US2621851A - Rotary impeller and the like - Google Patents

Description

R. G. VOYSEY ROTARY IMPELLER AND THE LIKE Dec. 16, 1952 2 SHEETS-SHEET 1 Filed May 28/ 1946 Inventor 5 Fig; IPEGZ/VHLD GE /FGE 1 0 31 1,

Attorney 7 Dec. 1%, 1952 R. G. VOYSEY ROTARY IMPELLER 2 SHEETS-SHEET 2 Filed May 28, 1946 1mm m 6 .R w a w 4 W F R Attorney Patented Dec. 16, 1952 UNITED STATES? RUTARY IMPELLER AND THE LIKE Appiication May 28, 1946, Serial No. 672,783 in Great Britain May 31, 1945 2 Claims.

This. invention relates tothe construction of impellers or compressors,- or similar vaned rotary structures. The object'of the invention is to afford constructional. features in impellers and the like which will extend their life in respect of fatigue. The kind of impeller or the like'to which this invention relates, is that invvhich vanes are arranged in circular series and in mainly-radial direction; springing from a mass of material which comprises in efiect a hub provided with andmerging into a disc-like portion, with which mass the vanes are integral or to which they are homogeneously united. Such impellers may have vanes to one'side. (for single-entry compressors) or to both .sides (for double-entry or bilateral compressors); The invention is applicable in either case, but the application to the latter case is especially desirable and is that to which the following more detailed remarks and description mainly apply.

IllplELCtlCG, there are "circumstances in which impellers are required to be made of light alloy, for exampl'eithe' impellers which are used in supercharger compressors fo'r reciprocating aero engines,v or in-centrifugalcompressors for gas turbine aero' engines and'which' are required 'to be as light as is practicable. They are, moreoveizxusually highly stressed, especially'as regards centrifugal loads. It'becomes important to haveregard to their vulnerability to fatigue failure, since a crack-may'develop very' rapidly by fatigue, and maylead to complete failure of theimpeller as well as'to the damage or destruction of adjacent components.

One of the most importantcauses of fatigue is the-fact that in the case where-the compressor delivery is led between diffuser vanes-past which the impeller. vanes rotate; each impeller vane receives an aero-dynamicbuffet orimpulseas it moves .past each. diffuser vane. Such an impulse may-be by increase or decrease of. the .aerodynamic load on the vane, but it maybe assumed to be of sufficient'magnitude' to constitute an exciting force fromv the viewpoint of vibration. Clearly, the'fundamental or some other natural frequency of vibration of the impeller vanes may correspond with the frequency of such impulses within the running range (i. e. the operational speed range) of the impeller. In a high-speed compressor under such resonance conditions it may'require only a short period of sustained rapid vibration to produce the number of reversals of strainnecessary' to cause fatigue failure.

This effect may be avoided to'some extent by so designing and constructing the impeller that the fundamental mode of: vibration- .of the vanes-that which is often the imost important from the point'of view of'failure-'is"produced" at a speed above the running range, that isfl'to: say, the product'of the numberiof impulsespert' revolution (or a submultiple thereof); and the" rate of revolutions per secondxdoesnot'in the design conditions of operation; correspond .to the natural fundamental frequency of vibration:of a vane. It is, however, often 'difficultto lift "the fundamental frequency well .out of the running range withoutiloss of performance'and this solu-.

tion may therefore be impracticable" for the; fundamental, and its practical achievement is likely to be evenzmore'idifiicult if other frequencies are also to be takencare of.:

The present invention provides "a construction of impeller in? which the impeller" vanes are at least to a great degree; immunized from the illeffects ofsyntony between external impulses 'and natural frequencies; whereby'their' fatigue 'life is greatly extended. While impellersiforcomprese sors with:..diffusers'-are under immediate considerati'oniihere, it is .to be understood that there may be other applications: The invention" is here; however, described as if :applied'to radially vaned impellers.ofrcompressors 'in which ".there' is a circular series-of equallyspaced "diffuser vanes and ch'annels,.to which'the major vibration-'exe citing impulses are attributable? The 'main' object of :the 'lllVBIltlOllliSthllS"the provision of 1211' impeller of a: centrifugalfcompressor wherein the :vibratoryeffect on" the impeller vanes of obstacles :to'fluidfiow'do'wnstream from the impeller (suchas diffuser: vanes; the separating walls 20f :delivery passages necessary structural parts; etc.) is recognised and dealt with in the construction of "the impeller'it's'elfzt.v

