said ,chamber communicating with said intermediate compressionstage through a peripheral opening in the outer peripheral Wall of the rotor, the chamber also communicating through a wall opening in one of saidaxially spacedyvllls nearer its axis of rotation than said peripheral; opening kwith a lowpressure zone in or incomrnuniation with amain working fluid flow path through thetengine A downstream of said turbine nozzle guide blades, inyvhich zone, ,duringrunning of the engine, the staticpressureis lowerthan the static .pressure at said peripheral opening, ,and s aid rotor alsobeingprovided infsaidk chamber, `with air-movement-restricting means -extending over a part only ofthe radial distance between said-peripheral opening and said wall opening whereby, during running of the engine lfree-vortex movement of air insaid chamber,.where provided with air-movementrestricting means, is prevented, the walls of the remainder of said chamber being surfaces of revolution substantiallywfree from features likely to hinder free-vortex movement of air therein, the radial extent of said airmovement-restricting means `being selected so that belowa predetermined speed of said rotor a free vortex of airis generated in said chamber, and above said predetermined speed a partially degenerated or totally degeneratedvortexis. generated in the chamber.

Two embodiments of the present invention willV now he .described byway of example with reference to the accompanying drawings whereof,

'".zFigure 1 is .a diagrammatic side elevation in crosssection of one gas turbine engine in accordance with the present invention, l

V'FigureZ isa diagrammatic side elevation in crosssection of another gas turbine engine in accordance with the present invention, land fzI-igure 3 is a diagrammatic showing of pressure ratios within .the disclosedfree vortex chamber, relative to rotaryspeeds.

.The gas turbine .engine shown diagrammatically in Figure .1 .comprises a compressor system 1,V having a rotor'2 with'fvestages of axial-flow blading and a stator 3,. the compressor system delivering compressed air through au outlet 1S into combustion equipment 4-into whichfuel is injected, for combustion, by burners rS to heat the air compressed in the compressor system, and a turbine .system 6 including a rotor 7 with axial-flow bladingandnozzle -guideblading 8, which turbine system receives al1-the yproducts of combustion of the ,combustionequipmentwhich products drive the turbine. The turbine rotor 7 is connected to the compressor rotor 2 bya .hollow shaft 10 so that these parts rotate together. The exhaust-.gases from the turbinel are discharged into the atmosphere .along an exhaust duct 9. y

Tolavoid lsurging inthe compressor 1 during starting and -slowspeed running of the engine it is necessaryrto blow off `air `from an intermediate compression stage. In this example, air is blown olf from the third compression stage through a number of similar equally spaced radially extending conduit means 11 each having an opening 12 at vone end into a chamber 13 formed in the interior of the compressor rotor 2 and an opening 14 at the other end into the working-fluid passage v15 of the engine .atthe third compression stage, the conduit means llfbeing positioned in the chamber 1 3. The blow-oir air leaves the chamber 13 through a central openlng 16 into the hollow shaft 10, and from the shaft passes through an opening 19 in` the turbine rotor?? :4 into a low pressure zone 20 which is in communication with the working-fluid-passage 15 downstream of the turbine nozzle guide blades 8 and of the turbine moving blades. The hollow shaft 10, therefore, constitutes a further conduit means connecting the opening 16 with the zone 20.

The chamber 13 is bounded in the forward axial direction by a wall 21, and lin the rearward axial direction by a wall 22, both these walls being surfaces of revolution relatively to the axis 17 and smoothly surfaced so as to be free ofrfe'atures likely to hinder free-vortex movement of air in the chamber 13.

During running of the engine the zone 20 has a lower static pressure than that which exists at the opening 14. Furthermore, the pressure ratio between a point in the working fluid passage of the engine in the region of the openings 14 and the zone 20, hereinafter referred to as P14/P20, has a drooping characteristic of the kind indicated by the curveA in Figure 3, in which compression Vratio R is plotted against non-dimensional compressor speed N/\/T. The reason for this is that as the compressor speed increases, the tlow through the nozzle guide blades 8 becomes, or tends to become choked, with the result that the pressure ratio between the outlet of the compressor and the zone 2t] (P18/P20) becomes, or tends to become, constant, while the pressure ratio between the outlet of the compressor and the region of the openings 14 (P18/P14) continues to rise approximately as the square of the compressor speed. Considering the ultimate state, in which n- LL l 2 Pza-A 'and YP 14T- A and B being constants,

it follows that Pa B NHT that is to saythatPM/Pzo varies inversely as the compres,sor`.speed, and the curve A in Fig. 3 changes progressively yfrom an approximation to a square-low curve to an approximation to a rectangular hyperbola.

