- Email: [email protected]
Experimental investigation on the thermal performance of a coiled heat exchanger using a new hybrid nanofluid
Accepted Manuscript Experimental investigation on the thermal performance of a coiled heat exchanger using a new hybrid nanofluid H.R. Allahyar, F. Hormozi, B. ZareNezhad PII: DOI: Reference:
S0894-1777(16)30069-3 http://dx.doi.org/10.1016/j.expthermflusci.2016.03.027 ETF 8732
To appear in:
Experimental Thermal and Fluid Science
Received Date: Revised Date: Accepted Date:
4 September 2015 26 March 2016 27 March 2016
Please cite this article as: H.R. Allahyar, F. Hormozi, B. ZareNezhad, Experimental investigation on the thermal performance of a coiled heat exchanger using a new hybrid nanofluid, Experimental Thermal and Fluid Science (2016), doi: http://dx.doi.org/10.1016/j.expthermflusci.2016.03.027
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental investigation on the thermal performance of a coiled heat exchanger using a new hybrid nanofluid
H.R. Allahyar, F. Hormozi, B, ZareNezhad* Faculty of Chemical, Petroleum and Gas Engineering, Semnan University, Semnan, Iran * Corresponding author (email: [email protected])
Abstract In this work the thermal performance of hybrid and single type nanofluid are investigated in a coiled heat exchanger at constant wall temperature and laminar flow operating conditions. The nanoparticle concentration is in the range of 0.1-0.4 vol% and the composition of the synthesized nanoparticle regarding hybrid nanofluid experiments is 97.5% alumina and 2.5% Ag. The maximum rate of heat transfer can be obtained by using the hybrid nanofluid at a concentration of 0.4 vol% which is 31.58% higher than that of the distilled water. Overall, the maximum thermal performance factor for hybrid nanofluid is about 2.55 suggesting the superior performance of the presented approach for energy intensification in heat exchangers.
Keywords: thermal performance; heat exchanger; hybrid; nanofluid; nanoparticle 1. Introduction Many researchers have been investigating about the heat transfer in the industry for many decades in which they have proposed various methods including using curved pipes. Another way to improve the heat transfer is to use fluids of transferring heat namely water, ethylene glycol and oil. In order to enhance the heat transfer and overcome the
1
limited capability of conventional fluids including water, ethylene glycol and oil, the researchers have proposed to use advanced fluids with higher thermal conductivity instead of conventional fluids. For the first time Maxwell [1873] [1], studying the performance
of
suspended
particles,
indicated
that
the
dispersion
of
millimeter/micrometer-sized particles in the base fluids leads to the improvement of the heat transfer. However, adding these particles results in some problems including erosion of the components due to the abrasion caused by the particles, clogging of narrow passages, settling of the particles and increased pressure drop. After several decades for the first time Choi et al. 1995 [2] at Argonne National Laboratory introducing nanofluid could tackle these problems. Eastman et al. (2001) [3] have also studied the effect of nanofluid thermal conductivity. They have shown that ethylene glycol-based copper nanofluid has a higher thermal conductivity in comparison with oxide form of particles. A 40 % increase in thermal conductivity due to existence of the nanofluid (containing ethylene glycol and 0.3% by volume of copper with a diameter less than 10 nm) has been observed. Zeinali et al. (2006) [4, 5] have also investigated the effects of aluminum oxide and copper oxide nanofluid at laminar flow and constant wall temperature conditions. They have concluded that an increase in nanoparticle concentration improves the heat transfer coefficient. They have also shown that aluminum oxide nanofluid gives better results in comparison with copper oxide regarding convective heat transfer. Duangthongsuk and Wongwises, (2010) [6] have studied the heat transfer and pressure drop
performance of water-based titanium dioxide nanofluid in turbulent flow conditions and shown that with increased Reynolds number and particle concentration, heat transfer coefficient increases by 26%. Sundar et al. (2010) [7] have conducted experimental
2
studies on the turbulent heat transfer and friction coefficient in the presence of alumina nanofluid (in a circular tubes with twisted tape inserts) and reported a 33.5% increase in heat transfer coefficient and a 1.096% increase in friction coefficient. Suresh et al. (2011) [8] have investigated the thermal performance of alumina and copper oxide nanofluid using the twisted tape inserts in tube under constant heat flux and laminar flow conditions. They have concluded that under constant thermal conditions in the twisted tape, copper oxide nanofluid shows better performance as compared to the alumina nanofluid. In addition, copper oxide nanofluid imposes higher pressure drop as compared to the alumina nanofluid and the use of twisted tape increases this pressure drop to a greater extent. Suresh et al. (2012) [9,10] have experimentally studied heat transfer characteristics and friction coefficient of water-based copper oxide and alumina nanofluid under laminar flow and constant heat flux conditions and concluded that increased concentration of nanoparticles in the solution increased Nusselt number (Nu) and friction coefficient. Additionally, the use of twisted tape instead of straight tubes significantly increases the Nusselt number. Tajik et al. (2013) [11] have investigated the heat transfer and pressure drop characteristics of water-based copper and aluminum nanofluid in spiral coils at constant wall temperature and laminar flow conditions and found that the thermal conductivity of copper nanofluid is about 18% higher than that of the aluminum nanofluid. They have also shown that an increase in the nanoparticle concentration leads to an increase in the thermal conductivity and pressure drop. Sajjadi et al. (2011) [12] have experimentally studied turbulent convective heat transfer and pressure drop of titanium oxide nanofluid in circular tubes under constant wall
3
temperature and concluded that the increased concentration of nanoparticles had no significant effect on the rate of heat transfer and pressure drop. Fakoor et al. (2012) [13] have conducted an experimental study on the pressure drop due to the flow of water-based carbon nanotubes in helical coils at laminar flow and constant wall temperature conditions. They have shown that pressure drop increased with increased nanoparticle concentration and Reynolds number, while pressure drop decrease occured with increased coil diameter with regard to tube diameter. Furthermore, the dependence of pressure drop on the step length was negligible, and they reported increased pressure drop by 3.5 times with the concentration of 0.45% compared to the base fluid in the regular tube. Although a lot of researches have been conducted on single-step nanofluids, but there are very limited information on the performance of nanocomposites. Synthesis and preparation of nanocomposites have been recently considered due to their special attraction. Nanocomposite generally refers to a nanofluid composed of two different materials with nanoscale particles. Suresh et al. (2012) [14] have studied the effect of water-based alumina-copper hybrid nanofluid on the heat transfer rate and pressure drop at laminar flow and constant heat flux conditions. The results indicate that at the 0.1% volume concentration of nanoparticles, the maximum increase in heat transfer rate and pressure drop are 13.6% and 16.97% respectively. Baghbanzadeh et al. (2012) [15] have studied the thermal conductivity of silica-nanotube hybrid nanofluid and shown that the thermal conductivity of multi-walled nanotubes nanofluid and silica nanofluid are about 23.3% and 8.85% more than the base fluid, respectively. The most and the least enhancement in the effective thermal conductivity of
4
the fluids were associated with MWCNTs (23.3%) and silica nanospheres (8.8%), while the enhancement for the hybrid nanomaterial was a value between MWCNT and silica nanoparticles. Furthermore, the hybrid consisting of higher percentage of MWCNTs showed more increase in effective thermal conductivity of the nanofluid, compared with the other hybrid. Sandar et al. (2014) [16] have also conducted an experimental investigation on heat transfer coefficient and friction coefficient at the conditions of fully developed turbulent flow and constant flux and reported 9.35% and 22.62% increase in heat transfer rate and friction coefficient respectively. In the present work, the thermal performance and pressure drop in a helical coil under laminar flow and constant wall temperature conditions employing a hybrid nanofluid (prepared by using alumina-silver nanocomposite) are experimentally investigated and the obtained results are compared to those of a single-step nanofluid (alumina).
