Design and installation of a wind tunnel fan

(1)

December, 1971.

DESIGN AND INSTALLATION OF A WIND TUNNEL FAN

by T.A.P.S. AppaRao

TECHNISCHE

HOGESCHOOl

.

~

.

yut<mOOI0UW'U~i)t ~OlllEEK

'13

JUli 19n

(2)

DESIGN AND INSTALLATION O

F

A WIND TUNNEL FAN

by

T.A.P.S. AppaRao

Submitted October, 1971.

(3)

•

ACKNOWLEDGEMENTS

The author wishes to thank Professor B. Etkin, who suggested the method of design described ·in thts report. His close supervision was of immense help in carrying out this work.

Mr.

H.

W.

Teunissen assumed responsibility for the manufacture of the blades while

Mr.

N

,

. O.

Umland carried out the installation of the fan. Their help in balancing the fan and in the measurement of performance of tne fan is also gratefully acknowledged. The author also wishes to thank

Mr.

R. Carter and

Mr.

G. Allen for the help they rendered in installing the fan .

(4)

SUMMARY

This repert describes the design and installatien ef a new

fan in the UTIAS lew speed wind tunne+. The previeus fan was insta+led .

in 1952·and has been eperating in an eff-design cenditien fer a leng

time. A few cracks were neticed en the fan blades in the fall ef 1968,

and the installatien ef a new fan was censid~red ·necessary.

The: ebjective ef the fan design was te. ebtain a maximum test

sectien wind velecity ef appreximately 220 fps while eperating the fan

at angles ef attack wellbelew the stall limit. This ebjective had te

,be achieved under certain re9trictiens: the meter HP, metor rpm, tunnel

less ceefficient, hub and tip diamete:rs ef the fan, etc. all had te

remain at their existing values and the existing straightener blades

were to remain if pessible. The metned ef design givenin this report

is a medified versien ef the ene given by Pepe

4 .

Details ef the manuf~cture ef blades, balancing ef t~e fan

and its installatien are alse given in this repert. The predicted and

achieved perfermance ef the fan is cempared and the rea sens fer the

discrepancy are suggested. Seme cemments are made en the eff-design

perfermance ef the fan.

(5)

Ackn9wledgements; Summary l. INTRODUCTION 2. DESIGN

3.

FABRICATION OF BLADES

4.

INSTALLATION

5.

PERFORMANCE OF THE FAN

6.

CONCLUDING REMARKS

7.

REFERENCES APPENDIX TABLE OF CONTENTS Page; 1 1 6 6 7 9 10 11

(6)

1 . INT-RODUCTION

A new fan was designed by the author for the .UTIAS low speed wind tun~el in the fall of 1968. A few cracks had been found on the old fan blades and it was considered necessary to replace them. The aId fan had been operating in an off-design condition (and possibly

in the stall region) for a.long time as a result of running the tunnel with tu.rbulence grids in i t; At present the wind tunnel is operated bath with and without a turbulence,g~id in it. The turbulence grid gives rise te an additional pressure loss and as a re sult the fan operates at a higher angle of attack. The new fan is designed to yield a fairly high test section speed while operating in an empty tunnel at a relatively low angle of attack. Th~s is expected to ensure a geod performance of·the fan even.in the off~design condition, iie.,

wi~h a turbulence grid in the tunnel,

The original fan wt;l.s designed by J. ;D. Stewart (Ref. 1) and wasinstalIed in 1952. It was designed for a tunnel loss coefficient

of.0.139. In 1965, J. B! Feir (Ref. 3) designed ~nd instalIed vortex generators in order to eliminate the flow separation in the diffuser section. He measured the tunnel loss coefficient bothbefore and af ter the installation of the.vortex generators and observed it to be 0.26 and 0.269 respectively. Th~s~ the fan which was.designed for a tunnel wi~h a loss cqefficient of 0.139 has been operating in a tunnel with nearly,twice the amount of ,losses. This caused the :fan blades to eperate at very high angles of attack (possibly in the stall'region). The new fan is designed to match the wind tunnel in its present

condition. It is designed to operate.at a low angle of attack resulting in a smoother, safer ope~ation.

This report also desçribes the fabrication of the fan blades, balancing and installation of the.fan and performance measurement. It

was not possible to balance the complete fan assembly.before installation.

As aresuIt, the .residual unbalanc~ af ter installation caused vibration

of the tunnel structure. A simple method has been deyised to determine the amount and location of unbalance witho~t disassembling the fan. ' No special balancing rigs were needed for this purpose. A series of acceleration measurements at a certain point on the tunnel.structure provided all the necessary information. Using this method, the fan

un~alance was brought down to a very small value resulting in an

extremely smooth operation of the fan. The balancing method is described

in Appendix A.

2. DESIGN _,

The design of the new fan is based on the basic.e~uations

given by Pope. (Ref.

4).

It was decided to use the existing electric motor to drive the new fan. Tt .was also decided to carry out the

design in such a.way as to eliminate;the necessity of anyalterations in th~ fan assembly. Hence the new design had to be carried out under the following restrictions.

