Iranian Journal of Chemical Engineering Vol. 11, No. 2 (Spring 2014), IAChE
Convective Heat Transfer Enhancement of CNT-Water
Nanofluids in Plain Tube Fitted with Wire Coil Inserts
M. Fallahiyekta1, M.R. Jafari Nasr2*, A. Rashidi2, M. Arjmand3
1- Department of Chemical Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran 2- Nanotechnology Research Center, Research Institute of Petroleum Industry (RIPI), Tehran, Iran 3- Department of Chemical Engineering, South Tehran Branch, Islamic Azad University, Tehran, Iran
Abstract
The turbulent convective heat transfer and pressure drop characteristics of CNT-water nanofluid in a horizontal tube fitted with wire coil inserts are studied experimentally. CNTs were synthesized by chemical vapor deposition (CVD) method with purity of more than 99%, functionalized by acid treatment and dispersed in distilled water in different concentrations. Also, the thermal conductivity and viscosity of synthesized nanofluids were measured experimentally. Convective heat transfer experiments are conducted with water and nanofluids in the range of 5000 < Re < 22000, CNT volume concentration 0 <ϕ< 0.1 % and wire coil with wire pitch of 2. The experimental results indicate that the convective heat transfer increases up to 23% in 0.05 vol% CNT-nanofluid and the heat transfer coefficient increases with CNT vol% and Reynolds number. Wire coil inserts increase the heat transfer coefficient of water and nanofluids up to 102% in Re=5700 but its performance decreases with Reynolds number. Experiments have shown that only use of wire coil inserts increases pressure drop of working fluid. Moreover, empirical correlations for Nusselt number and friction factor are proposed from nonlinear regression of the experimental data. Further, performance evaluation of enhanced tube is determined with considering opposing thermal resistance.
Keywords: CNT, Nanofluid, Heat Transfer Coefficient, Wire Coil Inserts
1. Introduction∗
From the past decades, researchers have studied development of enhancement techniques to reduce the size and costs of heat exchangers. Use of wire coil inserts, twisted tape inserts and other mechanical turbulators is one of the passive methods that change hydrodynamics and increase turbulence of the working fluid that leads to higher heat transfer coefficient [1-4].
∗Corresponding author: nasrmrj@ripi.ir
clogging in microchannels due to smaller size and larger interface area compared with micrometer-sized particles [5-8]. CNTs are excellent candidates as dispersions for preparing nanofluids, due to their very high thermal conductivity and very large aspect ratio [8]. Pak and Cho [9] studied TiO2-water and Al2O3-water nanofluids in turbulent convective heat transfer inside tubes. Yang et al. [10] studied laminar convective heat transfer of graphite nanofluids in a horizontal tube heat exchanger. Xuan and Li [11] considered convective heat transfer and flow features of CuO-water nanofluids. Wen and Ding focused on entry region under laminar flow condition using nanofluids containing Al2O3 and CNTs in horizontal tube [12, 13]. Ding et al. [14] reported significant enhancement in convective heat transfer of multi-wall carbon nanotube dispersion in water and found that amount of enhancement depends on the Reynolds number, CNT concentration, and pH. Xing et al. [15] found the laminar heat transfer enhancement of 70% and 190% for nanofluids containing 0.05 and 0.24 vol% CNT in water at Re=120. Also, Rashidi et al. [16] experimentally investigated heat transfer enhancement of CNT–water nanofluids in a horizontal shell and tube heat exchanger in laminar flow. They found an increase of overall heat transfer coefficient with Reynolds number. Using nanofluids and tube inserts together in horizontal tubes is considered by several researchers. Heat transfer enhancement for Fe3O4 nanofluid in a tube with twisted tape inserts has been experimentally investigated by Sundar et al. [17]. They found that 30.96% enhancement for 0.6 vol% of Fe3O4 in a plain tube and further enhancement of
18.49% in a plain tube with twisted tape (H/D=5) at the Re = 22000. The heat transfer enhancement of 23.69% for 0.1 vol% of Al2O3 nanofluid in a tube has been analyzed by Sharma et al. [18]. Further 44.71% heat transfer enhancement is observed with twisted tape insert with (H/D=5) inside a circular tube at Re = 9000. Sundar and Sharma [19] have observed 30.30% heat transfer enhancement for 0.5% of Al2O3 in a plain tube, further, 42.71% of heat transfer enhancement with twisted tape (H/D=5) is observed compared to water at Re = 22000. Laminar flow of CuO/base oil nanofluid in a tube with wire coil inserts has been estimated by Saeedinia et al. [20] experimentally and 45% enhancement was noted in heat transfer for 0.3 vol% CuO. Fully developed laminar flow of 0.1 vol% Al2O3 nanofluid in a tube with wire coil inserts has been analyzed by Chandrasekar et al. [21] who observed 21.53% heat transfer enhancement with wire coil pitch of 3. Noticeable increase of pressure drop of the nanofluids with inserts was found in all above-mentioned researches. Jafari Nasr et al. [22] investigated performance evaluation of heat transfer enhancement tubes fitted with turbulator devices such as twisted tape, wire coiled and so on. They developed a new performance index taking into consideration the effect of opposing thermal resistances such as fouling. Their results showed that the potential benefits of heat transfer enhancement cannot be analyzed from the effects of thermal resistances separately.
