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''%? PERFORMANCE EFFECT OF FULLY SHROUDING A CENTRIEUGAL or a/ ->& <$ ,s &$< &o lQao

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,\ - (3 c.9 By W i l l i a m K. Rit ter and Irving A. John~ien

Aircraft Engine Research Labomtory I Cleveland, Ohio

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WASHINGTON . I NACA WARTIME REPORTS a re reprints of papers originally issued to provide rapid distribution of advance research results to an authorized group requiring them for the war effort. They were pre- viously held under a security status but a re now unclassified. Some of these reports were not tech- nicr.11~ edited. All have been reproduced without change in order to expedite general distribution.

NACA Am NO, %5H23

By blilliam K. R i t t e r and Irving A. J o h s e n

A program of t e s t s was conducted t o determine the e f fec t of an integral front shroud on the performa~ce characteriotics of a centrif- ugal supercharger impsller. The impellers tes ted were a modification of a commercial semisnrouded impeller and a fu l ly shrouded impeller, which was the sane a s the senishrozCied one except Tor an integral front s h r o ~ ~ d . Tests were conducted i n a variable coapcnent t e s t setup i n conjunction with a vaneless diffuser of TL4C.A design a t irnpeller t i p speeds fron 800 t o 1200 fee t per second,

Results of the t e s t s indicated thz t the peak performance of the f u l l y shrouded impeller oscrjrred a t a larger flot7 quantity than t h a t of the semishrou3ed impeller en& tha t the performance character is t ics of the f~LLly shrouded impeller were s i igh t lg be t t e r than those of the corresponding seaSshrouded b p e l l e r a t t i p speeds of 800 and 900 f e e t per second. This ad~antage decreas9d with an increase i n t i p speed and, at t i p speeds of 1000 fee% per second a%d above, t he performance of the nemishrouded impeller was bet ter . Results indicate& tha t f u l l y shrouding an impeller of the type tested does not resu l t i n any s ignif icant improvement i n performance.

The superior perfomnance of a f u l l y shrouded centrifugal super- charger h p e l l e r , i n which the a i r passages a re c o ~ ~ l e k e l y conta5ned within the body of the impeller, has been a controversial question. This type of impelley has been used i n relat tvely few instances, although cer tain fvndwental advantages, such a s a reduction of axial thrust and an elimination of the losses tha t normally occur a s a r e su l t of in4;erpassa,ye flow through the impeller clearance space,

NACA 130, XSH23 2

have been cl.aimed f o r it. The p-esenco of an in tegra l f ront s'nroud, although it does reduce the bms"ung skrengti? of the inpel ler , makes t41-e s t ruc twe s t i f f e r and l e s s sub Ject t o v i b r a - t i o d f a i lu re ,

Reports giving performance character is t ics of experimental and c o ~ ~ e r c i a l fu l ly shrouded b p e l l e r s have been published, i n which claims of su-periority have been mde f o r f u l l y shrouded impellers. (See references 1, 2, and 3 . ) Plthough some of these impellers had outstanding performance, it m a not established t h a t the integral f ront shroud of the impeller was tile primrj. contributing factor. One i ~ p e l l e r , ~ ~ I c h has created widespread in t e res t and f o r which a nurri3er of claims have been aade, was aeveloped by the Deutsche Versuchsam-talk f;& L u f t f a M (DVZ) . Published r e su l t s by Werner von der i$.hll have claimed t h a t the DVL f u l l y shrouded im-peller has an ailvantage of as much as 5 polnts i n adiabatic efficiency over an equiva,lent seraisl?i.ouded iape l le r (references I. and 2) ; t h a t the shroucled impeller haa be t te r ;?erfornance throughout the opera t i r i . range (reference 2); an& t ha t an adiabatic efficiency of over 80 percent was obtained fo r a aupercl~arger using t h i s impeller (referehces 1 and 2 ) .