2. Experimental setup and procedure The schematic diagram of the employed setup is shown in Fig. 1. The system is kept at isothermal operating condition by using a PID control system. The tank is fully insulated with rock wool in order to avoid heat loss and a 2 kW heater is immersed in the tank to supply the required heat. For measuring of inlet and outlet pressures, very sensitive pressure transmitters (model PSCH 0.5BCIA SENSYS) have been employed. Two accurate T- type thermocouples are inserted so as to measure the inlet and outlet temperature of the fluid in the tube. Furthermore, six K- type thermocouples are installed at different positions to measure the surface temperature distribution of the tested part (tube). The flow rate is estimated by an ultrasonic system with an accuracy of 0.05 l/m. The physical characteristics of the copper coil are shown in Table (1). In this work a 5
water based single-step nanofluid containing alumina with an average diameter of 55 nm and a water-based hybrid nanoparticle containing alumina-sliver with an average diameter of 80 nm composed of 97.5% alumina and 2.5% silver (purchased from Nano Pooshesh Felez Co) have been employed, both of which have spherical shape. There are numerous ways to prepare nanoparticles, one of which is Sol-gel method. One of the advantages of this method is to prepare nancomposites with high purity. At first a homogenous suspension including solvent and precursor (which is going to form the final solution) are solved and then the homogenous solution is turned into Sol by hydrolysis. After provoking the particles in Sol by HCL and NaOH, they join together and form a wet gel. After separating the solution and drying it, the nanoparticles are formed. The produced nanoparticles are then dispersed in the base fluid by using an ultrasonic device for 30 minute. It is found that the prepared nanofluid preserves its stability for 48 hours. Fig. 2 shows that the TEM (transmission electron microscopy)
images of nanoparticles dispersed in the distilled water. The cooling system comprises cooling urban water as well as a fan for increasing the cooling system efficiency. After preparing and setting up all the equipments, the device is fully calibrated with pure water and then the main experiments are carried out with nanofluid at different concentrations. The experiments are started by turning on the heater, pump and cooling system simultaneously and the flow is controlled using the return system embedded after the pump. As soon as the tank temperature reaches saturation (constant temperature of 95°C) point, the pressure, inlet and outlet temperatures, and the surface temperature are recorded via a data logger system. The experiments have been performed at least twice to fully ensure the data accuracy. 3. Modeling section
6
The thermophysical characteristics of the nanofluid are calculated according to the following equations [17- 19]:
nf (1 ). f . P nf
(1)
f (1 ) 2.5
(2)
( .C P ) nf (1 ).( .C P ) f .( .C p ) P
(3)
Thermal conductivity is also calculated using the following equation [20]:
k nf kf
k P 2k f 2(k f k P ).
(4)
k P 2k f (k f k P ).
Since the fluid passes through a pipe at a constant temperature, the convective heat transfer coefficient and Nusselt number at thermal equilibrium is as follows: .
h(exp)
m.c p .( Tb1 Tb2 )
(5)
A.( Tw Tb )M
h(exp).d Nu (exp) k
(6)
Which TW Tb M is a logarithmic temperature difference. Several important parameters required for description of fluid flow through the coil include:
Re .U .d /
(7)
Pr c p . / k
(8)
De Re .(d / D)1 / 2
(9)
7
The theoretical equations for predicting the Nusselt number in straight and helical tubes are estimated by equations (10) and (11) [21, 22]:
Nu S 3.66
((d / L). Re . Pr).0.0668 1 0.04((d / L). Re . Pr) 2 / 3
(10)
Nu C 0.13.( f . Re 2 . Pr)1 / 3
(11)
Equation (12) can be used for determination of pressure drop in the straight tube where for laminar flow inside a tube, the theoretical values of Darcy friction factor (f) is evaluated by [21]: PS f S.( L / d )..U 2
f S ,la min ar
(12)
64 Re
(13)
After replacing f c with f S and also Lc with L, the above equation can be used to estimate the pressure drop in the coil. The following equation can be used for predicting friction coefficient in the coil [23]:
fC 0.47136.De1 / 4 fS
(14)
The criterion for transition from laminar to turbulent region is specified by the following equation [24]:
Re cri
12730
(15)
3.2
In this article, the critical Reynolds number is equivalent to 5600 such that for Re<5500 the laminar flow prevails.
8
4. Results and discussion The experiments have been performed at constant wall temperature and laminar flow conditions in a helical coil. The compositions of aluminum and silver in the employed nanocomposite are 97.5 % and 2.5 % respectively and the nanofluid concentrations are in the range of 0.1-0.4 vol%. In Fig. 3, the Nusselt number of nanofluid versus Reynolds number in helical coil at different concentrations of nanofluid is shown. The presented results clearly show that an increase in Nusselt number is accompanied by increasing concentration of nanoparticles and Reynolds number. It should be noted that the distribution and the proportional movement of nanoparticles near to the wall of the pipe due to Brownian movement and particles migration lead to a fast increase in heat transfer from the wall of the pipe to nanoparticles. Thus, an increase in concentration of particles leads to an increase in the heat transfer rate. The employed helical coil causes a centrifugal force and secondary flow such that a further dispersion of nanoparticles in the base fluid leads to a decrease in thickness of the boundary layer. Thus an increase in the heat exchange between the nanoparticles and the wall of the pipe enhances the Nusselt number. At a high flow rate, the effects of dispersion of nanoparticles in the base fluid will intensify the vacillation and consequently the heat transfer is increased. In general, the increase of heat transfer rate in the presence of hybrid nanofluid in helical coil can be attributed to the increasing rate of Brownian movement of nanoparticles, migration of the nanoparticles and reduction of thickness of boundary layer. According to Fig. 3 the employed hybrid nanofluid in this work leads to a higher Nusselt number at different operating conditions.
