The characteristics of the reverse ventilation of axial flow are analyzed. An s shaped airfoil with a double circular arc was tested in a wind tunnel. The experimental results showed that the characteristics of this new airfoil in reverse ventilation are the same as those in normal ventilation, and that this airfoil is better than the existing airfoils used on reversible axial fans

Gu, Chuangang

Li, Chaojun

Wei, Baosuo

English

NASA Technical Reports Server

553
Third International Conference on Inverse Design Concepts and OptimLz_don in Engineering Sciences
(ICIDES-III'I. Editor: G.S. Dulikravich. Washington D.C.. October 23-25. 1991 ._
F N92-139 '0
Study of a New Airfoil Used in Reversible Axial Fans
Li Chaojun Wei Baosuo Gu Chuangang
Xitan Jiaotong University
Xitan, People's Republic of China /
_:. .. L J
Abslract ,,
The characteristics of reversal ventilation of axial flow fan is analysed in the paper. In ac-
cording to the theory of flow around the airfoil, a new airfoil -- "s" shaped airfoil with doub-
le circular are is presented and experimented in the wind tunnel, the experimental results have
shown that the characteristics of new airfoil in reversal ventilation is the same as that in nor-
mal ventilation and is better than that of existing airfoils of reversible axial fans.
++ ..,
U
C
W
0
0
P
Nomerclalt,re
[
7
P
r
c
_ = _2_"
U I
C
#
C
k
b
F
?
tangential velocity of impeller
absolute velocity
relative velocity
enter angle of relative velocity of flow
installation angle of blade
theoretical total pressure
coefficient of theoretical pressure
dencity or Flow
radius
blade solidity
coefficient of flow rate
lift coefficient
drag coefficient
attack angle
slope of characteristic line of pressure coefficient
blade chord
half of the maximum thickness of airfoil
F
deflection of airfoil (f = _)
Subscript
L
https://ntrs.nasa.gov/search.jsp?R=19920004752 2020-03-17T14:30:15+00:00Z
554
Third International Conference on Inverse Design Concepts and Optimo.ation in Engineering Sciences
(ICIDES-[II). Editor: G.S. bulikravich. Washinzton D.C.. Oc./ob_r 23-25. 1991.
F
1 before blade cascade
2 after blade cascadc
u tangential direction
z axial direction
m for mean radius
o for e-, = 0
t tip
h hub
Introduction
The reversible ventilation of a fan is a problm which often arises in many places, for ex-
ample, in road and railway tunnels where it is required to drive the air in one or the other di-
rection depending upon the condition that exists at the time. Though an axial fan whose im-
peller blades are formed with conventional airfoil can drive the air in opposite direction by
simply reversing the rotor, it is found that the characteristics of reversal ventilation is much
lower than that of the normal ventilation, the efficiency of fan decreasing sharply and the []ow
rate being about 40-50% ornormal ventilation.
The characteristics of reversal ventilation of axial fan with single indcpcdent impeller is
analysed in the paper. In accordance with the theory ofl'low around an airfoil, a new airfoil--
"s" shaped airfoil with double circular arc is presented. If the impeller blades arc formed with
such airfoil, the axial fan can operate in each direction to provide a substantially equal but
opposite flow with a higher efficiency than can be obtained by existing fan with conventional
blades. The reversal ventilation can be achieved by simply reversing the rotor of fan. The %"
shaped airfoil presented in the paper provides a basis for constructing a new type reversible
axial fan with simple construction, easy control and better characteristics.
The characteristics of fan impeller durin_ normal and reversal rotation
According to the 2-D cascade theory, the velocity triangle of a blade cascade of the axial
fan with single indepedent impeller is shown in Fig.1.
From the Eular equation
P = pu(c2, -- cl,) -- pu(w,, -- w2_) (1)
then the coefficient of theoretical pressure is
--- 2 2
P =P,/pu, =u(w -w2,)/u, (2)
where u t is peripheral velocity of impeller.