aim of t turbulen pressure constan CNT-w Also, enhance inserts a
2. Expe
2-1. Exp
The exp convect schemat a test se tank, an tube of 15.8 mm
electrica of 2000 heat flu a thick R the heat section.
this paper is nt heat tr e drop in nt heat flu
ater nanoflu the perfo ed tube with are consider
erimental
perimental s
perimental tive heat tra
tically in Fi ection, a cen nd a coolin f 1100 mm
m OD is us al heater w 0 W was ad ux condition Rockwool i ter to preve
Seven
K-Figure 1. Pu
Bypass flow Reservoir
tank
s based on ransfer enh
a horizon ux using c
uids and w ormance h nanofluid red.
system
system for ansfer coeff ig. 1. It ma ntrifugal pu ng part. A length, 12 sed as the t wire with m dopted to su n along the c
insolating la ent heat los -type therm
Experimental Calming sction
ump
investigatio hancement
ntal tube combination wire coil ins
evaluation ds and wire
r measuring ficient is sh ainly consis ump, a reser
straight co 2.2 mm ID
test section maximum po
upply a cons copper tube ayer surroun ss from the mocouples w
l system for th on of
and with n of serts.
of coil
g the hown
ts of rvoir opper and . An ower stant e and nded e test were
m d th in th t s s e f o f s r h t o s in v p f r
he measureme Therm
mounted on distance to he tube nserted into he heat emperature steady state section is al exchanger w forced throu of constant flow is con section and reservoir tan hydrodynam o eliminate of pump is straight tub nterring in volumetric f pressure dr flowmeter a respectively
ent of convecti mocouples T1-T7 Cooling syst
the test sec measure w and two o the flow at transfer s of passin conditions llowed to c with cooling ugh the test
flow pum ntrolled by d additional nk through mically fully
e the entran s initially be in calm nto the tes
flow rate of rop are me and U-tube y.
ive heat transf 7
tem
ction in equ wall tempera
thermocou t the inlet an section to ng fluid. T
, outlet flow ool in a coi g water. Th t section w mp. Flow ra
y a valve l fluid is r
bypass line y developed nce effects,
passed thr ming secti st section.
f inlet liqui easured by mercury m
fer coefficient
ually spaced ature along uples were nd outlet of o measure To achieve w from test il tube heat he liquid is with the aim ate of inlet before test returned to e. To make d flow and outlet flow rough 2 m
ion before Also, the id and fluid y magnetic monometer,
t.
d g e f e e t t s m t t o e d w
m
2-2. Nanofl
Distilled w (CNTs) w Initially hi in Researc (RIPI) by supported 800-900oC methane as than 99%. spectroscop have avera
nm and 10 Fig. 2 that band ove confirms t samples o dispersing concentrati by wt %
percent of
converted t
(1 wt
φ = −
luids prepar
water (DW) were used t igh purity C ch Institute y CVD p MgO nan C, 45 min
s a carbon . Fig. 2 sh py of produ age diamete 0μm, respec the value o r D-band) the high q of nanoflui
CNTs ion of 0.03,
(0.0142 –
f CNTs in to volume p
f
p f
wt.
t) wt.
