heat transfer performance of different nanofluids flows in a helically coiled heat exchanger
TRANSCRIPT
Heat Transfer Performance of Different Nanofluids Flows in a Helically Coiled Heat Exchanger
M.A. Khairul1,a, R. Saidur1,b, Altab Hossain1,c, M.A. Alim1,d, I.M. Mahbubul1,e
1Department of Mechanical Engineering, Faculty of Engineering, University of Malaya 50603 Kuala Lumpur, Malaysia
[email protected], [email protected], [email protected], [email protected], [email protected]
Keywords: Coiled tube; Nanofluids; Heat transfer enhancement; Friction factor; Pressure drop.
Abstract. Helically coiled heat exchangers are globally used in various industrial applications for
their high heat transfer performance and compact size. Nanofluids can provide excellent thermal
performance of this type of heat exchangers. In the present study, the effect of different nanofluids
on the heat transfer performance in a helically coiled heat exchanger is examined. Four different
types of nanofluids CuO/water, Al2O3/water, SiO2/water, and ZnO/water with volume fractions 1
vol.% to 4 vol.% was used throughout this analysis and volume flow rate was remained constant at
3 LPM. Results show that the heat transfer coefficient is high for higher particle volume
concentration of CuO/water, Al2O3/water and ZnO/water nanofluids, while the values of the friction
factor and pressure drop significantly increase with the increase of nanoparticle volume
concentration. On the contrary, low heat transfer coefficient was found in higher concentration of
SiO2/water nanofluids. The highest enhancement of heat transfer coefficient and lowest friction
factor occurred for CuO/water nanofluids among the four nanofluids. However, highest friction
factor and lowest heat transfer coefficient were found for SiO2/water nanofluids. The results reveal
that, CuO/water nanofluids indicate significant heat transfer performance for helically coiled heat
exchanger systems though this nanofluids exhibits higher pressure drop.
Nomenclature
cp Specific heat capacity (J/kg K) f Friction factor
d Inner diameter of pipe (m) n No. of turns
dc Coil diameter (m) Greek symbols
pc Helical pitch of the coil (m) ϕ Particle volume concentration (%)
h Heat transfer coefficient (W/m2 K),
d/k.Nuh = ρ Density (kg/m
3)
V Velocity (m/s) µ Dynamic viscosity (N s/m2)
Re Reynolds number, µρ /Re Vd= δ d/dc
De Dean number, ( ) 50.
cd/dReDe = Subscripts
Pr Prandtl number, kc p /Pr µ= Nf Nanofluids
Nu Nusselt number Bf Base fluid
k Thermal conductivity (W/m K) np Nanoparticle
T Temperature (K) eff Effective
Introduction
Heat exchangers are universally used in different engineering applications, for example,
applications in food and chemical industry, power generation, waste heat recovery, refrigeration and
air conditioning. At present, different industries are motivated to employ energy saving policy as
much as feasible in their abilities due to the high cost of energy. In recent decades, attempts have
Advanced Materials Research Vol. 832 (2014) pp 160-165Online available since 2013/Nov/21 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.832.160
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been done to increase heat transfer of different heat exchangers, minimize the heat transfer time and
lastly try to make effectively utilize the energy with higher efficiency. Usually these efforts involve
active and passive methods such as extending the heat exchanger surface area or use such fluid
those have high thermophysical properties, and can create turbulence.
Micro/millimeter sized suspended particles were used at the beginning but it creates some
difficulties like clogging and blockage on the flow passage while trying to enhance the heat transfer.
Nanotechnology is introduced to overcome those difficulties. Choi [1] proposed the concept of
nanofluids as a promising heat transfer fluids with 1-100 nm sized nanoparticles suspended in base
fluids in 1995. Many researchers examined [1-4] the thermal conductivity of nanofluids relating
Al2O3 and CuO nanoparticles and compared the consequence with base fluid. They suggested that
the base fluid with nanoparticles contribute to the positive affect on heat transport properties.
Prabhanjan et al. [5] concluded that, helically coiled tubes are more effective compare to straight
tube when conventional fluids is used in heat transfer application, and the tube curvature have an
significant effect on raising the heat transfer rate. Moris and Nzkayana [6], Kalb and Seader [7]
considered the curvature effect of helically coiled tube and replaced Reynolds number by Dean
number. Jamshidi et al. [8] concluded that, the optimum condition for getting maximum heat
transfer enhancement is dc = 116 mm, pc = 18 m and fluid flow rate 3 LPM in case of shell and tube
heat exchanger. Generally, helical coiled exerts gravity force, inertia force and centrifugal force.