9
According to Fig. 4, the hybrid nanofluid shows a greater increase in heat transfer than single-step nanofluid. When the hybrid nanofluid is employed, a maximum increase of Nusselt number (~31.58%) with respect to pure water at the concentration of 0.4 vol% and Reynolds number 4687 is observed which is higher than that of the single-step nanofluid (~28.42%) at the same operating condition. With increasing nanoparticle contents and Reynolds number, physical properties of nanofluid (Prandtl number) undergo change too. As it is seen since in this figure the ratio of heat transfer is depicted only versus Reynolds number, some trends which are almost non-uniform trends are observed over experimental data because it was not taken into consideration in this figure. At lower Reynolds number, thermal conductivity is the main reason for heat transfer coefficient and consequently it leads to the increase in the heat transfer. With increasing Reynolds number, the importance of thermal conductivity in enhancement of heat transfer becomes less considerable. [25, 26]. On the other hand, the importance of other mechanisms including Brownian movement of nanoparticles, migration of the nanoparticles and reduction of thickness of boundary layer becomes more tangible in the increase in heat transfer.[27-29], In addition, at higher rate of Reynolds number the effects of dispersion and turbulent movement leads to increase in fluctuations resulting in increase in the heat transfer. This trend continues until the heat transfer of nanofluid reaches the maximum enhancement ratio in Reynolds number [4678] compared to base fluids. Then, with the increase in Reynolds number, the relative decrease is experienced. Since with the increase in Reynolds number, the collision of nanoparticles increases and it is likely to be aggregated which relatively decreases in the increase trend of heat transfer. However at higher Reynolds numbers, the advantage of using the nanofluid
10
gradually diminishes such that the Nu number in the presence of nanoparticles approaches to that of pure water at very high turbulent conditions as shown in Fig. 4 [30, 31]. For studying the thermal performance of industrial units, measuring pressure drop of nanofluid is necessary. In Fig. 5, the pressure drop in the presence of nanofluid is shown at different concentrations. The pressure drop is increased as the Reynolds number and concentration of particles are increased. This can be attributed to the increase of the nanofluid viscosity and density. The Brownian movement of particles and dispersion and vacillations of nanoparticles leads to increase the momentum transfer between the particles especially near the wall of the pipe such that the axial pressure drop is increased. It should be noted that an increase in pressure drop in the presence of hybrid nanofluid may be due to the formation of particle agglomerates and clusters which affect the suspension viscosity [32, 33]. The thermal performance factor is defined according to the following equation [34]:
Nu C ,nf
Nu S , f PC ,nf 0.1666 ( ) PS , f
(17)
The thermal performance factor of nanofluid is calculated at constant wall temperature and laminar flow in the helical coil with respect to the straight tube. As shown in Fig. 6, the performance factor is always greater than one. This means that by using the hybrid nanofluid in the helical, the thermal performance of the system improves significantly. As
11
shown in this figure, the maximum thermal performance factor for hybrid nanofluid at the nanoparticle concentration of 0.4 vol% and Re= 4687 is about 2.55.
5. Conclusion In the present work, the thermal performance and pressure drop in a helical coil under laminar flow and constant wall temperature conditions employing a hybrid nanofluid (prepared by using alumina-silver nanocomposite) are experimentally investigated and the obtained results are compared to those of a single-step nanofluid (alumina) and pure water. The maximum rate of heat transfer can be obtained by using the hybrid nanofluid at a concentration of 0.4 vol% which is 31.58% higher than that of the distilled water. Overall, the maximum thermal performance factor for hybrid nanofluid is about 2.55 suggesting the superior performance of the proposed hybrid nanocomposite for enhancing the thermal performance of heat exchangers in chemical and petroleum industries.
Nomenclature
A
inner surface of tube (m2)
b
Pitch of coil (m)
Cp
Specific heat (J. kg−1 .K−1)
d
Inside diameter of tube (m)
D
Diameter of coil (m)
De
Dean number
f
Fanning friction factor
h
Heat transfer coefficient (W. m−2 .K−1)
He
Helical coil number
K
Thermal conductivity (W .m−1 .K−1)
L
Length of tube (m) 12
m
Mass flow rate (kg .s−1)
N
Number of coil turns
Nu
Average Nusselt number
Pr
Prandtl number
Re
Reynolds number
T
Temperature (K)
U
Average velocity (m. s−1)
Greek letters ΔP
Axial pressure drop (Pa)
η
Thermal performance factor
ρ
Density (kg. m−3)
μ
Dynamic viscosity (Pa. s)
λ
Curvature ratio (=D/d)
φ
Nanoparticle volume fraction (%)
Subscripts C
Coiled tube
exp
exprimental
f
Base fluid
nf
Nanofluid
p
Particle
S
Straight tube
th
theoretical
w
Wall
References [1] J.C.Maxwell. Electricity and magnetism. Clarendon Press: Oxford; 1873. [2] S.U.S. Choi, Enhancing thermal conductivity of fluids with nanoparticle, ASME FED. 231 (1995) 99–105.
13
[3] J.A. Eastman, S.U.S. Choi, S. Li, W. Yu, L.J. Thompson, Anomalously increased effective thermal conductivities of ethylene glycol-based nanofluids containing copper nanoparticles, Applied Physics Letters. 78 (2001) 718–720. [4] S. Zeinali Heris, M. Nasr Esfahany, S.G. Etemad, Experimental investigation of convective heat transfer of Al2O3/water nanofluid in circular tube, Int. J. Heat Fluid Flow. 28 (2) (2007) 203–210. [5] S. Zeinali Heris, S.G. Etemad, M. Nasr Esfahany, Experimental investigation of oxide nanofluids laminar flow convective heat transfer, International Communications in Heat and Mass Transfer. 33 (2006) 529–535. [6] W. Duangthongsuk, S. Wongwises, An experimental study on the heat transfer performance and pressure drop of TiO2–water nanofluids flowing under a turbulent flow regime, Int. J. Heat Mass Trans. 53 (2010) 334–344. [7] L. Syam Sundar, K.V. Sharma, Turbulent heat transfer and friction factor of Al2O3 nanofluid in circular tube with twisted tape inserts, Int. Commun. Heat Mass Transf. 53 (2010) 1409–1416. [8] S. Suresh, K.P. Venkitaraj, P. Selvakumar, Comparative study on thermal performance of helical screw tape inserts in laminar flow using Al2O3/water and CuO/water nanofluids, Superlattice Microst. 49 (2011) 608–622. [9] S. Suresh, M. Chandrasekar, P. Selvakumar, Experimental studies on heat transfer and friction factor characteristics of CuO/water nanofluid underlaminar flow in a helically dimpled tube, Heat and Mass Transfer. 48 (4) (2012) 683–694. [10] S. Suresh, K.P. Venkitaraj, P. Selvakumar, M. Chandrasekar, A comparison of thermal characteristics of Al2O3/water and CuO/water nanofluids in transition flow through a straight circular duct fitted with helical screw tape inserts, Experimental Thermal and Fluid Science. 39 (2012) 37–44. [11] M. Tajik Jamal-Abad, A.H. Zamzamian, M Dehghan, Experimental studies on the heat transfer and pressure drop characteristics of Cu–water and Al–water Nanofluids in a spiral coil, Experimental Thermal and Fluid Science. 47(2013) 206–212. [12] A.R. Sajadi, M.H. Kazemi, Investigation of turbulent convective heat transfer and pressure drop of TiO2/water nanofluid in circular tube, Int. Commun. Heat & Mass Transfer. 38 (10) (2011) 1474–1478. [13] M. Fakoor-Pakdaman, M.A. Akhavan-Behabadi, P. Razi, An empirical study on the pressure drop characteristics of nanofluid flow inside helically coiled tubes, International Journal of Thermal Sciences. 65 (2013) 206-213.