ρ ρ + ρ
ration
) and carbo to produce CNTs were of Petrole process ov nostructured
n reaction source and hows TEM uced CNTs er and leng ctively. It is of IG/ID (Int ) is about quality of ids were p
in DW , 0.05, 0.1, – 0.095 vo
n nanoflui percent as fo
f
(a)
Figure 2. (a)
on nanotube e nanofluid e synthesize eum Industr ver Co-Mo d catalyst i
time wit purity mor and Rame s. The CNT gth of 10-2 s found from tensity of G t 2, whic
CNTs. Fiv prepared b with CN 0.15 and 0.
ol%). Weigh
ids can b follows:
(1
TEM. (b) Ram es ds. ed ry o, in th re en Ts 20 m
G-ch ve by NT .2
ht
be
1)
Whe are this Bec that DW trea acid hyd C, C the CNT are aggl DW aggl for 40 mad over or s
men spectrosc
ere wt is w density of D
work p
ρ is cause of hy t lead to ag W, CNTs atment with
d in a rati drophilic fun
C=O, and O CNTs surfa Ts in DW
entangled lomerates. W, to disent lomerates, n 1 hour at 1
kHz ultras de in this w
r 1 month ettling.
copy of produ
weight perc DW and CN
taken as 2. ydrophobic ggregation a are functi
mixture of o of 1:3. nctional gro O–H might
aces, which [23]. It wa and some After add angle nano nanofluids w 00% amplit sonic bath way were fou
with no vis
(b)
uced CNTs.
Wavelen
cent and
ρ
NTs respect1 g/cm3. surface o and precipit ionalized b f nitric and By acid tr oups such a be introduc h leads to di as found tha
are in the dition of C tubes and t were ultras tude using a . CNT na und to be st sible sedim
gth (cm-1)
f
ρ
andρp tively. Inof CNTs tation in by acid
sulfuric reatment as C–O– ced onto ispersing at CNTs form of CNTs to
3. Phys 3-1. The The th samples thermal Devices accurac the wor method DW as probe.
W / m.k instrum of each tempera conduct shown i of water vol% enhance achieve nanoflu Due to h in the noticeab works d on the nanoflu presente thermal water nanoflu were us heat tran 3-2. Visc Because 0.1%, Newton nanoflu Gallen ical proper ermal condu hermal con s were me
properti s, Inc., USA
y of 5%.Th rking princi . Initially,
a base flu For DW which prov ment. Theref h five samp atures. Fig.
tivity ratio in this figur
r was enhan and t ement of
d for 0 uids in 25 o high purity
experimen ble in co done by re ermal cond uids. More
ed by auth conductivi nanofluids uids therma
sed for each nsfer.
cosity
e CNT vol%
nanofluids nian fluid uids sample
Camp bath
rties of nan
uctivity
nductivity asured usin es analyz A) in 25oC he instrumen
iple of a tra thermal c id was mea kw was ob ves the acc fore therm ples was m 3 shows me
(knf/kf) of re, the therm nced with in temperature
25.5 % .2 wt%
o
C and 30oC of synthesi nts, this e omparison
esearchers ductivity o
details of hors in pre
ity enhance [27]. Th al conductiv h experimen
% in samp could be ds. The
es was m h viscometer
nofluids
of nanofl ng a KD2 zer (Deca
and 30oC nt was base ansient hot conductivity
asured by K btained as curacy of K al conduct measured in
easured the nanofluids mal conduct ncrease of C
. Maxim and 38.7%
(0.095 vo
C, respectiv ized CNTs u enhancemen with prev [5,13,14,24 of CNT-w this work vious work ement of C
he values vity in Fig nt of convec
les is less e assumed viscosity measured u
r in 20, 30 luids Pro agon with ed on wire y of KD2 0.6 KD2 ivity two rmal . As ivity CNT mum % is ol%) vely. used nt is vious 4-26] water k are k on CNT- of g. 3 ctive than d as of using and F D 4 f r p in s m v I v in w in f o s t n h n 3 T n l
Figure 3. The DW versus CN
40oC. In ea filled with n reach deter passing tim ndicators o stop watch multiplied b viscosity of Initially, to viscometer,
n 20 oC. Ob was 1.0042
n good agre for DW in 2 of about 2% samples wa
emperature nanofluids v heat transf nanofluids i 3-3. Specific To determin nanofluids, iterature we ermal conduct NT vol% and t
ach experim nanofluid a rminate tem me of nan
of viscome h. This m by viscome
nanofluid. o evaluate
viscosity o btained resu centi-stoke eement with 20oC (0.98 %. Thus visc as measured s accurate viscosity ar fer calcula
s taken from
heat and de
ne specific the foll ere used [28
tivity ratio of temperature.
