US2384000A - Axial flow fan and compressor - Google Patents

Description

iSept. 4, 1945. l F'. L.` wATTl-:NDORF 2,384,000

AXIAL FLOW FAN AND COMPRESSOR Filed May 4, 1944 7 sheets-sheet 1 f 76W T0 MEA/v caw/55 UNE.

l Hr L. E.

con/nen WINES /o V v {zjc (Q2/.ao Lc TAW ro Man/v cfmf/e UNE Hr L. E.

EUNNEE FZAA/K L Mfrs/v0 ce;

BY @nit-ym Sept 4, 1945. F. L.. WATTENDORF 2,384,000

AXIAL FLow FAN AND COMPRESSOR Fild May 4, 1944 7 sheets-sheet s UP 67H50 l 7h A lac EUNNEB @L H055 (F214 v auf' ' t INVENTOR @4A/v .lWrrEA/paef- BY Lib ATTOR:

Sept 4, 1945- F. l.. WATTENDORF AXIAL FLOW FAN AND COMPRESSOR Filed May 4, 1944 7 Sheets-Shea?l 4 -w -MHH F @www N A f Fen/VK L AXIAL FLOW FAN AND COMPRESSOR Filed May 4, 1944 sheets-sheet 6 Sept 4, 1945 F. L. WATTx-:NDORF 2,384,000

AXIAL FLOW FAN AND COMPRESSOR Filed May 4, 1944 7 Sheets-Sheet 7 I INVENTOR /sIeA/VA/ Z. Mrns/vooep BYMMW ATTORNEYJ Patented Sept. 4, 1945 l 2,384,001 sxw. now nm aan comnssoa Funk r.. wsctenam. Damn. ome Application my s, 1944, serai Ne. scares lo claims. een. zas-122i (Granted under the yact of March 3, 1883, as

. amended April 30, 1928; 370 0. G. 757) The invention described herein may be manuiactured and used by or for the Government for governmental purposes, without the payment to me o! any royalty thereon.

This invention relates to improvements in axial iiow fans, pumps, compressors.. and the like o! the curved lattice type, i. e., in which the pressure coeiilcient is in the neighborhood of .5 or greater and the blade chord to spacing ratio approaches or is greater than one.

In the design of axial ilow fans to suit definite operating conditions it has heretofore been the practice to employ the single airfoil theory as' in propeller design: In the `application of the single airioil theory. empirically corrected values of the lift coeilicient C1, and drag coeiiicient Cd, of the airioil section employed are used to determine the pitch distribution to give a desired de-r livery against a known pressure head at a xed design speed. By several trial computations the proper vblade angle settings at a selected number In my prior United states Patent No. 2,208,215,

I disclosed formulae for determining the blade angles and pitch distribution in axial flow ian blades or the curved lattice type in commotion with iixed guide vanes: and i'ans constructed in accordance with the disclosure of this patent gave a much better performance in the design operating range than any commercially available equipment capable of operating in a similar pressure and volume coeicient range and at comparable speed conditions, and new values of the of stations on the blade radius can be determined and, permitting the thrust for one blade and the total thrust'to be computed, the air delivery for 'a multiblade fan can be readily determined.

When'the design requirements are such that in order to obtain a given delivery against a'higher static pressure with a xed fan diameter and operating speed, it is necessary to increase the blade area and finally increase the number of blades so that the spacing ratio or ratio of blade chord to mean blade spacing approaches one. When the blade chord to blade spacing ratio reaches approximately six-tenths or seven-tenths, the interference eiiects between the blades so affect the values of lift and drag coemcient's that the conventional single airfoil theory breaks down, due to the fact that no criteria exist at pumping limit were obtained. The determination of the blade angles at entrance and exit, however. were ultimately, though not solely, determined inthe patent disclosure by reference to an assumed optimum value of lift coeiiicient for the particular airioil section in question.

Subsequentdesign and test work on axial flow fans or the character described has led to the discovery that when the ratio of blade chord to blade 'spacing approached or exceeded one, and for operating pressure coeilicients in the neighborhood of 0.5 o r greater, the line of relative iiow through the blade channels or cascade 'arrangement was such that the deviation at outflow from a line parallel with the zero lift line oi' the respective blade sections became very small as the design conditions were approached and for a considerable `range beyond the design point, This phenomena appeared to be substantially independent of the blade entrance angles provided the latter were such as to give minimum shock at entrance.