K o l h a m (reference 3) states tha t the DVL fuUy shrouded impeller is 'better than seraislwo~.ed i n ~ e l l e ~ s only a t low t i p s-peeds. The NACA tested a Dm su~ercharger incorporating a f u l l y slwoudea iinpeller (reference 4) and obtained perfomance about e q d t o the most e f f ic ien t production-type American superchargers. The B r i - s t 0 1 ,4eroplane ~olirpaG 1,iYaited of England aznufactui-es supor- chargers with fu l ly shrouded impellers, b ~ t unpublished r e su l t s of t e s t s of one oi' these impellers by the PIACA indicated no performance advantage over nore conventional types of bnpeller.

This report pr-esents the r e su l t s of an investicatlon t o i so la t e the e f fec t of an integral front shroud on the perfomance of one type of centrifugal supercharger impeller. The method chosen f o r t h i s investigation was t o cons t r~c- t a f u l l y shrouded irapeller t ha t was designed t o copy the passages of a production-tme semishrouded inyel ler which had good perfomance. This method, -though requiring pea - t machining co~iplexity, ha4 the advantage of establishing the possible ~ r c v e m e n t ?estiLting from tb.e addition of a sliroud t o an inyel ler t ha t was considered good rather than finding the los s which might res~iLt from renos~ing the shroud f roa a f u l l y shrouded impeller proportior-ed f o r ease of macliining . The perf ol-mance character is t ics of the f3~31y shrouded d p e l l e r an& the correspo~ding semishrouded impeller were compared a t iilrpellel. t i p speeds f roa 890 t o 1200 fee t per e e c c d , The perfomance t e s t s were g t a ~ t e d a t the NACA Langley Field la30ratcyy and conpleted a t Cleveland.

NACA ARX No, E5H23

A f u l l y shrouded b ~ e l l e r mts designed t o copy the passages of an existing conventi&& rac ia l impelier t h a t was considered t o have good performance characteris'tics. Because it xias not mechanically feasible t o machine a ft?ll;- sSzroudeO i n l e t section, the f u l l y shrotded impeller was cons-Lruzted with a separate in&ucer tha t had no f--on",hroud, In crder t o ~ a k e the smisixroucted impeller d i rec t ly comparable, a conventional impell.er mas modified by adding a similar s e ~ a r a t e inducer,

Fully Slwouded Impeller

A commercial seaishrouded impeller of conver;tlonal type (f ig . 1 ) wan ~ ~ e d a s tho basis f o r %he design of the full j ; shrouded impellel-. It i a a 12-inch dlaroe%e-i semishrouded vheel, with 22 rad ia l blades, and with ~ c a l l o p s arouxd the inpel ler per-iyherg. The f u l l y shrouded irapelier zz7as desip::d t o hm?e the same passage prof i les tzs t h i s con- ventional semisFzotr3ed 3qeZlax, and the passago i'il.lets ~wld the constant-thic'mass blades were proportionei? t o main3ain the sane paasage areas a s t h i s %;.,sic impeller. The scallops were orniLted on the f u l l y shrot~ded b~.pel ler . Ber\.diilg of the i n l e t blades .rms struc- tux-ally impracticable on tho f u l l 2 shrouded impellor, and it zms necessary t o use a separate in&uccr 5.n coaJmction with t h i s impellor, This separate ir,ducer section, which c o ~ i e d the Sending of the semi- sliroudsd impeUer i d e - c sesti on, was i?zounted on t h e f ront face of the f u l l y shrouded impeller ( f ig , 2 ) . Becauso the fully nhrouded impeller proper was of the s m e depth as the conventional semishroudcd impeller, the additior, of the inducer resliLted i n a longcr flow path f o r the fvJlg s!wo:;ded Sapeller, a s well a s i n differ ing passage-area charactar is t ics i n tho region of bending.

