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ethylene glycol nanofluid in electric field
International Journal of Thermal Sciences 62 (2012) 114e119
Contents lists available at SciVerse ScienceDirect
International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts
Natural convective heat transfer of Fe3O4/ethylene glycol nanofluid in electric field F. Asadzadeh, M. Nasr Esfahany*, N. Etesami Department of Chemical Engineering, Isfahan University of Technology, Isfahan 8415683111, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 March 2011 Received in revised form 9 November 2011 Accepted 10 November 2011 Available online 6 December 2011
Natural convective heat transfer of Fe3O4/Ethylene Glycol nanofluids was examined in the presence of electric field around a thin platinum wire. Results showed that addition of nanoparticles to ethylene glycol promoted heat transfer up to the volume fraction of 0.02%. Increasing the volume fraction further however, deteriorated heat transfer. Applying the electric field intensified natural convective heat transfer of both ethylene glycol and nanofluids. Enhancement increased with electric field intensity while decreased with Rayleigh number. Electric field delayed deterioration of heat transfer of nanofluids to greater volume fractions. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Natural convection Nanofluids Electrohydrodynamic Heat transfer enhancement
1. Introduction Heat transfer enhancement improves the performance of existing heat exchangers or results in considerable decrease in heat transfer area at the same performance in design stage. In general, enhancement methods can be classified into active and passive techniques. The former requires external forces, e.g. surface vibration and applying electric field while the latter uses additives or special changes in surface geometries. Boundary layers formed on the thermally active surfaces offer a significant resistance to transfer of heat. Enhancement techniques therefore, target the boundary layer structure to lessen the resistance of the flow of heat. One of the active techniques in heat transfer enhancement is electrohydrodynamic (EHD) in which a high-voltage low-current electric field is applied in the dielectric heat transfer medium. The presence of a high electric field between a discharged and a grounded electrode induces secondary flows, disturbs the resistive boundary layer and promotes heat transfer. EHD is furnished with many advantages such as ease of control of performance by varying the electric field intensity, low power requirements, and simple design. The ability of a high voltage electric field to enhance heat transfer in a dielectric fluid has been known since Chubb [1] performed his experiments with water. Jones [2] and Allen and Karayiannis [3] conducted reviews of electrohydrodynamically enhanced heat transfer. EHD enhancement of natural convection
* Corresponding author. Tel.: þ98 311 391 5631; fax: þ98 311 391 2677. E-mail address: [email protected] (M. Nasr Esfahany). 1290-0729/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ijthermalsci.2011.11.010
from horizontal heated wires and cylinders is represented by the work of Kronig and Schwarz [4] for gases, and Ahsmann and Kronig [5], for liquids. Macro and Grassi [6] studied natural convection in presence of electric filed under variable gravity filed during parabolic flights. Heat transfer intensification using nanofluids has attracted lots of interest recently. The published studies can be classified into three categories, namely, effective thermal conductivity under static conditions [7e15], convective heat transfer [16e26] and phase change heat transfer [27e30]. Most of the studies from the first category showed that nanofluids exhibited much higher thermal conductivities than those of base liquids. Several models have been proposed to explain such intensification of thermal conductivity [31e33]. Compared with research efforts in forced convective and phase change heat transfer of nanofluids, relatively few studies have been carried out on natural convective heat transfer. Khanafer et al. [34] numerically investigated heat transfer behavior of nanofluids in a two-dimensional horizontal enclosure and showed that suspended nanoparticles increased heat transfer at any given Grashof number substantially. Jou and Tzeng [35] investigated heat transfer performance of nanofluids inside twodimensional rectangular enclosures numerically and showed that heat transfer coefficient increased with the volume fraction of nanoparticles. Hwang et al. [36] have carried out a theoretical investigation of thermal characteristics of natural convection of an alumina-based nanofluid in a rectangular cavity heated from below using Jang and Choi’s model [33], for predicting the effective thermal conductivity of nanofluids (and various models for predicting the effective viscosity). Oztop and Abu-Nada [37]
