US2726806A

US2726806A - Axial compressor

Info

Publication number: US2726806A
Application number: US198779A
Authority: US
Inventors: Keast Francis Henry
Original assignee: AV Roe Canada Ltd
Current assignee: AV Roe Canada Ltd
Priority date: 1950-12-02
Filing date: 1950-12-02
Publication date: 1955-12-13
Anticipated expiration: 1972-12-13

Description

Dec. 13, 1955 F. H. KEAsT Axm. COMPRESSOR 2 Sheets-Sheet l Filed Dec. 2. 1950 //V VE N TOR FH KEA s T ibilan ma Nt. m3

mm 3. wn mw Q mm n mw E Dec. 13, 1955 F. H. KEAST AXIAL COMPRESSOR 2 Sheets-Sheet 2 Filed DSC. 2, 1950 I/VVENTO/ FHA/EAST El? United States Patent i MAL COF/WRESSOR Francis Henry Keast, Toronto, Ontario, Canada, assigner to A. V. Roe Canada Limited, Malton, Ontario, Canada, a corporation Application Becember 2, 1950, Serial No. 198,779

4 Claims. (Cl. 230-122) This invention relates to axial air compressors, and particularly to those used in aircraft gas turbine engines.

The main object of the invention is to provide in a compressor an annular air passage which is so arranged as to distribute electively the loading on the various blades, taking into consideration the high Mach numbers encountered in the first stage. Another object of the invention is to eliminate as far as possible the manufacturing diiculties involved in making fundamentally different blades for each stage of a compressor.

Other objects and advantages of the invention will be apparent during the course of the following description.

In the accompanying drawings forming a part of this pecitication, and in which like reference characters designate like parts throughout the several views:

Fig. l is a longitudinal cross-sectional View of the upper half of a compressor, the lower half which is not shown being a mirror image of the said upper half; and

Fig. 2 is a set of diagrammatic representations of the air velocities at various stages in the compressor.

For convenience, the axial compressor depicted in Fig. l is shown as having ten stages, but the claimed invention is not confined to compressors of any particular number of stages, and its principles may be applied to any axial compressor of the kind described. Each stage comprises one circumferential row of rotor blades mounted on the rotor and one circumferential row of stator blades mounted on the stator casing. In Fig. l the rotor blades of the rst stage are designated 1R, while the symbol 1S 4designates the correspondingy stator blades. This form of identification is used throughout the various stages, in the last of which the symbols K and 10S indicate the tenth or last stage of rotor and stator blades respectively. Immediately forward of the first row of rotor blades 1R is a circumferential row of inlet guide vanes 11, and at the other end of the compressor the stator blades 10S discharge the compressed air into difusers 12. All the rotor blades are mounted externally on the rotor drum 13 while the stator blades are mounted internally on the casing 14. It will be understood by those familiar with axial compressors of this type, that the front bearing of the rotor 13 is mounted in a housing 15 which is supported from the casing 14 by longchord streamlined struts 16, so that the inlet guide vanes 11 may be supported at their outer diameter on the casing i4 and at their inner diameter on the housing 15.

The velocities at which the air enters and leaves cer tain stages of the compressor, at some speciflc radius, are illustrated diagrammatically in Fig. 2 by means of vectors, each vector representing the corresponding air velocity in magnitude, direction and sense. The air emerges from the inlet guide vanes 11 with a velocity which is represented by the vector V1 and this vector may be resolved into axial and tangential components, namely the vectors Val and Vwi respectively. Now if u is the tangential velocity of the rotor blades 1R at the l radius under consideration, 1t is evident that the vectorv 2,726,806 Patented Dec. 13,. 1955 2 Vn, representing the relative velocity at which the air is introduced to the said rotor blades, is made up of two components V1 and u, the u vector being of equal magnitude and opposite sense to the tangential velocity of the blades.

The velocity relative to the rotor blades 1R at which the air leaves these blades is. depicted by the vector Vrz and it follows that the absolute velocity of this air is represented by the vector V2; made up of. the components Vrz and a vector of magnitude u, representing the tangential velocity of the blades. This absolute velocity V2 is the velocity at which the air is introduced to the first row of stator bla-des 1S and it may be resolved axially and tangentially into the components represented by the vectors Voz and Vwz respectively.