Writting F- r u (3)
r l I/I
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555
Third International Conference on Inverse Design ConccpL_ and Optimiz.adon in Engineering Sciences
.3-,.5, 1991(ICIDES-III). Editor: G.S. Dulikravich, Washington D.C,, October ") "_
r
C
q) = --&
fgl
(4)
thus the eq.(2) becomes
From fig. 1
P_= ?¢)(t#0_ - tg02) (5)
t£01 wl. u -- cl. }"
= c = c -_; t_,_, (6)
z
According to the cascade theory [4]
tgO 2 = Atgft t + B (7)
Where
T
1 - -_ (de, / dcOcosfl
,4 = (8)
't
1 + _ (de, / da)cos[J
_-(dc / da)sinB
2 r
B = (9)
,[
1 + -_ (dey / dot)cos[l
p--the entry angle of relative velocity during P,=0, i.e. angle o['zero lift line (Figl)
From eq. (5), (6), (7) we gel
p, = (I -- A)72 -- ?q)[B + (I - A)tg6_] (10)
The slope of characteristic line__oF_'i-is
dP
K dO _-[B + (!-A)tg61] (11)
When P-_= 0, the coefficient of flow rate • would have the maximum value
(1 - A#- (12)
q)o = B + (1 - A)tg_ 1
and the flow rate becomes maximum too
0o = u [" 2nr_odr (13)
If representing with mean radius r m
r
m rl
thus
®o,. -- (I - A)7_/[B + (I - A)tgS,] (12a)
0... = u ¢)., n(r - r A) (13a)
It is noted, that the preceding equations are all suitable for both normal and reversal ro-
tating or impeller.
For axial fan with single indepedent impeller no matter what the rotating direction may
be, the air flow into the blade cascade always with axial direction, i.c the angle 61 = 0. Making
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Third International Conference on Inverse Design Concepts and Optimization in Enginecnng Sciences
(ICIDES-III/. Editor: G.S. Dulikravich. Washineton D.C,, October 23-25. 1991.
F
superscript " ' ,I for condition of reversal ventilation, thus from the eq. (10), (1 I), (12a), (13a)
the following equations are getten:
1) the theoretical pressure coel'Ncient
P-t = (! --A)F 2 -- FCbB (14)
P' = (! - A')F 2 - _O'B' (15)
2) the ratio of K of reversal ventilation to that ol" normal ventilation
g K _ B'
- - (16)K B
3) the ratio of maximum theoretical reversal flow rate to maximum theoretical normal
flow rate during fir,--- 0
0'o,, qYo,, (1 - A')B (17)
Jo 0 q> (I -- A)B'
om pen
Assure that dc_/d _ maintains constant during normal and reversal ventilation, and in-
sert cq.(8), (9) into eq. (16), (17)
From fig 2
m
K=
T
sin//'[1 + -_(dc / da)cosfl]
(18)
T
sinf/[1 + 4 (dc _ / da)cosfl']
ctgfl'J = -- (19)
o ctgfl
fl=/_ -%,fl'=/?,+-10
then inserting into eq.(18).(19)
thus
sin(fl, + _'o)[I + 4(dcr / d_)cos(fl, - _,o)]
._= >/1 (18a)
T
sin(fl, -- a0)[I + _ (de / d_')cos(fl, + Co) ]
ctg(fl, -t _'o)
J - _< 1 (19a)
o ctg(_,-,,,o)
From preceding two cquations, it is found that J0 less than ! and K larger than 1 cxcept
//'=/_ (i.e. ao=-r/0). It follows that ir the impeller of fan rotates in opposite dircction, the
maximum flow rate of fan would decrease and the slope of characterstic line of P- would be-
come steep.
Because the _0, C0are relative with geometrical parameters of blade cascade, such as
airfoil deflection .f, blade solidity x, installation angle of blade 0,and blade thickncss c. there-
fore Jo and Kmust bc the function ol'such parameters
.t o = .f( ?,_,o, ,c)
g = f(.?,_,O,,c) (20)
From cq. (! 8a), (19a) if//= fl'(i.e =o+='0 = 0)
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557
Third International Conference on Inverse Design ConccpLs and Optimiz.auon in Engineering Sciences
(ICIDES-III). Editor: G.S. Dulikravich. Washington D.C.. October 23-25. 199l
F
then
J 0 = 1,._= I (21)
It means that the characteristics of fan during contrary rotation is the same as that dur-
ing normal rotation, therefore thc best effect of reversal ventilation is obtained.
It is clean that only following two condition can satisfy the eq.(21)
/ ---0(1) "0 = _ 0
It can be obtained that if the airfoildeFlection 7"=0, such as the nat plate airfoil, ellipse
airfoil, the characteristics oFsuch airfoils arc lower and not satisloctory for uc.
(2) So = -_'0(%:#0, _'0=J:O, I'_ O)
It can be obtained when thc zero lift line of cascade in rcvcrsal ventilation is parallcl to