When air passes into or out of the chamber 13 through the openings 12 or 16 a drop of pressure occurs across the opening concerned, and it will be appreciated that fora How of air to pass through the chamber such pressuredrops must be in the same sense, so that the mean of the pressures on the inner sides of the openingsy will be equal or approximately equal to the mean of the pressures on the outer sides of the openings according to whether the pressure drops through the openings are, equal or unequal. l.Having established this preliminary relation it will be convenient in considering theoperation ,ofltiie systemto compare the ratio lof the inner side pressures withthe ratio of the outer side" pressures, thesejratiosbeing non-dimensional, so'as to determine whetherow takes place, and if svo, in which direction.

If, when the engine uis running Aat a particular speed, the pressure'ratiobetween the outer sides of the openings 12"'and 16 as determined by the prevailing value of the pressure ratio P14/P20, is greaterthan the pressure ratio across aifree vortexifrmed in the chamber 13 between the inner Asides v 0f the -openings (the. whirl velocity of the vortex'at its outer periphery being equal to the angular velocity of the compressor rotor) then a How of air will take place intozthe chamber lthrough the openings 12 and out through the opening 16. As the speed of the engine increases, thepressure" ratio across the free vortex increases, butv therpressu're ratio across the outer sides of the openings *12 andl increases less rapidly or decreases, for the reasons already explained, so that a conditionis reached at which the two or equal. As this conditionfis approached the `flow through the chamber 13 falls Vrapidly land reaches a minimum value suiiicient only to keep the free vortex in existence. Further progressive falling of the external pressure ratio results in the free vortex progressively degenerating into a forced vortex from its inner periphery outwardly, a very small ow being maintained out through the opening 16 during this period. When a state is reached at which the free vortex has become wholly converted to a forced vortex, ow through the chamber 13 ceases, this being the case when the difference in pressure between the' outer sides of the openings 12 and 16 is equal to the dierence between the centrifugal head pressures corresponding to the radii at which the openings 12 and 16 are situated. If the external pressure ratio continues to fall below the forced vortex pressure ratio a small ow will occur into the chamber 13 through the opening 16 and out through the opening 12, but this flow will start to destroy the forced vortex since this latter is only maintained by the boundary layer friction against the walls of the chamber. In other words, the main body of air in the chamber 13 will progressively lose rotational speed relatively to the walls of the chamber as the air-ow through the chamber increases, so that the excess of centrifugal head over the external pressure difference remains small until the external pressure at the opening 16 exceeds the external pressure at the opening 12, but this condition is of course, never reached during the operation of the engine.

When the speedof the engine decreases again the process reverses, that is to say, first a forced vortex is generated and this gradually changes to a free vortex, and only when this had been fully established can any substantial flow of blow-off air take place,

It will be clear, therefore, that by suitably choosing the radial distances of the openings 12 and 16 from the axis of rotation it can be arranged that blowing off of a significant quantity of air occurs only at engine speeds below a selected value appropriate to guard against surging in the compressor. This is illustrated in Figure 3 in which the curves B1, B2 and B3 show for three values of the ratio of the inside and outside diameters of the free vortex chamber, the variation with speed of the pressure ratio between the inside and outside peripheries of a free vortex therein. Neglecting the centrifugal pressure head generated between the openings 12 and 14, which may if necessary be balanced by the provision of vanes 37 on the downstream face of the turbine rotor 7 in the zone 20, the point of intersection of a curve B with the curve A denotes a condition at which the pressure ratio between the blow-off openings 12 and the zone 20 is just equal to the pressure ratio between the inside and outside peripheries of a free vortex in the chamber 13. At this speed therefore, and -at all higher speeds, no significant blow-off flow can take place through the chamber 13, but at lower speeds there is an excess of pressure ratio available, as.

indicated, for example, by the ordinate 40 applicable to the curve B2, which will cause a blow-off ow of air through the chamber 13.