ment visco and set in t mperature. ofluids bet eter was re
measured eter constan
e the ac of DW was ult for visco
s (1.0016 c h the data i cP) with a cosity of al d by visco ely. The re shown in ations, vis m Fig. 4.
ensity
c heat and lowing re 8];
nanofluids to
Figure 4. M CNT vol% an
nf
ρ =ϕ ρ
p,nf
C =ϕC
Where Cp nanopartic
4. Results
4-1. Heat tr
The energ could be ca
1
Q = V× I
In which Voltage an the flowing
2 p o
Q =m C (T&
Where
m
&
while Tout a of outlet a section of t ExperimenMeasured Vis nd temperatur
p (1 )
ρ + −ϕ ρ
p,p
C + −(1 ϕ)
p,p and Cp,f
les and base
and discus
ransfer calc
gy supplied alculated as
V and I nd Ampere.
g liquid is d
out in
T −T )
is mass flo and Tin are
and inlet flu the experim nts show th
cosity of nan re.
f
ρ
p,f
C
f are speci
e fluid respe
ssion
ulations
d by heati s follows:
are recorde Also, heat defined as:
ow rate of e measuring t uid flows in mental setup
hat the hea
nofluids versu
(2
(3
ific heat o ectively.
ing elemen
(4
ed electrica absorbed b
(5
entering flui temperature nto test tub p.
at loss from
us
2)
3)
of
nt
4)
al by
5)
id es be
m
elem diffe aver
Q=
Hea
'' q =
Whe leng The coef
exp
h
w
T
that
f T
prop
4-2.
Initi in p exp com Dat
1
N u
400 k f
= =
ment to surr ference of Q rage of them
1 2
(Q +Q ) / 2
at flux per u
o Q d L
π
=
ere do and gth of the erefore the
fficient is ca
'' p
w
q (T T =
−
is average
t are measu
is average
perties of w
Validation
ially, exper plain tube erimental mpared with
a Unit) equ
(
1 2
f R 8
k k
00 R e 5 1 13.6 f (1.82 L og R
⎛ ⎞ ⎜ ⎟ ⎝ ⎠ =
⎛
+ ⎜
⎝ < < × = +
rounding is Q1 and Q2 is
m was used
unit area is d
d L are ou e test sec experime alculated as
e
f
, Nu T )
value of 7
ured during
of Tout an
working fluid
of heat tran
riments wer to evaluat
system. h ESDU (E
ation as foll
)
(
0.5 2 / 3
6
2 2
R e 1000 P r
f
P r 8
10 , 0.5 k R e 1.64)−
−
⎞ −
⎟ ⎠
× <
= −
s negligible s less than 2 in calculati
defined as fo
utlet diame ction resp ental heat
s:
exp
exp h di
k =
7 wall temp
g experime
nd Tin. All
d are taken
nsfer system
re done wit te the accu The resu Engineering
lows:
)
r
1
P r 2000 11.7 1.8 P
−
< +
and the 2%. The ions as:
(6)
follows:
(7)
eter and ectively.
transfer
i (8)
peratures
ents and
physical
inTf .
m results
th water uracy of ults are
Science
1 / 3
P r−
Where f
into the (Re =50 water an heat flu of hea compare error of rather th from Fi heat tran used for
Figure 5 coefficien
4-3. Ex inserts
To stud heat tra length w
in Fig. 6 shows t transfer found fr heat tr Reynold and clea could be
f is friction e test sectio 000 - 2000
nd tube wa ux of 1500 W
at transfer ed with ES f experiment
han ESDU ig. 5 it is o
nsfer system r further exp
5. Validation nt of water wi
xperiments
dy the effec ansfer coef wire coil wi
6) is inserte the enhanc r coefficient from this Fi ransfer dec ds number. arly from F e useful in l
n factor. W on in diffe 0) and all t all are recor W. Fig. 5 sh
r coefficie DU equatio tal heat tran equation is bserved tha m are valid periments o
of experime ith the ESDU
with water
ct of wire fficient of
th wire pitc
ed into the t ement of c t with wire c igure that a creases wit It is observ Fig. 8 that w low Reynol
Water is ent erent flow r
temperature rded in cons hows the re ent of w on. The ave nsfer coeffic
s about 8% at the result
and it coul of nanofluid
ental heat tra equation.