The air leaving the tirst row ofV stator blades 1S has a velocity represented by the vector Vs, having' a tangential component Vws, and this air is introduced to the second row of rotor blades 2R with a velocity, relative to the said rotor blades, represented by the Vector Vra. Thevelocity Vr.; is the resultant of the component V3 and, as in the case of Vri- (since all the rotor blades are mounted on a common rotor 13), a component u of equal magnitude and opposite sense to the tangential velocity of the rotor bla-des.

By similar reasoning, the vector diagram for each stage of the compressor may be developed: it is believed that from the foregoing description the series of suixes will be suiciently understood and therefore the diagrams for the intermediate stages are not shownl in the drawing;

Those familiar with the compressor art will. understand that the work done by the rotor blades 1R. on a unit mass (l 1b.) of air is gwn-Vw.)

where g is the acceleration due to gravity. As a result of this work' the air experiences a temperature rise and therefore the speed of sound inthat air is higher as the air leaves the rst rowrof stator blades 1S than it was when the air left the inlet guide vanes 11. Since the velocity at which the air may be introduced, to any row of rotor or stator blades is limited bythe critical Mach number, it will be evident that due to the aforesaid increase in the speed of sound it is permissible to introduce the air to the second row' of rotor blades atv a velocity greater than the velocity atl which it was introduced to the first row; in other words Vrs may' be' greater than Vri. By reference to the vector diagrams it will be seen that the magnitude of the relative velocity at which the air -is introduced to a row of rotor blades is dependent upon the difference between the tangential velocity ofV the said blades and the whirl velocity, that is the tangential component of the velocity' at which the air leaves the preceding stator blades or guide vanes; e. g'. for a given value of u', the magnitude of Vri will be reduced by an increase in the value` of Vwi. Hence, since as has been shown Vrs may be greater than Vri, andV since u is constant at a given. radius, Vw's maybe less than Vwi.

From similar considerations of the Mach number permissible on the stator blades, it may be deduced that V4 may be greater than V2 and the tangential component Vwi may be greater than Vwz. Thus the work done by the second row of rotor blades 2R, i. e.

may be appreciably greater than the work done by the first row of rotor blades, 1R, ie.

as stated hereinbefore.

In order to facilitate production it is desirable that identical blade forms should be employed at each stage,

It will be explained later Vthat the last stage must also be treated as lan exception; thus only the intermediate stages, namely the second to the ninth in the compressor described, may embody blading which, at any given radius, is identical in aerofoil section and incidence to the rotor blades 2R and the stator blades 2S of the second stage. Y Y

Since the axial velocity V a of the air stream is substantially constant, fromA considerations of mass ow it will be understood that the cross-sectional area of the annular air passage between the rotor drum and the stator casing is inversely proportional tothe density of the air. At any stage the ratio of the density at outlet to the density at inlet is proportional to a positive power (-1-) of the ratio of the temperature at outlet to the temperature at'inlet, the compression being adiabatic. Since an equalamount of work is done by each stage from the second to the ninth inclusive, the temperature rises by Y the same amount across each stage, and it follows that the ratio of the temperature at outlet to the temperature at inlet Vof each stage decreases progressively along the passage; therefore the ratio of the cross-sectional area of the inlet to the cross-sectional area of the outlet of each also decreases progressively. In consequence the annular air passage converges progressively from the second'to the ninth stage.V In the compressor described, the radius of the rotor drum is constant from the second to the sixth stage so Vthat identical rotor blades may be used by cropping the tips to suit the convergence of the annular passage; The corresponding stator blades are similarly identical to one Vanother b ut in this case it is necessary to crop the roots rather than the tips, a procedure which may present some diiculties depending upon the type ,of` blade construction employed, though still obviously advantageous, from the manufacturing viewpoint, overV the use of blades of different aerofoil section at each stage.