I

Another functional difference reaulted from the f a c t t ha t the saparate inducer had a constant li:~b diamoter a ~ d a constant outside dime-i;er, whereas the in le t section on %he conventional seaishrouded impeller had an increasing radlus along the axia l depth. Compression in a centrifugal s ~ p e ~ c h a r g e r is accomplished b3- increasing the moment of rncmentm of the air by means of an increase i n angda velocity a d an increase i n radSus of rot.atton of tho air as i . t flo'lis through the iapel ler . I n the b p e l l e r with a aeparate inducer, the ;zngd.ar velocity is theoret ical ly impartoi! t o the Eir Sefore the radius of rotat ion is increased, vhereas i n the i n l e t section of the conventional semishrou&ed irapeller, these two functions are par t ly superimposed.

IVACA Am, No. E5923 4

The -impeller front shroud T T ~ S designed a s e compromise between a design tha t would approach the e l a s t i c s t r a i n of the in-peller disk and blades and a shroud that wo~iLd not s t r e s s the blades at the entrance too highly. Similar elongation f o r the front shroud and the rear disk would reduce the load transference, which is the source of additional and complex stresses. The elolle;ation of the impeller disk and blades was computed along the radius by a modification of the stress-fmcti,on method given i n reference 5. A s e r i e s of hmer- bolas was :zsed t o define '.,he rear-sllroud profi le , and the impeller- blade load was consideyed as a density var iat ion i n the rear-shroud disk. The impeller f ront shroud was then computed on the basis of the t ~ r o op'posing requirements and the compromise made. The inducer section, because it constituted a weaker otructuxe than the inlet section of the basic ixpeller, was analyzed on the basis of strength, C e ~ t ~ i f u g a l s t resses c ~ ~ c - ; l a t e c i fo r the inducer were lower than the maximum calculated s t resses i n the inpe l le r dlsk.

The f u l l y shrouded impeller was machined by the NACA Langley Field machine shop from a so l id a l ~ m i n m forging. The inducer was fabricaY;ed by machining straighk blades and forming tilea t o the required contour wi5n special hending dies. The inducer was f i t t e d t o the impeller by ztachini~g the inducer blades t o match corres'ponding notches i n the impeller blades. \?lien assembled wiLz a pinch fit, t h i s mounting arrangemeill; asswed cllneaent of the blades cad provided support fo r the in&~xcer. blades.

Modif led SmishrouCcd Tmlpeller

A semiahrouded speller, t&e s m e a s the f u l l y shrouded impeller except f o r the front sh~oud, was obtained by modifying an unfinished model of the basic semishrouilod impeller. Thls raodlficntion was accomplished by mounting 2 seyarate inducer on the front of a basic impeller on which the i n l e t blades had not been bent o r beveled. The in6ucer was the s a e a s tne f u l l y shrot~ded impeller i r i e t section, except tha t the blades were rad ia l ly tapered rather than of uniform thickness a t the ilrpoller entrance. This modified semishrouded impeller was then eqxlvalent, i n passage form, t o the fvJly s;?rouded impeller, and comyarative t e s t s x~o~lld give a valid indication of the e f fec t of a f ront shroud on performclnce.

The moaificd scaisllrov.ded impeller with the added inducer i s shot",m i n figure 3. The inducer section was mczdc by the method used i n constructing the inclucer f o r the f u l l y shrouded !Impeller.

Tes'i Setup

The t e s t ins ta l la t ion a t the Laagley Field laboratory was made i n a variable camponent su.yereharger t e s t setup tha t incorporated a vaneless diffuser. Thc ins t a l l a t ion was made i n accorciance with reference 6, except that only one rad ia l o ~ t l e t pipe was used because of space l imitations. Previous t e s t s had shown no apprecia-ble &if- ference i n perfoi-mance when oEe ou%let pipe was used instead of two.

When the t e s t setup wzs reinstal led a t the Cleveland labo~atory , the single discharge pipe tias retained t o mike the ins ta l la t ions com- paraktle. The two ins ta l la t ions were the s&%e except thc t , i n the Laneley Field t e s t tmit, the out le t pipe discharged tlwouglh a duct system di rec t ly t o %he atmaphere, whereas i n the Cleveland laboratory setup, the out lot ~ i p e dischar$ed into the laboratory atnospheric- exhau.st~system i n which the pressure w a s maintained a t 3 inches of water below at~osp'neric pressure.