14
[14] S. Suresh, K.P. Venkitaraj, P. Selvakumar, M. Chandrasekar, Effect of Al2O3–Cu/water hybrid nanofluid in heat transfer, Exp. Thermal Fluid Sci. 38 (2012) 54–60. [15] M.A. Baghbanzadeh, A. Rashidi, D. Rashtchian, R. Lotfi, A. Amrollahi, Synthesis of spherical silica/multiwall carbon nanotubes hybrid nanostructures and investigation of thermal conductivity of related nanofluids, Thermochimica Acta. 549 (2012) 87– 94. [16] L. Syam Sundar, Singh, Manoj K. Singh, Antonio C.M. Sousa, Enhanced heat transfer and friction factor of MWCNT–Fe3O4/water hybrid nanofluids, International Communications in Heat and Mass Transfer. 52 (2014) 73–83. [17] B.C. Pak, Y. Cho, Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particle, Exp. Heat Transfer. 11 (1998) 151–170. [18] Y. Xuan, W. Roetzel, Conceptions for heat transfer correlation of nanofluids, Int. J. Heat and Mass Transfer. 43 (2000) 3701-3707. [19] H.C. Brinkman, The viscosity of concentrated suspensions and solution, J. Chem. Phys. 20 (1952) 571. [20] F.J. Wasp, Solid–liquid slurry pipeline transportation, Trans. Tech, Berlin, 1977. [21] F.P. Incropera, D.P. De Witt, Fundamentals of heat and mass transfer, fourth ed. John Wiley, New York, 1996. [22] R.A. SEBAN, E.F. McLAUGHLINt, Heat transfer in tube coil with laminar and turbulent flow, Heat and Mass Transfer. 6 (1962) 387-395. [23] M. Van Dyke, Extended Stokes series: laminar flow through a loosely coiled pipes, Journal of Fluid Mechanics. 86 (1978) 129–145. [24] V. Kubair, C.B.S. Varrier, Pressure drop for liquid flow in helical coils, Transactions of the Indian Institute of Chemical Engineers. 14 (1961) 93. [25] D. Wen, Y. Ding, Experimental investigation into convective heat transfer of nanofluid at the entrance region under laminar flow conditions, International Journal of Heat and Mass Transfer. 47 (24) (2004) 5181–5188. [26] H. Chen, W. Yang, Y. He, Y. Ding, L. Zhang, C. Tan, A.A. Lapkin, D.V. Bavykin, Heat transfer behaviour of aqueous suspensions of titanate nanofluids, Powder Technology. 183 (2008) 63–72. [27] A.A. Abbasian Arani, J. Amani, Experimental investigation of diameter effect on heat transfer performance and pressure drop of TiO2-water nanofluid, Experimental Thermal and Fluid Science. 44 (2013) 520–533.
15
[28] M. Chandrasekar, S. Suresh, A. Chandra Bose, Experimental studies on heat transfer and friction factor characteristics of Al2O3/water nanofluid in a circular pipe under laminar flow with wire coil inserts, Experimental Thermal and Fluid Science. 34 (2010) 122–130. [29] S. Suresh, M. Chandrasekar, S. Chandra Sekhar, Experimental studies on heat transfer and friction factor characteristics of CuO/water nanofluid under turbulent flow in a helically dimpled tube, Exp. Therm. Fluid Sci. 35 (2011) 542–549. [30] S. ZeinaliHeris, S.H. Noie, E. Talaii, J. Sargolzaei, Numerical investigation of Al2O3/water nanofluid laminar convective heat transfer through triangular ducts, Nanoscale Research Letters. 6 (2011) 179. [31] M. Kahani, S. Zeinali Heris , S.M. Mousavi, Comparative study between metal oxide nanopowders on thermal characteristics of nanofluid flow through helical coils, Powder Technology. 246 (2013) 82–92. [32] Y. Xuan, Q. Li, Investigation on convective heat transfer and flow features of nanofluids, J. Heat Transfer. 125 (1) (2003) 151–155. [33] U. Rea, T. McKrell, L.W. Hu, J. Buongiorno, Laminar convective heat transfer and viscous pressure loss of alumina–water and zirconia–water nanofluids, Int. J. Heat Mass Transfer. 52 (2009) 2042–2048. [34] H. Usui, Y. Sano, K. Iby a combination of internally grooved rough tube and twisted tape, International Chemicawashita, A. Isozaki, Enhancement of heat transfer l Engineering. 26 (1) (1996) 97–104.
16
Table 1: Physical Properties of the Helical Coil (mm)
Tube
d
t
L
D
λ
N
Helical Coil
5
1
2600
65
15
10
Table 2. Thermophysical characterizations of nanopowders
Particle/base
Average diameter
Purity
Actual density
CP
k
fluid
(nm)
(%)
(kg/m3)
(J/kg·K)
(W/m·K)
Al2O3
55
99
3690
780
30.5
Ag
25
99
9320
233
429
nanocomposite
80
99
3830.75
766
41
Table 3. Uncertainty of measurement instruments
Parameter
Instrument
uncertainty
Fluid flow rate
Flownetix® 100seriesTM
±1% of reading
Steam temperature sensors
PT-100Ω thermo resistance
0.1K
Wall temperature sensors
K-type Omega Thermocouples
0.1K
Inlet/outlet temperature sensors
PT-100Ω thermo resistance
0.1K
Pressure transmitters
Keller, Series 35X-Bullet type
±1% of reading
17
Fig. 1 The Device Used in the Experiment
18
Fig. 2 TEM images of hybrid nanoparticles.
19
24 distillated water 0.1 % hybrid 0.2 % hybrid 0.4 % Alumina/water 0.4 % hybrid
22 20
Nu
18 16 14 12 10 8 500
1000
1500
2000
2500
3000
3500
4000
4500
5000
5500
Re
Fig. 3 Nusselt Number versus Reynolds Number at different concentrations of nanofluids
20
0.1 % hybrid 0.2 % hybrid 0.4 % Alumina/water 0.4 % hybrid
1.32
Nu (nf)/Nu(water)
1.28 1.24 1.2 1.16 1.12 1.08 1.04 1 500
1000
1500
2000
2500
3000
3500
4000
4500
5000
5500
Re Fig. 4 Normalized Reynolds Number in the helical coil at different concentrations of nanofluids
21
16 Disstilat water 0.1 % hybrid 0.2 % hybrid 0.4 % Alumina/water 0.4 % hybrid
14 12
∆p
10 8 6 4 2 500
1000
1500
2000
2500
3000
3500
4000
4500
5000
5500
Re Fig. 5 The pressure drop versus Reynolds Number at different concentrations of nanofluids
22
2.6 distillated water 0.1 % hybrid 0.2 % hybrid 0.4 % Alumina/water 0.4 % hybrid
2.5 2.4
Ƞ
2.3 2.2 2.1 2 1.9 1.8 500
1000
1500
2000
2500
3000
3500
4000
4500
5000
5500
Re Fig. 6 Performance factor versus Reynolds number employing hybrid and single-step nanofluids
23
Highlights
Thermal performance in a helical coil employing a hybrid nanofluid are measured. The alumina-silver nanocomposite is used for nanofluid preparation. The hybrid nanofluid at a concentration of 0.4 vol% gives the best result. There is a 31.58% increase in heat transfer rate with respect to distilled water. A 25% increase in thermal performance as compared to that of DI water is observed.
