CA1154339A - Shroud arrangement for engine cooling fan - Google Patents

Abstract

SHROUD ARRANGEMENT FOR ENGINE COOLING FAN

Abstract of the Disclosure:

A bladed fan has an inlet shroud formed with a bell-mouthed inlet section which extends to an annular skirt portion affixed to the tips of the blades of the fan. The fan and the shroud are immersed within a:
shroud fixed to an engine cooling radiator. This fixed shroud has an ejector section that extends to or beyond the rotating shroud and fan blading. The bell-mouthed inlet section has sufficient arc to prevent separation so that the fan operates at high efficiency and discharges air outwardly therefrom including the blockade of the clearance between the two shrouds to thereby effectively reduce air recirculation.

Description

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SHROUD ARRANGEMEN~ FOR ENGINE COOLING FAN
.

This invention reIates to engine cooling fans and, more particularly, to a multibladed cooling fan having new and improved relativeIy rotatable shrouding to retard the recirculation of air into a specially contoured fan inlet provided by a rotatable shroud to improve fan efficiency and to reduce fan-generated noises~
Prior to the present invention, various shroud-ing designs have been devised for automotive engine cooling fans to reduce fan-generated noise and to make the fan pump air with higher efficiency. Among these are shroud assemblies that are fixed with respect to the engine heat-dissipating radiator which have cylindrical ejectors within which the fan is immexsed. However, with higher standards new and improved shrouding arrangements are required. In my prior patent, No. 3,937,192 issued February 10, 1976 for Ejector Fan Shroud Arrangement, a rotating shroud is attached to the tips of the fan blades, and immersed within the fixed shroud having a flared extension to provide a passage therebetween for the improved flow of the peripheral air from the inlet or suction side of the fan to the exhaus-t or pressure side. In that construction, the rotating shroud is provided with a , . . .

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diffuser that improves the ~ixing of fan-pu~ped air and entrained air being discharged between the two shrouds. While this prior construction provides an improvement over fixed shroud designs insofar as efficiency and noise control are concerned, undesired quantities of air pumped by the fan are recirculated through the passage between the rotating and fixed shrouds back into the inlet of the rotating shroud. Furthermore, when turning the leading edge or inlet of the rotating shroud, the inner layers of the recirculating air became detached from the inner wall of the rotating shroud. W:ith this~detachment, there was a transition from lam:inar to turbulent flow. With turbulent air enterlng the fan, the fan ; ;lS ~blades and, in particular, the outer radial portion thereof which received most of the turbulence, could not~efficiently handl~e t~e`recirculating air so that fan pu~ping efficiency~was adversely a~fected and fan~noise~was generated.
In the present invention, new and improvea fixed and rotating fan shroud construction is ~
provided with cooperative~action to achieve new 1evel&
of fan efficiency and fan~noise reduction.~T~e rotating shroud is formed~with a be1l-mouthed inlet o~
sufficient curvature to eliminate or sharply xeduce recirculating air sep~ration to according1y reduce , . . .
. : . : ' , :~

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turbulence. Importantly, in this invention, the rotatable shroud cooperates with an ejec-tor portion of the fixed shroud so that radial components of the air pumped by the fan effectively block or further restrict the passage between the two shrouds. with the passage between the shrouds reduced, the amount of recirculating air entering the rotating shroud is markedly reduced to improve fan efficiency, since less air is repumped by the fan.
In one embodiment of the invention, the fixed shroud has a cylindrical non-flaring ejector portion generally concentric with the bladed fan and the rotating shroud. This ejector shroud extends a fixed distance beyond the terminal edges of the fan blades and the rotating shroud fixed to the tips thexeof.
The radial component of the high veIocity air discharged by the powered~fan may impinge onto the `~ ~ inner wall of the fixed shroud to pneumatically restrict~
the clearance between the fixed and rotatlng shrouds so that the re~circulating flow of air from the high pressure or exhaust sid~of the fan to the low~
pressure or inlet side thereof is sharply reduced in volume. With such reduced recirculating flow, fan efficiency is sharply improved. The air whic~ is recirculated turns the corner of the bell-mou~hed inlet of the rotating shroud without separation from ~5~3~