The impellem were tested In conjunction with a vaaelcss d i f - fuser of N X X des5gn and construction. This vaneless ciiffuser has an outside diameter of 34 inches and a passage-mea divergeace (equiv- a lent ,to a cone with an apex angle cf 50 l y i q along the i&eal loga- r i t h a i c sp i r a l flow pa31 901- rated 91m) $ha% T ~ S fowl6 t o give a f l a t performance cul-ve i r ? r ) r e v i ~ ~ s Wpeller t e s t s . A vaneless dif- fuser was used i n the tes%s because it; has unifwcm o p e r a t j . ~ cllar- ac t e r i s t i c s over a wide range of a i r flows and is l e s s subject t o m a l l chmges In im-peller character is t ics than a vaned diffuser, tkereby giving a more valid coqr r i son of the cllaructeristics of the impellers. Schemt i c diagrams of -tifie tnstallacYions of the f 1 3 1 1 ~

shro~ded i a ~ e l l e r and the nodif Led se~nisilz-ouded i ape l l e r i n con junc- t i o n w i t h t11s vaneless diffuser are s b m in f i g ~ l r e 4.

The locat ion of the standard inatr~menks and the precision of instrmentat. ion was a s prescribed i n reference 7, except tha t the i n l e t measwing s ta t ion w a s at 4 instead of 2 pipe diameters from the impeller- ii~d..et plane. Total-presslure svzvey tubes were pro-~ided i n the diffuser , with the Toour tubes located a t diameters of 13; 16, 23, and 33 inches. The total-yressure suvey tubes were of 3/32-inch diameteT wit11 a lj32-inch-diameter hole d r i l l ed i n tile s ide of the tu3e m a r t3e plugged en4 for rrieasur-ine; the pressure. Survey pres- swe readfngs were accurate t o f 0.1 inch of mercury.

NACA AIiR Xo, ES23 6

Tests

Tests of the f K L l y shrouded rzrrpeller and the conventional semishro~ded impeller were conducted at the NACA Langley Field lab- oratory and t e s t s of the llnodified semishrouded impeller were made a t the NASA Cleveland laboratory. I n addition, t e s t s of the con- ventional semishrouded impeller were repeated i n the second t e s t i n s t a l l a t ion a t the Cleveland laboratory t o check the reproduci- b i l i t y of the t e s t r e s l l t s . Reyrodu.ction of the data was sat isfac- tcry, with peak efficiency c:lecking within f0 .5 percent a t a l l ~ p e e d s ,

The t e s t s of the fu l ly slwouded inpel ler and the moaified semishrouded impeller were conducted t o determine the e f fec t of an integral impeller f ront shroud on impeller performance. Results of t e s t s of the conventional h p e l l e r were used o d ~ r t o determine the e f fec t of the use of a separate inducer on the performance of the seiaishrouded impeller,

Over-Ezll t e s t s of the three impellers were conducted i n accord- ance with reference 7 a t t i p sgeede from 800 t o 120C fee t per second, In the Langley Field ins ta l la t ion , hcwever, the l imited capacity of the discharge system res t r l c t ed flow t o t l ~ e extent %hat out let pres- sures were above 40 inches of meretry absolute with wide-open th ro t t l e . The maximum flow values at ta ined i n the t e s t s of the fl l l ly shrouded impeller and the conventional seinishrouded impeller are therefore not d i rec t ly comparable with tho t e s t s of the modified semishrouded impeller, i n which t h i s e f fec t was not present. Mininum- flow v a l ~ e s i n a11 t e s t s represented the incipient surge point of the u n i t a t each speed. Tests were conauc.ted with i n l e t a i r of room temperature. '

Sixrveys a t the diffuser s ta t ions were taken i n conjunction with the over-all t e s t s . A total-pressure survey with a given tube con- s i s t ed i n rotat ing the tube u n t i l a maximum read-ing was obtained a t t he midpoint of each of four equal lengths across the diffuser passage.