The construction of fans such that exit angles of the zero litt lines of the respective blade sections corresponded to the necessary design point resultant velocity vector at each radial blade stapresent for determining the corrections to lift the,l

drag coeiiicients for cascade airfoil arrangements. It has been customary when the above limit has been reached to go to multiple stage design which, however, entails increased loss 0f eiiiciency.

Further diiiiculties arise in trying to develop axial flow fans for high pressure coelcient requirements due to the fact that the spacing between blades using conventional Vdesign criteria was such that the airow would not conform to the mean channel form between adjacent blades and burbling or air iiow separation would occur,- so that upon reaching a, limiting pressure coei'llcient, there would be an abrupt drop in delivery even though additional power was supplied, the

point of air iiow separation being known as the pumping limit.

tion gave a very considerable increase in performance and further extended the operating range of axial tlow fans beyond that attainable with the patented construction. A very marked improvement has also resulted in delaying iiow separa- It is a` further object of the invention to provide l ,means whereby the blade angles and pitch dis` tribution of a curved lattice axial flow fan stage can be determined when the fan stage includes as a part thereof, fixed guide vanes. located ahead 'or behind the fan stage, or both, or in combination with a contrarotating impeller or ian.

Other objects'of the invention will appear by reference to the detailed description hereinafter given, and to the appended drawings in which:

Fig. 1 is a schematic view illustrating the velocity vector diagram for a single stage fan in accordance with theinvention employing a series of xed radial inlet guide of contravanes positioned ance with the invention is followed by a similar,

but counterrotating stage;

Fig. 5 is a view similar to Fig. 4, showing the velocity diagrams for a contrarotating impeller arrangement employing both upstream and downstream contravanes;

Fig. 6 is a fragmentary view showing a construction similar to that outlined in Fig. l;

Fig. '1 is a diagrammatic longitudinal sectional view illustrating a multistage axial flow blower constructed in accordance with the invention and employing fixed guide vanes between successive I stages;

Fig. s is a view similar te Fig. '1, but illustrating the use of contrarotating fan stages; and

Fig. 9 is a view illustrating the mixed applicatp the dynamic pressure of air moving at the tip speed of the fan.

Similarly, the volume or delivery of the fan is expressed in terms of tip speed of the fan by means of the volume coeilicient which is a nondimensional coefficient or: n

or the ratio of the mean velocity of the fluid stream through the plane of the'fan to the fan tip speed.

Prior to considering the detailed aspects of the invention, a consideration will be made of the fundamental theory on which the invention is based. If we consider an axial ow multi-blade fan arranged in a conduit with axial entrance of the air into the fan assumed.. and the ian speed and blade angles so arranged that the discharge from the fan is `axial, then there will be no pressure increase in passing through the plane of the fan. since the relative path of the air will remain unchanged. In order to create a pressure rise the relative velocity of the air passing the fan disc must be changed even though the axial component o f the discharge remains constant, which means that in the fan blade channels a whirl or tangential component must be added to the airtion of the features of Figs. 'l and 8 to a wind v tunnel fan. n

For a. clear understanding of the detailed description of the invention, reference should be made to the following table of symbols and their deilnitions.

AP=total head developed by the ian or pump.

R=radius of the blade tips.

w=angular velocity of the fan.

p=mass density of the fluid (air. Water, oil, etc) in which the fan is operating.

`A=net area of the uuid.

In expressing fan characteristics it is customary to employ non-dimensional coeiilcients to define the operating speed-pressure and volume relationships so that one ian may be compared with another if the pressure and volume coefcients are the same. The pressure coefficient 1]/ may be.

expressed by the following iormula:

' AP l iicar. or he ratio of the total pressure rise in the fan Iiow, i. e., if the axial component of the flow is V both at inlet and discharge sides of the fan stage, the resultant velocity at discharge must also contain a tangential component up in order to develop pressure.

From aerodynamic considerations the change in the magnitude and direction of the resultant velocity of the air stream can only result because of the creation of lift by the fan blades, the axial component of the lift being the thrust which produces the acceleration of the air molecules to overcome the static yhead against which the fan vmust work. The lift over any radial section of a lan bladein turn is a result oi the superimposed eiTects of a circulatory flow and an axial ow, the circulatory now being of a. peculiar type similar to a mathematical free vortex and the strength of the circulation I being a function of the lift coemcient at the Aparticular section, the average velocity of the flow at the section and the blade chord. The circulation around the respective blade sections in turn produces the tangential component ur of the fan discharge.