r and wire
coil insert water, a ch of 2 (p / d
test tube. F convective coil inserts. augmentatio th increase ved from Fi wire coil in lds numbers
tered rates es of stant sults water erage cient and ts of ld be ds.
ansfer
coil
s on
full-d=2
ig. 7 heat It is on of e of
ig. 7 serts s and
h F o
F c
F c
F e in w th
h-ratio is eq Fig. 7 maxim observed in
Figuer 6. S
Figure 7. E coefficient wit
Figure 8. Hea coil to water v
From nonl experimenta n turbulent was propose he average
qual to unit mum enhan Re = 5700.
chematic diag
Enhancement th wire coil in
at transfer coe versus Re.
linear regr al data for
t flow an ed as a func
error of 2%
t in Re =20 ncement of
gram of wire c
of water h nserts.
efficient ratio
ression of water-wire empirical ction of Re a % as follows
0000. From 102 % was
coil inserts.
heat transfer
of water-wire
f obtained coil insets correlation and Pr with s:
m s
r
e
Nu
=
1.7
Presented c Nusselt nu inserts in R
4-4. Experi
Three nano 0.05, 0.1 a fluid and coefficient constant h the compar coefficient concentrati Fig. 9 that is enhance increase of transfer en number. nanofluids in all CN CNT- nan 23% was a
Figure 9. C coefficient o
An empiri nonlinear r of 0.1 wt%
of 2.5 % as
0.4028
795 Re
correlation umber in tu Re = 6000–1
iments with
ofluids with and 0.2 wt%
then con t was determ
eat flux of rison of exp t of wa
ion of nano t convective ed by use o f CNT conc nhancement Heat tra increases NT concentr
nofluid max achieved in
Comparison of f water and di
cal correlat regression
% nanofluid s follows:
0.1394
Pr
could be us ube fitted w
13000.
nanofluids
h CNT conc
% were used nvective h mined exper
1500 W. F perimental ater and ofluids. It is
e heat trans of nanofluid centration in t in the sam ansfer coe
with Reyno rations. Wi ximum enha
Re = 16000
f experimenta ifferent nanof
tion was ob of experim d with the a
(10
sed to predic with wire co
centration o d as workin heat transfe rimentally i Fig. 9 show heat transfe 3 weigh s found from sfer of wate ds. Also, th ncreases hea me Reynold efficient o olds numbe ith 0.1 wt%
ancement o 0.
al heat transf fluids.
btained from mental result
average erro 0)
ct oil
of ng er in ws er ht m er he at ds of er
%
of
fer
m ts or
Nu
Prop CNT
4-5. inse
Wir nano flow exp fluid obse heat pres the num heat with nano coil nano enh Mor adv high
Figu coeff wire
0.0119 R
=
posed corre T-water nan
Experimen erts
re coil was i ofluids of 0 w into te erimental h d in tube erved from t transfer o sence of wi
results with mber and C
t transfer h wire coil
ofluid and inserts enh ofluid by 7 ancement reover, it antage of w h Reynolds
ure 10. Comp fficient of wate
coil inserts.
0.8522 0.45
Re Pr
elation is v nofluid in R
nts with nan
inserted in t 0.1 and 0.2 est tube. heat transfer
are shown m this Figu of nanoflui ire coil inse h water in CNT concen
enhanceme and nanof wire coil hanced heat 72% in Re for water is found f wire coil ins
number.
parison of exp er, water-nano
572
valid for 0 Re = 6000 –
nofluid and w
test section 2 wt% are f
The res r coefficien n in Fig. 1 ure that co
ids is enha erts. Compa the same R ntration sho ent in com
fluids is mo individuall transfer of e = 6400 a
is about from Fig. serts is decr
perimental hea ofluid, water-n
(11)
0.1 wt%
19000.
wire coil
and two forced to
ults of nt of the 10. It is
nvective anced in arison of Reynolds ows that mbination
ore than ly. Wire 0.1 wt%
and this 100%. 10 that reased in
nanofluid-For 0.1 coil ano from n data in R
Nu =1.