(7)

(i) The m~imum op~~ating speed of the fan is 695 rpm (rateQ. speed

of.the electric moter),

(ii) Power required by the,fan i~ the ,design co~dition sboulQ. net:

exceed 60.HP. (An 0verlQad of 2'to 3 HP is permitted).

(iii) Themechanic~l design ,of-the blades should be such that no

alterations,áre needed te the ex~sti~g setup whileasse~bling

the new blades.

In order to be able to meet the first ,two requirements

gi ven "above, 'Etkin (Ref. 5) suggested a modif.icat ion of- Pepe' s method;

The de~ails of this method are.discussed in the following paragraph$.

Defini t'ions'

The 10ss c'oefficie~t K of the ,tunn~l duct 'is defined as the

ratio of thesta'tic pressure 'loss, in th~ tunnel to the :dynamic pressure

in thetest 'section, i.e."

6P

K -

(2.1)

Energl Ratio. It is the reciprocal of the tunnel 10ss

coefficient" ' " •

Velocitl Diagram

1

ER

=

K (2.2)

F~g. ;3 shows the :velocity diagram for the fan blade at ra,dius r .

U is the axial velocity of-,air at the "fan sectiçm, Cl' is the angle of

attackefthe blade, and.$ is the blade angle at radius r. Wl andW₂

are.the relative velocitie~ of air atthe leading and tra~ling edges,

of the blade,and W is th~ mean relative velecity. Th~ tangentiál

velocity of air relative.to the tun~el at the ,trailing edge,is wr.

R0t~tion of the flow is.defined by e

=

wr. Fig. 3'also shows the f~rces_ ~

acting on,the blade, The lift L is per~endic~lar to the W vector.

A number of formulae are given bëlow. These will be,used:

later in t~e design.

The f,ollewing relations can eastly be shown from Fig. 3.

"

~

=

,tan-1 _~21Tnr~~wr~u (2.3)

W =

J

U 2+ (

21T~-~wr

) 2 (2.4)

T = L C0S, ~ - D sin <I> (2.5)

(8)

A non~dimensiona1.force coefficient.may be defined·as

where.F. is.aforce ,acting on the b1ade. Using this definition Eq.

(2.5) and (2.6) may be rewritten for the thru~t coefficient Ct'a~d

-th~ torque force :_{coefficient Cx as:}

u at the

Ct = CR. cos ~ Cd sin </> C;C = _CR. sin ~ + Cd cos </> If VT isthe air speed in the test section,

fan section is given by

V~T ' U

=

-A-f

From the definitiön of energy ratio, we have

;'2PV 2 ~p

=

r T ~R (2.8) (2.9) the velocity

(2.10)

"

:for, tlte purpose of design calcu1ation, (using strip theory) the fan disk area'is' divided into a numper of annu1ar regions (sse Fig.

4).

'

If

A

~s ,the area of one of th~se regions an~ ~Pf is the fan pressure rise, then the e1ementa1 thrust deve1oped .by that part of the fan is.given by

~T

=

~P ·A

f (2.12)

Usi~g strip theory and,the definitiGln of thrust coefficient~

the chord length for that part of the fan ca~ be obtained as

From Ref.

4,

where y is defined as

dQc _ yxC

_x

dn - sin2</>

Torque coefficientQ is aefined by c

Q

=

~PU27TRt3Qc

The horse power,required.to drive the fan is _ 27TnQ

H .. P.- 550

(2.14)

(9)

The efficiency of the fan is given ~y From Ref.

4

where Method of Design

n

=

_AP.AfVf 550 HP wr e

=

_'_u (2.18) (2.19)

The iterative method used for .the .design .of,the fan is .

described below.

(i) The values of thefol1ewing parameters are known

design:

N = 695 rpm, ER = 3.84, ~ ,= 9-,76 sq. ft., Af =·33.2 sq. ft., ~

=

1.95 ft., Rt = 3.25 ft. In addition, t~e f0110wing parameters were chosen fer the

Test section speed V

_T

=

220. ~

Angles of·attack of the blade~: ar = 50, at

=

20. (a is, assumed to vary linearly from root to tip). Reynolds number ~

=

1. 5 x

Hl\

(This value is used fqr the first two iterations onlt. For s~bsequent iterations, a value based on the desig~ of second'iteration is used.)

It is proposed to use Section E airfoil sections for the blades. The characteristic curves of the airfoil section are given in Fig. 2. (Reproduced from Ref. 6)

(ii) For the purposes of design, five radial stations are. selected,along the 1ength of each b1ade'(x

=

0.6,0.7,0.8,0.9, 1.0). The annular areas ,A_{1 ,} A2' A3' A4 and A5 (~ee Fig.

4)

can now be .ca1cu1ated.

(iii) Knowing the en~rgy ratio ER and the. design tunne~ speed ' VT, the ,pressure rise acress th~ fan-str~ightener unit, 8p, is found.using Eq. (2.·11). For the firstiteration, i t may be assumed th~t 90% of this pressure rise takes p1ace in the fan and thus

(10)

The ~T due to the annular regions A of the fan may be

n n

found using Eq. (2.12).

(iv) For the ·first iteration w is assumed to be zero. ~ and W

may then be calculated using Eqs. (2.3) and (2.4).

