US2898031A - Vaneless diffuser for radial flow machines - Google Patents

Description

Aug. 4, 1959 W. VOIGT VANELESS DIFFUSER FOR RADIAL FLOW MACHINES 2 Sheets-Sheet 1 Filed Sept. 24, 1954 INVENTOP` WULDEMHE VO/GT U wm HTTOENEY Aug. 4, A1959 w. volGT VANELESS DIFFUSER FOR RADIAL FLOW MACHINES Filed Sept. 24, 1954 2 Sheets-Sheekl 2 uvz/ENTORA Wa DEM/ire Ifo/67' HTTO/ENEYS Unified States ,Patent O vANELEss DIFFUsnR Fon RADIAL FLow MACHINES Woldemar Voigt, Baltimore, Md. Application September 24, v1954, Serial No. 458,294

4 Claims. (Cl. 230-127) (Granted under Title 35, U.S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon.

This invention relates to a novel improved vaneless diffuser particularly applicable for use in supersonic flow. The invent-ion presents novelty in itsV use per se or in conjunction with one or more diffuser stages arranged as cascades of vanes. t

While the use of radial vaneless diffusers is known in the prior art, such prior use as practically defined in its constructed form comprises two plane and parallel walls arranged normal to the compressorY axis so as to include between them the annular discharge slot of the compressor wheel so that the flow from the compressor wheel is guided between the two walls to a collecting spiral encompassing the system. The desired flow deceleration in suchprior art is achieved in the spiral flow between the two walls by the increase in the radial direction of the available cross section of the flow. This prior art vaneless diffuser has sometimes been modified by arranging the two diffuser walls radially divergent with the angle of divergence notmore than six degrees (6) since it was heretofor considered that at divergence angles exceeding the amount of six degrees the flow in a diffuser would separate from the walls and the pressure recovery downstream of the flow separation point would not obtain.

The two basic disadvantages `of this prior art vaneless diffuser that have prevented its use to any large extent are first, that to influence the comparatively small radial flow component the diffuser had to be of rather great radial length due to the low limit of the divergence angle, resulting in a diffuser much too bulky and one incompatible with usual space limitations, and, secondly, that the large inner surfaces of the Walls resulting from the large radii employed caused undesirably high friction and efficiency losses.

However, the present invention eliminates the disadvantages of the prior art and in view of the known insensitivity of the vaneless annular diffuser to the difficulties generally arising in supersonic difusers such as pressure gradients, shocks and the like, this invention provides a development of the vaneless diffuser as presented herein which is of major importance and presents a distinct advance in the art.

The present invention introduces the use of divergence angles greatly in excess of six degrees, enabling smaller more compact dilfusers with low efficiency losses as well as low friction loss. And, moreover, in another form it broadens the scope oftavorable applicationsy of such vaneless difusers by employing cascade diffusers in combination therewith to make subsequent cascade difusers achieve higher pressure differentials and/0r work at higher Reynolds numbers, and consequently at higher efliciency.` Also, the improved vaneless diffuser renders subsequent cascade difusers suitable to Work efficiently at higher Mach numbers than they could handle without thearrangement of a previous diffuser stage according fc ICC to the invention as well as enabling the taking advantage of the flow deections caused by a cascade diffuser when arranged upstream of a vaneless diffuser.

The vaneless diffuser corresponding to this invention is based on two principles. First is that the effects which in the prior art have limited the divergence angle of a difuser to a maximum of about 6 are of very small importance in an annular vaneless diffuser and the optimum divergence angle therein is of a very much higher magnitude than previously assumed, namely, in the order` of magnitude of about 35 to 90 and more, and secondly, the pronounced reduction of the pitch angle of the spiral flow, which is caused by the decrease of the radial velocity component in such vaneless diffuser yields several highly desirable indirect results which manifest themselves in higher pressure differentials, higher Reynolds numbers and/or higher Mach number limits in a normal cascade diffuser arranged downstream of a vanelessdiffuser according to this invention.