F. Asadzadeh et al. / International Journal of Thermal Sciences 62 (2012) 114e119
Nomenclature C Cp D g hðexpÞ
Morgan correlation constant in Eq. (8) specific heat, J kg1 K1 diameter of wire, m acceleration of gravity, m s2 nanofluid experimental average heat transfer coefficient, W m2 K1 I applied current to the heater, A k thermal conductivity, W m1 K1 L heater length, m m mass, kg n Morgan correlation constant in Eq. (8) NuðthÞ average Nusslet number calculated from Morgan equation NuðexpÞ nanofluid experimental average Nusslet number q heat rate, W R resistance of the platinum wire, U Ra Rayleigh number T temperature, K investigated heat transfer and fluid flow due to buoyancy forces in a partially heated enclosure using nanofluids numerically and reported that heat transfer enhancement was more pronounced at lower aspect ratios. However, experimental results by Putra et al. [38] showed that natural convective heat transfer coefficient was lower than that of the base fluid. Similarly, Wen and Ding [39] reported that natural convection heat transfer deteriorated by addition of nanoparticles. Hwang et al. [36] reported that the ratio of heat transfer coefficient of nanofluids to that of the base fluid decreased with nanoparticles volume fraction. Polidori et al. [40] investigated the natural convection heat transfer of g-Al2O3ewater nanofluids and showed that the addition of nanoparticles deteriorated heat transfer. Review of the literature shows that there is still a controversy on the effect of nanofluids on natural convection heat transfer. Many experimental studies reported deterioration of natural convection heat transfer by addition of nanoparticles to the base fluid while numerical investigations reported the contrary. According to our literature review, natural convection of nanofluids around horizontal thin wires has not been studied yet. Besides, the effect of electric field on the natural convection in nanofluids has not been investigated so far. Therefore, the main aim of this study is to examine the natural convective heat transfer of Fe3O4 e ethylene glycol nanofluid around a thin wire in the absence and presence of electric field.
V v
115
applied voltage to the heater, V volume, m3
Greek symbols b thermal expansion coefficient, K1 h the ratio of the nanolayer thickness to the original particle radius m dynamic viscosity, N s m2 w kinematic viscosity, m2 s1 p Pi number, constant r density, kg m3 f volume fraction of nanoparticles Subscripts b bulk f base fluid film film condition nf nanofluid p nanoparticle s wire surface
HY3020) provided current to the platinum wire (6). Multimeters (model DEC330F) were used to measure both voltage and current supplied to the wire (7, 8). Pool temperature was measured with a calibrated thermocouple located 2 cm away from the wire and 1 cm below the free surface in the horizontal plane of the wire. A variable 0e30 kV high voltage power supply (9) was used to create and maintain electric field. The high voltage supplied to these electrodes was measured with a multimeter (model HIOKI/3010); (10). The platinum wire was submerged in nanofluid located 1 cm below the free surface. The breaking voltage for the discussed geometry was measured to be 15 kV.
2. Experimental set-up Fig. 1 shows schematic of the experimental set-up. Different parts of the experimental set-up as shown in the figure are explained with the numbers in the parentheses. The housing of the experimental apparatus was made from a 13 cm diameter, 18 cm long Pyrex cylinder (1). Poly Ethylene plate was used at the top of the housing to seal the cylinder from the atmosphere (2). A platinum wire, 0.32 mm diameter and 7.5 cm long, immersed in the nanofluid was used as the heat source (3). The platinum wire was attached to two 1 cm diameter copper rods (4), which were electrically isolated using Teflon spacers. Three stainless steel wires (0.8 mm diameter, 1.5 cm long) were soldered to a copper rod (5), located 3.5 cm above the platinum wire (2.5 cm above the free surface) and acted as high voltage charged electrode while the platinum wire (1.0 cm below the free surface) itself acted as grounded electrode. A direct current (DC) power supply (model
Fig. 1. Experimental set-up 1 e Pyrex beaker 2 e poly ethylene spacer 3 e platinume wire 4 e copper rod 5 e high voltage electrodes 6 e heater power supply 7 e Voltmeter 8 e Ampermeter 9 e high voltage power supply 10 e high voltage measurement multimeter.
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3. Nanofluid preparation
In equation (4), q is heat transfer rate calculated from equation (7).