(v)

Using the assumed values of angle of attack at various

radial stations and the Reynolds number,

C

i

and Cd for Section E air~

foil, are read from Fig.

2.

(vi) Now Ct ' c, y, C and dQ /

dx

may be eval.uated using the

approp-x c

riate equations (2.8) to (2.15).

(vii) Now the values of dQ

/dx

are known for x ranging from 0.6 to

c

l.O. A nlÁffierical integration of dQ

/dx

using trapezoidal rule yields

c

a good approxi.ma.tion to Q , the torque coefficient.

c

(viii} Now the torque Q, and the power (HP) required by the fan

may be calculated using Eqs. (2.16) and (2.17). Also, the efficiency

of the fan-straightener unit may be evaluated using Eq. (2.18).

This is the end of the first itera~ion. The geometry of the

blade (i.e., c and ~

=

~~ at various r), the power required by the fan

and the efficiency of the fan-straightener system are now known. Ho

w-ever, as indicated above, this design was based on certain assumptions.

In step (i),. the value of RN was assumed and in step (Hi), Eq. (2.21)

was assumed to hold good. In addition, in step (iv), w was assumed to be equal to zero. However, the results of the first iteration

pro-vide enough information to calculate w, RN and ~Pf' and these new values

may then be used to repeat steps (iv) through (viii). In the parti.cular

method used here, R was not corrected until the end of second iteration

and the corrected vthue of RN was us ed for all the sub sequent i tere.tions.

However, w and ~Pf are corrected at the end of each iteration. The

iterations are continued until the difference of chord lengths at x

=

0.7

for two successive iterations is less than 0.0001 ft.

The procedure for updating the values of R

N, w and ~Pf is indicated below.

Knowing d.Q

/dx

at various values of x, the rotation e(x) may

c

be calculated using Eq. (2.19). Now, w(x) may be calculated using Eq.

(2

.2

q)

.

Assuming a straightener efficiency ~s

=

0.7, the pressure rise

across the fan ~Pf(x) may be obtained using the following formul.a.

(2.22)

(Here, the straight ener efficiency is defined as the ratio of actual

pressure rise across the straight ener to the ideal pressure rise

(~ p w2r2)).

RN is updated at the end of the second iteration using the equation

R

=

N

5

(11)

The values of Wand C at x

=

0.9 are used in the above

e~uation.

Kncwing the updated value of RN, the corre~ponding values of CL'and L/D are read from Fig. 2 and tnese.values are used for all subsequent ite~ations.

Af ter the,firtal iteration, i~ is necessary .to, check ir the , HP 'neededby the fan isreasonably clos~ to the rated HP of the motor '" ' (vi(iS' 60 HF}. For the values of VT and~'s chosen in step (i) of the

design, the HP required was found te be 62.14. The fan efficiency is ,0~84. The design ié accepted.

The' design calculations were done on an IBM 1130 computer. The iteration conv~rged in four steps.

Fig.

5

s~ows the geometry of the fan blade. The geometry of·the new fan b~ade is compared with that of the original f~n blade in Fig.

6.

3. FABRICATION OF BLADES

The fan blades were made by Alcore Fabricating Ltd., Richmond Hil+, Ontario. The. blades were fabricated from G. P. polyester re~in a~d a wood flour-polyester filling material. The skin of the blades.is fi~erglas reinforc~d and is approximately. ,1/8" thick. The blades have a smooth Gelcoat finish. .

The specified toleranees 'on,the blades were ± 0.200 in angles and ± 0.010" in thick~esses. UnfortunateJ,.y, only half the .

angle measurements met·the specified tolerances.while·deviati9ns

as large as ,± 0.05" were observed in thicknes$ measurements. However, the blades were accepted.

4.

INSTALLATION

The installation of the new fan blades was done in October-November , 1970. This operation consisted of three steps:

(i) Removing the old fan blad~s, fan hub and,the aluminum rear

plate.from the tunnel. (Fig.

7

shows the fan-spinner assembly.) (ii) Static balancing of the new fan blades along with the hub and·

the aluminum rear plate (a new rear plate was designed a~d. made since the old plat~.was sJ,.ightly damaged).

(iii) Installation and assembly of new·blades, fan.hub, rear plate and,the spinner.

Static balancing of '. the fan blades had t0 be done outside the tunnel. The six blades and t~e rear plate were bolted to the fan hub,which was then,mounted on a dummy shaft. The dummy shaft

(12)

, ~.'

··waB suppor-"ted, at its ·two ends on two .pairs of s1:;ati9 balancing rol;Lers

'; ,', ; ; (se~, Fig.

8).

When the fan is left. free; i t cqmes ,to re~t af ter a . " .. while, with. the ,location or' unbalance pointing dO'Wnwards. Balancing

'-'-was, done by: adciing washérs as weights on the bolts used to fasten the blades .to.th~ hub. The.right amo~t of bàlancing ma~s was found by trial and; erro~. ' The end.of th~ ba~~nci~g operàtion was 'in~icated by the ,lack of preference;for any particular .0rie~tati.0n on the ,part , . of the fan, whel'l rot ~t ed : by ha~d 'and' let ,free."