The explanation of the basis for the invention and the novel improvement thereby, the causes which make divergence angles far in excess of 6 possible in the novel diffuser can be shown with reference to the drawings. With reference to the axial view in Fig. 2, at any certain point in a diffuser section according to thel invention are two velocity vectors shown. One vector VR represents a velocity component strictly radial in nature and the other vector VSP represents a velocity component due to induced spiral flow and forms a pitch angle rp with a circumferential direction. For purposes of our discussion the magnitudes of VR and VSP are chosen so VR equals the radial component of VSP and moreover is subsonic.

Due to the simultaneous increase in width and circumference of a highly divergent diffuser according to the invention, noting Fig. 3, in which VR is subsonic there will be a positive pressure gradient in the radially outward direction. Previously it has been believed that 6 is the critical divergence angle above which flow would separate from the'wall and pressure recovery would not obtain.

According to this invention there are several influences which prevent the harmful consequences` of flow separation beyond a divergence yof 10. First, the'inuenceeffective in purely radial and spiral flow acting on the boundary layer which is exerted by the increasing circumference of the diffuser according to the invention spreads the developing boundary-layer volume over an increasing surface keeping the boundary thickness small.. Second, since according to the invention structure Vthe spiral flow angle is different than and generally between 8f and 30, there is an influence exerted which acts to increase the friction between the free ow and the boundary layer which is proportioned Vto the true fiow velocity Vsp, and the radial component of that friction force which acts to overcome the counteracting pressure gradient in the boundary layer is proportional to VSp2.sin il). Since VSP=VR/sin tt, the radial friction force in spiral flow is proportional to VRz/sin 1p. That means that the friction force coordinated to a certain VR increases with a decreasing w. And since the radial pressure gradient in a certain given diffuser does not increase with decreasing rp, the very strongly increased radial friction forces acting on the boundary layer will prevent the boundary layer from reversing and the flow from separating in correspondingly much steeper diffuser angles than used vin normal straight diffusers.

Third, which is a consequence 0f a spiral angle x// 90, acts by increasing the effective viscosity of the boundary layer.v This effective or simulated viscosity is higher than that which would correspond to the radial velocity component alone because the turbulence in the boundarysatented Aug. 4, 1,959:y

layer and, therefore, the energy transfer within the boundary layer is actually controlled by the line velocity VSP=VR/sin rl/ and not by the radial component VR only. Y

Thefourth inuence is a consequence of the rotation of spiral flow and acts by contributing an additional force in the boundary layer which counteracts the positive pressure gradients. This influence is exerted by the centrifugal forces acting on the boundary layer particles in the rotating ow and acting primarily on the relatively faster layers within the boundary layer while transmitting their effect to the more stagnant layers by viscosity forces in the same way as friction forces are transmitted from the free fiow. j These influences prevent boundary-layer reversal and flow separation up to diffuser divergence angles exceeding 50 when the spiral angle is smaller than 45. In addition there is another influence which is active to make opening angles up to 180 and more usable.

An object of this invention is to provide a new and novel improved vaneless diffuser.

A further object of the invention is to provide a new and novel vaneless diffuser wherein the optimum divergence angle is in the order of magnitude of 35 to 90 and more to provide greater efficiency thereof.

An additional object of the invention is to provide in combination a novel arrangement of an improved vanless diffuser with cascade diffusers arranged downstream thereof whereby to obtain higher pressure differentials, higher Reynolds numbers, and/ or higher Mach number limits in a normal cascade diffuser.

Other objects and advantages of the subject invention will become readily apparent to those versed in the art from the following description taken in conjunction with the accompanying drawings wherein:

Fig. 1 shows schematically a radial flow compressor wherein two vaneless diEusers according 'to the invention are employed in conjunction with a cascade diffuser.

Figs. 2 and 3 are explanatory diagrams to show how the high divergence angles according to this invention are made possible in a radial flow system.

Fig. 4 shows the benefits of the invention by a cornparison of two cascades of vanes, one for high flow pitch angle and one for low pitch to show the benefits of low flow pitch angles, as effected in conjunction with and by a highly divergent diffuser according to the invention.