Fe3O4 nanoparticles from Sigma Aldrich (Germany) with spherical shape and mean diameter of less than 50 nm were suspended in the base liquid (ethylene glycol). Nanofluid was made in 0.2 L batches. To make 0.2 L nanofluid with specified volume fraction of nanoparticles, required volume and mass of nanoparticles were calculated by using Eqs. (1) and (2), respectively.
vp ¼ 2 104 f
(1)
mp ¼ rp $vp
(2)
Required mass of nanoparticles were weighed with a digital balance (accurate to 104 g). Nanoparticles were gradually added to the base fluid in the presence of vigorous mechanical agitation. The suspension was then sonicated for an hour in the ultrasonic bath (model UP200S). 0.6 L nanofluid was used in each experiment. The depth of the liquid pool was about 5 cm in all experiments. No sedimentation was observed after 12 h in the glass beaker containing nanofluid. Fresh nanofluid was prepared and used in each run. Four nanofluids with nanoparticle volume fractions of 0.015, 0.02, 0.05, and 0.1 were prepared. 4. Experiments
5. Data processing
R ¼ 0:00036T þ 0:013
(3)
Equation (3) was developed in the laboratory prior to conducting experiments. The wire was immersed in a constant temperature bath. By changing the temperature of the bath, resistance of the wire was measured by a Wheatstone bridge. Linear relation between resistance and temperature was then fitted to the measured data and equation (3) was obtained. The coefficient of determination, r2, for the fitting was 0.91. Average heat transfer coefficient (hnf), Nusselt number (Nu), and Rayleigh number (Ra) were calculated from Eqs. (4), (5), and (6), respectively.
Ra ¼
q
pDLðTs Tb Þ hnf $D knf
gðTs Tb ÞD3 b w2
Experimental uncertainty analysis was performed following the method proposed in [40]. Experimental uncertainties in hnf ðexpÞ, NuðexpÞ, and Ra are 1%, 1.5% and 1%, respectively. Experimental results were compared with Morgan correlation as presented in Eq (8).
NuðthÞ ¼ CRan
(8)
Nanofluid properties were calculated using the following correlations for density, specific heat, and thermal expansion coefficient at mean temperature [41e43].
rnf ¼ ð1 fÞrf þ frp ð1 fÞrf Cpf þ frp Cpp
Cpnf ¼
bnf ¼
rnf
ð1 fÞrf bf þ frp bp
rnf
mnf ¼ mf ð1 þ 2:5fÞ
(9)
(10)
(11)
(4)
(5)
(6)
(12)
Yu and Choi’s correlation was used for determination of the nanofluid effective thermal conductivity [43]:
kp þ 2kf þ 2 kp kf ð1 þ hÞ3 f knf ¼ kf kp þ 2kf kp kf ð1 þ hÞ3 f
(13)
In Eq. (13), h is the ratio of the nanolayer thickness to the original particle radius and in this investigation h ¼ 0.1 was used [18,19,26]. All properties were estimated at film temperature.
Tfilm ¼
The resistance of the platinum wire was calculated by dividing the measured voltage across the wire by the measured current. The temperature of the wire was then calculated from:
NuðexpÞ ¼
(7)
Viscosity was calculated using Einstein’s correlation [42].
A typical experiment involved washing the test beaker by deionized water and drying it in oven followed by filling with the nanofluid. Different voltages were applied to the platinum wire as heat source. The steady state readings of thermocouple and multimeters were recorded. The multimeter was accurate to 0.5% for currents greater than 1 A and 0.3% for the voltages less than 1 V. Resistance of the platinum wire varies linearly with temperature. This property was used to calculate the wire temperature. Experiments were performed with base fluid and nanofluids both in absence and presence of electric fields. Each run was repeated at least three times.
hnf ðexpÞ ¼
q ¼ VI
Ts þ Tb 2
(14)
6. Results and discussion 6.1. Base fluid e ethylene glycol Initial experiments were conducted with ethylene glycol to check the performance of the experimental set-up. Fig. 2 shows the experimental Nu number for ethylene glycol versus Ra number. In the same figure predictions by Morgan Correlation is plotted (C ¼ 1.02, n ¼ 0.148 taken from [44]). The maximum difference between the experimental results and predictions is less than 6%. Fig. 3 shows variation of natural convective heat transfer coefficient with Rayligh number for different supplied voltages for ethylene glycol. Symbols show the measurements while the dotted lines show the trends for different supplied voltages. It can be seen that presence of electric field promotes natural convection heat transfer. The intensification of heat transfer coefficient decreases with Ra at constant electric field. Buoyancy driven flow becomes more significant at higher Rayligh numbers making the contribution of secondary flows due to the electric field negligible therefore, less intensification of heat transfer is seen at higher Rayligh numbers. Morgan predictions are plotted in Fig. 3 for reference. Enhancement ratio which is defined as the ratio of the convective heat transfer coefficient in the presence of electric field
F. Asadzadeh et al. / International Journal of Thermal Sciences 62 (2012) 114e119
Fig. 2. Nu number versus Ra number for pure ethylene glycol.