24
S0894-1777(16)30069-3 http://dx.doi.org/10.1016/j.expthermflusci.2016.03.027 ETF 8732
To appear in:
Experimental Thermal and Fluid Science
Received Date: Revised Date: Accepted Date:
4 September 2015 26 March 2016 27 March 2016
Please cite this article as: H.R. Allahyar, F. Hormozi, B. ZareNezhad, Experimental investigation on the thermal performance of a coiled heat exchanger using a new hybrid nanofluid, Experimental Thermal and Fluid Science (2016), doi: http://dx.doi.org/10.1016/j.expthermflusci.2016.03.027
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental investigation on the thermal performance of a coiled heat exchanger using a new hybrid nanofluid
H.R. Allahyar, F. Hormozi, B, ZareNezhad* Faculty of Chemical, Petroleum and Gas Engineering, Semnan University, Semnan, Iran * Corresponding author (email: [email protected])
Abstract In this work the thermal performance of hybrid and single type nanofluid are investigated in a coiled heat exchanger at constant wall temperature and laminar flow operating conditions. The nanoparticle concentration is in the range of 0.1-0.4 vol% and the composition of the synthesized nanoparticle regarding hybrid nanofluid experiments is 97.5% alumina and 2.5% Ag. The maximum rate of heat transfer can be obtained by using the hybrid nanofluid at a concentration of 0.4 vol% which is 31.58% higher than that of the distilled water. Overall, the maximum thermal performance factor for hybrid nanofluid is about 2.55 suggesting the superior performance of the presented approach for energy intensification in heat exchangers.
Keywords: thermal performance; heat exchanger; hybrid; nanofluid; nanoparticle 1. Introduction Many researchers have been investigating about the heat transfer in the industry for many decades in which they have proposed various methods including using curved pipes. Another way to improve the heat transfer is to use fluids of transferring heat namely water, ethylene glycol and oil. In order to enhance the heat transfer and overcome the
1
limited capability of conventional fluids including water, ethylene glycol and oil, the researchers have proposed to use advanced fluids with higher thermal conductivity instead of conventional fluids. For the first time Maxwell [1873] [1], studying the performance
of
suspended
particles,
indicated
that
the
dispersion
of
millimeter/micrometer-sized particles in the base fluids leads to the improvement of the heat transfer. However, adding these particles results in some problems including erosion of the components due to the abrasion caused by the particles, clogging of narrow passages, settling of the particles and increased pressure drop. After several decades for the first time Choi et al. 1995 [2] at Argonne National Laboratory introducing nanofluid could tackle these problems. Eastman et al. (2001) [3] have also studied the effect of nanofluid thermal conductivity. They have shown that ethylene glycol-based copper nanofluid has a higher thermal conductivity in comparison with oxide form of particles. A 40 % increase in thermal conductivity due to existence of the nanofluid (containing ethylene glycol and 0.3% by volume of copper with a diameter less than 10 nm) has been observed. Zeinali et al. (2006) [4, 5] have also investigated the effects of aluminum oxide and copper oxide nanofluid at laminar flow and constant wall temperature conditions. They have concluded that an increase in nanoparticle concentration improves the heat transfer coefficient. They have also shown that aluminum oxide nanofluid gives better results in comparison with copper oxide regarding convective heat transfer. Duangthongsuk and Wongwises, (2010) [6] have studied the heat transfer and pressure drop
performance of water-based titanium dioxide nanofluid in turbulent flow conditions and shown that with increased Reynolds number and particle concentration, heat transfer coefficient increases by 26%. Sundar et al. (2010) [7] have conducted experimental
2
studies on the turbulent heat transfer and friction coefficient in the presence of alumina nanofluid (in a circular tubes with twisted tape inserts) and reported a 33.5% increase in heat transfer coefficient and a 1.096% increase in friction coefficient. Suresh et al. (2011) [8] have investigated the thermal performance of alumina and copper oxide nanofluid using the twisted tape inserts in tube under constant heat flux and laminar flow conditions. They have concluded that under constant thermal conditions in the twisted tape, copper oxide nanofluid shows better performance as compared to the alumina nanofluid. In addition, copper oxide nanofluid imposes higher pressure drop as compared to the alumina nanofluid and the use of twisted tape increases this pressure drop to a greater extent. Suresh et al. (2012) [9,10] have experimentally studied heat transfer characteristics and friction coefficient of water-based copper oxide and alumina nanofluid under laminar flow and constant heat flux conditions and concluded that increased concentration of nanoparticles in the solution increased Nusselt number (Nu) and friction coefficient. Additionally, the use of twisted tape instead of straight tubes significantly increases the Nusselt number. Tajik et al. (2013) [11] have investigated the heat transfer and pressure drop characteristics of water-based copper and aluminum nanofluid in spiral coils at constant wall temperature and laminar flow conditions and found that the thermal conductivity of copper nanofluid is about 18% higher than that of the aluminum nanofluid. They have also shown that an increase in the nanoparticle concentration leads to an increase in the thermal conductivity and pressure drop. Sajjadi et al. (2011) [12] have experimentally studied turbulent convective heat transfer and pressure drop of titanium oxide nanofluid in circular tubes under constant wall
3
temperature and concluded that the increased concentration of nanoparticles had no significant effect on the rate of heat transfer and pressure drop. Fakoor et al. (2012) [13] have conducted an experimental study on the pressure drop due to the flow of water-based carbon nanotubes in helical coils at laminar flow and constant wall temperature conditions. They have shown that pressure drop increased with increased nanoparticle concentration and Reynolds number, while pressure drop decrease occured with increased coil diameter with regard to tube diameter. Furthermore, the dependence of pressure drop on the step length was negligible, and they reported increased pressure drop by 3.5 times with the concentration of 0.45% compared to the base fluid in the regular tube. Although a lot of researches have been conducted on single-step nanofluids, but there are very limited information on the performance of nanocomposites. Synthesis and preparation of nanocomposites have been recently considered due to their special attraction. Nanocomposite generally refers to a nanofluid composed of two different materials with nanoscale particles. Suresh et al. (2012) [14] have studied the effect of water-based alumina-copper hybrid nanofluid on the heat transfer rate and pressure drop at laminar flow and constant heat flux conditions. The results indicate that at the 0.1% volume concentration of nanoparticles, the maximum increase in heat transfer rate and pressure drop are 13.6% and 16.97% respectively. Baghbanzadeh et al. (2012) [15] have studied the thermal conductivity of silica-nanotube hybrid nanofluid and shown that the thermal conductivity of multi-walled nanotubes nanofluid and silica nanofluid are about 23.3% and 8.85% more than the base fluid, respectively. The most and the least enhancement in the effective thermal conductivity of
4
the fluids were associated with MWCNTs (23.3%) and silica nanospheres (8.8%), while the enhancement for the hybrid nanomaterial was a value between MWCNT and silica nanoparticles. Furthermore, the hybrid consisting of higher percentage of MWCNTs showed more increase in effective thermal conductivity of the nanofluid, compared with the other hybrid. Sandar et al. (2014) [16] have also conducted an experimental investigation on heat transfer coefficient and friction coefficient at the conditions of fully developed turbulent flow and constant flux and reported 9.35% and 22.62% increase in heat transfer rate and friction coefficient respectively. In the present work, the thermal performance and pressure drop in a helical coil under laminar flow and constant wall temperature conditions employing a hybrid nanofluid (prepared by using alumina-silver nanocomposite) are experimentally investigated and the obtained results are compared to those of a single-step nanofluid (alumina).