that in normal ventilation, i.e. the airfoil must posses the reversal symmctcrical npofilc.
nS" SHAPED AIRFOIL WITII DOUBLE CIRCULAR ARC
On the basis of analysis in preceding section we present a new airFoil--"s" shaped airpoil
with double circular arc as shown in fig.3. It is a reversal symmeterical airfoil satisfying the
condition _0=-_'0.The centre line olthe airfoil posses the shape oF"s", which is connectcd by
two circular-arc lines ABC and CDE tangential to each other. At the middle points of two
arcs (B.D) the denection of centre line is maximum.
WhereF is thehalfofthcmaximumthicknessofairfoil.
The equations orccntre line of airfoil may bc written as
(b) 2 =R2 barc ABC x-_ + (y+a) 2 x_(0,_)
arc CDE x - + (y - a) 2 xe(_ ,b) (22)
2 F
wherea=(b /32F)-- (23)2
1 jb 2 b _ FR(radius ofcircular arc )=2---4" + ( 16 2) (24)
The formation of airfoil profile
Along the cenlrc line BCD a number of circles with radius r= F are drawn whose centre
points are at the arc BCD,thcn a number of another circles are drawn along the ares AB and
D_E,whosc centre arc at these arcs and whose radiuses arc identifcd with the perpendiculars
from center points to abscissa line x. Drawing envolope line around such circles, the profile of
the new airfoil is formed.
If the chord of the airfoil b is constant, taking different value of F , a number of'%"
shaped airfoil with different deJlcction/" would be obtained.
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558
Third Inu_matiorml Conference on Inverse Design Concepts and Optimization in Engineering Sciences
(']CIDES-III_. Editor: G.S. DulikJ'avich. Washington D.C.. October 23-25. 1991
V 7
L
EXPERIMENTAl. RESULT
Four "s" shaped airfoils with different /" are experimented in wind tunncl in
aerodynamic laboratory of Jiaotong University in october 1987. Their symbols are CS-3(/
= 3%), C-3.5(./'-3.5%) CS-4(/'-- 4"%) and CS-7(F-?%).
The cxpcrimcntal results of these airfoilsarc shown in Fig 4. From fig 4a) ]t is sccn that
with the increaring of attack anglec( thcliftcocfficientscy incrcascandrcach thcmaximum
valucthcndccrcascgradually, lnthcrangec(=0 ° _13 °, thccy of airfoil CS-4 is thc high-
cstamongothcrs(c_,,=0.87) and thccy oF airFoil CS-7 is lowcr than that of others. From
Fig4b) itisscenthatthcdragcocFficentcx ofal]fourairfoilsarcincreascdfromc(=0 ° and
thec x of airfoiI CS-T is much higher than that ofothcrs, ln thcrangcofattack ang]e 0(From
0 ° to I0 ° the cxof airfoil CS-4 is lower than that of others. It is followed that the
aerodynamic characteristic of airfoil CS-4 is better than that of" others. Furthermore whcn
the deflection of airfoil is too big (._7%), the drag coefficient % of airfoil would increase
sharply. The aerodynamic characteristic of" airfoil CS-4 is compared with the conventional
airfoil (NACA-64, NACA-66). From Fig 5 a), 6 a) it is shown, though the c_ of airfoil
CS-4 is lower than that oFNACA airlbil in normal ventilation, it is much higher than that in
reversal ventilation, expccialy at bigger attack angle. From Fig 5 b), 6 b) it is seen, in the
range c(= 0_10 ° thc drag coefficient cx oFairfoi]CS-4islowcr than thatoFNACA airfoil
in normal ventilation, while0( >I0 ° it is higher than that oFNACA air[bil, but in reversal
ventilation condition the cx of airfoil CS-4ismuch lower than that of NACA airfoil in all
range of attack angles in experiment.
In order to analyse the Flow around the %" shaped airloil , the pressure discribution
along the surface of airloil CS-3 at a =4 ° is measured as shown in Fig 7. There arc two arc-
as bounded by pressure coefficient cp line of airfoil. The area on the preceding half of airfoil
(x / b = 0-0.5) is positive, which indecates that the lift force on the airfoil is upward; The area
on the rear half of airfoil (x / b = 0.5-- 1.0) is negative, which indecates the lift force on the
airfoil is downward. Since the preceding area is bigger than the year area, so the summary lift
