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Experimental investigation on heat transfer characteristics of various nanofluids in an indoor electric heater
Renewable Energy 147 (2020) 1011e1018
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Experimental investigation on heat transfer characteristics of various nanofluids in an indoor electric heater n c Zhanxiu Chen a, Dan Zheng a, Jin Wang a, *, Lei Chen b, Bengt Sunde a
School of Energy and Environmental Engineering, Hebei University of Technology, Tianjin, 300401, China Key Laboratory of Thermo-Fluid Science and Engineering (Ministry of Education), Xi'an Jiaotong University, Xi'an, 710049, China c Department of Energy Sciences, Division of Heat Transfer, Lund University, P.O. Box, 118, SE-22100, Lund, Sweden b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 June 2019 Received in revised form 26 August 2019 Accepted 12 September 2019 Available online 12 September 2019
Heat transfer characteristics of an electric heater were experimentally investigated by using various fluids in this paper, including Cu-EGW (a mixture of ethylene glycol and DI-water), Al2O3-EGW, Fe3O4EGW nanofluids. A 4:6 mixture of ethylene glycol and deionized water was used as the base liquid. All these nanofluids were prepared by ultrasonic treatment, and nanoparticle mass concentration of samples varies from 0.5% to 2%. In addition, natural convective heat transfer of Fe3O4-EGW nanofluid in an electric heater was carried out by considering an effect of different magnetic fields. The results indicated that heat transfer performance of Cu-EGW nanofluid was significantly higher than the Al2O3-EGW and Fe3O4EGW nanofluids, and the heating efficiency of the Cu-EGW nanofluid increased with the mass concentration of Cu particles. Compared with that of the base fluid, equilibrium temperature values of electric heaters filled with 2.0% Cu-EGW, 1.0% Al2O3-EGW and 1.0% Fe3O4-EGW nanofluids increase by 13.18%, 3.77% and 4.52%, respectively. It was also found that the magnetic field had a positive effect on the heat transfer enhancement of the Fe3O4-EGW nanofluid. In addition, for the 0.5% Fe3O4 nanofluid under a magnetic intensity of 100 mT, the equilibrium temperature on the middle fin increases by 14.68%. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Electric heater Nanofluid Magnetic field Natural convection Equilibrium temperature
1. Introduction In recent years, traditional fluids applied in heat exchanges cannot meet the increasing demand for the heat dissipation of equipment. Considering that nanoparticle materials with higher thermal conductivity have received an extensive attention, various nanofluids with improved thermophysical properties are involved in many research fields, such as microelectronics, energy utilization, chemical engineering, vehicles, etc. By uniform dispersion of nano-scale metal or non-metal particles into the base fluid, the nanofluid as a two-phase medium is prepared to provide an enhancement of heat transfer [1]. Thermophysical properties of a nanofluid are related to many factors, such as material, concentration and size of nanoparticles. Ahmadi et al. [2] reviewed effects of temperature, concentration and shape of particles on thermal conductivity of nanofluids. It was
* Corresponding author. E-mail addresses: [email protected] (Z. Chen), zhengdan2018hebut@ 163.com (D. Zheng), [email protected] (J. Wang), [email protected] n). (L. Chen), [email protected] (B. Sunde https://doi.org/10.1016/j.renene.2019.09.036 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
concluded that thermal conductivity of nanofluids increased with the temperature and concentration. Sajid and Ali [3] summarized studies on thermal conductivity of hybrid nanofluids in recent years. They pointed out that thermal conductivity was affected by several crucial factors, including types of base fluid, concentration of nanoparticles, addition of surfactant, etc. Adding nanoparticles to a base fluid not only improves the thermal conductivity of the fluid, but also increases the viscosity of [4] reviewed effects of various the solution. Murshed and Estelle factors (such as concentration of nanoparticles and temperature) on the nanofluid viscosity. They found that it was necessary to develop a uniform and standardized procedure for preparation of nanofluids. Khodabandeh et al. [5] numerically investigated heat transfer enhancement of water-graphene/platinum hybrid nanofluids in a horizontal spiral coil. They found that the maximum Nusselt number was obtained for the coil with the elliptic cross section. Sorour et al. [6] experimentally studied heat transfer between a vertical free surface jet and a horizontal stainless steel heated plate by using water-SiO2 nanofluids with various volume fractions (0%4 8.5%). They found that the average Nusselt number for a case with volume fraction of 8.5% increased up to 80%
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compared to pure water. Nanofluids have been extensively studied in the field of solar energy. Loni et al. [7] experimentally conducted a comparison of heat transfer performances of nanofluids (Al2O3thermal oil and SiO2-thermal oil) in a cylindrical cavity. Compared with the SiO2-oil nanofluid and pure thermal oil, the cylindrical cavity receiver had the highest average thermal efficiency. Zeiny et al. [8] comparatively studied the vaporisation through direct absorption solar collectors with gold-water nanofluid and carbon black-water nanofluid. They found that carbon black nanofluids were more suitable for solar evaporation applications than gold nanofluids due to the high cost in the production process using gold nanofluids. They concluded that the increase of nanoparticle concentration or incident solar radiation would lead to a higher evaporation rate. Xu et al. [9] investigated the heat transfer characteristics of graphene oxide/water-ethylene glycol nanofluid in direct absorption solar collectors. They found that the receiver efficiency of this nanofluid increased by 70% compared with the base fluid with the same exposure time of 6000 s and a solar intensity of 1000 W/m2. Tong et al. [10] experimentally conducted comparisons of energy efficiency and entropy generation in a flat-plate solar collector by using various nanofluids. They reported that the highest efficiency of 77.5% was obtained when 1.0% Al2O3 nanofluid was used. Hassan et al. [11] used graphene/water nanofluid and phase change material RT-35HC to regulate temperature values of photovoltaic (PV) modules. It was concluded that the maximum reduction of the PV temperature is 29.3 C at 0.1% volume concentration of graphene nanoparticles and 40 LPM flow rate. At the same working condition, the thermal efficiency increased by 20.8% compared with water. Kiseev and Sazhin [12] studied the heat transfer characteristics of a loop thermosyphon filled with Fe2O3water nanofluid with mass concentrations of 0.5%, 1.0%, 1.5% and 2.0%. They showed that the Fe2O3-water nanofluid could enhance the heat transfer of the loop thermosyphon, and the heat transfer coefficient could be increased by 20%e25% when using 2.0% Fe2O3water nanofluid. A valuable way to improve the heat dissipation performance of the radiator is to develop new-type high-efficient heat transfer fluids [13]. Islam et al. [14] employed 50:50 water-EG nanofluid with ZnO nanoparticles to cool a Proton Exchange Membrane Fuel Cell (PEMFC). They found that for the ZnO nanofluid with volume fraction of 0.5 vol% as PEMFC coolant, the radiator size decreased by almost 27% compared to 50/50 water-EG rdenas Contreras et al. [15] experiwithout ZnO nanoparticles. Ca mentally analyzed heat transfer performance of an automotive cooling system and used graphene and silver nanofluids as automotive coolants. They found that heat transfer rate had an increase of 3.3% when 0.1 vol % graphene nanofluid was used at an inlet temperature of 85 C. Soylu et al. [16] conducted experimental research of heat transfer enhancement in an automobile radiator by using various EG/water-based hybrid nanofluids. Results indicated that the maximum enhancement ratio of the overall heat transfer coefficient was 11.094% for the nanofluid with 2% TiO2 and 0.3% Ag nanoparticles at a flow rate of 19 L/min. Ahammed et al. [17] investigated a multiport-minichannel heat exchanger with aluminum oxide-water nanofluid to remove heat from electronic devices. It was observed that the performance coefficient of the thermoelectric module was significantly improved by 40% when 0.2 vol% Al2O3-water was used. Kumar and Kumar [18] numerically studied heat transfer performance of Al2O3/water nanofluids based on an electronic chip with a microchannel heat sink. Compared to water as coolant, 70% reliability enhancement of the electronic chip was obtained by using 0.75 vol% Al2O3/water nanofluid. Most of the earlier research works are related to heat transfer characteristics of non-magnetic nanofluids, whereas magnetic nanofluids (ferrofluids) are a functional material with both flow characteristics and magnetic properties. Sundar et al. [19]