Towards the rear of the Vcompressor it is desirable to increase the mean radius of the annular air passage so that the diifusers 12 may'lead smoothly from the compressor into the combustion chambers 17, which, by reason of their size, must necessarily be situated on a fairly large Vpitch circle about thel axis of the engine. Therefore from the seventh to the ninth stages in this construction, the outer radius of the annulus, that is the inner. radius of the stator casing, is held constant and the radius of the rotor drum is progressively increased. This feature is apparent in Fig. l, from the change in the longitudinal is less than the work done by succeeding stages the passage must converge less rapidly over the rst stage than over the subsequent stages. Furthermore, considering the rst row of stator blades 1S, if Vwa is less than Vwr, the said stator blades must remove all the whirl or tangential velocity contributed by the rst row of rotor bladesV 1R together with some of the whirl originally contributed by the inlet guide vanes 11. Thus the pressure rise across the stator blades 1S is very high, that is the Vblades are heavily loaded. Now, as will be known tov those skilled in the art,facross any row of stator blades, neglecting the losses which are small,

where p is the pressure, p is the density, Va is the axial velocity, Vw is the whirl velocity, and HM) is a correction for Mach number M. Across the rst row of stator blades, Vw is reduced by a large amount, so that p rises by a large amount, and to keep this inevitable pressure rise to a minimum, the shape of the annular passage must be so arranged that Va is increased, or at leastnot reduced, in passing through this particular row of stator point 19. It will be appreciated that the cross-sectional Y area of the annular passage diminishes more rapidly across the row of stator blades 1S of the first stage than across the stages intermediate the rst stage and the last stage.

contour of the rotor drum immediately forward of the 1 blades 7R.

It will be understood that from the seventh to the ninth stages the blades used maybe of the same aerofoil section and incidence as those used from the second to the sixth stages, but from the seventh tothe ninth stages the tips of the stator blades and the roots of the rotor blades V must be croppedl to conform to the convergence of the annular passage. Y Y Y Reverting to the first stage, as has been indicated the rapidity of convergence of the Vannular air passage across any particular stage is determined by the ratio of the temperature at outlet to the temperature at inlet, that is by the amount of work done by the stage, and since the work done by the first stage, i. e.

The design of the last stage, namely the tenth stage in the compressorrdescribed herein, is dictated by the following considerations. Since equal worky is performed on the air passing through the compressor by eachV stage from the second to the ninth inclusive Vand since the said stages employ blades of identical aerofoil form and in-` cidence, it will be understood that the whirl velocity of v the air leaving the stator blades 9S is equal to Vws. It is necessary that this whirl velocity should be removed from the air stream by the last row of stator blades 10S, before the said air enters the ditusers 12, and therefore the last row of stator blades must not only eliminate the whirl velocity applied by thel rotor blades 10R but must also remove the residue of the whirl velocity originally imparted by the inlet guide vanes 11 and maintained throughout succeeding stages of the blading. In` Vconsequence there will be a considerable increase in pressure in the air passing through the stator blades 10S and, for reasons similar to those hereinbefore explained in connection with the first stage, it is desirable that the 'axial velocity Va should beincreased, or at least no t reduced, in passing through the said stator blades of the last stage: to this end it is obviously beneficial to reduce the axial velocity at Vwhich the air is introduced to the vstator blades and this is achieved by increasing the width 'of the annular air passage in the neighbourhood of thew leading edge of the said blades. It will be notedV in Fig. V1 that there isa substantial reduction in the radius of the rotor Adrum across the last row of rotor blades, the Vradius of the stator remaining constant, and consequently the cross-sectional area of the passage'increases across Ythe row of blades ofthe last stage; .thereby there is effected a departure from the uniform convergence of the f annular passage between the rotor and the stator.

In addition, the rotor blades IGR may be so shaped Athat the whirlvelocity whichthey impart to the air is Yless than the whirl velocity applied by other rows of rotor blades. The workv done by the tenth stage may be, fromV l5,.thiscause, materially less thanY the work done by prej vious stages, but it results in an increase in static pressure over the whole stage comparable to the increase in pressure over previous stages, the reduction in work being oifset by the total diffusion of the residual whirl velocity. It is realized that the residual whirl velocity Vwa could be eliminated progressively from stage to stage down the compressor, but this would entail a change in the blade design from stage to stage with attendant manufacturing complications: it is preferable to accept the aforesai-d high loading of the blades S and to ameliorate it by arranging that a substantial proportion of the diffusion occurs in the rotor blades 16K, thereby transferring some of the loading from the stator blades to the said rotor blades.