the inner wall of the bell-mouthed inlet. Without separation, turbulence of the inflow air into the fan is reduced. With reduced and smo~ther laminar flow entering the fan blades, the fan can pump the air at reduced noise levels anà with higher efficiency to minimize parasitic effect of the fan on fuel consumption. :~
In a second embodiment of the invention, a double radiused inlet section of the rotating shroud proves for smooth laminar flow .into..the :f:an. blading supporting this shroud.
~ In another embodiment of the invention, the fixed shroud is formed with an axially extending but . ~ ~.
shortened ejector which is substantîally concentric ; :
wit~ the outlet end of the:rotatable shroud. The : terminal ends of the ~ixed~and rotating shrouds are ;~ ~ generally coplaner but wîth variation occuring in accordance with allowable productîon tolerances and ~variations~in the ~an~and shroud mountings. In .:
this embodiment of the inventîonJ~some~radial ~ : : : ~ : . :: : . . , components of the d.ischarged air will restrict the opening between the fixed and rotatlng shroud so that the quantity of reclrculating air is reduced. `

While the pneumatic blockage of this embodiment of the .
25 invention is not as efficient as~that of the first ~ :
mentioned embodiment, clearance between these two shrouds is preferably held to a:practical minimum ' ' ~ .

1~4339 for quantity production so that the recirculating air passage is substantially equivalent in restriction as the first embodiment.
Accordingly, it is a feature, object and advantage of this invention to provide new and improved rotating and fixed shrouding for multibladed fans in which the rotatlng shroud is immersed within a fixed ejector shroud secured to a radiator and has a bell-mouthed inlet curved to match the flow of recirculating air entering the suction side of the fan so that air separation and resultant turbulence :~ . :
is sharply minimized.
~ nother feature, object and advantage of this invention is to provide new and improved fan ~construction with relatively rotatable ~hrouding in which the radial component of discharged air e~fectively reduces the clearance between the~ ~;
relatively rotating shrouds to thereby reduce amounts of~air~recircul0tlng from~the pre0sur0 to the suction side of the fan.
Ano~ther f0ature~,~0bject and~advantage of this invention is to provide new and improv0d rotating and fixed shrouding for an engine cooling fan in which~the rotating shroud, fixed to the tips

2~ of the fan blades, is immersed within a surrounding fixed shroud which has an ejector portion~ the extremity of which terminates in the same general " ..

~5~339 plane of the extremity of the rotating shroud.
Ano~er feature, object and advantage of this invention is to provide new and improved relatively rotating shrouding for a fan in whi h the clearance 5 between the fixed and rotating shrouds is preferably ~.
kept to an optimized minimum. With reduced clearance, the radial component of discharge air is effective to block passage between the fixed ana rotating ~ :
shrouds so that recirculating airflow is restricted.
10 The reduced.recirculating airflow is smoothly fed ~.
by the bell-mouthed construction of the rotating shroud ~-, . .
into the bladed fan without tur.hulence so that there .-~
is increased eficiency and recluced an generated noises.
lS These and other features, objects and .:
advantag 9 of this invention will be more apparent ~rom~ :
the ollowing detailed description and drawing, in whlch~
Figure 1 is an exploded perspective view of an automotive radlator, engine and enyine cooling fan and a fan shrouding arrangement illustrating a first embodiment of this invention; ~ :
.:
Figure 2 is a fragmentary cross-sectional view of a portion of the fan~and shroud of Figure l:
Fi~ure 3 is a fragmentary cross-sectional view :
similar to the view of Figure l illustrating a second embodiment o this invention.

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Figure 4 is an exploded perspective view of an automotive engine cooling radiatvr, cooling fan and shrouding package embodying a third embodiment of this invention;
Figure 5 is a top plan view partly in section of the third embodiment of this invention as viewed along line 5-5 of Figure 4;
Figure 6 is an enlarged fragmentary view partly in section of a pOrtiQn of the radiator and 10 fan shrouding of Figure 5. `
Referring now to the drawing in greater detail, Figure 1 illustrates an automotive internal combustion engine 10 powering a belt and pulley drive system 12 operatively mounted on the front end thereof for drlving accessories including a bladed cooling fan 14. A radiator 16 hydraulically onnected to~the vehicle engine dissipates engine~;generated heat as engine coolant is~circulated therethrough.
The radiator 16 is mounted separately~fr~m the engine immediately in flont of the cooling fan~14 and supports~
a thln-walled outer;shroud~}8 of plastic material or sheet metal. The outer shroud is a fixed or stationary shroud having a~generally rectan~ular shaped base 20~with a plurality o~ spac~d tab: 22 extending outwardly ~rom;the periphery and adjacent to the corneFs thereof. Tabs 22 are formed with openings ` ::