measured thermal conductivity of an Fe3O4 nanofluid by the transient hot wire method. A thermal conductivity enhancement of 46% for the 2% Fe3O4 nanofluid was observed compared to the base fluid (2:8 ethylene glycol and water mixture) at 60 C. They also found that for the Fe3O4 nanofluid under the same volume concentration and temperature, the viscosity showed a greater enhancement than the thermal conductivity [20]. Toghraie et al. [21] investigated viscosities of Fe3O4-water nanofluids with different volume fractions (0.1%, 0.2%, 0.4%, 1%, 2% and 3%) in a temperature range of 20e55 C. They found that the maximum viscosity enhancement was 129.7% by calculation of viscosity ratios. Khosravi et al. [22] studied effects of magnetic field (0e500 G) and particle concentration (1e4 vol%) on the heat transfer coefficient, and they found that the 4% ferrofluid under a magnetic field of 500 G showed the best thermal performance. Wang et al. [23] conducted an investigation of laminar convective heat transfer inside a pipe filled with ferrofluids under various external magnetic fields. Results indicated that compared to the ferrofluid without magnetic field, a heat transfer enhancement of 261% was obtained at Re ¼ 805 when five adjacent magnetic cannulas were used. Sha et al. [24] experimentally studied convective heat transfer of Fe3O4- water nanofluids under a parallel constant magnetic field. The experimental results showed that the parallel constant magnetic field resulted in a reduction of the heat transfer coefficient for the Fe3O4-water nanofluid. The effect of a vertical magnetic field on the convective heat transfer of Fe3O4-water nanofluid was also studied in Ref. [25]. It was found that the Fe3O4-water nanofluid under a gradient magnetic field of 800 G and at an inlet temperature of 40 C had a 8.1% enhancement of the convective heat transfer coefficient. Mehrali et al. [26] investigated heat transfer characteristics and entropy generation of a hybrid graphene-Fe3O4 nanofluid under a magnetic field. The results showed that the local convective heat transfer coefficient increased by 4% and the total entropy production decreased by 41% compared with water. Heat transfer characteristics of AgeMgO/water hybrid nanofluid in a channel with active heaters and coolers were numerically studied by considering the effect of a magnetic field in Ref. [27]. Results showed that the heat transfer rate decreased with the increase of Hartmann number. Huminic and Huminic [28] carried out an experimental study of heat transfer performances and entropy generation in a flattened tube with both MWCNT/Fe3O4-water and ND/Fe3O4-water hybrid nanofluids. According to their experimental results, for 0.3 vol% MWCNT/Fe3O4-water hybrid nanofluid, the heat transfer coefficient increased by 20.607% at Re ¼ 250, and the total entropy production decreased by 26.487% compared to water at 333 K. Sheikholeslami et al. [29] analyzed natural convection of a hybrid nanofluid (MWCNT-Fe3O4/H2O) inside a circular cavity with two circular heaters under a non-uniform magnetic field. Results revealed that an increment in Hartmann number increased the boundary layer thickness which weakened the natural convection. In the above relevant literatures, it is revealed that nanofluid as a new functional material can effectively enhance the heat transfer of the base fluid. A review on the relevant literature shows that although some researchers have investigated heat transfer characteristics of various nanofluids in heating equipment, there is no study for indoor electric heaters. The electric heater is one kind of heating equipment used in many industries due to its advantages (low operating costs and mobility heating). This paper will investigate heat transfer characteristics of various nanofluids inside an electric heater, which will improve both the heating efficiency and building thermal comfort. Compared with a 4:6 mixture of ethylene glycol and DI-water as the base fluid, the effects of nanoparticles Cu, Al2O3 and Fe3O4 on the heat transfer enhancement are analyzed in the present paper. In addition, heat transfer characteristics of the Fe3O4 nanofluid was also analyzed by a comparison of results with
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Table 1 Thermophysical properties of various nanoparticles and base fluid (25 C). Materials
Grain size (nm)
Density (kg/m3)
Specific heat (J/kg.K)
Thermal conductivity (W/m.K)
Al2O3 Fe3O4 Cu deionized water ethylene glycol
30 20 30 / /
3600 5180 8960 4180 1113.4
765 670 385 997 2350
25.08 [30] 80 401 [30] 0.613 0.224
and without a magnetic field. 2. Experimental investigation 2.1. Preparation of nanofluids In this research, nanofluids are synthesized by a two-step method. Nanoparticles (Cu, Fe3O4, and Al2O3) are dispersed into a 4:6 mixture of ethylene glycol and DI-water with specified mass fractions (0.5 wt%, 1.0 wt%, 1.5 wt%, and 2.0 wt%). The mass of the nanofluid is calculated by Eq. (1) and then the mass fraction of nanofluids can be calculated by Eq. (2).
W ¼ ry 4¼
(1)
u
(2)
WDIwater þ WEG þ u
where W, r, n, u and 4 represent fluid mass, fluid density, fluid volume, nanoparticle weight and mass fraction, respectively. WDIwater, WEG are masses of deionized water and ethylene glycol, respectively. All the nanoparticles were purchased by Beijing Deke Daojin Company of China. Ethylene glycol and deionized water were supplied by Yitian Company of China. Grain size, density and specific heat of nanoparticles were supplied by Deke Daojin Company of China. Thermal conductivities of Al2O3 and Cu nanoparticles are obtained through related literature, and the average values of results are listed in Table 1. In order to obtain a well dispersed nanofluid, many influential factors for stability of nanofluids were investigated, including surfactant (cetyl trimethyl ammonium bromide, gum arabic, sodium hexametaphosphate, trisodium citrate, sodium dodecyl benzene sulfonate), pH value and ultrasonication time (0.5 h, 1 h, 1.5 h, 2 h, 3 h). Optimum values of pH and oscillation time for preparation of stable nanofluids are listed in Table 2. Fig. 1 shows zeta potential values of stabilized nanofluids. Results indicate that all the nanofluids used in this paper have a zeta potential value of 33.4 mV, which proves that the prepared nanofluids are stable. Prepared nanofluid samples were used for experimental tests as shown in Fig. 2. 2.2. Anti-freezing test As a material with an excellent antifreeze property, ethylene glycol can be mixed with water in any ratio to obtain a low fluid viscosity and good anti-freezing performance. In this research, a 4:6 mixture of ethylene glycol and DI-water was used as the base liquid to meet a required freezing point below 20 C. Antifreeze testing
of nanofluids was carried out in a freezer with a setting temperature of 20 C, and the prepared nanofluids were placed in 50 mL calibration bottles to measure the central temperature of the samples by thermocouples. It was observed the measured temperature was kept below 20 C during the whole test. All the samples used in present experiments meeted the anti-freezing requirement. 2.3. Experimental setup An electric heater was used as the testing section, and a 500 W heating rod as a heat source is located on one side of the bottom horizontal tube. The electric heater consists of two horizontal tubes (with a diameter of 3 cm and a length of 66 cm) and seven vertical tubes (with a diameter of 2 cm and a length of 45 cm). An integrated fin was fixed on each vertical tube. For research of the magnetic nanofluid (Fe3O4-EGW nanofluid), a pair of homopolar magnets was fixed on both sides of each vertical tube. The detailed dimensions of the electric heater and the fins are shown in Fig. 3. In order to reduce effects of environmental conditions (like a wind flow due to persons’ walking) on the testing, the electric heater is placed in a square cavity with dimensions of 1 m 1 m 1.5 m. The underside of the cavity provides an outlet of most heat loss, and other walls of the cavity are made of insulating materials (rubber insulation cotton) with a thickness of 10 mm. Considering thermal expansion of the nanofluid during the heating process, 85% chamber volume (about 1.3 L) of the electric heater is full of the nanofluid, and 8 T-type thermocouples are arranged on the fins and bottom horizontal tube of the electric heater as shown in Fig. 4. The thermocouples on the fins were distributed symmetrically, and the distance (l) between the thermocouples and the centerline of the electric heater is 135 mm. Two thermocouples are located at the bottom horizontal tube: one is in the middle of the heating rod, and the other one is in the middle of the part without heating rod. Temperatures measured are recorded by a data collection instrument. 2.4. Analysis of experimental uncertainty Uncertainty of experimental tests is mainly analyzed based on testing accuracies of instruments. Some measuring instruments are involved, including an electronic scale (a resolution of 0.01 g), thermocouples (a resolution of 0.1 K) and graduated cylinder (a resolution of 10 mL). The accuracy of the heating power is ±2.5 W. In addition, environmental temperature changes within 1 C in every test and within 3 C throughout the testing period. An analysis of experimental uncertainty was conducted based on the
Table 2 Preparation conditions of nanofluids. Nanofluids
pH value
Ultrasonication time
Surfactant
Zeta potential
Al2O3-EGW Fe3O4-EGW Cu-EGW
6e7 6e7 8
2h 1.5 h 1.5 h
sodium hexametaphosphate trisodium citrate Arabic gum
37.4 mV 33.4 mV 35.6 mV
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Fig. 1. Zeta potential of nanofluids.