Centrifugal impellers ofeither the single or double sided type and comprising ardisc to which vanes are attached, may"be:made'to";vibratei'in a Wide range of modes .of varying frequenciesof which certainv modes are :mostdangeroustinas much as -they can (be: excited riby aerodynamic buffetingof the 1 impeller vanes as Ithey:=pass1the diffuser vanes. Thus" each impellervzinermay be made .to'vibrate as a whole'about it's fixing;to the disc 'and this "vibration may "be simple, .in which caseallparts of -the. vane haveithetsame phase angle and direction (this mode'of'.'vibra-. tion being: termedtherfundamental *mode), or

the vibration may; be 'of a: more complicated' nature with the' vane divided: into zitwoforrmore portions vibratingz'. in: 1 Opposite f phase s about" a nodal zone in the :avan'e (this 'mode :02 vibration.

beingytsrmed the :second-u-mode') These and other 'modes may be excited at almost any place on the vane and in particular buffets applied at the circumferential tip of the vane will cause motions compounded of several modes which may become dangerously large and cause rapid fatigue failure when a resonant condition is reached. Normally the motion of each impeller vane will be to a large extent distinct from the motion of the other vanes although by symmetry, similar amplitudes may be expected from each vane if all the vanes are similar and there will be a certain phase relationship between the vibrations of the various vanes. It can be shown that if the buffeting forces on each vane be of equal magnitude and similar wave form, and if the numbers of impeller and diffuser vanes are suitably chosen, then their vector resultant (made with due allowance for phase angle) will approximate to zero. Therefore although normally each impeller vane vibrates independently of the other vanes, if all the vibration-exciting forces and couples can be transmitted into the disc without substantial change in magnitude or relative change of phase, then since the vector sum of all the forces will be approximately zero, an impulse externally applied to any one impeller vane will be more or less completely counterbalanced by impulses transmitted through the disc from the other vanes.

By vector resultant here is meant the resultant of all the forces which resultant may be either zero (for a suitable number of impeller and diffuser vanes) or may be a force in the plane of or a plane parallel to that of rotation of the disc, or may be a couple which will produce torque about the axis of rotation of the disc. In the latter cases the resultant, if not zero, is a force or couple on the disc which in the normal methods of mounting and supporting impellers will be absorbed in the disc or disc mounting, and will thus relieve the vanes of the vibration-exciting impulses.

The above objects are achieved by the invention according to which there is constructed an impeller having substantially radially-extending vanes and rigidly connected along one edge to a mass comprising hub and disc, in which the dimensions of the vanes and the nature of their connection with the disc are such that, in the range of operating conditions, and having regard to the frequency of externally regularly applied vibration-exciting impulses on the impeller vanes and to the phase relationship between the said impulses, the vector resultant of all the forces due to said impulses and transmitted to the impeller disc is either approximately zero or acts on and is effectively dissipated in the disc or its mounting whereby at any given instant an impulse externally applied to any one vane and transmitted thereby to the disc is wholly or partly counterbalanced by impulses transmitted to said disc from the other vanes.

It is, of course, the region of the vane tips which is the most likely to be strongly excited by diffuser buifeting; it is proposed therefore to construct the impeller so that the junction between each vane and the disc in the region of the tip is more rigid and has a lower mechanical vibration impedance than that which applies in the structure of the vane between this tip region and the remainder of the vane. The term mechanical impedance is an established technical term which is almost self-explanatory since it is the impedance to the setting up of vibratory movement. Mathematically, it is the quantity by which the applied force must be divided, to calculate the resulting amplitude of vibration. By

tip region is implied a region corresponding to a length of blade-to-disc conjunction of the order of 10-20% of the total length of this conjunction or attached edge. The vibration of the tip region of a vane is thus transmitted to and opposed by the disc rather than transmitted to the remainder of the vane. In a convenient construction for the purpose the thickness of the tip of each vane at the root is at least of the root thickness of a typical section which will be hereinunder defined.

The foregoing features are concerned primarily with immunization against fatigue by a reduction in the intensity or amplitude of the excitation for all vibrational modes of the impeller vanes. According to a still further feature of the invention. provision may be made for judicious adjustment of the values of the resonant frequencies produced in relation to the running range, and in particular for the fundamental mode to be produced within this range, preferably 60-70% of maximum speed or lower, while the second mode is produced above maximum speed.