force on the airfoil is upward. Varying the attack angle a, the two areas and the summary lift
torce would be changed.
In the Fig 7. c = (P - P o) /1
-}peon, where Pi is the pressure at the surface of airloil.
P, is the ambient pressure, co. is the flow velocity in the wind tunnel.
Conclusion
1) If the impeller of axial fan with conventional airfoil rotates in reversal direction, the
maximum theoretical flow rate would decrease, the characteristic line of P, become steep, the
/
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Third International Conference on Inverse Design Concepts and Optimization in Engine__ring Sciences
(I¢IDES-Ill), Editor: G.S. Dulikravich, Washington D.C., October 23-25, 1991
F
characteristics of fan would be worse.
2) The characteristic of impeller in reversal ventilation is identical with that in normal
ventilation, when the deflection or airfoil .I'= 0 or ._ 0 but the profile of airfoil is reversal
symmeterical.
3) The tls" shaped air[oil presented in the paper is a reversal symmeterieal airfoil. When
the impeller whose blade is formed with such airfoil rotates in opposite direction, its charac-
teristic would be the same as that in normal ventilation.
4) The synthetical characteristic of axial fan whose impeller blade is formed with "s"
shaped airfoil would be better than that formed with conventional airfoil, especially in rever-
sal ventilation condition, therefore the new airfoil presented in the paper is more available for
reversible axial fan.
Reference
1. Radha krishna, H.C. Aswa Thanarayana, P.A. and Ramachandran, Rm. "Some
Flow Studies on "s" Cambered Aerofoils" Proceeding of Conference on Fluid Ma-
chinery, 1979 Budapest.
2. Ravindram, M. and Radha Krishna, H.C. "Influence of Blade Profiles on the Per-
formance of a Fully Reversible Axial Pump-turbine" Proceeding of Conference of
Fluid Machinery, 1979 Budapest.
3. Ravindram, M. and Radha Krisbna, H. C. "Characteristics of a Fully Reversible
Axial Pump-turbine for Application in Tidal Power Plants" Paper presented at the
Conference on Future Energy Concept, Orgaized by IEE, London, Jab-Fab 1979.
4. Longhua Li '_Feasibility study of inverse ventilation in Chinese coal-mines by sim-
ple reversing the axial fan" The 2nd China-Japan Conference on Fluid Machinery,
1989 Xi'an China.
]
L 1
560
Third International Conference on Inverse Design Concepts and Opumtzation in Engineering Sciences
(ICIDE5-III'). Editor: G.S. Dulikravich. Washington D.C.. October 23-25. 1991.
F
/
_C_
_ _A, "l
Fig.l. The velocity triangle of a blade caseade
-]
?_ Or ,,
Fig.2. Normal and contrary flow
around an airfoil
k
Y
i
| t/
r b
Fig.3. _S" shaped airfoil with double-circular arc J
561
Third Intematiorml Conference on Inverse Design Concepts and Optima.anon in Engineenng Sciences
OCIDES-III'). Editor: G.S. Dulikravich. Washington D.C.. October 23-25. 1991.
F
cy c._,
_.C
,2j +
.:1.2
,l.o
,Ib
.08
.O
I I I
o + ,_ /.+ +'bo<°
(<X
Fig.4.
----i---(-S-_
--_---cs-_'._" (b
--+,..-- cS_ _
--w-- c.,5- "P
The experimental results of CS-4 airfoil series
]
@
I hC
,8
,6
4
.:t
.0
I
f
/
/
t
/111/
2/'
' I I I I
• 4, $ I_ tb
Cx
°j¢
.t6
,Ix
O( o
--_--- cs-a b)
a
, --o..,---.. NAcA-_ (ReY)
HAcA-_ CwoP)
-_'.
h ,It1
• q- _P /_- tg ¢g °
Fig.5. Comparision of airfoil CS-4 with conventional aisfoil NACA-64 in normal and
reversal ventilation
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562
Third International Conference on Inverse Design Concepts and Optimization in Engineering Sciences
(ICIDES-IID. Editor: G,S. Dulikravich. Washineton D.C.. October 23-25. 1991.
F ]
cy
1,o
.8
,6
.4.
/
/
t ; I ; I
a) ..... o---
,)0
16
,t2
,o8
.04-
fl e'.
///
I i I I
C:; -_
/'CA cA -66 Cwor)
Fig.6. Aerodynamic characteristic comparision of CS-4 airfoil with airfoil NACA-66
in normal and reversal ventilation
c?
- 1.0
"'! *S_
D
"'¢
0
f*-
./,
.8
• i
•o ,z . ,6 .9 I.o 3L
b
Fig.7. The pressure discribution along
the surface of airfoil CS-3.
L J


Study of a new airfoil used in reversible axial fans

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