It can be sh'own from minimum energy considerations that for efficient operation there should be no radial component of flow in the fan discharge, which results only when the circulation about the blade sections is substantially constant, which in turn is only satised when the circulation is independent of radius and when the function rUr is constant and the value of Uf at any radius r is the value of ur at the blade tip at radius R times the ratio R/r.

In accordance with the conventional blade element theory, the value of up is determined fora given static pressure at the rear of the ian which must be known or assumed and from the velocity diagrams which are then determined for any radius; the blade sections as a whole are adjusted so that each section makes an angle with respect Vto the resultant velocity equal t0 th'e angleof attack for maximum eiliciency or maximum L/D. Knowing the value of the lift coeffi-` cient for the given angle oi' attack, the thrust and delivery are computed and compared with the desired delivery and, if much in error, new

values of up must be computed or alterations y made in the angie oi' attack oi each section.

In accordance with the present invention, however, the assumption is made that the fan in -cordunction with guide vanes must develop a certain ideal total head or total pressure (AP ideal) greater than the actual or working total pressure (AP) or APi 11- where n is the emciency. j

Now from the well-known Euler turbine law the maximum value of the ideal total head is AP (ideal) =p((-R) Ur-)m discharge angle at the tip is so made that the rotational component ur so calculated is obtained and the value of sie at any other radius is obtained by multiplyingv by the value of the ratio R/r at the radial station in question. The value of ur for any radial station of la fan blade is, however, only capable of unique determination when the guide vane arrangement is known, i. e., whether prerotation guide vanes are employed or whether the i'an stage proper is followed by guide vanes or a con.. trarotating fan stage designed to convert the kinetic energy of rotation of the ilow into static pressure. The present invention therefore is ,radial stations in both guide and fan blades as-l semblies is accomplishedby warping theblade section, as distinguished from setting the blade as a whole, and leaving the original airfoil section undisturbed, as generally practiced by the prior The general application of the principles of the invention will be made clear by its application to typical cases as follows:

Casa I.-Snior.a rtorsnon wn'n Comasvwas Ursi-assu f In this case, as illustrated in the velocity ilow diagrams of Fig. l, the reference numeral I9 illustrates the development of a radial section of a series of guidev Yanes placed upstream o! an axial ilow fan or impeller having vanes Ii o! air-A foil section. The guide vanes i0 are designed so' as to impart a rotational component to the inilowing air such thatthe rotational component .ue is of a magnitude suilicient to develop the desired total head AP. The discharge from the entrance or prerotation guide van'es enters the fan or impeller and, in passage through the fan blade channels, the kinetic energy off the rotational component 'of the inflow is converted into pressure' by imparting a contraspin or rotation to the vow so that the delivery from the exit of the impelleris axial. t

Referring to Fig. 1, the tangent to the mean .camber line at the leading edge of each blade section i0 must -be parallel with the Aaxis of the inlet air orl the-entrance angle measured from a plane normal to the longitudinal axis of the inguide vanes are not rotatable or: (pimsiiu.

- The'subscripts uc and de respectively, indicate I upstream and downstream contravanes.

Now from the velocity diagram o! Fig. 1, it is seen that the angie o! discharge (prius. 'at any radius with respect to a piane normal to the iiow and which is also the ansie oi' the zero litt line ottheguide vaneaisdeterminedby un (anwhere V is the axial now velocity and Us is the tangential component o! the discharge at any radius. But in the case or non dimensional coemcients o and it AP (the totalvhead) #fp/2 (coli.)s

and as previously noted from Euler's turbine equation AP=pU(wR)n and equating I v I aU(df)n=W/s(:)