(hEHD) to the convective heat transfer coefficient in the absence of electric filed (ho) versus Rayligh number is plotted for ethylene glycol in Fig. 4. Symbols show the measurements and dotted lines show the trends. Enhancement ratio increases (decreases) with voltage (Rayligh number). Pascual et al. [45] studied natural convection around a thin platinum wire and reported similar decreasing trend in enhancement ratio with Rayligh number.
117
Fig. 4. Enhancement ratio for ethylene glycol.
Experiments with nanofluids were performed both in presence and absence of electric field. Enhancement ratio which is the ratio of the heat transfer coefficient of nanofluid to that of the base fluid for different volume fractions of nanoparticles in the absence of electric field is plotted in Fig. 5 as a function of Rayligh number. Symbols show the measurements while the dotted lines show the trends. It can be seen that enhancement ratio is greater than one for low Rayligh numbers (Ra < 3) for all volume fractions and remains greater than one for volume fractions of 0.0002 and 0.00015 at higher Rayligh numbers. For volume fractions of 0.0005 and 0.001 however, enhancement ratio is less than one at higher Rayligh numbers. In other words for low Rayligh numbers nanofluids show higher heat transfer coefficient than pure ethylene glycol but for higher Ra numbers heat transfer coefficients of nanofluids with volume concentrations of 0.05 and 0.1% are less than pure ethylene glycol. The decreasing trend of Nu number with volume fraction of nanoparticles has been previously reported [38,39]. However, our
experiments showed that deterioration of natural convective heat transfer occurs for concentrations greater than a specific value (0.02% in this work) while intensification was observed for smaller volume fractions. Deterioration of natural convective heat transfer at greater volume fractions of nanoparticles could be due to the enhanced viscosity of more concentrated nanofluids. Increased viscosity increases the flow resistance thus natural convective heat transfer deteriorates. On the other hand nanoparticles may deposit and form a thin layer on the surface of the heater resulting in extra thermal resistance, decreasing heat transfer. For low concentration nanofluids (0.015, 0.02%) enhancement ratio is greater than one since nanofluids are less susceptible to deposition and viscosity enhancement is not that significant. Fig. 6 shows heat transfer coefficients for 0.02% nanofluid in presence of electric field. Trends are plotted with dotted lines in the figure. Heat transfer augmented in the presence of electric field. Similar behavior was observed for nanofluids with different volume fractions (not shown). The enhancement ratio which is the ratio of the heat transfer coefficient of nanofluid under electric field to that of the nanofluid in absence of electric field is plotted in Fig. 7 for 0.02% nanofluid. Again, dotted lines show the trends. Similar behavior was observed for different volume fractions (not shown). Enhancement ratio increased with applied voltage while decreased with Rayligh number. Electric body force in the presence of electric field disturbs the boundary layer and decreases the heat transfer resistance. Buoyancy driven flow intensifies with Rayligh number. Reduction of heat transfer resistance due to the disturbances
Fig. 3. Nusselt number for base fluid under electric fields.
Fig. 5. Enhancement ratio of nanofluids versus Ra number.
6.2. Nanofluids e Fe3O4/ethylene glycol
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F. Asadzadeh et al. / International Journal of Thermal Sciences 62 (2012) 114e119
Fig. 6. Heat transfer coefficient for 0.02% nanofluid.
induced in the flow by electric field becomes less significant at higher Rayligh numbers. Thus enhancement ratio decreases with Rayligh number. Fig. 8 shows the natural convective heat transfer coefficient of nanofluid in the presence of electric filed (supplied voltage ¼ 12.5 kV) versus Rayligh number. Dotted lines represent the trends. It can be seen that in the presence of electric field heat transfer intensifies by addition of the nanoparticles up to 0.02%. Increasing the volume fraction of nanoparticles after 0.02% deteriorates heat transfer. Recall that heat transfer coefficients of 0.05% nanofluids were less than that of the pure ethylene glycol in the absence of the electric field (Fig. 5). In the presence of electric field however, heat transfer coefficients of 0.05% nanofluids is greater than that of pure ethylene glycol. Although addition of nanoparticles with the volume fraction of 0.0005 deteriorates heat transfer due to the intensified viscosity and probable deposition of nanoparticles on the heating surface in the absence of electric field, electric body forces introduce disturbances in the boundary layer promoting heat transfer that overcomes heat transfer deterioration by the former mechanisms. For 0.1% nanofluids however, disturbances by electric body forces although intensify heat transfer, cannot overcome heat flow deterioration therefore, heat transfer coefficients in 0.1% nanofluids in the presence of electric field were less than that of pure ethylene glycol. The maximum heat transfer coefficient was measured for 0.02% nanofluids with a supplied voltage of 12.5 kV at very low Rayligh number.