Finally, the total uncertainty of this research is 4.29%. 3. Results and discussion 3.1. Heat transfer performance of the base fluid
Fig. 2. Nanofluid samples: (a) EG/DI-water (b) Al2O3-EGW (3) Cu-EGW (4) Fe3O4-EGW nanofluids.
The heat flux released by the heat source is transferred to the wall of the electric heater through the nanofluid. As a reference to the heating performance of the electric heater, a basic case is conducted by using the 4:6 mixture of ethylene glycol and DI-water (EGW), and the wall temperature distribution on the electric heater is shown in Fig. 5. For different parts of the heater, it is evident that the heating rate has a similar trend, and the temperature on the heating rod is higher than on other parts. This is because the thermocouple position 7 is closer to the heat source than the other thermocouple positions. Due to an asymmetric position of the heat source in the bottom tube, it is found that the fin close to the heat source has a higher heating rate. Temperature distributions on different parts of the electric heater indicate that the heating rate of the electric heater is related to the position of the heat source, structure of the electric heater, and thermophysical properties of the fluid.
method in Ref. [31]:
(
dR ¼
N X i¼1
vR vX vXi i
3.2. Heat transfer performances of various nanofluids
2 )12 (3)
where variable R is a function of parameters X1, X2, …Xn. dX1, dX2, … dXn are uncertainties of these parameters. The total error consists of accuracies of instruments and repeatability uncertainty (3.66%).
It is observed that the base fluid (EGW) shows the lowest heating efficiency compared with other nanofluids with a nanoparticle mass concentration of 0.5% as shown in Fig. 6. For the CuEGW, Al2O3-EGW and Fe3O4-EGW nanofluids, the average temperature values on the middle fin increase by 9.92%, 2.93%, 4.19%, respectively. It is also found that for the same mass concentration,
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Fig. 3. Dimensions of the testing section (unit: mm).
Fig. 5. Local temperature of the electric heater filled with the base fluid.
Fig. 4. Schematic of the experimental setup.
the Cu-EGW nanofluid has both higher heating rate and higher equilibrium temperature compared to the other two. Considering that the thermal conductivity of the nanoparticle Al2O3 is lower than that of the nanoparticle Fe3O4, Fe3O4-EGW nanofluid shows a higher heating efficiency than the Al2O3-EGW nanofluid. This indicates that a small particle size of nanoparticles and high thermal conductivity can strengthen the Brownian motion, which enhances the heat transfer inside the nanofluid. Fig. 7 shows temperature values on the fins for various nanofluids with different mass concentrations. It can be seen that
Fig. 6. Fin temperature tests with time for different nanofluids.
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Fig. 7. Fin temperature for various nanofluids with time.
nanoparticle concentration has a small effect on heat transfer performance of the base fluid (the mixture of ethylene glycol and DI-water). It is easily found that the equilibrium temperature of the nanofluid is rising due to an increase of nanoparticles Cu and Fe3O4. This is because the thermal conductivity of the nanofluid increases when nanoparticles are added into the base fluid (a mixture of ethylene glycol and DI-water).
Table 3 Middle fin equilibrium temperature of Cu-EGW nanofluid, Fe3O4-EGW nanofluid, Al2O3-EGW nanofluid. Case
Equilibrium temperature ( C)
Enhancement (%)
EG/DI-water 0.5% Cu-EGW nanofluid 1.0% Cu- EGW nanofluid 1.5% Cu- EGW nanofluid 2.0% Cu- EGW nanofluid 0.5% Fe3O4- EGW nanofluid 1.0% Fe3O4- EGW nanofluid 1.5% Fe3O4- EGW nanofluid 2.0% Fe3O4- EGW nanofluid 0.5% Al2O3- EGW nanofluid 1.0% Al2O3- EGW nanofluid 1.5% Al2O3- EGW nanofluid 2.0% Al2O3- EGW nanofluid
82.47 90.65 91.58 92.16 93.34 85.93 86.20 84.08 85.46 84.89 85.58 83.72 84.60
/ 9.92 11.05 11.74 13.18 4.19 4.52 1.94 3.62 2.93 3.77 1.51 2.58
Note: the time required to reach the equilibrium temperature is 100 min.