The above-mentioned and further features of the invention will be better understood from the following description made with reference to the accompanying drawings in which:

Fig. 1 is a fragmentary perspective view of a normal double sided centrifugal impeller;

Fig. 2 is a diagrammatic view of a portion of said impeller in relation to the corresponding diffuser vanes;

Figs. 3, 4 and 5 are sectional views of a single vane showing different modes of vibration;

Fig. 6 is a fragmentary perspective view of the tip region of an impeller according to the invention;

Fig. 7 is a plan view of Fig. 6;

Fig. 8 is a diagrammatic representation of a mechanical system;

Fig. 9 is a section on an enlarged scale of an impeller vane according to the invention;

Fig. 10 is a section on the line XY of Fig. 9.

In Fig. 1 is shown in perspective view, part of a normal double sided centrifugal impeller of the kind which may for example be used as a compressor in a gas turbine engine. The said impeller is provided with a disc portion I formed integral with or attached to a hub or boss I 3 and with vanes 2, each having an eye vane portion 3 of which the ear or leading edge 4 is preferably bent over, as shown, for aerodynamic reasons.

The entry or leading edge 4 is approximately radial. for axial entry, and (as shown more clearly in Figs. 3-5 and Fig. 9) the vane edge I8 sweeps around a bend through about degs. whilst progressively (though not necessarily uniformly) diminishing in breadth from the leading edge 4 to the vane tip 6 so that the inlet height 4 of the vanes 2 at the impeller eye is about two to five times the outlet width 6. As shown in Fig. 2 the impeller vanes 2 rotate past a circular series of uniformly spaced diffuser vanes 1 which define diffuser passages 8 through which the compresser air delivery flows.

As each vane 2 passes a diffuser vane I the former receives an aerodynamic buffet and if the speed of rotation and number of diffuser vanes I in the complete series is suitable, the frequency of the impulses on any one vane 2 may be in resonance with its fundamental or higher natural mode of vibration with consequent danger of fatigue failure after a relatively short period of operation.

In the example of Fig. 2 the impeller has 29 vanes and'the diffuser I vanes so that each vane 2" receives ten impulses per revolution.

Figs; 3 4 and 5 represent the two' most dangerousmodes of vibration of an impeller vane of this type, the-modal Zonebeing shown dotted. Fig; 3 represents the fundamental mode in which a modal zone is formed along the line-of attachment of the vane 2 to the disc I andhub I3 and in this case the vane'vibrates as a whole about this line, all parts of the vane having the same phase angle. With this modeof vibration fatigue failure is liable-to occur along the line 9 in Fig. 1. Fig; 4 shows one form-oi-secondmode vibration in which case the vane 2 is dividedbythe modal zone into two portions I 5, I5 which vibrate in opposite phase. 'Fig. 5 shows another form of secondmode vibration in which vane 2 is divided into two oppositely vibrating parts It, ll. InJthe second mode vibration; fatigue failure occur 'alongthe line I 9 or IQ- of Fig.1.

VVith regularly spaced similar impeller vanes 2 and difiuser vanes I the vector sum at any instant of the whole series of vibrational forces on the affected vanes 2 is (having regard to the difierent phase angles), dependent on the number of impeller vanes and diffuser vanes. It can be shown that for the vector sum of all the vibration exciting forces to approximate to zero, the following relationships: must 'hold:

S.1'+M.n or Mai-.1

where S 'numberof difiuser vanes,

r=harrnonic order of the vibration exciting force, M :number of impeller vanes,

n= an integer.

the greater partthereof) may be transmitted in to the disc I without substantial relative change of phaseor change in magnitude to'give little net force on the disc I or vanes 2.

Assuming that the buffeting forces are concentrated close to the rim Ii of disc (asmust necessarily be the case) the desired objects may be achieved by thickening the tip of the vane 2 especially in the zone I2 where it is in contact with the rim I I as shown in the thickened wedge shaped tip. 28 in'Fig. 6.

In this way the greater'part of the bufieting forces are transmitted directly to the disc I near the rim .II rather than being allowed to excite the remainder of the vane 2 in any of its possible modes. Put in another way, the mechanical vibrational impedance of the connection 23 between the excited tip of the vane 2 and thedisc I is deliberately made smaller compared with the impedance of the whole vane 2 viewed from the said excited tip, so that within the running rangeof buiietingfrequencies, the major portion of the buffeting energy is directed into the disc I and each vane 2 is now helping to counterbalance'the vibrations of all the other vanes 2 by supplyingv anchoring or neutralising forces to the tips which would otherwise be strongly excited by the diffuser bufieting. At teach xtip 20 -'the aerodynamic buffeting is thus iwhollyfor partially neutralised by forces suppliedirom other vane tips 29 :through the rim region :I I. of the disc I.