1t would,have been preferabIe tO,balançe the assembly with the large spinner includeq.,. "Howeve~, th~ hatch in, the mot0r "sectien of the .wind ',tunnel wasn0t, big enough ·t0 ,take the spinner' eut and,

.hence it couldn0t b~ balanced.

Af ter balancing the fan and the ,rear plate they were moved into the tun!J.Eü ar,td ',mounte~ on t1;le metor s1).aft. Then the ,spinner WaS

.installed and the assembly wasproperly secured.

Aftel' installing the ,fan it was run at speeds up t0 approx-imately 600 rpm. At that spèed the :tunnel'structure WaS fotmd tQ vibrate ,rather severé+y. 1t was considered ess,ential t~ investigate ~

the vibrationproblem and solve it befqre proce~ding further. Te

start:with, it was obvieus that the ,caus~ of,t1).e vibrations,was an unbalance in,the fan assembly. The presence of unba~anc~ could be due t~, one,or both of,the fOllow,ing reasQns:

(i) small residual unbalance: due,to the inaçcuracies of the static ,balancing methpd cQuld give ,rise to fairly,larg~

forces at high ~pe~ds.

( i i) there coUld have be~n an unbaJ,.anc ~(. in the spinner whi ch ,was not 'part; of·the assembly balanced o~tside the tunnel.

Fulldetails of the ,tech~iques used to measure tunne+ vibratiçms and balanc~ the fan assembly are 'given in the iAppendix. Essentially the method consists of making a series of acceleration melfsurements at ,a point on the , tunnel structl:l,!'e where . the vi bratiGm .',

level was ",C0ns idered tö bè high. Fir st ,the measurement was made withthe ,fan running aS,installed at a,low speed~ Then six more accelerati0n measurements ,were ; made ,wi ththe fan runnin~ at the same 'speed and with a kno'Wn amount of,massadded to th

_e

fan hub at

si~ circumferential locations (co~resp'onding to the .six blade~).

An analysis of the data then'yielded the ,necessary infqrmation about the 'amount anti the 10cationof unbalance. The f~n was then balance~ by adding the required,aIDount 'of,mass.at the proper location. The above procedure was ,then repeated "wi th ,the fan running at a, high,

speed, t~ obtain bette~ accuracy. Th~ balancing operat:i.,on reduced'

the, tunqel vibration to a negligibly. small level.

.

5. '

PERFORMANCE, OF THE FAN

The 'most important perfo~an~e parameter from the user's ,

point of view i~ the .maximum air speed in the test: section of the ,

wind :tunneL The present fan was designed te. give a maximum tes1:;, -7

(13)

.seGtton.speed of 220·fps in.an empty tunneJ,.. " Howeve~, the fan was found·to deliver a maxi~um spee4 of only:194 fps. A tent~t~ve ~exp1anati0n .of the-poer·perfermance.ef the' fan is·giyen b~low.

'"

The pressure,rise acress the fan straightener.system was measureGj."anà.:it sh0~ed that the tunne+ pressure less ceefficient is ncw,·,9%,highen. thall the ,value measRreQ. by Feir (Ref. 3) in .1965. The tunnel l'reesure less coe'fficient ~ is new 0.283 as against ·the 1965 va1ue ef· 0.260 . . , ('rhe latter v~iugo was "used :in ,the design of the fan.)

, ,A"simple : calclf-lat ion s~ewed that the maximum test sectiçm sp~ed would 1

""" ':have, been ,202 fps, had the tunnellosses remained unchanged af ter 1965. A numl;ler of leaks are present, around the hatches .of the wind tunnel and lea.kpredfing the tunnel,is expected tO,resu+t ina

substantialredu~ticn cf the tunnel,losses.

It may be seen fr9m th~ figures ;given above that the increase in t1.1,nneJ,. less ceefficient explai;ns ,only : part ·.of the

discrepancy:between th~ 'designed'and measured values of the maximum t~st section velecity. Thè remaining part may be .:due to the fel.lowing causes:

(1)- As mentionedearlier err~rs in blade ma~ufactur~ were rat~er

l~rge. These errers~ especially the errors in thicknesses

n.e~r the tra~ling edge, ceulèl. have a , subs1;.antial 'effect en

the aerody~amics

Of

the blade.sectie~s.

(2) The fan design was based,on strip theery and ne three-dimensional effects were : taken ioto account. Such. effect s may' be :' expected ;

te re~uce,the performance 'of the fan.

(3) The aeredyn~ic,ferces en the fan pl~des.were calculated using Fig. 2 whose accuracy is ,unknown.