A comparison of the equilibrium temperature for Cu-EGW, Al2O3EGW and Fe3O4-EGW nanofluids is conducted in Table 3. For the CuEGW nanofluid with 2% mass fraction, it is found that the maximum equilibrium temperature is obtained for a heating time of 100 min, corresponding to the maximum temperature of 93.34 C. It is revealed that the 2% Cu-EGW nanofluid has an increase of 13.18% in heating efficiency compared to the base fluid. Addition of Cu nanoparticles into the ethylene glycol-water mixture improves the thermophysical properties of the fluid, which results in an increase in Brownian motion of the base fluid. The fluid Brownian motion enhances movement behaviors of nanoparticles in the nanofluid, which destroys the boundary layer close to the tube wall. The 1% Al2O3-EGW and Fe3O4-EGW nanofluids show the highest value of heat transfer enhancement, which indicates that the heating efficiency of the nanofluid does not always increase with increasing particle concentration. The reason behind these phenomena is related to effects of nanoparticle thermal conductivity and nanofluid viscosity. This is because an increase of the particle concentration will increase both the thermal conductivity and viscosity of the nanofluid. Usually, a high nanofluid viscosity will weaken the natural convective heat transfer inside the electric heater. However, these results indicate that the effect of the increase in thermal conductivity on the heat transfer of the electric heater is significantly greater than that of the increase in viscosity, when more Cu nanoparticles are added into the base fluid. Compared with the
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heating efficiency of an electric heater filled with various nanofluids. In order to obtain detailed information of heat transfer inside the electric heater, temperature distributions on fins of the electric heater were studied by experimental tests. Moreover, heat transfer performance of various nanofluids is analyzed by considering effects of nanofluid material, nanoparticle concentration and intensity of magnetic field. Main conclusions of this research are summarized as follows:
Fig. 8. Effect of magnetic field on the rib temperature. Table 4 Middle fin equilibrium temperature of 0.5% Fe3O4-EGW nanofluid with different magnetic field intensity. Case
Equilibrium temperature ( C)
Enhancement (%)
EG/DI-water without magnetic field low-intensity magnetic field (100 mT) high-intensity magnetic field (200 mT)
82.47 85.93 94.58 87.03
/ 4.19 14.68 5.53
nanoparticle Cu, both nanoparticles Al2O3 and Fe3O4 have lower thermal conductivity when adding nanoparticles, which cannot offset the effect of the increase of the nanofluid viscosity. It is concluded that the 2% Cu-EGW nanofluid has the highest heating efficiency compared with other nanofluids. 3.3. Effect of magnetic field on heat transfer In order to improve the heating efficiency of the electric heater filled with Fe3O4-EGW nanofluid, an investigation of effect of a magnetic field on heat transfer performance of the nanofluid is conducted as shown in Fig. 8. Compared with the base fluid without a magnetic field, with low-intensity (100 mT) and high-intensity (200 mT) magnetic fields, the equilibrium temperature of the 0.5% Fe3O4-EGW nanofluid increases by 4.19%, 14.68% and 5.53%, respectively. It can be found that the 0.5% Fe3O4-EGW nanofluid is slightly affected by a strong magnetic field. This is because a magnetic field of 200 mT will cause nanoparticle aggregation on the tube wall, which weakens the heat transfer and Brownian motion of the particles inside the base fluid. However, a lowintensity magnetic field can promote movement of magnetic nanoparticles. Compared with cases without a magnetic field, it is concluded that the 0.5% Fe3O4-EGW nanofluid obtains about 14.68% improvement of the heating efficiency by adding an external magnetic field of 100 mT as shown in Table 4. From Tables 3 and 4, the nanoparticle Fe3O4 under a magnetic field of 100 mT shows the highest enhancement of heat transfer. So the low-cost nanoparticle Fe3O4 is recommended to be used in electric heaters. Note: the time required to reach the equilibrium temperature is 100 min. 4. Conclusions The purpose of this study is to experimentally investigate
(1) Compared with the base fluid, a maximum increase of 13.18% in average equilibrium temperature is obtained using 2% CuEGW nanofluid as working fluid. An increase in heating efficiency was obtained by increasing the mass fraction of nanoparticles. (2) Both the Al2O3-EGW and Fe3O4-EGW nanofluids with 1% nanoparticle mass fraction have the highest heat transfer enhancement, and the heating efficiency of the nanofluid has optimal enhanced values of 3.77% and 4.52% for Al2O3-EGW and Fe3O4-EGW nanofluids, respectively. (3) Compared with the base fluid without a magnetic field, a magnetic field of 100 mT increases average equilibrium temperature of fin 3-4 by about 14.68%. Compared with a case with a magnetic intensity of 200 mT, the equilibrium temperature of the 0.5% Fe3O4-EGW nanofluid increases by 5.53%. Finally, it is concluded that the 0.5% Fe3O4-EGW nanofluid with a magnetic field of 100 mT provides the best heating performance of the electric heater. Acknowledgment This work is supported by the National Natural Science Foundation of China [Grant numbers 51876161 and 51576059] and Project of Innovation Ability Training for Postgraduate Students of Education Department of Hebei Province [Grant number CXZZSS2019012]. References [1] Z. Said, S.M.A. Rahman, M. El Haj Assad, A.H. Alami, Heat transfer enhancement and life cycle analysis of a Shell-and-Tube Heat Exchanger using stable CuO/water nanofluid, Sustain. Energy Technol. Assessments. 31 (2019) 306e317. [2] M.H. Ahmadi, A. Mirlohi, M. Alhuyi Nazari, R. Ghasempour, A review of thermal conductivity of various nanofluids, J. Mol. Liq. 265 (2018) 181e188. [3] M.U. Sajid, H.M. Ali, Thermal conductivity of hybrid nanofluids: a critical review, Int. J. Heat Mass Transf. 126 (2018) 211e234. , A state of the art review on viscosity of nanofluids, [4] S.M.S. Murshed, P. Estelle Renew. Sustain. Energy Rev. 76 (2017) 1134e1152. [5] E. Khodabandeh, M.R. Safaei, S. Akbari, O.A. Akbari, A.A.A.A. Alrashed, Application of nanofluid to improve the thermal performance of horizontal spiral coil utilized in solar ponds: geometric study, Renew. Energy 122 (2018) 1e16. [6] M.M. Sorour, W.M. El-Maghlany, M.A. Alnakeeb, A.M. Abbass, Experimental study of free single jet impingement utilizing high concentration SiO2 nanoparticles water base nanofluid, Appl. Therm. Eng. 160 (2019), 114019. [7] R. Loni, E. Askari Asli-Ardeh, B. Ghobadian, A.B. Kasaeian, E. Bellos, Thermal performance comparison between Al2O3/oil and SiO2/oil nanofluids in cylindrical cavity receiver based on experimental study, Renew. Energy 129 (2018) 652e665. [8] A. Zeiny, H. Jin, G. Lin, P. Song, D. Wen, Solar evaporation via nanofluids: a comparative study, Renew. Energy 122 (2018) 443e454. [9] X. Xu, C. Xu, J. Liu, X. Fang, Z. Zhang, A direct absorption solar collector based on a water-ethylene glycol based nanofluid with anti-freeze property and excellent dispersion stability, Renew. Energy 133 (2019) 760e769. [10] Y. Tong, H. Lee, W. Kang, H. Cho, Energy and exergy comparison of a flat-plate solar collector using water, Al2O3 nanofluid, and CuO nanofluid, Appl. Therm. Eng. 159 (2019), 113959. [11] A. Hassan, A. Wahab, M.A. Qasim, M.M. Janjua, M.A. Ali, H.M. Ali, T.R. Jadoon, E. Ali, A. Raza, N. Javaid, Thermal management and uniform temperature regulation of photovoltaic modules using hybrid phase change materialsnanofluids system, Renew. Energy 145 (2020) 282e293.