2. Experimental setup and procedure The schematic diagram of the employed setup is shown in Fig. 1. The system is kept at isothermal operating condition by using a PID control system. The tank is fully insulated with rock wool in order to avoid heat loss and a 2 kW heater is immersed in the tank to supply the required heat. For measuring of inlet and outlet pressures, very sensitive pressure transmitters (model PSCH 0.5BCIA SENSYS) have been employed. Two accurate T- type thermocouples are inserted so as to measure the inlet and outlet temperature of the fluid in the tube. Furthermore, six K- type thermocouples are installed at different positions to measure the surface temperature distribution of the tested part (tube). The flow rate is estimated by an ultrasonic system with an accuracy of 0.05 l/m. The physical characteristics of the copper coil are shown in Table (1). In this work a 5
water based single-step nanofluid containing alumina with an average diameter of 55 nm and a water-based hybrid nanoparticle containing alumina-sliver with an average diameter of 80 nm composed of 97.5% alumina and 2.5% silver (purchased from Nano Pooshesh Felez Co) have been employed, both of which have spherical shape. There are numerous ways to prepare nanoparticles, one of which is Sol-gel method. One of the advantages of this method is to prepare nancomposites with high purity. At first a homogenous suspension including solvent and precursor (which is going to form the final solution) are solved and then the homogenous solution is turned into Sol by hydrolysis. After provoking the particles in Sol by HCL and NaOH, they join together and form a wet gel. After separating the solution and drying it, the nanoparticles are formed. The produced nanoparticles are then dispersed in the base fluid by using an ultrasonic device for 30 minute. It is found that the prepared nanofluid preserves its stability for 48 hours. Fig. 2 shows that the TEM (transmission electron microscopy)
images of nanoparticles dispersed in the distilled water. The cooling system comprises cooling urban water as well as a fan for increasing the cooling system efficiency. After preparing and setting up all the equipments, the device is fully calibrated with pure water and then the main experiments are carried out with nanofluid at different concentrations. The experiments are started by turning on the heater, pump and cooling system simultaneously and the flow is controlled using the return system embedded after the pump. As soon as the tank temperature reaches saturation (constant temperature of 95°C) point, the pressure, inlet and outlet temperatures, and the surface temperature are recorded via a data logger system. The experiments have been performed at least twice to fully ensure the data accuracy. 3. Modeling section
6
The thermophysical characteristics of the nanofluid are calculated according to the following equations [17- 19]:
nf (1 ). f . P nf
(1)
f (1 ) 2.5
(2)
( .C P ) nf (1 ).( .C P ) f .( .C p ) P
(3)
Thermal conductivity is also calculated using the following equation [20]:
k nf kf
k P 2k f 2(k f k P ).
(4)
k P 2k f (k f k P ).
Since the fluid passes through a pipe at a constant temperature, the convective heat transfer coefficient and Nusselt number at thermal equilibrium is as follows: .
h(exp)
m.c p .( Tb1 Tb2 )
(5)
A.( Tw Tb )M
h(exp).d Nu (exp) k
(6)
Which TW Tb M is a logarithmic temperature difference. Several important parameters required for description of fluid flow through the coil include:
Re .U .d /
(7)
Pr c p . / k
(8)
De Re .(d / D)1 / 2
(9)
7
The theoretical equations for predicting the Nusselt number in straight and helical tubes are estimated by equations (10) and (11) [21, 22]:
Nu S 3.66
((d / L). Re . Pr).0.0668 1 0.04((d / L). Re . Pr) 2 / 3
(10)
Nu C 0.13.( f . Re 2 . Pr)1 / 3
(11)
Equation (12) can be used for determination of pressure drop in the straight tube where for laminar flow inside a tube, the theoretical values of Darcy friction factor (f) is evaluated by [21]: PS f S.( L / d )..U 2
f S ,la min ar
(12)
64 Re
(13)
After replacing f c with f S and also Lc with L, the above equation can be used to estimate the pressure drop in the coil. The following equation can be used for predicting friction coefficient in the coil [23]:
fC 0.47136.De1 / 4 fS
(14)
The criterion for transition from laminar to turbulent region is specified by the following equation [24]:
Re cri
12730
(15)
3.2
In this article, the critical Reynolds number is equivalent to 5600 such that for Re<5500 the laminar flow prevails.
8
4. Results and discussion The experiments have been performed at constant wall temperature and laminar flow conditions in a helical coil. The compositions of aluminum and silver in the employed nanocomposite are 97.5 % and 2.5 % respectively and the nanofluid concentrations are in the range of 0.1-0.4 vol%. In Fig. 3, the Nusselt number of nanofluid versus Reynolds number in helical coil at different concentrations of nanofluid is shown. The presented results clearly show that an increase in Nusselt number is accompanied by increasing concentration of nanoparticles and Reynolds number. It should be noted that the distribution and the proportional movement of nanoparticles near to the wall of the pipe due to Brownian movement and particles migration lead to a fast increase in heat transfer from the wall of the pipe to nanoparticles. Thus, an increase in concentration of particles leads to an increase in the heat transfer rate. The employed helical coil causes a centrifugal force and secondary flow such that a further dispersion of nanoparticles in the base fluid leads to a decrease in thickness of the boundary layer. Thus an increase in the heat exchange between the nanoparticles and the wall of the pipe enhances the Nusselt number. At a high flow rate, the effects of dispersion of nanoparticles in the base fluid will intensify the vacillation and consequently the heat transfer is increased. In general, the increase of heat transfer rate in the presence of hybrid nanofluid in helical coil can be attributed to the increasing rate of Brownian movement of nanoparticles, migration of the nanoparticles and reduction of thickness of boundary layer. According to Fig. 3 the employed hybrid nanofluid in this work leads to a higher Nusselt number at different operating conditions.