The secondary force is induced when fluid passes through a helical coil due to the curvature effect
of the coil. This secondary flow allows appropriate mixing to improve the heat transfer.
Based on the literature, there are studies available about shell and tube heat exchanger with
nanofluids [8, 9] and there is limited relative analyse was reported on the helically coiled tube heat
exchanger with nanofluids. Most of these studies were conducted with a single nanofluid and
compared the results with base fluid. However, there is no study available about helical coiled heat
exchanger operated with different nanofluids and compared the results with one another. Therefore,
the objective of this investigation is to compare the heat transfer coefficient, friction factor and
pressure drop of helically coiled heat exchanger by flowing CuO/water, Al2O3/water, SiO2/water,
and ZnO/water nanofluids.
Methodology
In this study, the thermal performance of different nanofluids flow through a helical coil heat
exchanger is studied analytically. In the current analysis, it is assumed that the nanofluids flows
through the coiled tubes are incompressible and turbulent flow regime. Calculations have been done
by taking inner diameter of the helical tubes is d = 9 mm, helical pitch of the coils is pc = 18 mm,
and the coil diameter is dc = 116 mm, because enhanced heat transfer was found in these
dimensions [8]. CuO/water, Al2O3/water, SiO2/water, and ZnO/water nanofluids with 1% to 4%
volume fraction of nanoparticle have been used as working fluid in this analysis and water was the
base fluid for all nanofluids.
The thermophysical properties of nanoparticle and water were taken from literature [10] for 300
K temperature. The thermophysical properties as thermal conductivity, viscosity, density, and
specific heat of four different nanofluids have been calculated by using Maxwell [11], Einstein
[12], Gherasim et al. [13], and Xuan and Roetzel [14] equations, respectively.
Effective thermal conductivity and viscosity of nanofluids
( )( )bfnpbfnp
bfnpbfnp
bf
eff
kkkk
kkkk
k
k
−−+−++
=ϕϕ
2
22 (1)
( ) bfnf . µϕµ ×+= 521 (2)
Density and specific heat of nanofluids
npbfnf )( ϕρρϕρ +−= 1 (3)
Advanced Materials Research Vol. 832 161
nf
nppbfp
nf,p
)c()c)((c
ρρϕρϕ +−
=1
(4)
Nusselt number is a function of Dean number, Prandtl number and basic geometrical parameters of
corrugation and can be calculated by using Eq. (5) [8].
40864005510 .. PrDe.Nu = (5)
Friction factor of a helical tube is found from Eq. (6) [15]. The pressure drop is obtained from Eq.
(7) [16]. It involves the friction factor, velocity of flowing medium.
0860104680062 ...De.f ϕδ −−= (6)
2
2
8δπρ∆ nVf
P = (7)
Results and discussion
Fig. 1 shows the Reynolds number with different nanoparticle volume concentration for four
different nanofluids. From Fig. 1, a noticeable rise in Reynolds number is observed by increasing
the particle volume concentration of CuO/water and ZnO/water nanofluids and negligible increase
in Reynolds number is found for Al2O3/water nanofluids, while SiO2/water nanofluids show the
reverse character. Normally Reynolds number depends on the density, the volume flow rate, the
hydraulic diameter and the dynamic viscosity of the nanofluids. Density and viscosity have an
indicative effect on Reynolds number among all these factors. In compare to other nanofluids, the
density of CuO/water is significantly greater which guides to a higher Reynolds number [17].
Elsewhere, the trend of SiO2/water is reversed due its lower density.
Fig. 1, Variation of Reynolds number with nanoparticles volume concentration for four different
nanofluids flow inside the helically coiled tube
Fig. 2 clarifies that, the variation of the heat transfer coefficient for CuO/water, Al2O3/water,
SiO2/water and ZnO/water nanofluids with four distinct particles volume concentrations at constant
volumetric flow rates. The maximum heat transfer coefficient is found for CuO/water nanofluids
with 4% volume fraction compare to others nanofluids. However, similar trends are observed for
Al2O3/water and ZnO/water nanofluids. On the other hand, SiO2/water nanofluids exhibited reverse
behavior due to its lower density and thermal conductivity.