4-6. Pr working
The fric horizon Darcy r
exp
2
f
=
ρ
Where calculat rate and Paramet diamete pressure evaluate
exp
P
12.55
Δ
=
=
To valid entered rates an tube m compari experim equation as follow
0.3 f
Re
=
wt% nano
other corre onlinear re Re = 6000–
0.3801
.591Re
ressure dro g fluid
ction factor ntal tube cou
elation as:
2
2 D
P
L
u
Δ
⋅ ⋅
ρ
u is averag ted from e d knowing t
ters of L,
er of the tu e drop of fl ed by U-tub
Hg w
w Hg
(
5
h
=
γ
−
γ
γ
date the pre into test nd pressure manometer. ison of mental frict
n for turbu ws:
0.25
316
e
fluids in pr lation coul egression o –19000 as fo
1 0.2865
Pr
op and fric
of a fluid e uld be deter
exp
2
ge velocity experimenta the cross ar
D are len ube, respec fluid and is be Mercury
w Hg
w w
)h
(13
,
=
=
γ
ρ
essure drop section in e drop mea
Fig. 11 obtained tion factor ulent flow in
resence of ld be prese of experime
ollows:
ction facto
entering into rmined from
y that could al volume
rea of test t ngth and in ctively.
Δ
Pexperimen manometer
w
w
3.55 1)
g
−
γ
system wat different sured using 1 shows
results r with Bla
n smooth p wire ented ental
(12)
r of
o the m the
(13)
d be flow tube. nside
P is
ntally r as:
w
h
Hg(14)
ter is flow g
U-the for asius pipes
(15)
A r r i w in
F
o
P w r c o o w th w
F
o
As shown results are in relation of B
s suitable working flu n this exper
Figure 11. Co of water with B
Pressure dro wire coil ex results of compared w observed fro of water inc wire coil ins
he needed working flui
Figure 12. Co of wire coil ins
in this Fig n good agre Blasius and for measur uids. The av
riment was
omparison of e Blasius equati
op of wate xperimenta
friction fa with plain t om this figu crease by 2
serts. This l pumping p id.
omparison of e serts with wat
gure the ex eement with d pressure d ring friction
verage obse less than 6%
experimental f ion.
er in tube lly measure actor calcu
tube in Fig ure that pre .5-3 times w leads to an power for t
experimental f ter in plain tub
xperimental h theoretical drop system n factor of erved error %.
friction factor
filled with ed and the ulation are g. 12. It is essure drop when using increase in the flowing
friction factor be.
l l m f r
r
h e e s p g n g
Nonlinear with wire with averag
exp
f
=
0.9
Presented water frict in Re = 50
The result nanofluids increase in correlation could be u wire coil fr
4-7. Perfor
To evalua transfer sy performanc performanc [22];
P.I ( 1 R
= +
Where Ropp
is heat tran is friction f Performan coil insert nanofluids 0.0001 are this figure Reynolds n with nan performanc the same analysis, resistance
regression coil inserts ge error of 2
0.26
527 Re
−correlation tion factor i
00 – 20000
ts of meas in all cases n pressure n for water– used to pre riction facto
mance Eval
ate the ad ystem using ce was det ce index (P
1.5
opp t
1 ) R ⋅h
p is opposin
nsfer coeffi factor, and S
ce index of ts and com
– wire co shown in F that P.I dec number. M nofluids–wir
ce than wa Reynolds n the effect on P.I was
of the resu s leads to a
2% as follo
626
is effectiv in presence
.
sured press s have show
drop of w –wire coil fr ediction of or.
luation
dvantages o enhanced t ermined wi P.I) of Jafar
3
St f
ng thermal r icient of flu
St is Stanton f tube enhan mbination o oil inserts Fig. 13. It is
creases with Moreover, th
re coil ater–wire co
number. Ba of oppos studied. De
ults for wate a correlatio ows:
(16
ve to predic of wire co
ure drop o wn negligibl water. So th riction facto f nanofluids
of the hea tube, therma ith propose ri Nasr et a
(17
resistance, h
uid in tube, n number. nced by wir
of 0.1 wt%
with Ropp
s found from h increase o he tube fille
has highe oil inserts i ased on tha
ing therma ecrease of P
er on
6)
ct oil
of le he or s–
at al ed
al.