9
According to Fig. 4, the hybrid nanofluid shows a greater increase in heat transfer than single-step nanofluid. When the hybrid nanofluid is employed, a maximum increase of Nusselt number (~31.58%) with respect to pure water at the concentration of 0.4 vol% and Reynolds number 4687 is observed which is higher than that of the single-step nanofluid (~28.42%) at the same operating condition. With increasing nanoparticle contents and Reynolds number, physical properties of nanofluid (Prandtl number) undergo change too. As it is seen since in this figure the ratio of heat transfer is depicted only versus Reynolds number, some trends which are almost non-uniform trends are observed over experimental data because it was not taken into consideration in this figure. At lower Reynolds number, thermal conductivity is the main reason for heat transfer coefficient and consequently it leads to the increase in the heat transfer. With increasing Reynolds number, the importance of thermal conductivity in enhancement of heat transfer becomes less considerable. [25, 26]. On the other hand, the importance of other mechanisms including Brownian movement of nanoparticles, migration of the nanoparticles and reduction of thickness of boundary layer becomes more tangible in the increase in heat transfer.[27-29], In addition, at higher rate of Reynolds number the effects of dispersion and turbulent movement leads to increase in fluctuations resulting in increase in the heat transfer. This trend continues until the heat transfer of nanofluid reaches the maximum enhancement ratio in Reynolds number [4678] compared to base fluids. Then, with the increase in Reynolds number, the relative decrease is experienced. Since with the increase in Reynolds number, the collision of nanoparticles increases and it is likely to be aggregated which relatively decreases in the increase trend of heat transfer. However at higher Reynolds numbers, the advantage of using the nanofluid
10
gradually diminishes such that the Nu number in the presence of nanoparticles approaches to that of pure water at very high turbulent conditions as shown in Fig. 4 [30, 31]. For studying the thermal performance of industrial units, measuring pressure drop of nanofluid is necessary. In Fig. 5, the pressure drop in the presence of nanofluid is shown at different concentrations. The pressure drop is increased as the Reynolds number and concentration of particles are increased. This can be attributed to the increase of the nanofluid viscosity and density. The Brownian movement of particles and dispersion and vacillations of nanoparticles leads to increase the momentum transfer between the particles especially near the wall of the pipe such that the axial pressure drop is increased. It should be noted that an increase in pressure drop in the presence of hybrid nanofluid may be due to the formation of particle agglomerates and clusters which affect the suspension viscosity [32, 33]. The thermal performance factor is defined according to the following equation [34]:
Nu C ,nf
Nu S , f PC ,nf 0.1666 ( ) PS , f
(17)
The thermal performance factor of nanofluid is calculated at constant wall temperature and laminar flow in the helical coil with respect to the straight tube. As shown in Fig. 6, the performance factor is always greater than one. This means that by using the hybrid nanofluid in the helical, the thermal performance of the system improves significantly. As
11
shown in this figure, the maximum thermal performance factor for hybrid nanofluid at the nanoparticle concentration of 0.4 vol% and Re= 4687 is about 2.55.
5. Conclusion In the present work, the thermal performance and pressure drop in a helical coil under laminar flow and constant wall temperature conditions employing a hybrid nanofluid (prepared by using alumina-silver nanocomposite) are experimentally investigated and the obtained results are compared to those of a single-step nanofluid (alumina) and pure water. The maximum rate of heat transfer can be obtained by using the hybrid nanofluid at a concentration of 0.4 vol% which is 31.58% higher than that of the distilled water. Overall, the maximum thermal performance factor for hybrid nanofluid is about 2.55 suggesting the superior performance of the proposed hybrid nanocomposite for enhancing the thermal performance of heat exchangers in chemical and petroleum industries.
Nomenclature
A
inner surface of tube (m2)
b
Pitch of coil (m)
Cp
Specific heat (J. kg−1 .K−1)
d
Inside diameter of tube (m)
D
Diameter of coil (m)
De
Dean number
f
Fanning friction factor
h
Heat transfer coefficient (W. m−2 .K−1)
He
Helical coil number
K
Thermal conductivity (W .m−1 .K−1)
L
Length of tube (m) 12
m
Mass flow rate (kg .s−1)
N
Number of coil turns
Nu
Average Nusselt number
Pr
Prandtl number
Re
Reynolds number
T
Temperature (K)
U
Average velocity (m. s−1)
Greek letters ΔP
Axial pressure drop (Pa)
η
Thermal performance factor
ρ
Density (kg. m−3)
μ
Dynamic viscosity (Pa. s)
λ
Curvature ratio (=D/d)
φ
Nanoparticle volume fraction (%)
Subscripts C
Coiled tube
exp
exprimental
f
Base fluid
nf
Nanofluid
p
Particle
S
Straight tube
th
theoretical
w
Wall
References [1] J.C.Maxwell. Electricity and magnetism. Clarendon Press: Oxford; 1873. [2] S.U.S. Choi, Enhancing thermal conductivity of fluids with nanoparticle, ASME FED. 231 (1995) 99–105.
13
[3] J.A. Eastman, S.U.S. Choi, S. Li, W. Yu, L.J. Thompson, Anomalously increased effective thermal conductivities of ethylene glycol-based nanofluids containing copper nanoparticles, Applied Physics Letters. 78 (2001) 718–720. [4] S. Zeinali Heris, M. Nasr Esfahany, S.G. Etemad, Experimental investigation of convective heat transfer of Al2O3/water nanofluid in circular tube, Int. J. Heat Fluid Flow. 28 (2) (2007) 203–210. [5] S. Zeinali Heris, S.G. Etemad, M. Nasr Esfahany, Experimental investigation of oxide nanofluids laminar flow convective heat transfer, International Communications in Heat and Mass Transfer. 33 (2006) 529–535. [6] W. Duangthongsuk, S. Wongwises, An experimental study on the heat transfer performance and pressure drop of TiO2–water nanofluids flowing under a turbulent flow regime, Int. J. Heat Mass Trans. 53 (2010) 334–344. [7] L. Syam Sundar, K.V. Sharma, Turbulent heat transfer and friction factor of Al2O3 nanofluid in circular tube with twisted tape inserts, Int. Commun. Heat Mass Transf. 53 (2010) 1409–1416. [8] S. Suresh, K.P. Venkitaraj, P. Selvakumar, Comparative study on thermal performance of helical screw tape inserts in laminar flow using Al2O3/water and CuO/water nanofluids, Superlattice Microst. 49 (2011) 608–622. [9] S. Suresh, M. Chandrasekar, P. Selvakumar, Experimental studies on heat transfer and friction factor characteristics of CuO/water nanofluid underlaminar flow in a helically dimpled tube, Heat and Mass Transfer. 48 (4) (2012) 683–694. [10] S. Suresh, K.P. Venkitaraj, P. Selvakumar, M. Chandrasekar, A comparison of thermal characteristics of Al2O3/water and CuO/water nanofluids in transition flow through a straight circular duct fitted with helical screw tape inserts, Experimental Thermal and Fluid Science. 39 (2012) 37–44. [11] M. Tajik Jamal-Abad, A.H. Zamzamian, M Dehghan, Experimental studies on the heat transfer and pressure drop characteristics of Cu–water and Al–water Nanofluids in a spiral coil, Experimental Thermal and Fluid Science. 47(2013) 206–212. [12] A.R. Sajadi, M.H. Kazemi, Investigation of turbulent convective heat transfer and pressure drop of TiO2/water nanofluid in circular tube, Int. Commun. Heat & Mass Transfer. 38 (10) (2011) 1474–1478. [13] M. Fakoor-Pakdaman, M.A. Akhavan-Behabadi, P. Razi, An empirical study on the pressure drop characteristics of nanofluid flow inside helically coiled tubes, International Journal of Thermal Sciences. 65 (2013) 206-213.