Page n and for any radius U :329012) 1 21j r substituting values o! V and Us in terms o! and gb respectively f l tan 21p 1' Runner blade anales-It will be apparent from' the velocity diagram lor the entrance tothe impeller or runner that the entrance angle Br at any radius r must conform to the relative velocity of the discharge from the inlet vanes `which is seen to-be the resultantof the axial ow V and the sum of the tangential components rw and ue so that:

nil-M but V=(wR) .and s UxifwR! 1 2 21p r so tan Bl 21; r and dividing numerator and denominator by (WR.) gives mm1+ ie R 217 r Similarly the outow from the fan must be axial, so that the discharge portions of the blades But since Ur must equal Us (wR) R I'his case, as illustrated in Fig. 2 and accompanying vector or flow diagrams, is seen to diller from Case I in that no prerotation guide vanes are employed, the necessary tangential component being developed in the fan and governed by the same law as for the prerotation vanes I ot Case I, namely, that rrc must be constant. The fan blades, Fig. 2, are indicated by reference numeral I2 and are followed by stationary guide vanes I3 designed to remove the tangential component from the discharge of the fan and deliver the air in a pure axial now. In the downstream guide or contravanes I3 the same law is applicable as for the fan namely that (few) must be constant. The inlet and exit angles in this combination are determined as follows:

Impeller blade angles Since there is no prerotation of the inow to the fan, the inlet angle of the blades i, at any radius, can be determined from the inlet vector diagram such that which when substituting for V and Us in terms of the pressure coeillcient and cancelling the term (oR) gives v tan 52:

tan il l! R 211 r Contravane angles 'I'he contravanes must remove the residual tangential rotation from the discharge of the impeller and this results in a tangential velocity change Uc2 and from the vector diagram, Fig. 4,

.v for the entrance of the after rotation vanes I3, it

is seen that:

V tall d= such that but Ue: isjrelated lto the pressure coeilicient it lt 0@ E. v Uca- 211 r and =(WR) so;

tan (vllc-A i i 211,1 v- 'I'he negative sign indicates that the guide vanes must introduce a contrary spinningleliect on the airow from that imparted inthe impeller which requires that the inclination of the-vanes v be opposite that of the impeller.

The discharge angle of the guide vanes must be such that the velocity vector V is parallel to the longitudinal axis and hence (z) dc=90.

.Case IIL-Smet.: RorA'rIoN wrrn CoNrRAvANns UPSTREAM AN D DOWNSTREAH This combination, as illustrated schematically in Fig. 3, involves the use of upstream guide or contravanes I5, which are designed to impart a definite tangential component Uuc to the air stream flowing into the fan, this component being some predetermined percentage K of the total rotation necessary to develop the necessary total head. The fan or impeller I6 is designed to impart unit rotation to the iiow in the opposite sense so that a balance of (i-k) of the required tangential component remains in the fan discharge and stationary .downstream contravanes I1 remove the tangential component from the fan discharge and deliver the air in pure axial ilow.

Upstream contravane angles (Inlet angle) (51) wc: 90

(Discharge angle) tan (,)u= R Ki I 21) 7* Fan blade angles 'I'he entrance angles of the fan blade sections at each radius are determined in the same manner as for Case I, with the exception of the introduction of the percentage tangential rotation -factor k so that the entrance angle is determined by:

l Similarly the discharge angles ofthe fan blade sections are determined in the same manner as for Case II, Vwith the exception of* the introduction of the factor (i-k) winch expresses the percent of tangential velocity residual in the fan discharge' so that:

Downstreamv contravane angles The inlet angles ofl the downstream contravanes are determined 'by the same formula as for factor (1-K) which expresses the percentage of rotation required to be removed by the guide vanes to give axial flow and the minus sign indicating that the vane inclination is opposite from the vertical to the fan blades so that:

The discharge ends of the downstream guide vanes must be aidal so that (z) dc=90.

This combination, as illustrated schematically in Fig. 4, employs two fans or runners coax-lally positioned and rotating in opposite directions, the upstream and downstream runners being indicated respectively by the reference characters i9 and 20.l It will beseen by reference to the vector diagrams, Fig. 4, that the loperating conditiens for the upstream :an is is identical with that of the fan I2, ,Case II, so thatthe sameformulae apply:

(Entrance angles) tall (uf =17a 1ral For the downstream fan impeller or runner the condition is similar to the fan Il receiving the rotational vdischarge of the prerotation guide vanes I0 of Fig. 1 (Case I) and the same formulae are applicable with the exception of the negative sign which indicates that the vanes are sloped in the opposite sense than in the upstream fan. The blade section angles are then determined by:

(Inlet angles) tan (50d a+?,

R (Discharge angles) tan (2)df= -r In the formulae above relating to Case IV and as employed in Case V to be described below, the subscripts uf and d! refer respectively to the upstream and downstream fans or runners.