At presellt tqe UTIA,S'lew speed aeredynamics laboratory is engaged primarily in turbulence oriented research. Turbulence is .

gene~ated in tqe wind tunnelby means of woeden grids. When a .grièl. is 'plac~~ in,the t~pnel, the less coefficient .of the tun!lel is

inçreased an~ this cau~es the fan to run in anoff-design ,cenditien. '

Tp~ felleVfin~ table compares the pressure rise across the fan ' and '

straight,ener, ana. ~ximum test,sectipn speed.with and wj,theutthe

grid. . : . '

With Grid Without Grid Áp _fan _' 83.7 mIn water 53.0 mIn water

Áp _-9.~mIn water 8.5 mm water

s

ÁPf_s 74.3 mm water 61.5 mm water

V 132.0 f:ps 194.0 fps

From the ,abeve table it:is obvieus that in the eff-design cend~tien the straj,ghtener~ are ,not serving their purpose. Instead'

of yielding a pre~sur~ rise, they are causing

a

pressure drep. This, cf course, is ,duetc the presence .of excessive swirl in .the flew at the fan exitwhen a grièl.is in the tunnel and the straightener~ are,

(14)

.. .; .,. ~

operating in a stalled condition., A measureIp.ent wi th a yaw probe indicated that the flow velocity at.the fan exit makes an angle of 300 _{with the tunnelaxis. This would imply that the straighteners}

are operating at a h~gh angle .of attac~ of 300 _and_·_{hence are}_._{stalled .}

. (Th'e straighteners have a symmetric airfoil secti9n with their chords

, , .' -pal'allel. to the tunnel axis.) It should be, possible to improve the eff-design.performànce substantially without sacrificing the perf-",." om.ance in the dei;lign condition by ;r-edesigning the straighteners .

Mr.

R. Carter, a graduate stud~nt at the,Institute, is now working

.on' this· problem.

When the fan was first run at the maximum speed, t~e blade tips 'began.to contact thetunnel walls af ter .a fewminutes of

running. It was deduced, that this was primarily due to the thermal expansion of the fan blades as the tunnel heated up. When a .grid is present in t~e tunnel, theair temperature rises rather fast and,thé blades began tO ,touc~ the tunn~l walls in less time. To solve this problem the high spots on th~ steel tunnel -wall were

ground down. Af ter this·operation it was possible to run the tunnel. at maximum speed, for about l~ ho~s with a grid in the tunnel, ' , before the,blades touche~ the tunnel 'walls. Due to some practical prQblems no more.grinding was done. At present the tunqel is hardly eyer r~ contin~ous~y at maximum speed,for more than an hour, sono significant operational.limitation is experienced.

,

6.

CONCLUDING REMARKS

A,detailed description of the various phases of the design and installation of a,fan for a low speed wind t~nne~ are discussed in this r~port: The method of design given.in th~sreport, is

applic~ble ,in situations where a fan has to be designed;to match a.

given wind tunnel and'a driving motor. Designers of fans may find this discus sion in Chapter

5

partic~larly useful where the perfor-mance,of the fan was compared with design objectives. They may aiso find the cOInIqents on off-design pe~formance usefu1.The novel , .balancing technique' described in this report should prove useful in

sl,tuations wher~ i t is n0~ ,possibleto accurately balance t1;le fan before it is instalIed, or where ,it 'isnecessary to balancethe fan

in situ.

(15)

'7.

REFERENCES 1. 2. 3.

4.

5.

6.

Stewa!.'t, J .. D. LAundry, Wm. E. Feir, J.B. Pope~ A. -Etkin, B. Patterson, G.N.

The Design.of a Wind T~nnel Fan and Straight ener Syste~. M.A.Sc. Thesis,

Univ~ of Toranto,

1949-50.

Calibra~i9n af,the U~iver~ity of Terente

Low Speed Wipd·Tu~nel. M.A;Sc. Thesis, University of Terento, March

1954 .

• The ~ffects of an Arrangement of Vort ex

Generaters Installed to 'Eliminate Wind

T~nnel piffuser Separation ." UTI-AS TN No.

87,

J~n~

1965.

Wind-~unnel Testing, Sec:orid Edit ien.

John Wiley

&

Sons, New York. Pri vate ,Comm\lnic~tian.

Ducted Fans~ Design for High -Efficiency.

~eport ACA-7. Australian Council tor

(16)

APPENDIX

MEASUREMENT ANDREDUCTION OF TUNNEL VIBRATION

As menti0ned,in Chapter

4

the tunnel vibratións wer~ rather large.at high ·fan speeds. It was considered essential to reduce .

the vibration by balancing the fan.

Before installation the fan was statically balanced along with its hub and the aluminum rear plate. (See Fig.

7

for details of the fan and.spinner assembly.) However, the ,spinner ~ould,not be balanced due,to some'practical problems. The tunnel vibràtions could be due to either the,inaccuracies of the static balancing method'or anunbalance in the spinner.

The problem therefore was to measure the amount and loc-ation ·of unbalance in the ,fan-spinner assembly without 'disassembling it andremoving it from the, tunnel. Disassembly would .have i required .

muchadditional ti~e and effort. Further, it would have been necessary

tobu~ld a .special rigto .. balance tbe fan-spinner assembly static~lly.

The meth~d us~d tO,balance the fan-spinner assembly consisted '

of making a series of vibrat.ion measurements, of the tunI}.el. These

.measurements were so planned'as,to yield the necessary.information -the amount and location of unbalance. Once',this',is known, it ,is.a simple matter to b~lance the fan. This method requires the use of an accelerometer and no special balancing rigs are ne~ded.