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[12] V. Kiseev, O. Sazhin, Heat transfer enhancement in a loop thermosyphon using nanoparticles/water nanofluid, Int. J. Heat Mass Transf. 132 (2019) 557e564. [13] J. Xu, K. Bandyopadhyay, D. Jung, Experimental investigation on the correlation between nano-fluid characteristics and thermal properties of Al2O3 nanoparticles dispersed in ethylene glycol-water mixture, Int. J. Heat Mass Transf. 94 (2016) 262e268. [14] R. Islam, B. Shabani, J. Andrews, G. Rosengarten, Experimental investigation of using ZnO nanofluids as coolants in a PEM fuel cell, Int. J. Hydrogen Energy 42 (2017) 19272e19286. rdenas Contreras, G.A. Oliveira, E.P. Bandarra Filho, Experimental [15] E.M. Ca analysis of the thermohydraulic performance of graphene and silver nanofluids in automotive cooling systems, Int. J. Heat Mass Transf. 132 (2019) 375e387. _ Atmaca, M. Asiltürk, A. Dog an, Improving heat transfer perfor[16] S.K. Soylu, I. mance of an automobile radiator using Cu and Ag doped TiO2 based nanofluids, Appl. Therm. Eng. 157 (2019). [17] N. Ahammed, L.G. Asirvatham, S. Wongwises, Thermoelectric cooling of electronic devices with nanofluid in a multiport minichannel heat exchanger, Exp. Therm. Fluid Sci. 74 (2016) 81e90. [18] P.C. Mukesh Kumar, C.M. Arun Kumar, Numerical study on heat transfer performance using Al2O3/water nanofluids in sixcircular channel heat sink for electronic chip, Mater. Today Proc. (2019). https://doi.org/10.1016/j.matpr. 2019.04.220. [19] L.S. Sundar, M.K. Singh, A.C.M. Sousa, Thermal conductivity of ethylene glycol and water mixture based Fe3O4 nanofluid, Int. Commun. Heat Mass Transf. 49 (2013) 17e24. [20] L.S. Sundar, M.K. Singh, A.C.M. Sousa, Investigation of thermal conductivity and viscosity of Fe3O4 nanofluid for heat transfer applications, Int. Commun. Heat Mass Transf. 44 (2013) 7e14. [21] D. Toghraie, S.M. Alempour, M. Afrand, Experimental determination of viscosity of water based magnetite nanofluid for application in heating and
cooling systems, J. Magn. Magn. Mater. 417 (2016) 243e248. [22] A. Khosravi, M. Malekan, M.E.H. Assad, Numerical analysis of magnetic field effects on the heat transfer enhancement in ferrofluids for a parabolic trough solar collector, Renew. Energy 134 (2019) 54e63. n, Experimental investigation on [23] J. Wang, G. Li, H. Zhu, J. Luo, Bengt Sunde convective heat transfer of ferrofluids inside a pipe under various magnet orientations, Int. J. Heat Mass Transf. 132 (2019) 407e419. [24] L. Sha, Y. Ju, H. Zhang, J. Wang, Experimental investigation on the convective heat transfer of Fe3O4/water nanofluids under constant magnetic field, Appl. Therm. Eng. 113 (2017) 566e574. [25] L. Sha, Y. Ju, H. Zhang, The influence of the magnetic field on the convective heat transfer characteristics of Fe3O4/water nanofluids, Appl. Therm. Eng. 126 (2017) 108e116. [26] M. Mehrali, E. Sadeghinezhad, A.R. Akhiani, S. Tahan Latibari, H.S.C. Metselaar, A.S. Kherbeet, M. Mehrali, Heat transfer and entropy generation analysis of hybrid graphene/Fe3O4 ferro-nanofluid flow under the influence of a magnetic field, Powder Technol. 308 (2017) 149e157. [27] Y. Ma, R. Mohebbi, M.M. Rashidi, Z. Yang, MHD convective heat transfer of AgMgO/water hybrid nanofluid in a channel with active heaters and coolers, Int. J. Heat Mass Transf. 137 (2019) 714e726. [28] G. Huminic, A. Huminic, The heat transfer performances and entropy generation analysis of hybrid nanofluids in a flattened tube, Int. J. Heat Mass Transf. 119 (2018) 813e827. [29] M. Sheikholeslami, S.A.M. Mehryan, A. Shafee, M.A. Sheremet, Variable magnetic forces impact on magnetizable hybrid nanofluid heat transfer through a circular cavity, J. Mol. Liq. 277 (2019) 388e396. [30] C.J. Ho, Y.H. Chiou, W.M. Yan, M. Ghalambaz, Transient cooling characteristics of Al2O3-water nanofluid flow in a microchannel subject to a sudden-pulsed heat flux, Int. J. Mech. Sci. 151 (2019) 95e105. [31] R.J. Moffat, Describing the uncertainties in experimental results, Exp. Therm. Fluid Sci. 1 (1988) 3e17.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Experimental investigation on heat transfer characteristics of various nanofluids in an indoor electric heater n c Zhanxiu Chen a, Dan Zheng a, Jin Wang a, *, Lei Chen b, Bengt Sunde a
School of Energy and Environmental Engineering, Hebei University of Technology, Tianjin, 300401, China Key Laboratory of Thermo-Fluid Science and Engineering (Ministry of Education), Xi'an Jiaotong University, Xi'an, 710049, China c Department of Energy Sciences, Division of Heat Transfer, Lund University, P.O. Box, 118, SE-22100, Lund, Sweden b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 June 2019 Received in revised form 26 August 2019 Accepted 12 September 2019 Available online 12 September 2019
Heat transfer characteristics of an electric heater were experimentally investigated by using various fluids in this paper, including Cu-EGW (a mixture of ethylene glycol and DI-water), Al2O3-EGW, Fe3O4EGW nanofluids. A 4:6 mixture of ethylene glycol and deionized water was used as the base liquid. All these nanofluids were prepared by ultrasonic treatment, and nanoparticle mass concentration of samples varies from 0.5% to 2%. In addition, natural convective heat transfer of Fe3O4-EGW nanofluid in an electric heater was carried out by considering an effect of different magnetic fields. The results indicated that heat transfer performance of Cu-EGW nanofluid was significantly higher than the Al2O3-EGW and Fe3O4EGW nanofluids, and the heating efficiency of the Cu-EGW nanofluid increased with the mass concentration of Cu particles. Compared with that of the base fluid, equilibrium temperature values of electric heaters filled with 2.0% Cu-EGW, 1.0% Al2O3-EGW and 1.0% Fe3O4-EGW nanofluids increase by 13.18%, 3.77% and 4.52%, respectively. It was also found that the magnetic field had a positive effect on the heat transfer enhancement of the Fe3O4-EGW nanofluid. In addition, for the 0.5% Fe3O4 nanofluid under a magnetic intensity of 100 mT, the equilibrium temperature on the middle fin increases by 14.68%. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Electric heater Nanofluid Magnetic field Natural convection Equilibrium temperature
1. Introduction In recent years, traditional fluids applied in heat exchanges cannot meet the increasing demand for the heat dissipation of equipment. Considering that nanoparticle materials with higher thermal conductivity have received an extensive attention, various nanofluids with improved thermophysical properties are involved in many research fields, such as microelectronics, energy utilization, chemical engineering, vehicles, etc. By uniform dispersion of nano-scale metal or non-metal particles into the base fluid, the nanofluid as a two-phase medium is prepared to provide an enhancement of heat transfer [1]. Thermophysical properties of a nanofluid are related to many factors, such as material, concentration and size of nanoparticles. Ahmadi et al. [2] reviewed effects of temperature, concentration and shape of particles on thermal conductivity of nanofluids. It was
* Corresponding author. E-mail addresses: [email protected] (Z. Chen), zhengdan2018hebut@ 163.com (D. Zheng), [email protected] (J. Wang), [email protected] n). (L. Chen), [email protected] (B. Sunde https://doi.org/10.1016/j.renene.2019.09.036 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
concluded that thermal conductivity of nanofluids increased with the temperature and concentration. Sajid and Ali [3] summarized studies on thermal conductivity of hybrid nanofluids in recent years. They pointed out that thermal conductivity was affected by several crucial factors, including types of base fluid, concentration of nanoparticles, addition of surfactant, etc. Adding nanoparticles to a base fluid not only improves the thermal conductivity of the fluid, but also increases the viscosity of [4] reviewed effects of various the solution. Murshed and Estelle factors (such as concentration of nanoparticles and temperature) on the nanofluid viscosity. They found that it was necessary to develop a uniform and standardized procedure for preparation of nanofluids. Khodabandeh et al. [5] numerically investigated heat transfer enhancement of water-graphene/platinum hybrid nanofluids in a horizontal spiral coil. They found that the maximum Nusselt number was obtained for the coil with the elliptic cross section. Sorour et al. [6] experimentally studied heat transfer between a vertical free surface jet and a horizontal stainless steel heated plate by using water-SiO2 nanofluids with various volume fractions (0%4 8.5%). They found that the average Nusselt number for a case with volume fraction of 8.5% increased up to 80%