CAsr: V.-DUAL Ro'rA'rroN wr'rHCoNrnAvANns This combination, schematically illustrated in Fig. 5, employs upstream contravanes 2|, oppositely rotating upstream and downstream fans or runners 22 and 23, respectively, and downstream contravanes 2l. The upstream contravanes 2| are designed'to impart a desired percentage K of the tangential rotation necessary to develop the required total head in the upstream fan, the upstream fan 22 again imparts unit rotation in the opposite direction leaving .a residualspin ln the a upstream fan discharge of (l-K).

stream fan 23 puts in a rotation of unity in thel The downopposite direction from that of the upstream fan 22 leaving a residual spin in the discharge of the downstream fan of (-K) which must be overcome in the downstream contravanfes 24 by imparting an equal and opposite spin or +K thus giving an axial discharge from the downstream contravanes. ing criteria. results in the following blade angle formulae:

Upstream contravene anales (Inlet) (l)1.ic- 90 Downstream vrunner blade angles Downstream coatravane angles Case v when K=50% since for this condition the blade angles for both fans are equal but opposite direction.

It should be further noted that in arrangements where upstream contravanes are employed, the spin component may be in the same sense as that added in the impeller and the downstream contravanes would then be required t0 take out the rotation equal to the sum of the spin components imparted in the upstream contravanes; and the impeller rather than to remove the residual spin equal to the difference in tangential components imparted by the upstream contravanes and impeller, respectively.

Fig. 6, a fragmentary view, illustratesan'arrangement of prerotation guide vanes and impeller similar to that described above with reference to Case I, Fig. 1, and the same reference The application of these governnumerals are accordingly applied. The prerotation guide vanes I0 have an overall radius R, the position of any radial station being indicated by radius 1'. The vanes lilv are seen to be of airfoil section having rounded leading edges for shock free entrance with a chord length b and the spacing between the blades at any radius r is indicated by s so that S the spacing ratio= where n: the number of blades. As seen in this figure, the inlet angle is determined as being the angle between the mean camber line C anda plane normal to the flow axis while the discharge angle is determined by the angle of the zero liftline as indicated with a plane normal to the flow axis. A The zero lift linecanbe determined avproximately by joining the point of the mean cambe une at the fifty percent chord point by a straight line passing through the trailing edge. The required discharge angles are obtained by bending or curving the blade section so that the zero lift line makes the necessary anglewith the plane normal to the axis of rotation. The same part a tangential velocity to the inflowing airstream which upon flowing into the vane free portion of the inlet passage forms a vortex which automatically causes the proper radial distribution ot' the tangential velocity prior to entrance into the iirst set of impeller` vanes, this feature.

forming no part of the present invention being claimed in the copending applicationof Frank L.

Wattendorf and Frank W. Williams Serial No.4

520,922, A rotor hub l of cylindrical cross section is mounted within the housing 50 in bearings 55 and 58 provided in end plate 5| and a similar end plate 51 respectively, and the hub is adapted to be rotated by a driving shaft 58 connected to a suitable source of power not shown. The rotor hub 54 has a plurality of sets of impeller vanes 80 mounted thereon with a corresponding number of sets of stationary guide vanes Ei interposed between adjacent sets of impeller vanes to form separate blower stages. 'Ihe stationary guide vanes are secured to the blower casing and the impeller and guide vane height decreases from the inlet toward the outlet passage |52 to compensate for 'the decrease in volume as the pressure is increased. In the design of a blower as illustrated the pressure boost per stage may `be such that the pressure coefficient, volume coefficient, and blade spacing aresuch as to come within the contines of the present invention, and,

accordingly, the guide vane and impeller vane' angles may be readily determined for each sepa-f casing 10 is provided with dished end plates 1|' and 12 which, `together with kthe casing, deflne inlet and discharge passages 13 and 14, respectively, which may belprovided with controllable inlet and discharge guide vanes 15 and 16, respectively, the latter being optional and indicated in dotted lines. Hub members 11 and 18 support sets of impeller vanes which at their outer periphery are secured to an annular shroud 8|, to which other sets of impeller vanes 80 arev suitably secured. 'I'he sets of vanes 80 are axially spaced and intermediate sets of oppositely rotating impeller vanes 82 are positioned therebetween and secured at their radially innerends to a cylindrical rotor 85. A driving shaft 8B rotatably journailed in a bearing. 81 fixed in end plate 12 is connected to the hub member 18 and the hub member 11 is rotatably mounted on a bearing 88 in turn mounted on a driving shaft 80 journalled as at 8| in the end plate 1| and the shaft con- 5 nected at its inner end to the rotor 85 to drive the same. The rotor 85 at its other end is provided with a pilot 82 journalled in a bearing 88 housed within a -bore 84 in the hub 18. The driving shafts 86 and 80 are respectively driven 10 in opposite directions by power means not shown.