THEORY

Suppos.e the unbalance. in th~ fan asseIllbly is equivalent to ·

a mass m located at radius r. Further, assume that it is located somewhere between the blades l .and 2.of the fan. This is shown in Fig. 9(a) where the numbers 1 to

6

refer to the six blades and a+l angles :are measured with respeçt to blade No. 1. At any angular veloci ty w: 'of the fan. the force .due to unbalance is

F

=

m w2 r.. If the ,tunnel struct1:lre is assumed to be. a linear .system, the amplitude of acceleration at an arbitrarily chosen point on the structure is given.by A

=

KF where K is

a

cqnstant. The acceleration varies sinusoidally with time, the frequençy being the rotational frequency of the fan.

Now suppose.an additional unba+anc~ is introduced in the fan assembly by fastening a mass M to,the fan hub at r~dius R. .

Then the force due to this,mass is B =:M

w

2 R. The resultan~ force of unbalance is given,by Fl when M is located at blade number 1, F2 when M is located at blade.number 2, etc. (See Figures 9(b) to 9(g).) In all these cas~s F is the vector sum of the force U due to the.original unbalanc~ and:the forc~ B due to the added mass. The variation ,of F with the location of,the mass M is shown qualit-atively in Fig. 9(h). The'maximum force of unbalance corresponds to the locatiQn of original unbalance andthe fan may be balanced by adding the proper amount ofbal~ncing mass at a diametrically opposite point. The amount, of unbal~ce U .is equal to the .mean

(17)

,/ . \

value of F. Thus th~ amo1,lnt a~d' location of unbalance can, be found by measuring the amplitude of.acceleration at a point on the tun~el structure

corr.espendi~g to thé cases shown in Figs. 9(a) to 9(g).

.1NSTRUMENTATION

The wind ,tunnel vibr.atiéns were measured. by aStath.am ST 2902 . aooeleremeter ... The accel~rometer ha,s · a range , of 2g which was .;far aQove .

, ' , "the- vibr.ation",level of the tunnel ~ Noise posed a "big probl~m in, the

m~asurement and it was necessary tG use,a low pass ,filter and a correl-ator to extractth~ signal from t~e noise. It ·was possible, tG m~e very aceurate.acceleration measurements even,though noise c9nstitutedmo~e .than 90% of the accelerometer output. '

The noi~e was partly randoIp. and partly sinusoidal, the, latter '

having a frequency.of 60 Hz. The sinusoidal co~ponent was due to the '

.pr.~ximity o~ the ,mote~ power cables to th~ acc~lerometer;

Tne electrical, ,connections, w~re m,,!-de as sho\fIl in, Fi,g. 10.,

The acc,elerometer wasmounted on the tunnel, structure at a, point

whel\,e the vibration level is censidered te>, be high. 'The low-pass filter. shewn in,Fig. 10 consists e>f two multimetr~cs variabIe filters co~nec~ed i~ sertes and set te operate as low-pass filters ·with a cut~off of 20 Hz.

Theaccelératio~ signals have,frequenc~es l~ss than 12 Hz. The three '

amplifiers (each with ,a gain of 10) shewn in the circuit diagram are thosé of the l'abo:ratory Pace analog computer ~ A potentiometer was used tO,remove the d.c, from the accelerator outpu~. Th~

autocorrel-atio~ of the amplifier 01,ltput was,obtained:using an available,Princeton Applied ,Research ~Correlator anà. was ,displaye~ on an oscilloscope~

In the circuit diagram shown in Fig. 10 ,the lew pass filter helped r~move the ,60 Hz sinusoid, fr om the accelerometer output while

t~e correlaterextracted the ,acceleration signal frqm th~ noise present in the amplifier output . . This made it possible to measure very low .acce+erations with a high amount ,of accuracy.

As sho\fIl in,Fig. 11 ,the autocor~elation of a sinusQid is a , cosine .wave "whose amplitu,!e is equal to the mean square value of the sinusoid; Th,e autocorrelation of random noise falls rapidly to zero as ,the time delay T is increased. Thus the output.of the correlator

in Fig. 10 shows clearly the sinusoidal ·acceleration signal'which can then be ,measured: accurateJ,.y .

. MEASUREMENT .

As mentioned,under 'Theory' above, a series of properly ,planned aqceleration m~asur~ments yield the necessary information

about the location and amçmnt' of unbalance. Each, time the amplitude of the' sinusoidal ~art of the corr~lation curve (c) is measured. Knowing that the amplifier gain is 1000, the amplitude of acceler-ation A may be calculated fr om

(18)

•

tI"'-~

-A

=

K (A.l)

where K is ealibration constant of the accele,rom~ter.in g's/volt. Since the accelerations themselves were of no interest, a~l calculations were made using the correlator outputs (c) directly.

, .... ~,

With the fan running at abo:ut 350 rpm, th~ cor:relator O\ltput . c, was measureà. first -wi th no addi tiona,l mass on, the fan hub and, th en ,

." -",·with, a· mass· of.,· ~-2 Ibs. attlitched to the fan hub at six different loc-ations., A plot ef, these results showed that the unbalance in the fan is at location 4.75 (i.e., alocation 450 _{from position 4 and 15° from}

position 5. (See Fig. 9(a)). It must be noted .~hat:the accuracy of the determination of loc at ion of unbalance is not very high (probably

± 100 ) because the number o:f observations is rather smal1. Hawever, the accuracy may be impro~ed later by taking more ,points in the region

.of int~rest.