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compared to pure water. Nanofluids have been extensively studied in the field of solar energy. Loni et al. [7] experimentally conducted a comparison of heat transfer performances of nanofluids (Al2O3thermal oil and SiO2-thermal oil) in a cylindrical cavity. Compared with the SiO2-oil nanofluid and pure thermal oil, the cylindrical cavity receiver had the highest average thermal efficiency. Zeiny et al. [8] comparatively studied the vaporisation through direct absorption solar collectors with gold-water nanofluid and carbon black-water nanofluid. They found that carbon black nanofluids were more suitable for solar evaporation applications than gold nanofluids due to the high cost in the production process using gold nanofluids. They concluded that the increase of nanoparticle concentration or incident solar radiation would lead to a higher evaporation rate. Xu et al. [9] investigated the heat transfer characteristics of graphene oxide/water-ethylene glycol nanofluid in direct absorption solar collectors. They found that the receiver efficiency of this nanofluid increased by 70% compared with the base fluid with the same exposure time of 6000 s and a solar intensity of 1000 W/m2. Tong et al. [10] experimentally conducted comparisons of energy efficiency and entropy generation in a flat-plate solar collector by using various nanofluids. They reported that the highest efficiency of 77.5% was obtained when 1.0% Al2O3 nanofluid was used. Hassan et al. [11] used graphene/water nanofluid and phase change material RT-35HC to regulate temperature values of photovoltaic (PV) modules. It was concluded that the maximum reduction of the PV temperature is 29.3 C at 0.1% volume concentration of graphene nanoparticles and 40 LPM flow rate. At the same working condition, the thermal efficiency increased by 20.8% compared with water. Kiseev and Sazhin [12] studied the heat transfer characteristics of a loop thermosyphon filled with Fe2O3water nanofluid with mass concentrations of 0.5%, 1.0%, 1.5% and 2.0%. They showed that the Fe2O3-water nanofluid could enhance the heat transfer of the loop thermosyphon, and the heat transfer coefficient could be increased by 20%e25% when using 2.0% Fe2O3water nanofluid. A valuable way to improve the heat dissipation performance of the radiator is to develop new-type high-efficient heat transfer fluids [13]. Islam et al. [14] employed 50:50 water-EG nanofluid with ZnO nanoparticles to cool a Proton Exchange Membrane Fuel Cell (PEMFC). They found that for the ZnO nanofluid with volume fraction of 0.5 vol% as PEMFC coolant, the radiator size decreased by almost 27% compared to 50/50 water-EG rdenas Contreras et al. [15] experiwithout ZnO nanoparticles. Ca mentally analyzed heat transfer performance of an automotive cooling system and used graphene and silver nanofluids as automotive coolants. They found that heat transfer rate had an increase of 3.3% when 0.1 vol % graphene nanofluid was used at an inlet temperature of 85 C. Soylu et al. [16] conducted experimental research of heat transfer enhancement in an automobile radiator by using various EG/water-based hybrid nanofluids. Results indicated that the maximum enhancement ratio of the overall heat transfer coefficient was 11.094% for the nanofluid with 2% TiO2 and 0.3% Ag nanoparticles at a flow rate of 19 L/min. Ahammed et al. [17] investigated a multiport-minichannel heat exchanger with aluminum oxide-water nanofluid to remove heat from electronic devices. It was observed that the performance coefficient of the thermoelectric module was significantly improved by 40% when 0.2 vol% Al2O3-water was used. Kumar and Kumar [18] numerically studied heat transfer performance of Al2O3/water nanofluids based on an electronic chip with a microchannel heat sink. Compared to water as coolant, 70% reliability enhancement of the electronic chip was obtained by using 0.75 vol% Al2O3/water nanofluid. Most of the earlier research works are related to heat transfer characteristics of non-magnetic nanofluids, whereas magnetic nanofluids (ferrofluids) are a functional material with both flow characteristics and magnetic properties. Sundar et al. [19]
measured thermal conductivity of an Fe3O4 nanofluid by the transient hot wire method. A thermal conductivity enhancement of 46% for the 2% Fe3O4 nanofluid was observed compared to the base fluid (2:8 ethylene glycol and water mixture) at 60 C. They also found that for the Fe3O4 nanofluid under the same volume concentration and temperature, the viscosity showed a greater enhancement than the thermal conductivity [20]. Toghraie et al. [21] investigated viscosities of Fe3O4-water nanofluids with different volume fractions (0.1%, 0.2%, 0.4%, 1%, 2% and 3%) in a temperature range of 20e55 C. They found that the maximum viscosity enhancement was 129.7% by calculation of viscosity ratios. Khosravi et al. [22] studied effects of magnetic field (0e500 G) and particle concentration (1e4 vol%) on the heat transfer coefficient, and they found that the 4% ferrofluid under a magnetic field of 500 G showed the best thermal performance. Wang et al. [23] conducted an investigation of laminar convective heat transfer inside a pipe filled with ferrofluids under various external magnetic fields. Results indicated that compared to the ferrofluid without magnetic field, a heat transfer enhancement of 261% was obtained at Re ¼ 805 when five adjacent magnetic cannulas were used. Sha et al. [24] experimentally studied convective heat transfer of Fe3O4- water nanofluids under a parallel constant magnetic field. The experimental results showed that the parallel constant magnetic field resulted in a reduction of the heat transfer coefficient for the Fe3O4-water nanofluid. The effect of a vertical magnetic field on the convective heat transfer of Fe3O4-water nanofluid was also studied in Ref. [25]. It was found that the Fe3O4-water nanofluid under a gradient magnetic field of 800 G and at an inlet temperature of 40 C had a 8.1% enhancement of the convective heat transfer coefficient. Mehrali et al. [26] investigated heat transfer characteristics and entropy generation of a hybrid graphene-Fe3O4 nanofluid under a magnetic field. The results showed that the local convective heat transfer coefficient increased by 4% and the total entropy production decreased by 41% compared with water. Heat transfer characteristics of AgeMgO/water hybrid nanofluid in a channel with active heaters and coolers were numerically studied by considering the effect of a magnetic field in Ref. [27]. Results showed that the heat transfer rate decreased with the increase of Hartmann number. Huminic and Huminic [28] carried out an experimental study of heat transfer performances and entropy generation in a flattened tube with both MWCNT/Fe3O4-water and ND/Fe3O4-water hybrid nanofluids. According to their experimental results, for 0.3 vol% MWCNT/Fe3O4-water hybrid nanofluid, the heat transfer coefficient increased by 20.607% at Re ¼ 250, and the total entropy production decreased by 26.487% compared to water at 333 K. Sheikholeslami et al. [29] analyzed natural convection of a hybrid nanofluid (MWCNT-Fe3O4/H2O) inside a circular cavity with two circular heaters under a non-uniform magnetic field. Results revealed that an increment in Hartmann number increased the boundary layer thickness which weakened the natural convection. In the above relevant literatures, it is revealed that nanofluid as a new functional material can effectively enhance the heat transfer of the base fluid. A review on the relevant literature shows that although some researchers have investigated heat transfer characteristics of various nanofluids in heating equipment, there is no study for indoor electric heaters. The electric heater is one kind of heating equipment used in many industries due to its advantages (low operating costs and mobility heating). This paper will investigate heat transfer characteristics of various nanofluids inside an electric heater, which will improve both the heating efficiency and building thermal comfort. Compared with a 4:6 mixture of ethylene glycol and DI-water as the base fluid, the effects of nanoparticles Cu, Al2O3 and Fe3O4 on the heat transfer enhancement are analyzed in the present paper. In addition, heat transfer characteristics of the Fe3O4 nanofluid was also analyzed by a comparison of results with
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Table 1 Thermophysical properties of various nanoparticles and base fluid (25 C). Materials
Grain size (nm)
Density (kg/m3)
Specific heat (J/kg.K)
Thermal conductivity (W/m.K)
Al2O3 Fe3O4 Cu deionized water ethylene glycol
30 20 30 / /
3600 5180 8960 4180 1113.4
765 670 385 997 2350
25.08 [30] 80 401 [30] 0.613 0.224
and without a magnetic field. 2. Experimental investigation 2.1. Preparation of nanofluids In this research, nanofluids are synthesized by a two-step method. Nanoparticles (Cu, Fe3O4, and Al2O3) are dispersed into a 4:6 mixture of ethylene glycol and DI-water with specified mass fractions (0.5 wt%, 1.0 wt%, 1.5 wt%, and 2.0 wt%). The mass of the nanofluid is calculated by Eq. (1) and then the mass fraction of nanofluids can be calculated by Eq. (2).