6600
6900
7200
7500
7800
0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5
Rey
nold
s N
um
ber
Nanoparticle Volume concentration (%)
CuO/water Al2O3/water SiO2/water ZnO/waterCuO/Water Al2O3/Water SiO2/Water ZnO/Water
162 Nanoscience, Nanotechnology and Nanoengineering
Fig. 2, Variation of heat transfer coefficient with nanoparticles volume concentration for four
different nanofluids flow inside the helical tube
Usually enhanced heat transfer coefficient is observed by adding nanoparticles in base fluids. It
is on account of increasing CuO/water nanofluids thermal conductivity compared to other three
nanofluids and it plays an important role on increment of the heat transfer coefficient. Although the
increment of nanofluids effective thermal conductivity by adding the small amount of nanoparticles,
changes the flow and thermal fields that improve the heat transfer coefficient. Moreover, chaotic
motion of the nanoparticles in flow will disturb the thermal boundary layer formation on the tube
surface. This disturbance causes slow thermal boundary layer formation and greater heat transfer
coefficients are achieved of fluid flow in a helical coil heat exchanger [18]. Thus, using the CuO,
Al2O3 and ZnO nanoparticles in the coiled-tube can cause an additional enhancement in the heat
transfer coefficient.
Fig. 3, Variation of friction factor with nanoparticles volume concentration for four different
nanofluids flow inside the helical tube
Fig. 3 shows the friction factor variation for four different nanofluids at constant volumetric flow
rate. According to the friction factor equation (Eq. (6)), friction factor mainly depends on the
Reynolds number as well as the density of the nanofluids. By increasing the nanoparticles volume
5400
5600
5800
6000
6200
6400
0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5Hea
t T
ran
sfer
Co
effi
cien
t (W
/m2
K)
Nanoparticle Volume concentration (%)
CuO/Water Al2O3/Water SiO2/Water ZnO/Water
0,048
0,051
0,054
0,057
0,060
0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5
Fri
ctio
n f
act
or
Nanoparticle Volume concentration (%)
CuO/Water Al2O3/Water SiO2/Water ZnO/Water
ZnO/Water Al2O3/Water SiO2/Water
CuO/Water Al2O3/Water SiO2/Water ZnO/Water
CuO/Water
Advanced Materials Research Vol. 832 163
fraction, both the fluid density and friction factor rises [15]. The effect of particles volume
concentration on the friction factor is more at high density while at low density the effect is
insignificant.
Fig. 4, Variation of pressure drop with nanoparticles volume concentration for four different
nanofluids flow inside the helical tube
From Fig. 4, the results show that there is a noticeable increase in pressure drop of nanofluid
with 4 wt.% particle concentration. This enhancement trend tends to continue for the nanofluids
with higher weight fractions. According to the pressure drop equation (Eq. (7)), pressure drop
mainly depends on the friction factor as well as the density of the nanofluids. By increasing the
nanoparticles volume fraction, both the fluid density and the pressure drop rises. Because solid
nanoparticles with base fluid usually rise the dynamic viscosity compare to the base fluid. Since, the
dynamic viscosity have direct relation with pressure drop, the greater viscosity value conducts to
enhanced the amount of pressure drop. The migration and chaotic motion of nanoparticles is
another reason which are responsible for increasing the pressure drop [18].
Conclusion
In the present analysis, the thermal performance of a helically coiled heat exchanger using four
different types of nanofluids at various volume concentrations was analyzed. Based on the findings,
the following points are concluded:
• A significant increase in Reynolds number was found for CuO/water nanofluid because of its
higher density than the other three nanofluids.
• Similarly, CuO/water nanofluids indicate significant heat transfer performance by increasing
the heat transfer coefficient and reducing the friction factor for helically coiled heat exchanger
systems compare with other nanofluids. But on the other hand, it exhibits higher pressure drop
due to its higher density. Except SiO2/water nanofluids, other nanofluids show higher heat
transfer coefficient by increasing volume fraction of nanoparticles.
• SiO2/water nanofluids shows lower heat transfer coefficient by increasing volume fraction of
nanoparticles. This is due to the lower density of SiO2/water nanofluids. However, lower
pressure drop is observed by using SiO2/water nanofluids flow in a helical heat exchanger.
Acknowledgement
The authors are indebted to the High Impact Research Grant (HIRG) scheme (UM-MOHE)
project (UM.C/HIR/MOHE/ENG/40) for financial support to carry out this research.
20
22
24
26
28
0,5 1 1,5 2 2,5 3 3,5 4 4,5
Pre
ssu
re d
rop
(M
Pa)
Nanoparticle Volume concentration (%)
CuO/water Al2O3/water SiO2/water ZnO/waterCuO/Water Al2O3/Water SiO2/Water ZnO/Water
164 Nanoscience, Nanotechnology and Nanoengineering
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