Fig. 7. Enhancement ratio for 0.02% nanofluid.
Fig. 8. Heat transfer coefficient of nanofluids in presence of electric field (supplied voltage ¼ 12.5 V).
7. Conclusions This paper is concerned with natural convective heat transfer of Fe3O4/ethylene glycol nanofluid from a thin platinum wire under electric field. Experiments showed that EHD enhanced natural convection heat transfer of pure ethylene glycol. Enhancement increases with supplied voltage while decreases with Ra number. Nanoparticles up to the volume fraction of 0.02% augmented natural convection heat transfer while further increase in volume fraction of nanoparticles deteriorated heat transfer. It is suggested that there is a particular volume fraction that for less and more concentrated nanofluids than that heat transfer promoted and deteriorated, respectively. In the presence of electric filed enhancement of heat transfer for ethylene glycol and nanofluids were observed. Increasing electric field intensity and Rayligh number increased and decreased heat transfer enhancement, respectively. Maximum natural convection heat transfer coefficient obtained for 0.02% nanofluids under 12.5 kV supplied voltage at low Rayligh numbers.
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Contents lists available at SciVerse ScienceDirect
International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts
Natural convective heat transfer of Fe3O4/ethylene glycol nanofluid in electric field F. Asadzadeh, M. Nasr Esfahany*, N. Etesami Department of Chemical Engineering, Isfahan University of Technology, Isfahan 8415683111, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 March 2011 Received in revised form 9 November 2011 Accepted 10 November 2011 Available online 6 December 2011
Natural convective heat transfer of Fe3O4/Ethylene Glycol nanofluids was examined in the presence of electric field around a thin platinum wire. Results showed that addition of nanoparticles to ethylene glycol promoted heat transfer up to the volume fraction of 0.02%. Increasing the volume fraction further however, deteriorated heat transfer. Applying the electric field intensified natural convective heat transfer of both ethylene glycol and nanofluids. Enhancement increased with electric field intensity while decreased with Rayleigh number. Electric field delayed deterioration of heat transfer of nanofluids to greater volume fractions. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Natural convection Nanofluids Electrohydrodynamic Heat transfer enhancement
1. Introduction Heat transfer enhancement improves the performance of existing heat exchangers or results in considerable decrease in heat transfer area at the same performance in design stage. In general, enhancement methods can be classified into active and passive techniques. The former requires external forces, e.g. surface vibration and applying electric field while the latter uses additives or special changes in surface geometries. Boundary layers formed on the thermally active surfaces offer a significant resistance to transfer of heat. Enhancement techniques therefore, target the boundary layer structure to lessen the resistance of the flow of heat. One of the active techniques in heat transfer enhancement is electrohydrodynamic (EHD) in which a high-voltage low-current electric field is applied in the dielectric heat transfer medium. The presence of a high electric field between a discharged and a grounded electrode induces secondary flows, disturbs the resistive boundary layer and promotes heat transfer. EHD is furnished with many advantages such as ease of control of performance by varying the electric field intensity, low power requirements, and simple design. The ability of a high voltage electric field to enhance heat transfer in a dielectric fluid has been known since Chubb [1] performed his experiments with water. Jones [2] and Allen and Karayiannis [3] conducted reviews of electrohydrodynamically enhanced heat transfer. EHD enhancement of natural convection
* Corresponding author. Tel.: þ98 311 391 5631; fax: þ98 311 391 2677. E-mail address: [email protected] (M. Nasr Esfahany). 1290-0729/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ijthermalsci.2011.11.010