The *invention, moreover, :sets outv to cover practicalmethods of so proportioning the dimensions of the vane 2, tip 20 and the disc rim 1 I that the desired effect can reasonably-bezexpected to occur. For this purpose it is not 'onlytnecessary that all the 'buffeting'impulses be substantially identical .in magnitude and .harmonic' content .(ie. wave :form) but also that they. be transmitted to'the disc I between the vanes 2 Lin'such away as to sufierno serious relative change of phase or change inmagnitude. iBelow the .limit of acoustic velocity effects in the .metal which effects are usually excluded from. the limitations of present practice, the necessary condition" for this is obtained as follows. The mass of the'tip 20 and theadjacent vane portion may be considered to form in conjunction with thexmass and stifiness of the disc I in the region of the rim II a mechanical filtersystem of the potentially low pass variety as illustrated in Fig. 8which1represents a series of. masses M connected by springs S, the former corresponding tc-the vane region near tips 20 while the latter correspond to the portion of disc I between vane tips 20. Matters are so arranged that the cut-off frequency of this filter "system i. e. the .frequency above which the vibrations are attenuated in transmission from vane 2 to disc I is considerably higher than the frequency or any sizeable fundamental or harmonic contentd-ue to the diffuser bufieting forces. .Asa practical method-to this end, especially when the" disc is of markedinwardly divergent section,the'outerradial'(say) 10-20% of the impeller, is examined, as illustrated in Fig. '7 where the portion of impeller considered lies between the rim II and curved line 2I concentric therewith, the radial width of this portion being about 10-20% of the radius of the impeller. In

particular the mass of the vane 2 in this region is considered in conjunction with the stiffness and mass of the corresponding part of disc l between rim II and line 2| regarded as a strut in the circumferential direction and, by known methods, the resonant frequency of this system is calculated. This resonant. frequency in the case of such a chain of systems gives a'working indication of the cut-ofi frequency of the system beyond which .there will be attenuation of the buifets in the disc I between the vanes 2 and aphasedistortion' with consequent loss of the bufiet-balancing efiect of the vanes 2 onone another. :It is usually'sufiicient toconsider only this outer radial region since the radial taper of section of the disc I (amongst other factors) ensures that the outer 10-20% or so is the most critical part.

The importance of this method of construction lies .inithe fact that though itis desirable to strengthen the attachment-of the vanes 2 to the disc I by adding metal to form'thewedge shaped tip' '20, it is also important toensure that this metal be added-for the sake'of stifiness andnot for any mass or inertia becausesuch added mass or inertia will lower the cut-off frequency of the-mechanical filtersystem comprised by the vanes 2 and disc I.

The*thicl vane tip 26' gives the advantage which might be expected of say a shroud ring but turbance to the aerodynamic properties of the impeller since the extra metal used is not placed in the stream of working fluid.

A further advantage of the thickened tip 20 is that, in conventional side-entry centrifugal impeller vane shapes such as vane 2 whose radial height 4 at inlet is substantially greater than its width 6 at the circumferential outlet, the adding of thickness in the tip region has little effect on the frequency of the fundamental but has a marked effect on the more complex modes. In consequence, the thickening may be exploited to force apart the frequencies of the first and second modes so that they may be arranged to be produced at any desired point relative to the useful running range of the machine.

In particular it is advantageous to arrange for the fundamental mode to be produced within and preferably low down in the operational running range where aerodynamic buffets are less intense (e. g. at 60-70% of maximum speed or less) while the second mode frequency which is generally between 1.4 and 3 times the fundamental frequency is produced at a speed above the maximum speed.

One form of general construction of an impeller vane according to the invention is shown in Fig. 9 where the chain dotted lines are contour lines of constant thickness. The value of the thickness appropriate to each contour line progressively increases from the inner edge I8 and leading edge 4 which have equal and minimum thickness to the radially directed portion PQ of edge 23 which has maximum thickness.

In order to permit the invention to be applied in practice it is necessary to be able to calculate the fundamental frequency of a vane from given dimensions within a reasonable degree of accuracy.