The design of the blading of the blower of Fig. .7 can be handled stage by stage by the methods described with reference to Case IV (Fig. 4) above, each adjacent set of vanes 80 and 82 15 being considered as an individual stage.

Fig. 9 illustrates the application of the principles of the invention to a wind tunnel fan unit in which the tunnel walls form a conduit shown only in part and indicated by the reference nu- 20 meral |00, the inflow being as indicated by arrow A and outflow as indicated by arrow B with guide vanes |0| and |02 positioned so as to guide the ilow around the corners of the conduit. Separate ings |05 and |06, respectively, within the tunnel and are adapted to be driven in opposite directions by external power means not shown. The driving shaft. |03 has a rotor or hub |01 mounted thereon with two sets of impeller vanes |09 mounted thereon in axially spaced relation, each set of impeller vanes being preceded by a cooperating set of stationary guide vanes ||0 supported' by the tunnel walls to thereby form two separate blower or fan stages of the single rotation type. At a point axially spaced from the rotor |01 is a second rotor l| I2 which is mounted on the shaft |04 and is provided with two sets of impeller vanes H4 which cooperate with stationary guide vanes ||5 supported from *the tun- 40 nel walls. In this arrangement, if the discharge from the second set of impeller vanes |09 is purely axial, each fan unit may be designed by considering each as comprising two stages each including prerotation contravanes, and hence the for- 'y mula of Case I (Fig. l) would'be applicable. By

the use of contrarotating fan wheels it is posssible to obtain a large pressure boost with fewer stages than with a single rotation unit because larger spin components can be converted to pressure in a rotating vaned wheel than in stationary guide vanes and, accordingly, such'an arrange` ment is indicated for wind tunnels or drainage pumps.

The principles of impeller or contravane design set forth above are of general application since by drawing the velocity diagram for any combination of impellers and guide vanes the vane angles and pitch distribution can be readily determined. It "should be further understood thatthe principles of the invention are also applicable to-thin sheet metal vanes since such vanes have determinable airfoil characteristics.

'It will be apparent to those skilled in the art that many variations and modifications may be made falling within the scope of the invention as defined by the appended claims.

I claim: 1. In an axialv flow fanor compressor stage, a varied axial iiow impeller, a plurality of radially arranged inlet guide vanes, the design pressure4 coeiiicient for the stage being equal or in excess of I .5 and the ratio of the vane chord to vane spacing in both guide vanes and impeller being one or greater than one, the guide vanesand impeller 76 vanes having their discharge portions bent 'so driving shafts |03 and |04 are supported by bearas to produce tangential components in the fluid ilow through the fan vstage having a magnitude equal, or in the case of the guide vanes, less than the amount required to give the desired total head, the discharge angles in the guide vanes and impeller being such that they impart rotation to the uid iiow respectively therethrough in opposite directions the guide vane and impeller vane cross-sections being in the form of a. basic airfoil section modiiled by bending the discharge portions of the vanes until the zero lift line of the basic section of each vane lies parallel at each radius to dischargelines making an `angle with a plane normal to the axis of flow such vthat the product of the tangential velocity at any radius times the radius is substantially constant.

2. The structure as claimed in claim 1, in which the guide vanes constitute a second vaned impeller positioned upstream in the direction of flow from the rst named impeller and rotatable in a direction opposite that of the rst named impeller.

3. The structure as claimed in claim l, in which the inlet guide vanes are iixed and in which the vanes are bent at their discharge portions such as to impart a rotation to the fluid iiow therethrough less than the amount required to develop the design total head for the ian stage and a plurality of radial guide vanes positioned downstream of the impeller, the curvature of the inlet portions of the downstream guide vanes being such as to remove any residual rotation in iluid discharge from the impeller and the discharge from the last named guide vanes being axial.