from horizontal heated wires and cylinders is represented by the work of Kronig and Schwarz [4] for gases, and Ahsmann and Kronig [5], for liquids. Macro and Grassi [6] studied natural convection in presence of electric filed under variable gravity filed during parabolic flights. Heat transfer intensification using nanofluids has attracted lots of interest recently. The published studies can be classified into three categories, namely, effective thermal conductivity under static conditions [7e15], convective heat transfer [16e26] and phase change heat transfer [27e30]. Most of the studies from the first category showed that nanofluids exhibited much higher thermal conductivities than those of base liquids. Several models have been proposed to explain such intensification of thermal conductivity [31e33]. Compared with research efforts in forced convective and phase change heat transfer of nanofluids, relatively few studies have been carried out on natural convective heat transfer. Khanafer et al. [34] numerically investigated heat transfer behavior of nanofluids in a two-dimensional horizontal enclosure and showed that suspended nanoparticles increased heat transfer at any given Grashof number substantially. Jou and Tzeng [35] investigated heat transfer performance of nanofluids inside twodimensional rectangular enclosures numerically and showed that heat transfer coefficient increased with the volume fraction of nanoparticles. Hwang et al. [36] have carried out a theoretical investigation of thermal characteristics of natural convection of an alumina-based nanofluid in a rectangular cavity heated from below using Jang and Choi’s model [33], for predicting the effective thermal conductivity of nanofluids (and various models for predicting the effective viscosity). Oztop and Abu-Nada [37]
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Nomenclature C Cp D g hðexpÞ
Morgan correlation constant in Eq. (8) specific heat, J kg1 K1 diameter of wire, m acceleration of gravity, m s2 nanofluid experimental average heat transfer coefficient, W m2 K1 I applied current to the heater, A k thermal conductivity, W m1 K1 L heater length, m m mass, kg n Morgan correlation constant in Eq. (8) NuðthÞ average Nusslet number calculated from Morgan equation NuðexpÞ nanofluid experimental average Nusslet number q heat rate, W R resistance of the platinum wire, U Ra Rayleigh number T temperature, K investigated heat transfer and fluid flow due to buoyancy forces in a partially heated enclosure using nanofluids numerically and reported that heat transfer enhancement was more pronounced at lower aspect ratios. However, experimental results by Putra et al. [38] showed that natural convective heat transfer coefficient was lower than that of the base fluid. Similarly, Wen and Ding [39] reported that natural convection heat transfer deteriorated by addition of nanoparticles. Hwang et al. [36] reported that the ratio of heat transfer coefficient of nanofluids to that of the base fluid decreased with nanoparticles volume fraction. Polidori et al. [40] investigated the natural convection heat transfer of g-Al2O3ewater nanofluids and showed that the addition of nanoparticles deteriorated heat transfer. Review of the literature shows that there is still a controversy on the effect of nanofluids on natural convection heat transfer. Many experimental studies reported deterioration of natural convection heat transfer by addition of nanoparticles to the base fluid while numerical investigations reported the contrary. According to our literature review, natural convection of nanofluids around horizontal thin wires has not been studied yet. Besides, the effect of electric field on the natural convection in nanofluids has not been investigated so far. Therefore, the main aim of this study is to examine the natural convective heat transfer of Fe3O4 e ethylene glycol nanofluid around a thin wire in the absence and presence of electric field.
V v
115
applied voltage to the heater, V volume, m3
Greek symbols b thermal expansion coefficient, K1 h the ratio of the nanolayer thickness to the original particle radius m dynamic viscosity, N s m2 w kinematic viscosity, m2 s1 p Pi number, constant r density, kg m3 f volume fraction of nanoparticles Subscripts b bulk f base fluid film film condition nf nanofluid p nanoparticle s wire surface
HY3020) provided current to the platinum wire (6). Multimeters (model DEC330F) were used to measure both voltage and current supplied to the wire (7, 8). Pool temperature was measured with a calibrated thermocouple located 2 cm away from the wire and 1 cm below the free surface in the horizontal plane of the wire. A variable 0e30 kV high voltage power supply (9) was used to create and maintain electric field. The high voltage supplied to these electrodes was measured with a multimeter (model HIOKI/3010); (10). The platinum wire was submerged in nanofluid located 1 cm below the free surface. The breaking voltage for the discussed geometry was measured to be 15 kV.