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24 for receiving threaded fasteners 26 used to secure the outer shroud 18 to the radiator.
In addition to the rectangular base portion ~ :
20, the outer shroud 18 includes an annular and .:
convexly curved inte~mediate extension 28 and a cylindri~
cal ejector 30 projecting inwardly from the e.xtension ~
28 and terminating in an~annular edge 32 downstream - ~ :
of the trailing edges of the blades 36 of the fan.
: As shown, the blades 36 are arcuately spaced from one another and extend radially outwardly from the .
: hub portion of a fan pulley 40 rotatably mounted on ~.
. .
a projecting shaft 42 supported by engine lO. ~s will be appreciated, the blades 36 a.re pitched to pump a flow of cooling air through the radiator for engine : : 15 cooling purposes when the fah :pulley 4~ is driven by the engine through the belt and pulley system 12.
Attached to the outer extremity of the radial ::
an blades 36~i~ an annular thin~walled shroud 44 which cooperates with the outer shroud to provide an increase in fan pumping efficiency while allowing the fan to operate at a low noise level. The shroud 44 is:a rotating shroud having~aD:annu1ar bell-mouthed in7 et section 46 disposad forwardly and radially ou.twardly of the leadlng edges of t~s fan blades 36.

This outwardly flaring inlet sectLon has a smooth inner surface and:preerably describes an arc of -~.
about 90 degrees or more and terminates in an annular :

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outwardly extending edge 47. Wi~h this radial edge, recirculating air represented by flow arrow A flowing from the pressure to the suction side of the fan, can enter the bell-mouthed section without separation from the inner walls of this section. The recirculating air is subsequently funneled in a laminar flow pattern by the inner walls of the bell mouth into the rotating blades of the fan. Since air turbulence is .
avoided or sharply minimized in the recirculating air, the fan can pump alr supplied thereto with high efficiency and with reduced noise levels.
The ~ell-mouthed inlet: section 46 of the rotating shroud 44 is housed within the larger diameter intermediate portion 28 of the fixed outer shroud 18 to provide sufficient clearance between these relatively rotating shroud sections. This also allows for the reduction in clearance ~etween the ejector portion 30 of the outer shroud and the rotating shroud illustrated as clearance "C" in Figure 3. The annular clearance "C" between the concentric extending portions of the fixed and rotating~shrouds is preferably he1d to a minimum to raduce recirculating -airflow from the pressure exhaust side of the fan.
However, this olearance must be sufficiently large to accommodate engine oscillations relative to the fixed shroud and size and ~lounting variations occurring in quantity production.

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' As shown best by Figure 2~ the skirt portion 48 of the rotating shroud extends inwardly from the bell-mouthed inlet section and is secured to the tips of the fan blades. ~he annular skirt portion 48 terminates in an annular end edge 51 within the confines of the larger diameter ejector 30. The distance "I" that edge 51 of the rotating shroud is located from the trailing edge 32 of the ixed shroud represents the amount of axial immersion of the rotating shroud into the fixed shroud.
With an immersion of 3.18 mm, a clearance of 5.56 mm has been found to be acceptable~ However, smaller and larger clearances may be effectively employed when appropriataly matl-hed with immersion depths. Thus, if clearance "C" is larger than 5 56 mm, ~ -the amount of immersion "I" should he accordingly increased to provide for high-efficiency, low-noise fan operation. If the clearance between the shroud ; ;
1s reduced ~from 5.56 mm,~ the~amount of shroud 20, immersion should be accordingly reduced for the .
improved fan performance. ~
~ Wlth the helL-mouthed inlet section 46 outwardly flared through an axc of about 90 degrees or more and with the outwardly extending annular edg~
47, a flow control construction is pxovidad that -orresponds with inlet curvin~ laminar flow of recirculating air illustrated in Figure 2 by flvw , 10 ~54~39 arrow "A". With this match, there is little or no separation of the inner layers of air from the walls of the rotating shroud so that inlet induced turbulence is held to a minimum. With a smooth flow of air enterlng the rotatably driven fan blades, the fan can pump air at a low noise level and with high efficiency.
With the rotatable fan shrouding appropriately immersed within the stationary shrouding, the discharged air pumped by the fan will have a radial component which is directed onto the inner wall of the fixed shroud, This portion of the discharged air partially blocks the clearance "C" and consequently inhibits the recirculation of air from the discharge side of the fan through clearance "C" back into the suction side of the fan and, in particular, into the bell-mouthed section o~ the~rotating shroud. ~his blockage or restriction is illustrated in Figure 2 .
by flow arrow "B" whlch extends between ~he terminal ~ ~ -edges of the~fixed and rotatlng shrouds.
~ ccordingly/ the rotating shroud provides an inlet section in which airflow separation is minimized. Furthermore, with the rotating shroud , immersed within the~fixed shroud an appropriate di9tanoe, the fan and rotating æhroud effectively provide a discharge which cooperate~ with the ~ixed shroud which, in effect,further reduces the ~ ~.