As schematically shown in Fig. 1 of the drawings there is provided a compressor casing or housing 1 supporting bearings 2 which mount the shaft 3 of the compressor rotor 4. Flanges 5 circular in nature are provided on the housing 1 thereabout adjacent the annular opening therein. Bolted to housing flanges 5 are the anges 6 of the novel diffuser assembly which consists of an annular casing 7 having a separately detachable front wall forming a first vaneless diffuser stage or section 8 and a second vaneless diffuser stage or section 7' attached thereto. At the point of connection of said detachable front wall section 8 to the second vaneless diffuser stage or section 7' is provided an annular recess 9 in which is seated a cascade of skewed vanes 10 which acts as an intermediate diffuser stage. The other side of the cascade of vanes is secured to the annular casing 7 by pins 11. The aforesaid second vaneless diffuser stage or secl tion 7 opens into a collecting spiral 12 through an annular opening 13 which is bridged in spaced relation by narrow webs 14 cast integral with the annular casing 7. Outer flanges 15 serve to be bolted to connect the collecting spiral to the diffuser housing flanges 16.

As seen in Fig. 1 ofthe drawings, the detachable front wall section 8 forms a front portion of the annular dif- -fuser casing 7 which defines the previously mentioned rst vaneless diffuser stage or section as being positioned, `according to the invention, between the compressorrotor -4 and the cascade of vanes 10. The divergence as shown issuch that the distance between the 'walls is Vincreased v .less stages and one cascade diffuser is shown.

by about 2.3 times their original value and taking th simultaneous increase inicorresponding radii into account, the total reduction of radial flow velocity component in this first stage vaneless diffuser will be roughly about 65% which implies that this diffuser stage will recover about to 90% of the kinetic energy originally contained in the radial component of flow.

Owing to the fact that this vaneless diffuser actson the radial component only of the flow, the total pressure recovery attainable in it is only limited; under conditions as prevail usually in compressors, this pressure recovery will rarely exceed 10% of the total Ykinetic energy of the flow. On the other hand, two additional effects are attained together with the pressure recovery which increases the value of such a vaneless diffuser above that which is indicated by its moderate pressure recovery limits above; one of these effects is a reduction of the flow Mach number at the entry into the cascade of vanes 10; the other effect is a decrease of the pitch angle of the flow.

Quantitatively, the Mach number at the cascade entry is lowered by about 5% to 10% under usual conditions, the upper limit in appropriate cases being about AM=.4; the flow pitch angle is decreased somewhat more than corresponding to the decrease of the radial flow velocity, that is in the example of Fig. 1, by about 70%.

The decrease of the flow Mach number at the cascade entry, as achieved by a vaneless diffuser corresponding to this invention, makes it possible to operate the compressor at higher `internal flow velocities than hitherto possible, and to attain higher mass flow and pressure ratios from a machine of -a certain size.

The vdecrease of the ow pitch angle gives four favorable design possibilities in addition to the above mentioned Mach number reduction, for a cascade diffuser arranged downstream of a vaneless diffuser corresponding to thisinvention.

The smaller initial pitch angle of the flow requires smaller flow deflection in the cascade (see Fig. 4) wb2 and Aimplies the use ofless cambered vanes profiles which can work efficiently at higher Mach numbers than strongly cambered vanes and which, therefore, permit one more step forward to higher mass flow and pressure ratios; or, instead of using small camber and high flow Mach numbers, normal camber and iiow Mach numbers can be used, and a greater cross section increases cascades; and/or, the lower pitch angle yields automatically a 'cascade consisting of a smaller number of widechord vanes (see Fig. 4), which is very desirable, especially in smaller machines, as a means to obtain higher Reynolds numbers, and, consequently, reduced friction Vlosses between flow and walls; and the reduction of the magnitude of the flow pitch-angles also reduces, of course, the variation of the pitch angle under varying operating conditions, and thereby decreases the adverse effects of varyingoperating conditions on thediffuser efliciency.

Downstream of the cascade of vanes 10 in Fig. l, another vaneless diffuser corresponding to this invention is arranged by providing again a very pronounced divergence between the walls of the casing 7.

The provision of a vaneless diffuser corresponding to this invention subsequent to a cascade of vanes has a special advantage. Any cascade diffuser consisting of either skewed or unskewed vanes always discharges the flow at a greater pitch angle than that of the oncoming flow and, therefore, in a direction especially suitable for a vaneless diffuser.