2. Experimental set-up Fig. 1 shows schematic of the experimental set-up. Different parts of the experimental set-up as shown in the figure are explained with the numbers in the parentheses. The housing of the experimental apparatus was made from a 13 cm diameter, 18 cm long Pyrex cylinder (1). Poly Ethylene plate was used at the top of the housing to seal the cylinder from the atmosphere (2). A platinum wire, 0.32 mm diameter and 7.5 cm long, immersed in the nanofluid was used as the heat source (3). The platinum wire was attached to two 1 cm diameter copper rods (4), which were electrically isolated using Teflon spacers. Three stainless steel wires (0.8 mm diameter, 1.5 cm long) were soldered to a copper rod (5), located 3.5 cm above the platinum wire (2.5 cm above the free surface) and acted as high voltage charged electrode while the platinum wire (1.0 cm below the free surface) itself acted as grounded electrode. A direct current (DC) power supply (model
Fig. 1. Experimental set-up 1 e Pyrex beaker 2 e poly ethylene spacer 3 e platinume wire 4 e copper rod 5 e high voltage electrodes 6 e heater power supply 7 e Voltmeter 8 e Ampermeter 9 e high voltage power supply 10 e high voltage measurement multimeter.
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3. Nanofluid preparation
In equation (4), q is heat transfer rate calculated from equation (7).
Fe3O4 nanoparticles from Sigma Aldrich (Germany) with spherical shape and mean diameter of less than 50 nm were suspended in the base liquid (ethylene glycol). Nanofluid was made in 0.2 L batches. To make 0.2 L nanofluid with specified volume fraction of nanoparticles, required volume and mass of nanoparticles were calculated by using Eqs. (1) and (2), respectively.
vp ¼ 2 104 f
(1)
mp ¼ rp $vp
(2)
Required mass of nanoparticles were weighed with a digital balance (accurate to 104 g). Nanoparticles were gradually added to the base fluid in the presence of vigorous mechanical agitation. The suspension was then sonicated for an hour in the ultrasonic bath (model UP200S). 0.6 L nanofluid was used in each experiment. The depth of the liquid pool was about 5 cm in all experiments. No sedimentation was observed after 12 h in the glass beaker containing nanofluid. Fresh nanofluid was prepared and used in each run. Four nanofluids with nanoparticle volume fractions of 0.015, 0.02, 0.05, and 0.1 were prepared. 4. Experiments
5. Data processing
R ¼ 0:00036T þ 0:013
(3)
Equation (3) was developed in the laboratory prior to conducting experiments. The wire was immersed in a constant temperature bath. By changing the temperature of the bath, resistance of the wire was measured by a Wheatstone bridge. Linear relation between resistance and temperature was then fitted to the measured data and equation (3) was obtained. The coefficient of determination, r2, for the fitting was 0.91. Average heat transfer coefficient (hnf), Nusselt number (Nu), and Rayleigh number (Ra) were calculated from Eqs. (4), (5), and (6), respectively.
Ra ¼
q
pDLðTs Tb Þ hnf $D knf
gðTs Tb ÞD3 b w2
Experimental uncertainty analysis was performed following the method proposed in [40]. Experimental uncertainties in hnf ðexpÞ, NuðexpÞ, and Ra are 1%, 1.5% and 1%, respectively. Experimental results were compared with Morgan correlation as presented in Eq (8).
NuðthÞ ¼ CRan
(8)
Nanofluid properties were calculated using the following correlations for density, specific heat, and thermal expansion coefficient at mean temperature [41e43].
rnf ¼ ð1 fÞrf þ frp ð1 fÞrf Cpf þ frp Cpp
Cpnf ¼
bnf ¼
rnf
ð1 fÞrf bf þ frp bp
rnf
mnf ¼ mf ð1 þ 2:5fÞ
(9)
(10)
(11)
(4)
(5)
(6)
(12)
Yu and Choi’s correlation was used for determination of the nanofluid effective thermal conductivity [43]:
kp þ 2kf þ 2 kp kf ð1 þ hÞ3 f knf ¼ kf kp þ 2kf kp kf ð1 þ hÞ3 f
(13)
In Eq. (13), h is the ratio of the nanolayer thickness to the original particle radius and in this investigation h ¼ 0.1 was used [18,19,26]. All properties were estimated at film temperature.
Tfilm ¼
The resistance of the platinum wire was calculated by dividing the measured voltage across the wire by the measured current. The temperature of the wire was then calculated from:
NuðexpÞ ¼
(7)
Viscosity was calculated using Einstein’s correlation [42].