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clearance "C" so that the quantity of recirculating air is held to a minimum. With the amount of recirculating air minimized by the shrouding and with the 90 bell-mouthed inlet, pumping efficiency of a fan is increased and the fan blading operates at-a low noise levelO
Figure 3 is similar to the construction of Figure 2 but has, for some installations, an improved rotatable shroud having a double-radiused, arcuate inlet Rl and R2 which is substantially greater than 90. The leading annular edge 47' of the bell mouth is directed rearwardly and outwardly with respect to recirculation airflow through the shrouding. Thesa refinements provide improved fan operation w.ith ~furt~er minimized airflow separation.
In Figure 4, incorporating another embodiment~
of the invention, there is shown an engine cooling radiator 61 connected through brackats 62 to elongated upper and lower supports 63 and 65. Tha radiator is prefarably positioned at the front of the vehicle on the nutboard side of a transvexsely mountad internal combustion engine 67 hydraulically connected ;
: : :
to the radiator. Disposed behind ~he radiator 61 is an electric cooling;assembly 69 in which a three-armed mounting brackat 71, generally Y-shaped in configuration, providas a central support ~ox an electric motor 73. The lower arm 75 of the bracket 71 _ ~ . . . . . . . . .. . . . . ~ . ..

~ 339 has a doglegged end portion 77 that seats in the mounting grommet 79 disposed in a vertical opening in the lower radiator support 65. The hracket 71 has upper arms 81 and 83 having forwardly extending ends 5 85 and ~7 for attachment to the upper radiator support by threaded fasteners 89. The electric motor 73 drives a fan 91~ the blades 93 of which are preferably unequally spaced and e~tend radially from the hub of the fan to a terminal rotating shroud 95~
10 This shroud,corresponding to the rotating shroud of the ~:
first embodiment of the invention~ has a bell-mouthed :
inlet section 97 which leads rearwardly into a cyl.indrical skirt portion 99 tllat is secured to the tips of the blades 93 of the fan. Di:sposed about the l5:;~rotatlng shroud is a stationary shroud~101 which incorporates:a generally rectangular~shell-like body . .
~ 103 having rearwardly extending projections 105 ~:
.
~: : formed thereon which contact the arms of the bracket 71. Stitching wires 107 clinched over on the shroud ~; 20 side of the bracket~secure the fixed shroud~to the bracket 71 and thus to upper and:lower radiator:
supportsO~ The fixed shroud prevents tail :winds from overpowering the fan at idle and causi~n~ reduced airflow in the area of -the air conditioner heat exchange not ~ : :
25 shown.
As best shown in Figure 5, the fan assembly is angulated with respect to the plane of the ~L~5~3~9 radiator. With this construction the fan is backed from the face of the radiator and without interference from engine 67 so that the fixed shroud is opened up and is more effective in funneling air through the S radiator to the fan Eor engine cooling purposes. The cylindrical ejector portion extends rearwardly from the fixed shr4ud and has a terminal, annular edge 109 disposed ideally in the same plane as the annular edge 111 of the rotating shroud. However, variations in this alignment occur in view of allowable tolerances and differences in mountings. In any event, the annular clearance 113 between the fixed and rotating shrouds is limited as much as practical. With such limited clearance, the radial component of air pumped by the fan will at least partially block the restricted recirculating air passage 113 with substantially the same beneficial effect as in ~he~embodlments of .
Figures 2 and 3. In Figure 6, for example, the recirculating airflow illustrated by flow arrows "D" and "E" is reduced~by the blockage of passage 113 and the interference from discharge air represented by flow .
arrow "Fl'.
While a preferred embodiment of the invention has been shown and described to illustrate the invention, other partial fixed shrouding and other modifications will become apparent to those skilled in the art. Accordingly, the scope of this invention is set forth in the following claims.