W ¼ ry 4¼
(1)
u
(2)
WDIwater þ WEG þ u
where W, r, n, u and 4 represent fluid mass, fluid density, fluid volume, nanoparticle weight and mass fraction, respectively. WDIwater, WEG are masses of deionized water and ethylene glycol, respectively. All the nanoparticles were purchased by Beijing Deke Daojin Company of China. Ethylene glycol and deionized water were supplied by Yitian Company of China. Grain size, density and specific heat of nanoparticles were supplied by Deke Daojin Company of China. Thermal conductivities of Al2O3 and Cu nanoparticles are obtained through related literature, and the average values of results are listed in Table 1. In order to obtain a well dispersed nanofluid, many influential factors for stability of nanofluids were investigated, including surfactant (cetyl trimethyl ammonium bromide, gum arabic, sodium hexametaphosphate, trisodium citrate, sodium dodecyl benzene sulfonate), pH value and ultrasonication time (0.5 h, 1 h, 1.5 h, 2 h, 3 h). Optimum values of pH and oscillation time for preparation of stable nanofluids are listed in Table 2. Fig. 1 shows zeta potential values of stabilized nanofluids. Results indicate that all the nanofluids used in this paper have a zeta potential value of 33.4 mV, which proves that the prepared nanofluids are stable. Prepared nanofluid samples were used for experimental tests as shown in Fig. 2. 2.2. Anti-freezing test As a material with an excellent antifreeze property, ethylene glycol can be mixed with water in any ratio to obtain a low fluid viscosity and good anti-freezing performance. In this research, a 4:6 mixture of ethylene glycol and DI-water was used as the base liquid to meet a required freezing point below 20 C. Antifreeze testing
of nanofluids was carried out in a freezer with a setting temperature of 20 C, and the prepared nanofluids were placed in 50 mL calibration bottles to measure the central temperature of the samples by thermocouples. It was observed the measured temperature was kept below 20 C during the whole test. All the samples used in present experiments meeted the anti-freezing requirement. 2.3. Experimental setup An electric heater was used as the testing section, and a 500 W heating rod as a heat source is located on one side of the bottom horizontal tube. The electric heater consists of two horizontal tubes (with a diameter of 3 cm and a length of 66 cm) and seven vertical tubes (with a diameter of 2 cm and a length of 45 cm). An integrated fin was fixed on each vertical tube. For research of the magnetic nanofluid (Fe3O4-EGW nanofluid), a pair of homopolar magnets was fixed on both sides of each vertical tube. The detailed dimensions of the electric heater and the fins are shown in Fig. 3. In order to reduce effects of environmental conditions (like a wind flow due to persons’ walking) on the testing, the electric heater is placed in a square cavity with dimensions of 1 m 1 m 1.5 m. The underside of the cavity provides an outlet of most heat loss, and other walls of the cavity are made of insulating materials (rubber insulation cotton) with a thickness of 10 mm. Considering thermal expansion of the nanofluid during the heating process, 85% chamber volume (about 1.3 L) of the electric heater is full of the nanofluid, and 8 T-type thermocouples are arranged on the fins and bottom horizontal tube of the electric heater as shown in Fig. 4. The thermocouples on the fins were distributed symmetrically, and the distance (l) between the thermocouples and the centerline of the electric heater is 135 mm. Two thermocouples are located at the bottom horizontal tube: one is in the middle of the heating rod, and the other one is in the middle of the part without heating rod. Temperatures measured are recorded by a data collection instrument. 2.4. Analysis of experimental uncertainty Uncertainty of experimental tests is mainly analyzed based on testing accuracies of instruments. Some measuring instruments are involved, including an electronic scale (a resolution of 0.01 g), thermocouples (a resolution of 0.1 K) and graduated cylinder (a resolution of 10 mL). The accuracy of the heating power is ±2.5 W. In addition, environmental temperature changes within 1 C in every test and within 3 C throughout the testing period. An analysis of experimental uncertainty was conducted based on the
Table 2 Preparation conditions of nanofluids. Nanofluids
pH value
Ultrasonication time
Surfactant
Zeta potential
Al2O3-EGW Fe3O4-EGW Cu-EGW
6e7 6e7 8
2h 1.5 h 1.5 h
sodium hexametaphosphate trisodium citrate Arabic gum
37.4 mV 33.4 mV 35.6 mV
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Fig. 1. Zeta potential of nanofluids.
Finally, the total uncertainty of this research is 4.29%. 3. Results and discussion 3.1. Heat transfer performance of the base fluid
Fig. 2. Nanofluid samples: (a) EG/DI-water (b) Al2O3-EGW (3) Cu-EGW (4) Fe3O4-EGW nanofluids.
The heat flux released by the heat source is transferred to the wall of the electric heater through the nanofluid. As a reference to the heating performance of the electric heater, a basic case is conducted by using the 4:6 mixture of ethylene glycol and DI-water (EGW), and the wall temperature distribution on the electric heater is shown in Fig. 5. For different parts of the heater, it is evident that the heating rate has a similar trend, and the temperature on the heating rod is higher than on other parts. This is because the thermocouple position 7 is closer to the heat source than the other thermocouple positions. Due to an asymmetric position of the heat source in the bottom tube, it is found that the fin close to the heat source has a higher heating rate. Temperature distributions on different parts of the electric heater indicate that the heating rate of the electric heater is related to the position of the heat source, structure of the electric heater, and thermophysical properties of the fluid.
method in Ref. [31]:
(
dR ¼
N X i¼1
vR vX vXi i
3.2. Heat transfer performances of various nanofluids
2 )12 (3)
where variable R is a function of parameters X1, X2, …Xn. dX1, dX2, … dXn are uncertainties of these parameters. The total error consists of accuracies of instruments and repeatability uncertainty (3.66%).
It is observed that the base fluid (EGW) shows the lowest heating efficiency compared with other nanofluids with a nanoparticle mass concentration of 0.5% as shown in Fig. 6. For the CuEGW, Al2O3-EGW and Fe3O4-EGW nanofluids, the average temperature values on the middle fin increase by 9.92%, 2.93%, 4.19%, respectively. It is also found that for the same mass concentration,
Z. Chen et al. / Renewable Energy 147 (2020) 1011e1018
Fig. 3. Dimensions of the testing section (unit: mm).
Fig. 5. Local temperature of the electric heater filled with the base fluid.
Fig. 4. Schematic of the experimental setup.
the Cu-EGW nanofluid has both higher heating rate and higher equilibrium temperature compared to the other two. Considering that the thermal conductivity of the nanoparticle Al2O3 is lower than that of the nanoparticle Fe3O4, Fe3O4-EGW nanofluid shows a higher heating efficiency than the Al2O3-EGW nanofluid. This indicates that a small particle size of nanoparticles and high thermal conductivity can strengthen the Brownian motion, which enhances the heat transfer inside the nanofluid. Fig. 7 shows temperature values on the fins for various nanofluids with different mass concentrations. It can be seen that
Fig. 6. Fin temperature tests with time for different nanofluids.
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Fig. 7. Fin temperature for various nanofluids with time.
nanoparticle concentration has a small effect on heat transfer performance of the base fluid (the mixture of ethylene glycol and DI-water). It is easily found that the equilibrium temperature of the nanofluid is rising due to an increase of nanoparticles Cu and Fe3O4. This is because the thermal conductivity of the nanofluid increases when nanoparticles are added into the base fluid (a mixture of ethylene glycol and DI-water).
Table 3 Middle fin equilibrium temperature of Cu-EGW nanofluid, Fe3O4-EGW nanofluid, Al2O3-EGW nanofluid. Case
Equilibrium temperature ( C)
Enhancement (%)
EG/DI-water 0.5% Cu-EGW nanofluid 1.0% Cu- EGW nanofluid 1.5% Cu- EGW nanofluid 2.0% Cu- EGW nanofluid 0.5% Fe3O4- EGW nanofluid 1.0% Fe3O4- EGW nanofluid 1.5% Fe3O4- EGW nanofluid 2.0% Fe3O4- EGW nanofluid 0.5% Al2O3- EGW nanofluid 1.0% Al2O3- EGW nanofluid 1.5% Al2O3- EGW nanofluid 2.0% Al2O3- EGW nanofluid
82.47 90.65 91.58 92.16 93.34 85.93 86.20 84.08 85.46 84.89 85.58 83.72 84.60
/ 9.92 11.05 11.74 13.18 4.19 4.52 1.94 3.62 2.93 3.77 1.51 2.58
Note: the time required to reach the equilibrium temperature is 100 min.