It will be understood that the form of the invention herein shown and described is to be taken as a typical example of the same, and that various changes in the relative size and arrangement of the parts may be resorted to without departing from the spirit of the invention or the scope of the subjoined claims.

What I claim as my invention is:

l. An axial ow compressor having a stator, a rotor journalled coaxially within the stator and defining therewith an annular passage, and blades extending across the said passage and mounted in circumferential rows alternately upon the stator and the rotor, each row of rotor blades with the row of stator blades next downstream constituting a stage of the compressor, adjacent rows of blades interacting aerodynamically so that on rotation of the rotor a stream of air is drawn through the passage and progressively compressed therein, the characteristics of the ow of air entering each successive stage after the first stage depending on the blade characteristics of the preceding stages, the cross-sectional area of the passage at the stages intermediate the first and last stages diminishing progressively in a downstream direction, the cross-sectional area of the passage at the first stage diminishing at a rate which is less than the rate of diminution of the cross-sectional area of the passage at the said intermediate stages, and the cross-sectional area of the passage at the last stage diminishing at a rate which is less than the rate of diminution of the cross-sectional area of the passage through the said intermediate stages in order to impress on the air which has been given ow characteristics in the preceding stages desired flow characteristics on finally leaving the compressor.

2. An axial compressor as claimed in claim 1 in which the cross-sectional area of the passage diminishes more rapidly across the row of stator blades of the first stage than across the stages intermediate the first and the last stages.

3. An axial flow compressor having a stator, a rotor journalled coaxially within the stator and dening therewith an annular passage, and blades extending across the said passage and mounted in circumferential rows alternately upon the stator and the rotor, each row of rotor blades with the row of stator blades next downstream constituting a stage of the compressor, adjacent rows of blades interacting aerodynamically so that on rotation of the rotor a stream of air is drawn through the passage and progressively compressed therein, the characteristics of the ilow of air entering each successive stage after the iirst stage depending on the blade characteristics of the preceding stages, the cross-sectional area of the passage at the stages intermediate the iirst and last stages diminishing progressively in a downstream direction, the cross-sectional area of the passage at the first stage diminishing at a rate which is less than the rate of diminution of the cross-sectional area of the passage at said intermediate stages, the cross-sectional area of the passage at the last stage diminishing at a rate which is less than the rate of diminution of the cross-sectional area or" the passage through the said intermediate stages in order to impress on the air which has been given ow characteristics in the preceding stages desired iiow characteristics on finally leaving the compressor, the diameter of the stator at the first stage and at least two consecutive stages upstream of the last stage being uniform, while the diameter of the rotor progressively increases in a downstream direction at the irst stage and at at least one of the stages next preceding the last stage, and the diameter of the rotor at the other stages excepting the last stage being uniform while the diameter of the stator at the stages other than the previously mentioned stages of uniform diameter and other than the last stage decreases in a downstream direction.

4. An axial compressor as claimed in claim 3 further characterized in that the diameter of the rotor gradually decreases in a downstream direction across the last stage.

References Cited in the tile of this patent UNITED STATES PATENTS 2,213,940 Jendrassik Sept. 3, 1940 2,326,072 Seippel Aug. 3, 1943 2,361,887 Traupel Oct. 31, 1944 2,435,528 Barlow Feb. 3, 1948 2,452,782 McLeod Nov. 2, 1948 2,487,842 Whiteman et al. Nov. 15, 1949 2,530,477 Ostmar Nov. 21, 1950 2,532,721 Kalitinsky et al. Dec. 5, 1950 2,548,858 Benedict Apr. 17, 1951 2,548,886 Howard Apr. 17, 1951 2,638,744 Price May 19, 1953