A simple calculation baseQ. on t~e measurements also showed,

that the amount of unbala.nceis equivalen~ to a massof 4-1/3 oz. at

.hub radius.

A

balancing mass of 4-1/3 oz. was then attached tó tnefan hub at positien 1.75 (i.e., diametrically 0pposite to position 4.75). Now the t~nnel vibration wasconsiderably smaller,and m9re refined'

balancing c~n be,carried out 'only at:a higher fan speed. Running the fan at t~e highest spe~d (695 rpm) acceleration me~surements ,were

made ' fo~ a range Of values of balancing mass and its ,location. The optimum values of the balancing ~ss and its location are determined frem these meas~ements. These values are then used to balance the fan . . With the fan balanced this way, it was observed to run very smoothly'and the tunnel vibration was ve:ry small even at the,maximum fan speed.

(19)

rcOl'W"'" ...,. " ~ 4TH _DIFFUSER 13./-~~.o.v 00011 11 .&VA-45'RtU'r 54'_10* 1 2 - ''"''''''''''''_ Ctu,.&rAA/r"llllWA, 1 13 - '

DI'Ft.<S~'"

~'!u12-7· ~.111.ACle J:AA/ 60~ ~PMN.s/oN

/$"" ~ grid I I Isr DlÇJi:'(/SIr~ 5.0·clU'llMf.sl~ working section 2N"'a::w",,~ I'~

(20)

CL

Co

1.8

0 .

18 80.0 1.6

0.16

1.4 '1=

CL

CD

60.0

5

10

15 oRe

=

0.312

X

10

6

vR

_e

=0.791

X

10

6

oRe

=

1.50 x 10

6

DRe

=

3.10

X

10

8

öRe

=

5.07 x 10

8

20

(21)

• T

u

21Tnr

(22)

FIG.

4 R

x

=

Rtip

5

XI

=

0.6,

x

₄

=

0.9

J

ANNULI USED

AI

:

0.6Sx

<

0.65 AI

:

0.65 S x

<

0.75 A. :0.75 S x

<

0 .

85 A4

:0.85

S x

<

0.95 AI

:0.95Sxs

1.00

X I

= 0.7 ,

XI

= 1.0

X.

=0.8

DIVISION

OF

THE

PROPELLER

DISC

AREA

INTO

ANNULAR

REGIONS

(23)

T. E. <tc.g.

,

-I

-C .

1

loE. <tc.g. 3890" R ±.oo

r

'

.02 o .E.

+

B

--.

_C

t

o

----

--.

I r

6.00"

_1<---

._.

~

FIG. 5 PROPELLER BLADE

E I

' t

I

F I

ft

I G

I . .

H c A-A K - KL...--_-t-_--I L-L ' - - - t - - - '

(24)

51

18 t

17+

<:>

New

Fan

49

[::J Old

Fan

-

«I»

_t

_IS"

•

47 !

i'

-c

₄₅

_«I»

-

.&.

•

W U c: ...J

_{43 -}

₁₄

(!)

z

0 <I _0:

41

0 1.&.1 J:

""

"""'-

~ Chord

c

0 <I

39

...J

m

37 "

35

10

33

9

31

8

29

7

27

6

25

5

I

0.6

0.7

0.8

0.9

1.0 R

~

Rtip

FIG. 6

THE

GEOMETRY

OF OlO

a

NEW

FAN

BlAOES

(25)

~--+---14~··~---~~

REAR SUPPORT PLATE

HUB PLATE

.

FAN MOTOR

AG.7 FAN

a

SPINNER ASSEMBLY

FAN BLADE

SPINNER FASTENED TO HUB WITH 1/4" SCREWS

FAN BLADES. HUB PLATE a

REAR SUPPORT PLATE FASTENED TO HUB WITH 1/2" BOLTS

(26)