It can be shown that, as regards the fundamental mode, and with a vane of the shape illustrated in Fig. 9 it is sumcient to consider only the dimensions of the broader region near the leading edge 4. It can further be shown that such a vane can be treated (for the purpose of considering the fundamental frequency) as if it consisted of a typical cantilever element 22 (Fig. 10) having its free end in the tip Y of the inlet edge 4 and its fixed end at the point X in the line of attachment between vane 2 and disc fundamental frequency of this simple element calculated on cantilever theory assuming its wide end to be fixed and the narrow end free, when multiplied by an empirical constant (in the present case about 8.2) is found to give a reasonably realistic estimate of the fundamental frequency for impeller vanes of various proportions, when of the general shape above described. The mass distribution and edge anchorage stiffness of an impeller vane may therefore be adjusted to be such that the fundamental frequency of vibration of the vane determined as above, is produced at any desired point relative to the running range at least within the possible limits of error of this method of approximation.

Although up to the present it has been found that the angle XYZ in Fig. 9 is substantially 55 degrees, it is to be understood that over a widely varying type of impeller vane the value of this angle may vary slightly e. g. from about 50 to .60 degrees.

The thickness of the wide end 24 of the wedge element 22 of Fig. 10 is herein termed the root thickness of the typical section while the thickness of the tip at the point P is termed the root thickness at the tip. In normal designs the vane thickness at P is generally not more than 50% of the thickness at X but in impellers constructed according to the present invention the former thickness is at least 75% of the latter thickness and may be as much as 200%.

In applying the invention to the construction of an impeller, the first step is accordingly to calculate the fundamental frequency on the basis of the typical element defined as above, and multiplying the frequency so obtained by a constant depending on the vane design and generally between 1.5 and 3 gives the limits of frequency of the second mode according to the design used. After deciding on the shape and thickness of the vanes 2 and the increased thickness of the vane tip 20, application of the mechanical filter concept to the outer radial Ill-20% of the disc I then establishes whether the cut off frequency is sufficiency high to enable the vibrations to pass into the disc I without substantial relative change of phase or magnitude.

In the case for example when the diameter of the impeller is about 20 inches it is convenient to use a thickness of tip root of about .375 inch and in order to determine the cut off frequency it is sufficient to consider only the outer inch of the disc I and vanes 2. The leading edge 4 may in this case conveniently be 0.05 inch thick with the outer edge l8 also of this value increasing to a thickness of 0.375 inch along the straight part PQ of the outer edge 23 of the vane. Such an impeller vane may have a fundamental frequency of approximately 1625 cycles per second which in the case of a circular series of ten diffuser vanes corresponds to a speed of 9750 R. P. M. The cut off frequency of the mechanical filter system constituted by the outer inch of disc and vane may then be calculated to be 27000 cycles per sec. The second mode frequency in this case is 3840 cycles per second corresponding to 23,040 R. P. M. so that the fundamental frequency is well down in the running range while (assuming a top speed of 19000 revs. per m.) the second frequency is 20% above this. At the same time the cut off frequency is also sufficiently well above the fundamental frequency to enable the vibrational impulses on the vanes 2 to be transmitted into the disc I without substantial attenuation.

With a double sided impeller of such construction having 29 vanes and 10 diffuser vanes it was found possible to run the impeller at the speed which resonated to the fundamental frequency for a period of 30 hours without fatigue failure which in practice would correspond to a life of several thousand hours since the impeller would not normally be run for any considerable period at this speed. On the other hand when impellers not constructed to counterbalance the vibrational impulses according to the invention were run at resonant frequency, fatigue failure was produced after only a few hours.

Where the impeller is singlesided, i. c. has only one circular series of vanes, the disc itself is made sufliciently stiff to resist the tendency of vane vibrations to cause unacceptable deflections in the disc in the sense of bending out of its proper plane and this will also be the case in a bilateral impeller in which the two series of vanes are disposed in peripherally staggered rela- 9 tionship. In the case of a double sided impeller, however, the disc may be relatively thin.

I claim:

1. A centrifugal impeller having a plurality of similar radially extending vanes each rigidly connected along one edge of a mass comprising hub and disc wherein the thickness of the connection between the tip region of each vane and the adjacent disc portion is such that the mechanical impedance of the system comprising said tip region and said disc portion is low relative to the mechanical impedance of the remainder of said vane, the thickness of the vane tip at the root being between three-quarters and twice the root thickness of the typical section of said vane.

2. A centrifugal impeller according to claim 1 wherein the connection between the tip region of each vane and the adjacent disc portion is of wedged shaped formation having one face substantially perpendicular to the plane of the disc.

REGINALD GEORGE VOYSEY.

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

UNITED STATES PATENTS

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