14
[14] S. Suresh, K.P. Venkitaraj, P. Selvakumar, M. Chandrasekar, Effect of Al2O3–Cu/water hybrid nanofluid in heat transfer, Exp. Thermal Fluid Sci. 38 (2012) 54–60. [15] M.A. Baghbanzadeh, A. Rashidi, D. Rashtchian, R. Lotfi, A. Amrollahi, Synthesis of spherical silica/multiwall carbon nanotubes hybrid nanostructures and investigation of thermal conductivity of related nanofluids, Thermochimica Acta. 549 (2012) 87– 94. [16] L. Syam Sundar, Singh, Manoj K. Singh, Antonio C.M. Sousa, Enhanced heat transfer and friction factor of MWCNT–Fe3O4/water hybrid nanofluids, International Communications in Heat and Mass Transfer. 52 (2014) 73–83. [17] B.C. Pak, Y. Cho, Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particle, Exp. Heat Transfer. 11 (1998) 151–170. [18] Y. Xuan, W. Roetzel, Conceptions for heat transfer correlation of nanofluids, Int. J. Heat and Mass Transfer. 43 (2000) 3701-3707. [19] H.C. Brinkman, The viscosity of concentrated suspensions and solution, J. Chem. Phys. 20 (1952) 571. [20] F.J. Wasp, Solid–liquid slurry pipeline transportation, Trans. Tech, Berlin, 1977. [21] F.P. Incropera, D.P. De Witt, Fundamentals of heat and mass transfer, fourth ed. John Wiley, New York, 1996. [22] R.A. SEBAN, E.F. McLAUGHLINt, Heat transfer in tube coil with laminar and turbulent flow, Heat and Mass Transfer. 6 (1962) 387-395. [23] M. Van Dyke, Extended Stokes series: laminar flow through a loosely coiled pipes, Journal of Fluid Mechanics. 86 (1978) 129–145. [24] V. Kubair, C.B.S. Varrier, Pressure drop for liquid flow in helical coils, Transactions of the Indian Institute of Chemical Engineers. 14 (1961) 93. [25] D. Wen, Y. Ding, Experimental investigation into convective heat transfer of nanofluid at the entrance region under laminar flow conditions, International Journal of Heat and Mass Transfer. 47 (24) (2004) 5181–5188. [26] H. Chen, W. Yang, Y. He, Y. Ding, L. Zhang, C. Tan, A.A. Lapkin, D.V. Bavykin, Heat transfer behaviour of aqueous suspensions of titanate nanofluids, Powder Technology. 183 (2008) 63–72. [27] A.A. Abbasian Arani, J. Amani, Experimental investigation of diameter effect on heat transfer performance and pressure drop of TiO2-water nanofluid, Experimental Thermal and Fluid Science. 44 (2013) 520–533.
15
[28] M. Chandrasekar, S. Suresh, A. Chandra Bose, Experimental studies on heat transfer and friction factor characteristics of Al2O3/water nanofluid in a circular pipe under laminar flow with wire coil inserts, Experimental Thermal and Fluid Science. 34 (2010) 122–130. [29] S. Suresh, M. Chandrasekar, S. Chandra Sekhar, Experimental studies on heat transfer and friction factor characteristics of CuO/water nanofluid under turbulent flow in a helically dimpled tube, Exp. Therm. Fluid Sci. 35 (2011) 542–549. [30] S. ZeinaliHeris, S.H. Noie, E. Talaii, J. Sargolzaei, Numerical investigation of Al2O3/water nanofluid laminar convective heat transfer through triangular ducts, Nanoscale Research Letters. 6 (2011) 179. [31] M. Kahani, S. Zeinali Heris , S.M. Mousavi, Comparative study between metal oxide nanopowders on thermal characteristics of nanofluid flow through helical coils, Powder Technology. 246 (2013) 82–92. [32] Y. Xuan, Q. Li, Investigation on convective heat transfer and flow features of nanofluids, J. Heat Transfer. 125 (1) (2003) 151–155. [33] U. Rea, T. McKrell, L.W. Hu, J. Buongiorno, Laminar convective heat transfer and viscous pressure loss of alumina–water and zirconia–water nanofluids, Int. J. Heat Mass Transfer. 52 (2009) 2042–2048. [34] H. Usui, Y. Sano, K. Iby a combination of internally grooved rough tube and twisted tape, International Chemicawashita, A. Isozaki, Enhancement of heat transfer l Engineering. 26 (1) (1996) 97–104.
16
Table 1: Physical Properties of the Helical Coil (mm)
Tube
d
t
L
D
λ
N
Helical Coil
5
1
2600
65
15
10
Table 2. Thermophysical characterizations of nanopowders
Particle/base
Average diameter
Purity
Actual density
CP
k
fluid
(nm)
(%)
(kg/m3)
(J/kg·K)
(W/m·K)
Al2O3
55
99
3690
780
30.5
Ag
25
99
9320
233
429
nanocomposite
80
99
3830.75
766
41
Table 3. Uncertainty of measurement instruments
Parameter
Instrument
uncertainty
Fluid flow rate
Flownetix® 100seriesTM
±1% of reading
Steam temperature sensors
PT-100Ω thermo resistance
0.1K
Wall temperature sensors
K-type Omega Thermocouples
0.1K
Inlet/outlet temperature sensors
PT-100Ω thermo resistance
0.1K
Pressure transmitters
Keller, Series 35X-Bullet type
±1% of reading
17
Fig. 1 The Device Used in the Experiment
18
Fig. 2 TEM images of hybrid nanoparticles.
19
24 distillated water 0.1 % hybrid 0.2 % hybrid 0.4 % Alumina/water 0.4 % hybrid
22 20
Nu
18 16 14 12 10 8 500
1000
1500
2000
2500
3000
3500
4000
4500
5000
5500
Re
Fig. 3 Nusselt Number versus Reynolds Number at different concentrations of nanofluids
20
0.1 % hybrid 0.2 % hybrid 0.4 % Alumina/water 0.4 % hybrid
1.32
Nu (nf)/Nu(water)
1.28 1.24 1.2 1.16 1.12 1.08 1.04 1 500
1000
1500
2000
2500
3000
3500
4000
4500
5000
5500
Re Fig. 4 Normalized Reynolds Number in the helical coil at different concentrations of nanofluids
21
16 Disstilat water 0.1 % hybrid 0.2 % hybrid 0.4 % Alumina/water 0.4 % hybrid
14 12
∆p
10 8 6 4 2 500
1000
1500
2000
2500
3000
3500
4000
4500
5000
5500
Re Fig. 5 The pressure drop versus Reynolds Number at different concentrations of nanofluids
22
2.6 distillated water 0.1 % hybrid 0.2 % hybrid 0.4 % Alumina/water 0.4 % hybrid
2.5 2.4
Ƞ
2.3 2.2 2.1 2 1.9 1.8 500
1000
1500
2000
2500
3000
3500
4000
4500
5000
5500
Re Fig. 6 Performance factor versus Reynolds number employing hybrid and single-step nanofluids
23
Highlights
Thermal performance in a helical coil employing a hybrid nanofluid are measured. The alumina-silver nanocomposite is used for nanofluid preparation. The hybrid nanofluid at a concentration of 0.4 vol% gives the best result. There is a 31.58% increase in heat transfer rate with respect to distilled water. A 25% increase in thermal performance as compared to that of DI water is observed.
24