In-Fig. 1, avthree-stage diffuser consisting of two vane- In the same way, more than threestagescan be used, .arranging cascade stages, which essentially reduces the circumferential ow component, alternating with vaneless stages, which reduce the other components only.

The vaneless diffuser corresponding to this invention can be used alone, of course, as well as in combination with cascade difusers consisting of either skewed or unskewed vanes.

While a particular practical embodiment of the invention is shown herein, other modifications and applications thereof will be readily apparent to those versed in the art and such is considered to be Within the scope of the invention.

I claim:

1. In combination, a first vaneless diffuser, a radial flow compressor having a main housing and a rotary impeller positioned in said housing, said vaneless diifuser comprising a front wall section having opposed surfaces of revolution connected at their inner ends to said main housing to dene the annular outlet thereof therebetween, the said surfaces of revolution diverging outwardly from the periphery of said rotary impeller at a gradually increasing angle not less than whereby the radial component of flow velocity will remain subsonic regardless of the absolute velocity of ow, a second vaneless diffuser in cornmunication with said rst vaneless diluser and incorporating an inwardly converging section and an outwardly diverging section downstream thereof connected and in communication with a collecting spiral, and an intermediate diiuser stage positioned at the juncture of said first and second vaneless diifusers.

2. A vaneless diffuser for use in combination with a radial ow compressor comprising two opposed surfaces of revolution defining an annular passage therebetween Iand a cylindrical opening therethrough having its axis coaxial with the comon axis of said surfaces of revolution, said surfaces of revolution at their inner limits being adapted to engage the housing of a radial flow compressor at their inner opening so as to dene the annular outlet thereof, said surfaces of revolution diverging outwardly from each other at an angle of no less than 10 to a maximum point of divergence whereby maximum ow ei`nciency may obtain with a minimum radial extent of said surfaces of revolution, and a plurality of skewed vanes extending transversely between said surfaces of revolution at the said maximum point of divergence.

3. A vaneless diffuser comprising a pair of spaced opposed surfaces of revolution adapted to be secured at their CID inner limits to the housing of a radial ow compressor to define the annular outlet thereof, the said surfaces of revolution diverging outwardly from said outlet at a gradually increasing degree above a predetermined minimum to substantially reduce the radial llow velocity thereby, an annular cascade of vanes secured to the outer edges of said surfaces of revolution and extending transversely therebetween to deect the flow and increase the velocity thereof, and a second pair of opposed spaced surfaces of revolution extending from a converging section positioned at said cascade of vanes to a diverging section gradually increasing outwardly from said vanes at an angle of not less than 10 whereby diifuser eiciency is at a maximum.

4. A diffuser system for use with a radial flow compressor comprising a housing Wall dening one side of the compressor outlet, a removable arcuate wall section delining the other side of said compressor outlet and cooperating therewith to dene a first vaneless diffuser stage having a gradually increasing divergence of not less than ten degrees beginning at the periphery of said outlet and extending to a predetermined maximum divergent section, a cascade of vanes constituting a second diffuser stage and releasably retained by said housing wall and removable wall section in transverse relation thereto at the predetermined maximum divergent section, said housing wall having an extension forming a third vaneless diffuser stage having a divergence of not less than ten degrees between its Walls and disposed in angular relation to the rst diffuser stage whereby a highly efficient diffuser particularly applicable in supersonic ow obtains.

References Cited in the le of this patent UNITED STATES PATENTS 705,347 Harris July 22, 1902 1,617,133 Moss et al. ...7 Feb. 8, 1927 1,670,065 Eisenwinter May 15, 1928 FOREIGN PATENTS 636 Great Britain of 1889 132,189 Great Britain Mar. 19, 1918 152,673 Great Britain of 1922 264,614 Great Britain Jan. 27, 1927 338,025 Germany June 11, 1921 484,954 France Oct. 31, 1917 537,282 `Great Britain June 16, 1941 636,290 Great Britain Apr. 26, 1950 722,059 Germany June 29, 1942

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