7)
ht f
re
%
= m of ed er in at al P.I
with resu
Figu
wire
Figu
with
5. C Hig proc Aqu prep 0.1 cond exp show enh wor nano
h increase o ults of Fig. 1
ure 13. Comp coil and NF-w
ure 14. Perfor different Opp
Conclusions h purity C cess and fun ueous nano pared in f
vol % (0 ductivity a erimentally w highe ancement i rks done b ofluids and
of Ropp was
14.
parison perfor wire coil with
rmance index posing therma
s
CNTs was p nctionalized ofluids of five concen 0.2 wt%) and viscosi y. The ex er therm
in compari by research increase of
s obtained f
rmance index h Ropp=0.0001
of NF-wire c al resistance.
produced b d by acid tr
these CNT ntrations le and their ity were m xperiment's mal cond
ison with p ers on CN f nanofluids
from the
of water-.
coil inserts
by CVD eatment. Ts were
conductivity with CNT vol% and temperature. In 25oCwith 0.1 vol% (0.2 wt%) CNT in water, maximum thermal conductivity enhancement of 25% was observed in experiments. Also, obtained results of viscosity experiments in 20 – 40 oC show that viscosity of nanofluids increase with CNT vol% and temperature reduction. Turbulent convective heat transfer coefficient and pressure drop of water and nanofluids entering the plain tube and tube fitted with wire coil inserts were measured separately in constant heat flux of 1500 W for electrical element. To validate the experimental system, the result of Nusselt number of water was compared with ESDU equation and good agreement was achieved. Enhancement of convective heat transfer of water with wire coil inserts was observed from the experiment results. Heat transfer coefficient of water increases with Reynolds number but thermal enhancement by wire coil decreases in high Re.
Nanofluids with CNT concentration of 0.05, 0.1 and 0.2 wt% are entered into the test tube and enhancement of heat transfer with nanofluids was observed from experimental results. Also, heat transfer coefficient of nanofluids increases with Reynolds number and CNT concentration. With 0.1 wt% CNT-nanofluid maximum enhancement of 23% was achieved in Re = 16000. Nonlinear regression of experimental data leads to an empirical correlation for prediction of Nusselt number of 0.1 wt% CNT-water nanofluids. Using wire coil inserts in combination with nanofluids enhanced convective heat transfer more than wire coil and nanofluids alone, especially in low Re. The results of measuring pressure drop were
showed that presence of CNTs in water had negligible effect on friction factor of water. Also, wire coil inserts increase pressure drop by 2.5-3 times that of the increase needed for pumping power. Performance index of heat transfer system by using wire coil and combination of wire coil and nanofluid decreases with Re and performance of wire coil–nanofluid is higher than wire coil–water in the same Re. Also, performance index of enhanced tube decreases with increase of opposing thermal resistance.
Acknowledgements
The authors would like to express their gratitude toward Research Institute of Petroleum Industry (RIPI) for financial supported of this research and preparation of required facilities and the test rig.
References
[1] Godson, L., Raja, B., Mohan Lal, D. and Wongwises, S., "Enhancement of heat transfer using nanofluids—An overview", Renewable and Sustainable Energy Reviews, 14, 629, (2010).
[2] Jafari Nasr, M.R., Habibi Khalaj, A. and Mozaffari, S.H.," Modeling of heat transfer enhancement by wire coil inserts using artificial neural network analysis", Applied Thermal Engineering, 30, 143, (2010).
[4] Promvonge, P. and Eiamsa-ard, S.," Heat transfer enhancement in a tube with combined conical-nozzle inserts and swirl generator ", Energy Conversion and Management, 47, 2867, (2006).
[5] Assael, M.J., Metaxa, I.N., Arvanitidis, J., Christofilos, D. and Lioutas, C., "Thermal conductivity enhancement in aqueous suspensions of carbon multi-walled and double-multi-walled nanotubes in the presence of rwo different dispersants ", Int. J. Thermophys., 26, 647, (2005).
[6] Chen, L. and Xie, H., "Silicon oil based multi walled carbon nanotubes nanofluid with optimized thermal conductivity enhancement ", Colloids and Surfaces A: Physicochem. Eng. Aspects, 352, 136, (2009).
[7]. Murshed, S.M.S., Leong, K.C. and Yang, C., "A combined model for the effective thermal conductivity of nanofluids ", Applied Thermal Engineering, 29, 2477, (2009).
[8] Vajjha, R.S. and Das, D.K., "Experimental determination of thermal conductivity of three nanofluids and development of new correlations ", Int. J. Heat and Mass Transfer, 52, 4675, (2009).
[9] Pak, B.C., and Cho, Y.I., "Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles ", Exp. Heat transfer, 11, 151, (1998).