4. In an axial ow compressor, a iirst axial now multivaned impeller, a second multivaned impeller positioned downstream in the direction of uid ilow from the first impeller, said impellers being arranged for rotation in opposite directions respectively, and the vanes in each impeller deiining fluid flow passages therethrough and the mean passage length at any radius being equal to or greater than the mean width of the passage, the impeller vanes having cross-sections in the form of a basic airfoil section modiiied by bending the discharge portions of the vanes so that the zero lift line of the basic section at any radius makes an angle with the plane of rotation such that a tangential component is imparted to the iiuid flowing through the respective impeller capable of developing the design total head in the impeller discharge and the variation in vane discharge angle with respect to the radius being such that the product of the tangential velocity at any station times the radial distance of the station from the axisof rotationis substantially constant, and the vanes of the second impeller imparting a tangential component to the fluid received from the first impeller opposite in direction to that imparted to the fluid in the rst impeller so that the discharge of lthe second impeller is substantially free from rotation.

5. In an axial ow compressor stage for peration at a design pressure coeilicient of .5 or

greater, a vaned impeller, the impeller vanes hav- I ing an airfoil section and denning iiuid flow passages in which the ratio of mean pressure width to passage length at any radius is one or less than one, said impeller vanes having their inlet portions inclined to give shock free entrance under the design speed and flow conditions and said vanes being of an airfoil section derived from a basic airfoil section modiiied by bending the discharge portion of the vanes to deilect the zero lift lines of the basic section at each radius such that the discharge flowv relative tothe impeller will at any radius have a rotational velocity which in terms of pressure corresponds to the design total head to be developed in the compressor stage and the variation in the inlet and discharge angles with respect to the radiusbeing such that the product of the tangential velocity of the flow times the radius of -any station is constant.

6. I'he structure as claimed in claim 5, including stationary prerotation guide vanes located upstream in the direction of iiow to the impeller and adapted to impart a rotationto the now therethrough inthe opposite sense to that imparted in the impeller and the discharge portions of the vanes being curved so that the product of the radius at any radial station on the vanes times the tangential component of velocity imparted to the fluid ow at that station by the vanes is substantially a constant.

7. The structure as claimed in claim 5, including stationary prerotation guide vanes located upstream in the direction of ilow to the impeller and the guide vanes being constructed and arranged to impart a rotational spin to the fluid discharged from the guide vaneswith a radial Velocity distribution such that the product of the tangential velocity at any radius times the radius is substantially constant, and downstream guide vanes effective to remove residual spin from the impeller discharge.

8. In a curved lattice axial now fan or compressor stage for developing pressures such that the operating pressure coelcient equals or exceeds .5, a multivaned impeller, a set of prerotation guide vanes positioned upstream of the impeller and a set of guide vanes positioned downstream of the impeller, the space between ad- Jacent vanes forming uid ow passages in said sets of guide vanes and impeller respectivelyeach passage having a mean length equal to or greater than the mean passage width, said prerotation guide vanes and said impeller vanes being or an airfoil section derived from a basic airfoil section modiled by bending the discharge portions of the vanes to deect the zero lift line of the basic section at each radius so that at any radial station the direction of relative outflow, as measured by the angle of the zero lift line of the vane section with a plane normal to the flow axis, shall produce a spin component in the flow capable of developing a pressure 4substantially equal to the design total head and a set of guide vanes positioned downstream from the impeller and effective to remove any residual' spin in the discharge ofthe impeller.

9. The structure .as claimed in claim 5, m

which the design tangential velocity at any radius is determined by the expression UF@ 1 2 21p r where 10. In an axial flow fan or compressor stage having a design operating pressure coemcient i 0f the :basic section at each radius so that the zero 'lift line of any vane section makes an angle with a plane normal to the axis 0i iiow such that ilow velocity parallel with the zero lift line of the section produces a tangential or spin com'- ponent having a magnitude determined by the formula: y

Ut: xxl/(51B) where V=the desired tangential or spin velocity,

K=a constant expressed in percent and defining the amount of spin with respect to the total required to develop the design total head to be imparted in a respective set of vanes.`

up is the design pressure coemcient,

(WR) is the tip speed of the impeller,

R is the tip radius, and

r the radius of the vane station point in question.

FRANK L. WA'ITENDORF.

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