Com~utations of ove r -d l character is t ics , inrrluding adiabatic e f f i cbncy , pressure soefficie:lt qad, and load coefficient Q were mado i n accordance with reference 7. The values of

-qad at the diffuser s ta t ions were compu-bed using the total-prcssvxe readings of the diffuser surveys and the t o t a l temporatlare as deter- mined i n the out let pipe. The average t o t a l pressure a t any diffuser

NACA ARE? No. EaT23 7

s t a t ion was obtained by arithmetically averaging the pressures obtained across the diffuser passage, Check computations f o r a number of t e s t conditicns verified the f a c t that the difference between m arithmetic average and a mass-flow average w a s negligible. It was assumed tha t there was no heat t ransfer i n the lagged instal- la t ion , and therefore the t o t a l temperature i n the diffuser was taken as equal t o tha t a t the ou.tlet measuring s tat ion.

RESULTS AND DISCUSSICIN

Effect of Xercishroud.ed Impeller Modification

Comparison of the character is t ics of the conventional semi- shrouded impeller with those of the modified senishrouCed speller ( f ig . 5) shows Cvhe effect of the modification on the performance of the basic impeller. A t t i p speeds ~p t o 1090 f e e t pey second, the addition of the separate inducer had l i t t l e e f fec t on the perform- ance of the semisl.lrouded b p e l l e r . I n t h i s speed range, the charac- t e r i s t i c curves seeraed t o in8icate tha t the moclified seinishro~~ded h p e l l e r had a greater range than the conventional impeller, Tne apparent advantage i n ruaxim.m load coefficient, however, Fias due t o the difference i n t e s t - ~ l g installat,ions, A s previously discussed, the small capacity of the disc!iarge system I n the Langley Field ins t a l i a t ion limited the naxiam flow i n the t e s t s of the conven- t iona l sem9shiou.ded inpel ler ,

4.t t i p speeds of 1100 and 1200 f e e t Fer second, the modified .impel.ler had unstable flow characte2-ist i ccs axd, an i r regular perf om- ance curve, whereas the conventional impeller had a re la t ive ly f l a t character is t ic curve. The ins t ab i l i t y of flow with the modified semishrouded impeller was probably the resu l t of the inlet-blade f o m and the uncontrolled passage divergence.

I n general, except f o r the ins t ab i l i t y of operation a t t i p speeds of 1100 and 1209 fee t per second, the e f fec t of the modifi- cation on the perfcmance charactar is t ics of the semfs'nro~~ded imyelLer was sinall. This small change i n performance i~&fca-Led tha t conparative t e s t s of t h s nodified seaiskrou~ed b p e l l e r and the f @ l y shrouaed impeller xoxld establ is!~ the e f fec t of an integral f ront shroucl on the performance of a semrlshrouded impeller of con- ventional t n e .

f?P-CA L4m No. ES23 8

Effect of the Impeller Front Shroud on Performance

Over-all p e r f o m - c e c h ~ r a c t e r i s t i c s of the modified semishrouded impeller and the f u l l y shro~~ced imyeller, i n conj~uzction xsith a mele less diffuser , were used a s the Sasis of conyariscn f o r tile two bapellers. Ferforaance '~ased on total-pressixe measurements at the impeller dischai-ge and a t s ta t ions t:wougLo~~t the diffuser sho~ed t3a t %he t;e~ t installation was s-fitajl-e fo r siiot;ing irnpcller e f fec ts on the basFs of over-all perfomance.