The quantity of air which is blown o at any particular engine speed depends upon the excess pressure difference represented by the ordinate 40 available to cause the flow and upon the resistance of the ow path, as determined by thecross-sectional areas of the openings and ducts through which the ow takes place and any other sources of pressure loss.

The conduit means or tubes 11 are of such a radial extent into the rotating chamber 13 that the centrifugal head within the chamber 13 at the critical speed, when surging begins, is .such as to allow by-passing of uid through tubes 11, hollow shaft and openings 19 to the zone of lower pressure 20.

Figure 2 shows a somewhat different arrangement of the vortex chamber in an engine which is basically the same as that shown in Figure 1 and in which the same parts are denoted by the same reference numerals.

In this arrangement, the compressor rotor 2 carrying upon its outer periphery five stages of axial-How blading is provided with an interior chamber 13 communicating with the working-iuid-passage 15 at the third compression stage through a ring of peripheral openings 30 in the outer peripheral wall of the rotor. The chamber 13 also communicates through a wall opening 31, concentric with the axis of rotation, and in the rear wall 22 of the chamber and through a hollow bore 32 in the shaft 10 with la zone 20 which is in communication with the workingiiuidpassage 15 of the engine downstream of the turbine 6 and the ring of turbine nozzle guide blades 8 and, therefore, during running of the engine has a lower static pressure than that which exists at the peripheral openings 30. The rotor 2 is also provided in the chamber 13 with a number of radial vanes 34 which extend from the axis of rotation over a part only of the radial distance between the openings 31 and the openings 30. The vanes 34 may also have extended parts 41 located in the bore 32 of the shaft 10. The remaining parts of the side walls 35 and 36 and ofthe outer peripheral wall are surfaces of revolution substantially free from projections or other features likely to hinder free-vortex movement of air in that part of the chamber not provided with vanes.

The operation of this arrangement is very similar to that of Figure 1 in that the ratio of the inside and outside diameters of the free vortex chamber, which may be adjusted by varying the radial extension of the vanes 34, is so chosen that the free vortex breaks down at an engine speed at which blowing-otf of air from the compressor is no longer required.

If desired, radially extending vanes 37 may be provided on the downstream face of the turbine rotor 7 to balance the centrifugal pressure head generated through the vanes 34.

We claim:

1. A gas turbine engine comprising a multi-stage compressor having a rotor and an intermediate pressure zone, combustion equipment and a turbine through which a working uid passes in sequential flow and having a critical speed below which surging takes place in the compressor and a restriction in said sequential flow down stream of said intermediate pressure zone so constructed as to become progressively more choked as the speed of the compressor increases providing a pressure zone downstream of the restriction and having a pressure less than said intermediate zone, blow 0E path means comprising a chamber encircling the axis of rotation of said compressor and rotated by the compressor at the speed of the compressor, said chamber having an inlet communicating with the intermediate pressure zone, having a passage through said rotor Iand having an outlet communicating with the lower pressure zone, said inlet being positioned at a substantially greater radial distance from said axis than said outlet, and means attached to said chamber and rotating with said chamber for selecting the centrifugal head developed on the air within said rotating chamber to such a predetermined value that when the engine is running below its critical speed the iiuid from the intermediate pressure zone enters the chamber and forms a free vortex therein and ows through the outlet to the lower pressure zone and when the engine is runnin at or above its critical speed a forced vortex is formed the chamber the centrifugal head of which is suflicient to prevent the working uid from entering the chamber.

2. A gas turbine engine as claimed in claim l, wherein said communication between said outlet and lower pressure zone means comprises a hollow shaft by which said blades.