Claims (7)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In combination, an air-cooled radiator, a rotatable fan spaced rearwardly of said radiator, said fan having a plurality of radially extending spaced fan blades for pumping an air stream through said radiator for the cooling thereof, motor means operatively con-nected to said fan for rotatably driving said fan, a rotatable shroud secured to the outer tips of said blades including a cylindrical skirt section disposed around said fan blades and an annular bell-mouthed air inlet section flared radially outwardly from said skirt section at an arc of about 90 degrees as a minimum,a fixed shroud circumferentially surrounding said rotatable shroud fastener means securing said fixed shroud behind said radiator, said fixed shroud having an intermediate portion disposed radially outwardly of said outwardly flared inlet section of said rotatable shroud, said fixed shroud having a cylindrical ejector extending axially from said inter- :
mediate portion and throughout its axial dimension disposed at a substantially constant radlal distance from said skirt section of said rotatable shroud to thereby define a re-stricted recirculation air passage therebetween for the flow of air recirculating from the discharge side of said fan, said cylindrical ejector extending axially a pre-determined distance beyond said blades of said fan and the end of said skirt section, said ejector having a terminal end with a radius no greater than the other portions of said ejector so that a portion of the air stream discharged by said fan will be radially directed onto the inner wall of said ejector to thereby partially block said recirculation air passage and limit the quantity of recirculation air flowing through said recirculation passage and into said inlet section of said rotating shroud to improve the pumping efficiency of said fan while allowing said fan to operate at a low noise level.

2. In combination, an air-cooled radiator, a rotatable fan spaced rearwardly of said radiator, said fan having a plurality of radially extending spaced fan blades for pumping an air stream through said radiator, motor means operatively connected to said fan for rotatably driving said fan, a rotatable shroud secured to the outer tips of said blades including a cylindrical skirt section disposed around said fan blades and an annular bell-mouthed air inlet section flared outwardly from one end of said skirt portion through an arc of at least 90 degrees, said air inlet section ter-minating in an outwardly oriented annular edge encircling a portion of said rotating shroud, a fixed shroud fastener means securing said fixed shroud downstream of said radiator, said fixed shroud having a large diameter intermediate portion surrounding said outwardly flared inlet section and having a smaller diameter cylindrical ejector extending axially from said intermediate portion and disposed radially outwardly of and at a substantially constant radial distance from said skirt section of said rotatable shroud to define a restricted air passage therebetween adapted to conduct flow of air recirculating from the discharge side of said fan into said inlet section, said ejector extending axially a predetermined distance beyond said blades of said fan and the end of said skirt section to a terminal end with a maximum radius no greater than the other parts of said ejector so that a portion of the air stream discharged by said fan will be directed outwardly and radially towards the end of said ejector to effectively pneumatically block said air passage to thereby limit the quantity of recirculation air traversing said air passage and entering said bell-mouthed section of said rotat-ing shroud, and to improve the pumping efficiency of said fan while allowing said fan to operate at a low noise level.

3. In combination an air-cooled radiator, a rotatable fan spaced rearwardly of said radiator, motor means for rotatably driving said fan, said fan having a plurality of radially extended and arcuately spaced fan blades for pumping an airstream through said radiator, a rotatable shroud secured to the outer tips of said blade including cylindrical skirt section disposed around said fan blades and an annular air inlet section extending radially outwardly from a forward portion of said skirt section through a predetermined arc to a terminal outer annular edge of a diameter greater than the diameter of said skirt section and presenting a smooth inlet surface curved and tapering inwardly 50 that recirculating air remains attached to the walls of said air inlet section to thereby eliminate unsteady flow interaction characteristics between the recirculating flow of air and the blades of said rotatable fan, a fixed shroud entirely immersing said rotatable shroud therein and secured with respect to said radiator and having an intermediate portion disposed around said rotatable shroud, a cylindrical ejector extending axially from said intermediate portion and disposed radially outwardly of said skirt section of said rotatable shroud to define a restricted recirculation air passage therebetween for the flow of air recirculating from the discharge side of said fan, said ejector extend-ing axially so that a portion of the airstream discharged by said fan will be radially directed to partially block said recirculation air passage and thereby limit the quantity of recirculation air entering said inlet section of said rotating shroud to improve the pumping efficiency of said fan while allowing said fan to operate at a low noise level.