Over-all perfoxmince. - The over-all adiabatic eFficiencies and --- - presswe coefficients f o r the two i q e l l e r s , t es ted i n conjunction with the vaneless diffuser, a re shown i n fip;ures 6 and 7, respec- t ive ly . A t impeller t ip speeds from 000 -to 1000 fee t per second, tke curves of adiabatic efficier,cy and pressure c o e f f i c i e n X o r the modified semishrouded impeller had a f l a t character is t ic , peaking i n a lower load- coef f ic ient range than the f ~ d l y shrocdec2 i ~ p e l l e r , wi tk t the r e s - d t t ha t the fu l ly shl-o~ded impeller had be t t e r perfom- ance i n tlie uyper load-coefficient range, a& tlie semishrouded hpe?.ler, ha,d the advantage i n the lower load.-coef f f cient range. The range over vhich the fx~ l ly shrouded impeller had be t te r perfomance was a maximum a t 80b f e e t per seco:ld, d ro~p ing with a speed increase mtil a t 1060 fee t per second the advantage existed ofil_y i n the load- coefficient range from 0.31 t o 0.37. Tile advantage i n maxjlllum load coefficient indicated f o r the semishrouded h p e l l e r was again due t o the difference i n t e s t - r ig instal la t ions.

A t t i p speeds of 1190 and 1200 fee t -per seconcl, both the f u l l y shrouded impeller and the modified semishro'aded i m ~ e l l e r had unstable flow characteristics, (See f igs . 6 and 7 . ) This condition was especially t rue of the mnodified senishrouded b p e l l e r , wliich exhib- i t e d heavy pressure pulsations i n the operating range, I n general, a t high t i p speeds, the performance curves Tor the two iapel lers had similar c?larac"cerstics, wFth flow pulsation affecting the perfom- a x e of both.

Peak adiabatic efficiency, plotted over the rmge of t i p speeds i n f - igu~e 0 , sho:is that the perromance of the f u l l y sl:roude& h p e l l e r was be t t e r than tha5 or' the semlohrouded i ~ p e l l e r a t the low speeds; t h e a$-$antage t ~ a s 2 .2 p o i n h a% a t i p speed of 800 fee t per second and 1.0 p0i;l-L 8% 930 fee t per second. The semislirovded im-peller performancs i~cs be t te r a t speeds of 1902 fee t pe?- second and above, with a m i m m advantage of 4.0 points a t 1100 f e e t per second. This trend of the ful-ly shrouded impeller t o have an advantage only a t low t i p speeds is i n general agreeirtent with the r e su l t s publLshcd i n reference 3. Tile peak efficiency cf the fu l ly shouded inpellor

decreased rapridly a,n hcreese i n speea, whereas the peak e f f i - cSency of the semishrov.ded impeller had a sl-ower decline up t o 1100 fee t per second. aril a sv.dden drop a t 1200 f e e t ger second. A t 1200 fee t per second, there vas l i t t l e difference i n the character- isz5cs of the iapel lers , proba%ly the r e su l t of the unstable flow character is t ics of each impeller,

Perf o-mance based on diffuser surveys. - The ad?-abatic eff i- - ciencyes determined f roa total-pressur-e~asurement;s i n the diffuser are shown i n figure 9 f o r t i p speeds of 800, 1000, an& 1200 fee t per second, These curves indicate the s m e re la t ive performance char- a c t e r i s t i c s for the fu l ly sl?roxded iinpeller znd the modified semi- shrouded impel-ler a s do the curves of over-all efficiency. Because of the functamental d i f f icLf i t t of 05talning 3recise measurements i n turbulent flow, the absolute vaJ.v.es of efficiency aay be i n error especially i n the regior, of t h s irnpeller t i p ; however, the curves do indicate the re?,ntive mer-its of the two impellers and suh tan - t i a t e the over-all efficiency r e s f i t s .

A t , a t i p speed of 800 f e e t per secoi& ( f ig . 9(a) ) the f l d l y shrouded b p e l l e r operated a t a higher efficiency than the s e m i - shrouded impeller over nearly the en t i r e range. The ~ -e l a t ive 521yel- l o r character is t ics , a s established at tSe 13-inch-dimeter s t a t ion were maintained throughout the en t i re diffuser and i n the out let pipe,

A t a t i p speed of 1OOO f ee t per second ( f ig . 9(b)) the e f f i - ciency curves for the f u l i y s'rrrouded inyel ler and the se~ishrouded inpel ler , a s established a% the &iffuser sta--lions, had tlie same re la t ive character is t ics a s the curves of o7rer-all efficiency. There was a s l igh t s1;iPt i n Kie re la t ive posit ions of the curves from the impeller t i p t o the 23-inch s tat ion, but tke r e a d i n ~ s a t the 23-inch and 33-inch diame"Jr s ta t ions gave efficiency curves of the same form as those i n the out let pipe.