speedbelow which A'surging taires place inthe compressor i and a' restrictionin said sequential flow downstream .oi said intermediate p eslsfurezone soconstructed as to become progressively'rnore chokedas 'the speed .of the compressor increases providing apressure zone downstream of the restriction and having a pressure'rless than said intermediete, 1201.1@ blOW Oft flaw' Path means @mariana .ehember encirclingithe vajisnof` rotation of said `compressor and retated by the c .ornp'res'sorrat the speed of lthe compressor, saidn chamber having Nan inlet communicating withthe intermediate pressure zone'and an outletl communicating pressure zone, Asaid inlet being positioned at a substantially 'greaterl radial distance from s aid anis than'said outlet whereby when the engine is runningbelow 'its critical speed'the fluid lfrom the intermediate pressure zone enters the 4chamber and'forms a free vortex therein i and'ows through lthe outlet to the lower pressure zone and whenthe engine is running at or above its critical speed a forced vortex is formed in the. chamber the vcentrifugal head of which is suicient to prevent the working iiuid from entering the, chamber, a ring of turbine nozzle guide blades on the turbine, s aid rotor comprising axial-how blading carried upon the outer periphery of the rotor, saidiehamber comprising the outer peripheral wall of the rotor ,and walls spaced apart axially of the rotor', said inlet comprising av peripheral openingv in -the outer peripheral wall of the rotor, said outlet being formed in one of said axially spaced wallsrsaid low pressure zone in or inr communication with a main Working fluid flow path being downstream of said turbine nozzle guide blades'jin which low pressure zone, during running of theengine, the static pressure'isvlower than the static pressure at said peripheral opening, air-movement-restricting means on said' rotor within said chamberand entend- ,ing over a part only of the radial distance between said peripheral opening and said wall opening whereby, duringl running of the engine the free-vortex movement of air in said chamben'where provided with air-movement'- restricting means, is prevented, the Walls of the remainder of said chamber being surfaces of revolution substantially free from features likely to hinder freeQvorteX'rnovem'ent of air therein, the vradial extent of Vsaid air-movementrestricting means being selected so that below a predetermined s'peedlof said 'rotor a nfree vortex vof air is vgenerated' in said chamber, and above said predetermined speed a partially'degenerated or'totally degenerat'edvori teX isl generated inthe chamber.

gas turbine engine as claimed in claim 4, wherein saidl `airmoverrient-restricting means comprises 1* illy extending varies.

s V "ddjlarf'sjwhijeaar'e l'cated in 'said eomm'u cation betvveen ."said outlet an'dl'o'werA ss'u're ,zm. ,l f7. A` gas turbine engine asclaimed in claim 4,.,whevrein said turbine is .provided with a .rotor and said rotorfris providedon its downstream face with a plurality of radially extending vanes which balance the centrifugal`vpres- Vsure head developed through said air-movement-restricting means. l i

3. A gas turbine engine comprising a multi-stage cornpresser having a rotor and anintermediate pressurezonc, combustion equipment and a turbine through which a working yfluid passes in sequential tlow and having a critical speed below which surging takes place in ,the comprcssorand a restriction in said sequential ow drownstream of lsaid intermediate pressure zone so constructed asl to become progressivelymore choked v,as the speed of the compressor increases providing a pressure zone Vdownstream of the restriction'and having a pressurless thanlsaidintermediate Zone, blow offilow path 'm is Ycomprising aI chamber encircling the axis of rotation `of said compressor .and rotated by the compressorffat `the lspeed of the compressor, said chamber having aninlet communicating with the intermediate pressure zon'efa'nd an outlet communicating with the lower pressurejzlojne, said inlet being positioned at a substantiallyl .greater radial distance from said axis than said outlet whereby when the engine is running below its critical speed the fluid from the intermediate pressure zone enters. the chamber and forms a free vortex thereinfandoivs through the outlet to the lower pressure zoneandwhen the engine is running at or above its `critical Vspeed a. forced vvortexis formed in theichamber the. centrifugal head yof which isl suicient to prevent the working from entering the chamber, air-movementfrestrieting means on'said rotor within said chamber and ektending over a part 4only ofthe radial distance between saidinlet andoutlet whereby, during running of the .engine'the free-vortex movement of air inl said chamber, where priovided with air-movement-restricting means, is prevented, the .inner surfaces of the remainder of said chamber being surfaces of revolution substantially free fromfe'atures likely to hinder free-vortex movement Aof air therein, the radial extent of said air-movement-restrictingi means being selected so that below a predetermined speed of said rotor a freevortex of air is generated in said lcharnber, and above said predetermined speed a partially degenerated or totally degencrated vortex is generated in thechamber.

-References Cited in the file of lthis patent UNTTED STATES PATENTS 2,483,616 `Bergstedt OCLV, 1249 2,k6\`1 8f433 Loos'ietall Nov. 18,' i952 2,636,665 remis-ard Apr. 28,' 1953 2,680,001 Barr rune 1,' A19,54

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