4. In combination, an air-cooled radiator, a rotatable fan spaced rearwardly of said radiator, said fan having a plurality of radially extending spaced fan blades for pumping an airstream through said radiator, motor means operatlvely connected to said fan for rotat-ably driving said fan, a rotatable shroud including a cylindrical skirt section secured to the tips of at least some of said fan blades and an annular air inlet section means comprising a flared portion extending radially outwardly from said skirt section through an arc sufficient to eliminate separation of the boundary layer of air from the inner wall thereof as air discharged by said fan re-circulates into said inlet section, a fixed shroud disposed about said fan, said fixed shroud having an intermediate portion surrounding said outwardly flared inlet section and having a generally cylindrical ejector disposed radially outwardly of said skirt section of said rotatable shroud to define a restricted recirculation air passage therebetween for the flow of air recirculating from the discharge side of said fan, said restricted re-circulation air passage being partially blocked by a radial component of the airstream discharged by said fan to thereby limit the quantity of recirculation air entering said inlet section of said rotating shroud to improve the pumping efficiency of said fan.

5. A rotatable fan spaced adjacent to a radiator for dissipating heat, said fan having a plurality of radially extending spaced fan blades for pumping an airstream through the radiator, motor means operatively connected to said fan for rotatably driving said fan, a rotatable shroud including a cylindrical skirt section secured to the tips of at least some of said fan blades and an annular air inlet section means comprising a flared portion extending radially outwardly from said skirt section through an arc sufficient to eliminate separation of the boundary layer of air from the inner wall thereof as air discharged by said fan recirculates around said rotatable shroud into said inlet section thereof, a fixed shroud disposed about said fan and in which said rotating shroud is fully immersed, said fixed shroud having an intermediate portion surrounding said outwardly flared inlet section and having an annular ejector disposed radially outwardly of said skirt section of said rotatable shroud to define a restricted recirculation air passage therebetween for the flow of air recirculating from the discharge side of said fan into said inlet section of said rotatable shroud, said ejector and said skirt section having generally co-extensive terminal ends disposed closely adjacent to one another, said restricted recirculation air passage being partially blocked by a radial component of the airstream discharged by said fan to thereby limit the quantity of recirculation air entering said inlet section of said rotating shroud to improve the pumping efficiency of said fan.

6. In combination, an air-cooled radiator, a rotatable fan spaced rearwardly of said radiator, said fan having a plurality of radially extending spaced fan blades for pumping an airstream through said radiator, motor means operatively connected to said fan for rotatably driving said fan, a rotatable shroud secured to the outer tips of said blades including a cylindrical skirt section disposed around said fan blades and an annular air inlet section flared radially outwardly from said skirt section at an arc of about 90° as a minimum, a shroud secured to said radiator having an intermediate portion surrounding said outwardly flared inlet section and having a cylindri-cal ejector extending axially from said intermediate portion and disposed radially outwardly of said skirt section of said rotatable shroud to define a restricted recirculation air passage therebetween for the flow of air recirculating from the discharge side of said fan, said ejector extending axially a predetermined distance beyond said blades to a terminal annular edge substantially coplanar with the end of said skirt section so that a portion of the airstream discharged by said fan will be radially directed towards said ejector to thereby partially block said recirculation air passage and thereby limit the quantity of recirculation air entering said inlet section of said rotating shroud to improve the pumping efficiency of said fan while allowing said fan to operate at a low noise level.

7. In combination, an air-cooled radiator, a rotatable fan spaced rearwardly of said radiator said fan having a plurality of radially extending spaced fan blades for pumping an airstream through said radiator, motor means operatively connected to said fan for rotatably driving said fan, a rotatable shroud secured to the outer tips of said blades including a cylindrical skirt section disposed around said fan blades and an annular air inlet section flared outwardly from one end of said skirt portion through a predetermined arc and terminating in an outwardly oriented annular edge encircling a portion of said rotating shroud, a fixed shroud secured with respect to said radiator having a large diameter intermediate portion surrounding said outwardly flared inlet section and having a smaller diameter cylindrical ejector extending axially from said intermediate portion and disposed radially outwardly of said skirt section of said rotatable shroud to define a restricted air passage therebetween adapted to conduct flow of air recirculating from the discharge side of said fan into said inlet section, said ejector extending axially to an annular terminal edge generally coextensive with the end of said skirt section so that a portion of the airstream discharged by said fan will be directed radially outwardly from said skirt section to thereby partially pneumatically block said air passage to limit the quantity of recirculation air traversing said air passage and entering said bell mouthed section of said rotating shroud, resultantly improving the pumping efficiency of said fan while allowing said fan to operate at a low noise level.