The comparison curves a t a t i p speed of 1200 f e e t per second (f ig . 9 ( c ) ) were rather inccmplete a s a r e su l t of the pressure p7d- sat ions tha t occurred i n the operating range of the modified semi- sAbouc?ed imyeller. A s ~ho~yrl? i n f iguxe 9(c) , su-r7;eys i n the diffuser were ~m-obtainable i n the mild pressure pvLsation rmge, although flow i n the discharge duct ~lms suffici.ently s tab i l ized t o determine over-all character is t ics , The points tha t were obtainable, however, indicated that the conclusions reached from the over-all perfomance curves were valid.

NACA P-RR Mo, ES923 10

In gelreral, the trends established in these d i f fuser surveys correlated with the coyresponding over-al.1 performance cliaracter- i s t i c s , just i f ied the choice of a vaneless diffuser f o r the compara- t i v e t e s t s , anrl indicated tha t the characteristics determined from outlet-pipe neaswements weye adequate t o establ ish the re la t ive merits of the two impellers and de-temine the e f fec t of an impeller f ront shroud.

SUMMARY OF RESULTS

Comparative t e s t s of a semishroud~d impeller of comentional t;rpe and tke correspon8,ing f u l l y shrouded impeller established tho following resul ts :

1. The presence of a front shroud shifted the range of peak perforriance t o a higher load coefficient.

2. The performance of the fu l l $ shrouded impeller was be t t e r than tha t of the coTrespo:ldi:x semish~ou3ed impeller over most of the load-coefficient range at t i p speeds of 800 and 900 fee t per second., The range over which the f ~ l l y ar?,ro~dod inpel ler had the advantage decreased with speed; and at t i p speeds of 1000 foet per second and above, the s e ~ i s i ~ ~ o u d e d iape l le r had tho advantage over most of the flow range.

3. The presence of the fro,-rt shrolul increased the peak over- a l l efficiency by 2.5 points at 600 fee t per second and 1.0 point at 900 f e e t pel* second, with the semishrouded impeller h a ~ i n g the adva-rtage a t t i p speeds of 1000 f e e t por second and above. The maximum advantage of the semishrouded inpel ler was 4.0 points at 1100 f e e t per second.

Fully shrouding a conventional centrifugal supercharger impel- l e r of the type tes ted does not r e su l t i n any signif icant improve- ment i n performance characteristics.

Aircraft Engine Research Laboratory, National ,4dviaory Comit%oe f o r Aeronautics,

Cleveland, Ohio,

NACA ARR No. E5H23

I I

1, von der N u l l , Werner: The Design of Airplane-Engine Super- chargers. KACA Tfvl no. 839, 1937.

f l

2, von der N u l l , W.: The Maximum Dellvery Pressure of Single-Stage Radial Superchargers f o r Aircraft w i n e s . NACA TM No. 949, 1940.

3. Kollmann, K.: Limits of Single-Stage Compression i n Centrifugal Super chargers f o r Aircraft . PTACA TM No. 954, 1940.

4. Eing, 3. Austin, and Klein, Ra-rold: P e r f o m c e Characteristics of a Junkers Jmo 211F Ehgine Supercharger with a DVL Fufly Shrouded 74npeller- and Scrol l Diffuser. NACA ARR 80. E5B03, 1945.

5. Timoshenko, S.: Theory of I l las t ic i ty . McGraw-Hill Book Co., Inc., 1934, pp. 66-70.

6. ~l leGbrock, Herman B., Jr., an& Goldstein, Arthur W. : Principles and Methodls of Rat in& and Testing Cen-trif ugal Superchargers, NRCA AEB, Feb, 1942.