[10] Yang, Y., Zhang, Z.G., Grulke, E.A., Anderson, W.B. and Wu, G., "Heat transfer properties of nanoparticle-in-fluid dispersions (nanonanoparticle-in-fluid) in laminar
flow ", Int. J. Heat Mass Transfer, 48, 1107, (2005).
[11] Xuan, Y. and Li, Q., "Investigation on convective heat transfer and flow features of nanofluids", J. Heat Transfer, 125, 151, (2005).
[12] Wen, D. and Ding, Y., "experimental investigation into convective heat transfer of nanofluids at the entrance region under laminar flow conditions", Int. J. Heat Mass Transfer, 47, 5181, (2004).
[13] Wen, D. and Ding, Y., "Effective thermal conductivity of aqueous suspensions of carbon nanotubes (carbon nanotube nanofluids)", J. Thermophys Heat Transfer, 18 (4), 481, (2004).
[14] Ding, Y., Alias, H., Wen, D. and Williams, R.A., "Heat transfer of aqueous suspensions of carbon nanotubes (CNT nanofluids) ", Int. J. Heat Mass Transfer, 49, 240, (2006). [15] Wang, J., Zhu, J., Zhang, X. and Chen,
Y., "Heat transfer and pressure drop of nanofluids containing carbon nanotubes in laminar flows", Experimental Thermal and Fluid Science, 44, 716, (2013).
[16] Lotfi, R., Rashidi, A.M. and Amrollahi, A., "Experimental study on the heat transfer enhancement of MWNT-water nanofluid in a shell and tube heat exchanger", International Communica-tions in Heat and Mass Transfer, 39, 108, (2012).
nanofluid inside a plain tube: An experimental study", International Journal of Heat and Mass transfer, 55, 2761, (2012).
[18] Sharma, K.V., Sundar, L.S. and Sarma, P.K., " Estimation of heat transfer coefficient and friction factor in the transition flow with low volume concentration of Al2O3 nanofluid flowing in a circular tube and with twisted tape inserts", International Communications in Heat and Mass transfer, 36, 503, (2009).
[19] Sundar, L.S. and Sharma, K.V., "Turbulent heat transfer and friction factor of Al2O3 nanofluid in in a circular tube with twisted tape inserts", International Journal of Heat and Mass transfer, 53, 1409, (2010).
[20] Naik, M.T. and Sundar, L.S., "Investigation into thermophysical properties of glycol based CuO nanofluid for heat transfer applications", World Academy of Science, Engineering and Technology, 59, 440, (2011).
[21] Chandrasekar, M., Suresh, S. and Chandra, B., " Experimental studies on heat transfer and friction factor characteristics of Al2O3 – water nanofluids in a circular pipe under laminar flow with wire coil inserts", Experimental Thermal and fluid Science, 34, 122, (2010).
[22] Jafari Nasr, M.R., Polley, G.T. and Zoghi, A.T., "Performance evaluation of heat transfer enhancement (THE technology)", Proceeding of 14th International Chemical and Process Engineering Congress, CHISA, Parha,
Czech Republic, August 3-4, 27, (2000).
[23] Chen, H., Ding, Y. and Lapkin, A.A., "Rheological behavior of nanofluids containing tube/rod-like nanoparticles", Powder Tech., 194, 132, (2009).
[24] Xie, H., Lee, H., Youn, W. and Choi, M., "Nanofluids containing multi walled carbon nanotubes and their enhanced thermal conductivities", J. Appl. Phys., 94 (8), 4967, (2003).
[25] Murshed, S.M.S., Leong, K.C. and Yang, C., "Investigations of thermal conductivity and viscosity of nanofluids", Int. J. Thermal Sciences, 47, 560, (2008).
[26] Papari, M.M., Yousefi, F., Moghadasi, J., Karimi, H. and Campo, A., "Modeling thermal conductivity augumentation of nanofluids using diffusion neural network", Int. J. Thermal Sciences, 50, 44, (2011).
[27] Fallahiyekta, M., Jafari Nasr, M.R., Rashidi, A.M. and Arjmand, M., " An experimental study on thermal conductivity of nanofluids containing new produced carbon nanotubes in distilled water: Empirical model development", Proceeding of International Conference on Mechanical Engineering and Mechatronics, Toronto, Ontario, Canada, August 8-9, 42, (2013).