.---Aluminum Rear Plate

End -view of

the Statie Balancing

Rollers

Dummy Shaft

Statie Balancing

_~::=::+

_-+--_

~~~~~

Rollers

FIG.8

ARRANGEMENT FOR

STATIC BALANCING

(27)

Fo= u -

mw1r

u=

mw1r

4 M

B=McJlR~

(0) ( b)

cp ..

0 B U

~

u

F.~

(c)

cp-

600 (d)

cp

-1200

u

U B M B

~

F.et

(e)

cp ..

1800 _B (f)

cpa

2400

U F Location of Original _Unbalance

~

.

₀ ₆₀ ₁₂₀ _{180 240 300 360}

B

r

(g) CP-3000 _{( h) Variatlon of} _{Force oue to}

Unbalance Wlth

cp

(28)

Pace Analog

Computer

I

_l

L. P. F.

_PAR

cut off ot

I

Correlator

20 Hz.

Scope

I

-I

±IOOV

-I

Stotham

St-2902

L

Flo

Accelerometer

~

(29)

t

_x

I I /

"

t

R)tx

t ----.

104

T

--I Signal:

x

=

X sin

(wt

+

8 )

T - - '

Auto Correlation of the Signal

14

T

- - - -

....

~I

X

2

(30)

•

1JrIAS TECHNICAL NOTE NO. 170

Institute for Aerospace Studies, University of T oronto DESIGN AND INSTALLATION ar A WIND TUNNEL FAN

AppaRao, T.A.P.S. 13 pages 11 figures

1. Fan-design and installation of. 2. Wind Tunnel 3. Balancing

I. AppaRao, T.A.P.S. Il. IlTIAS Technical Note No. 170

This report describes the design and insta11ation of a new fan in the 1JrIAS low

speed wind twme1. The previous fan was installed in 1952 and has been operating

in an off-design conditlon tor a long time. A few cracks were notleed on the fan

blades in the fall of 1968, and the installation of a new fan was consldered

necessary. The obJective of the fan design was to obtaln a maximum test sectlon

wind velocity of approximately 220 fps while operating the fan at angles of attack

weU below the staU limit. This objective had to be achieved under certain

res-trictions: the motor HP, motor rpm, twmel loss coefficient, hub and tip diameters

of the fan, etc. aU had to remain at their existing values and the existing

straightener blades were to remain if possible. The methoe of design given in this report is a modified version of the one given by Pope.. Details of the

manufacture of blades, balancing of the fan and lts lnstallatlon are a160 glven in th1a report. The predicted and achieved performance of the fan is compared and the reasons for the discrepancy are suggested. Some comments are made of

the off-design performance of the fan.

~

Institute for Aerospace Studies, University of T oronto

DESIGN AND INSTALLATION ar A WIND TUNNEL FAN

AppaRao, T .A.P.S. 13 pages 11 figures

I. AppaRao, T.A.P.S. Il. IlTIAS Technical Note No. 170

This report describes the design and installation of a new fan in the 1JrIAS low

speed wind tunnel. The previous fan was installed in 1952 and has been operating

in an of! -design condition for a long time. A few cracks were noticed on the fan

blades in the faU of 1968, and the instaUation of a new fan was considered

necessary. The objectlve of the fan design was to obtain a maximum test sectlon

wind velocity of approximately 220 fps while operating the fan at angles of attack weU belaw the stall limit. This objective had to be achieved under certain

res-trlctions: the motor HP J motor rpm, tunnel loss coefflcient, hub and tip diameters

of the fan, etc. all had to rema.ln at their exlsting values and tbe exlsting

straightener blades were to remain 1f possible. The methoe of design given in

this report is a modified version of the one given by Pope. Details of the

manufacture of blades, balanc1ng of the fan and its installation are also given

in this report. The predicted and achieved performance of tbe fan ls compared

Md the reasons for the dlscrepancy are suggested. Same comments are made of

the off -design performance of the fan.

~

Available copies of this report are limited. Return this card to UTIAS, if you require a copy. Available copies of th is report are limited. Return this card to UTIAS, if you require a copy. 1JrIAS TECHNICAL NOTE NO. 170

AppaRao, T.A.P.S. 13 pages Ü figures

1. AppaRao, T.A.P.S. Il. IlTIAS Technical Note No. 170

This report descrlbes the design &nd installation of a new fan in the trrIAS low

speed wind tunnel. The previous fan was lnsta.l.led 1n 1952 and has been operatlng

in an off -design condition for a long time. A rew cracks were notlced on the fan

blades in the faU of 1968, and the installation of a new fan was considered

necessary. The objective of the fan design was to obtain a maximum test section

weU below the staU limit. This objective had to be achieved under certain·

res-trictions: the motor HP, motor rpm, tunnel loss coefficient, hub and tip diameters

of the fan, etc. all had to remain at their existing values and the existing straightener blades were to remain if posslble. The methoe of design glven ln

this report ls a modlfied version of the one given by Pope. Details of the manufacture of blades, balancing of the fan and its insta11ation are a1so glven in this report. The predlcted and achieved performance of the fan ls compared

and the reasons for the discrepancy are suggested. Same comments are made of

the off -design performance of the fan.

~

Available copies of th is report are limited: Return this card to UTIAS, if you require a copy.

AppaRao, T.A.P.S. 13 pages 11 figures

1. Fan-design and insta11ation of. 2. Wind Tunnel 3. Balancing

1. AppaRao, T.A.P.S. Il. 1JrIAS Technical Note No. 170

This report describes the design and installation of a new fan in the 1JrIAS low

speed wind tunnel. The previous fan was lnstal1ed in 1952 a.nd has been operatlng

in an off-design conditlon for a long time. A few cracks were notlced on the fan blades in the fall of 1968, and the installation of a new fan was cODladered

necessary. The objectlve of the fan design was to obtain a maximum test sectlon

weU below the stall limit. This objective had to be achieved under certain

res-trictions: the motor HP, motor rpm, tunnel loss coeffic1ent, hub and tip diameters

of the fan, etc. aU had to remain at their existing values and the existing

straightener blade. were to remain if possible. The methoe of design given in

th!s report is a mod1fied version of the one given by Pope. Details of the

manufacture of blades, balanclng of the fan and lts lnstallation are also given in this report. The predicted. and achieved performance of the fan is compared

and the reasons for the discrepancy are suggested. Some comments are made of

the off-design performance of the fan.