A comparison of the equilibrium temperature for Cu-EGW, Al2O3EGW and Fe3O4-EGW nanofluids is conducted in Table 3. For the CuEGW nanofluid with 2% mass fraction, it is found that the maximum equilibrium temperature is obtained for a heating time of 100 min, corresponding to the maximum temperature of 93.34 C. It is revealed that the 2% Cu-EGW nanofluid has an increase of 13.18% in heating efficiency compared to the base fluid. Addition of Cu nanoparticles into the ethylene glycol-water mixture improves the thermophysical properties of the fluid, which results in an increase in Brownian motion of the base fluid. The fluid Brownian motion enhances movement behaviors of nanoparticles in the nanofluid, which destroys the boundary layer close to the tube wall. The 1% Al2O3-EGW and Fe3O4-EGW nanofluids show the highest value of heat transfer enhancement, which indicates that the heating efficiency of the nanofluid does not always increase with increasing particle concentration. The reason behind these phenomena is related to effects of nanoparticle thermal conductivity and nanofluid viscosity. This is because an increase of the particle concentration will increase both the thermal conductivity and viscosity of the nanofluid. Usually, a high nanofluid viscosity will weaken the natural convective heat transfer inside the electric heater. However, these results indicate that the effect of the increase in thermal conductivity on the heat transfer of the electric heater is significantly greater than that of the increase in viscosity, when more Cu nanoparticles are added into the base fluid. Compared with the
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heating efficiency of an electric heater filled with various nanofluids. In order to obtain detailed information of heat transfer inside the electric heater, temperature distributions on fins of the electric heater were studied by experimental tests. Moreover, heat transfer performance of various nanofluids is analyzed by considering effects of nanofluid material, nanoparticle concentration and intensity of magnetic field. Main conclusions of this research are summarized as follows:
Fig. 8. Effect of magnetic field on the rib temperature. Table 4 Middle fin equilibrium temperature of 0.5% Fe3O4-EGW nanofluid with different magnetic field intensity. Case
Equilibrium temperature ( C)
Enhancement (%)
EG/DI-water without magnetic field low-intensity magnetic field (100 mT) high-intensity magnetic field (200 mT)
82.47 85.93 94.58 87.03
/ 4.19 14.68 5.53
nanoparticle Cu, both nanoparticles Al2O3 and Fe3O4 have lower thermal conductivity when adding nanoparticles, which cannot offset the effect of the increase of the nanofluid viscosity. It is concluded that the 2% Cu-EGW nanofluid has the highest heating efficiency compared with other nanofluids. 3.3. Effect of magnetic field on heat transfer In order to improve the heating efficiency of the electric heater filled with Fe3O4-EGW nanofluid, an investigation of effect of a magnetic field on heat transfer performance of the nanofluid is conducted as shown in Fig. 8. Compared with the base fluid without a magnetic field, with low-intensity (100 mT) and high-intensity (200 mT) magnetic fields, the equilibrium temperature of the 0.5% Fe3O4-EGW nanofluid increases by 4.19%, 14.68% and 5.53%, respectively. It can be found that the 0.5% Fe3O4-EGW nanofluid is slightly affected by a strong magnetic field. This is because a magnetic field of 200 mT will cause nanoparticle aggregation on the tube wall, which weakens the heat transfer and Brownian motion of the particles inside the base fluid. However, a lowintensity magnetic field can promote movement of magnetic nanoparticles. Compared with cases without a magnetic field, it is concluded that the 0.5% Fe3O4-EGW nanofluid obtains about 14.68% improvement of the heating efficiency by adding an external magnetic field of 100 mT as shown in Table 4. From Tables 3 and 4, the nanoparticle Fe3O4 under a magnetic field of 100 mT shows the highest enhancement of heat transfer. So the low-cost nanoparticle Fe3O4 is recommended to be used in electric heaters. Note: the time required to reach the equilibrium temperature is 100 min. 4. Conclusions The purpose of this study is to experimentally investigate
(1) Compared with the base fluid, a maximum increase of 13.18% in average equilibrium temperature is obtained using 2% CuEGW nanofluid as working fluid. An increase in heating efficiency was obtained by increasing the mass fraction of nanoparticles. (2) Both the Al2O3-EGW and Fe3O4-EGW nanofluids with 1% nanoparticle mass fraction have the highest heat transfer enhancement, and the heating efficiency of the nanofluid has optimal enhanced values of 3.77% and 4.52% for Al2O3-EGW and Fe3O4-EGW nanofluids, respectively. (3) Compared with the base fluid without a magnetic field, a magnetic field of 100 mT increases average equilibrium temperature of fin 3-4 by about 14.68%. Compared with a case with a magnetic intensity of 200 mT, the equilibrium temperature of the 0.5% Fe3O4-EGW nanofluid increases by 5.53%. Finally, it is concluded that the 0.5% Fe3O4-EGW nanofluid with a magnetic field of 100 mT provides the best heating performance of the electric heater. Acknowledgment This work is supported by the National Natural Science Foundation of China [Grant numbers 51876161 and 51576059] and Project of Innovation Ability Training for Postgraduate Students of Education Department of Hebei Province [Grant number CXZZSS2019012]. References [1] Z. Said, S.M.A. Rahman, M. El Haj Assad, A.H. Alami, Heat transfer enhancement and life cycle analysis of a Shell-and-Tube Heat Exchanger using stable CuO/water nanofluid, Sustain. Energy Technol. Assessments. 31 (2019) 306e317. [2] M.H. Ahmadi, A. Mirlohi, M. Alhuyi Nazari, R. Ghasempour, A review of thermal conductivity of various nanofluids, J. Mol. Liq. 265 (2018) 181e188. [3] M.U. Sajid, H.M. Ali, Thermal conductivity of hybrid nanofluids: a critical review, Int. J. Heat Mass Transf. 126 (2018) 211e234. , A state of the art review on viscosity of nanofluids, [4] S.M.S. Murshed, P. Estelle Renew. Sustain. Energy Rev. 76 (2017) 1134e1152. [5] E. Khodabandeh, M.R. Safaei, S. Akbari, O.A. Akbari, A.A.A.A. Alrashed, Application of nanofluid to improve the thermal performance of horizontal spiral coil utilized in solar ponds: geometric study, Renew. Energy 122 (2018) 1e16. [6] M.M. Sorour, W.M. El-Maghlany, M.A. Alnakeeb, A.M. Abbass, Experimental study of free single jet impingement utilizing high concentration SiO2 nanoparticles water base nanofluid, Appl. Therm. Eng. 160 (2019), 114019. [7] R. Loni, E. Askari Asli-Ardeh, B. Ghobadian, A.B. Kasaeian, E. Bellos, Thermal performance comparison between Al2O3/oil and SiO2/oil nanofluids in cylindrical cavity receiver based on experimental study, Renew. Energy 129 (2018) 652e665. [8] A. Zeiny, H. Jin, G. Lin, P. Song, D. Wen, Solar evaporation via nanofluids: a comparative study, Renew. Energy 122 (2018) 443e454. [9] X. Xu, C. Xu, J. Liu, X. Fang, Z. Zhang, A direct absorption solar collector based on a water-ethylene glycol based nanofluid with anti-freeze property and excellent dispersion stability, Renew. Energy 133 (2019) 760e769. [10] Y. Tong, H. Lee, W. Kang, H. Cho, Energy and exergy comparison of a flat-plate solar collector using water, Al2O3 nanofluid, and CuO nanofluid, Appl. Therm. Eng. 159 (2019), 113959. [11] A. Hassan, A. Wahab, M.A. Qasim, M.M. Janjua, M.A. Ali, H.M. Ali, T.R. Jadoon, E. Ali, A. Raza, N. Javaid, Thermal management and uniform temperature regulation of photovoltaic modules using hybrid phase change materialsnanofluids system, Renew. Energy 145 (2020) 282e293.
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