7. PJACA Subcomaittee on Supercharger Coapressors: Standard Pro- cedures fo r Ratiag and Testing Centrifugal Compressors. NACA ARB PJo. E5F13, 1945,

NACA A R R No . E5H23 F i g . I

F i g u r e I . - C o n v e n t i o n a l s e m i s h r o u d e d i m p e l l e r .

M A C A ARR N O . E5H23 F i g . 2

F i g u r e 2 , - F u l l y shrouded i m p e l l e r ,

N A C A A R R No. E5H23 F i g . 3

F i g u r e 3 . - M o d i f i e d s e m i s h r o u d e d i m p e l l e r . NACA C.4668

Vaneless-d i f f user p l a t e s / LzYh l n t e g r a l impe 1 l e r

f r o n t shroud

S t a t i o n a r y f r o n t s hr bud

S e p a r a t e inducer

I m p e 1 l e r r e a r s h r

V a n e l e s s - d i f f u s e r p l a t e s

S c a l l o p s

S t a t i o n a r y f r o n t shroud

S e p a r a t e inducer

I m p e l l e r r e a r shroud

NAT I ONAL AOV l SORY ,CWM!TTEE FOR AERONAUTICS

( a ) F u l l y shrouded impe1ler. ( b ) M o d i f i e d ~ e m i s h r o u d e d impeller.

F i g u r e Y . - Installation of fully shrouded and m o d i f i e d assmishrouded impellers.

N A C A A R R No. E5H23 F i g . 5 T ip speed

90 ( fps )

80

70

80

90

80 Semishrouded impeller

0 Conventional 70

50

Load c o e f f i c i e n t , Ol/n, cu f t / revo lu t ion

Figure 5. - Comparjson o f over-al l ad iabat ic e f f ic iency o f conventional semishrouded ~mpel l e r and mod1 f i e d sem~ohrouded impel l a r .

N A C A

Fully shrwded A Modified semishmuded

Load c o e f t l c i e n t , Ql /n , cu f t / revolut ion

Figure 6. - Comparison o f over-al l ad iabat ic e f f i c i e n c i e s o f f u l l y shrouded impeller and modified semishrouded impeller.

N A C A A R R No . E5H23 F i g . 7

Tip speed ( fps)

I NAT I ONAL ADV ISOR1 I 800 - - COMMITTEE FOR AERONAUTICS

80

70 Fully shrouded

6 0 A Modified seaishroudad

50

. I 2 .20 .24 .28 .32 .36 .40 .44 Load coeff icient , Ql/n, cu f t / revolut ion

Figure 7. - Comparison of over-all pressure coeff icients of fu l l y shrouded impeller and modlfied semlshrouded ~mpel ler .

Impel ler t i p speed, fps

Figure 8:- Comparison o f peak over -a l l ad iabat icef f ic iencles .of f u l l y shrouded impel ler and modif ied R semishrouded impel l e r . &a

N A C A A R R NO. E5H23

Dif fuser d i atneter

( i n . )

F i g . 9a

60.

Load coef f ic ien t , Ol/n, cu f t / revolut ion

100

90

80

( a ) T ip speed, 800 fee t per second. Figure 9. - Cotnearison o f adiabat ic e f f ic ienc ies o f f u l l y shrouded iage l le r and modified

semishrouded ~ m p e l l e r a t various measuring stations.

70 I I 1 I 1 NATIONAL ADVISORY

COMMI TTEE FOR AERONAUT I CS

N A C A A R R No. E5H23 . F i g . 9b D j f fuser dlaneter

( in. )

100 Over-at I,,dischar~e

- 90 • Fullyshmuded A Mudified semishrarded

80

70

60

. I 2 .20 .24 .28 .32 .36 .40 .44 Load coefficient, Ol/n, cu ft/revolution

( b ) Tip speed, 1000 feet per second. FiQure 9. - Continued.

N A C A A R R No. E5H23 F i g . 9c

Diffuser diameter

( i n . ) 100

90

80

70

60

50

Load coeff ic ient , Qa/n, cu ft /revolution

( c ) Tip speed, 1200 feet per second. Figure 9. - Concluded.