A typical experiment involved washing the test beaker by deionized water and drying it in oven followed by filling with the nanofluid. Different voltages were applied to the platinum wire as heat source. The steady state readings of thermocouple and multimeters were recorded. The multimeter was accurate to 0.5% for currents greater than 1 A and 0.3% for the voltages less than 1 V. Resistance of the platinum wire varies linearly with temperature. This property was used to calculate the wire temperature. Experiments were performed with base fluid and nanofluids both in absence and presence of electric fields. Each run was repeated at least three times.
hnf ðexpÞ ¼
q ¼ VI
Ts þ Tb 2
(14)
6. Results and discussion 6.1. Base fluid e ethylene glycol Initial experiments were conducted with ethylene glycol to check the performance of the experimental set-up. Fig. 2 shows the experimental Nu number for ethylene glycol versus Ra number. In the same figure predictions by Morgan Correlation is plotted (C ¼ 1.02, n ¼ 0.148 taken from [44]). The maximum difference between the experimental results and predictions is less than 6%. Fig. 3 shows variation of natural convective heat transfer coefficient with Rayligh number for different supplied voltages for ethylene glycol. Symbols show the measurements while the dotted lines show the trends for different supplied voltages. It can be seen that presence of electric field promotes natural convection heat transfer. The intensification of heat transfer coefficient decreases with Ra at constant electric field. Buoyancy driven flow becomes more significant at higher Rayligh numbers making the contribution of secondary flows due to the electric field negligible therefore, less intensification of heat transfer is seen at higher Rayligh numbers. Morgan predictions are plotted in Fig. 3 for reference. Enhancement ratio which is defined as the ratio of the convective heat transfer coefficient in the presence of electric field
F. Asadzadeh et al. / International Journal of Thermal Sciences 62 (2012) 114e119
Fig. 2. Nu number versus Ra number for pure ethylene glycol.
(hEHD) to the convective heat transfer coefficient in the absence of electric filed (ho) versus Rayligh number is plotted for ethylene glycol in Fig. 4. Symbols show the measurements and dotted lines show the trends. Enhancement ratio increases (decreases) with voltage (Rayligh number). Pascual et al. [45] studied natural convection around a thin platinum wire and reported similar decreasing trend in enhancement ratio with Rayligh number.
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Fig. 4. Enhancement ratio for ethylene glycol.
Experiments with nanofluids were performed both in presence and absence of electric field. Enhancement ratio which is the ratio of the heat transfer coefficient of nanofluid to that of the base fluid for different volume fractions of nanoparticles in the absence of electric field is plotted in Fig. 5 as a function of Rayligh number. Symbols show the measurements while the dotted lines show the trends. It can be seen that enhancement ratio is greater than one for low Rayligh numbers (Ra < 3) for all volume fractions and remains greater than one for volume fractions of 0.0002 and 0.00015 at higher Rayligh numbers. For volume fractions of 0.0005 and 0.001 however, enhancement ratio is less than one at higher Rayligh numbers. In other words for low Rayligh numbers nanofluids show higher heat transfer coefficient than pure ethylene glycol but for higher Ra numbers heat transfer coefficients of nanofluids with volume concentrations of 0.05 and 0.1% are less than pure ethylene glycol. The decreasing trend of Nu number with volume fraction of nanoparticles has been previously reported [38,39]. However, our
experiments showed that deterioration of natural convective heat transfer occurs for concentrations greater than a specific value (0.02% in this work) while intensification was observed for smaller volume fractions. Deterioration of natural convective heat transfer at greater volume fractions of nanoparticles could be due to the enhanced viscosity of more concentrated nanofluids. Increased viscosity increases the flow resistance thus natural convective heat transfer deteriorates. On the other hand nanoparticles may deposit and form a thin layer on the surface of the heater resulting in extra thermal resistance, decreasing heat transfer. For low concentration nanofluids (0.015, 0.02%) enhancement ratio is greater than one since nanofluids are less susceptible to deposition and viscosity enhancement is not that significant. Fig. 6 shows heat transfer coefficients for 0.02% nanofluid in presence of electric field. Trends are plotted with dotted lines in the figure. Heat transfer augmented in the presence of electric field. Similar behavior was observed for nanofluids with different volume fractions (not shown). The enhancement ratio which is the ratio of the heat transfer coefficient of nanofluid under electric field to that of the nanofluid in absence of electric field is plotted in Fig. 7 for 0.02% nanofluid. Again, dotted lines show the trends. Similar behavior was observed for different volume fractions (not shown). Enhancement ratio increased with applied voltage while decreased with Rayligh number. Electric body force in the presence of electric field disturbs the boundary layer and decreases the heat transfer resistance. Buoyancy driven flow intensifies with Rayligh number. Reduction of heat transfer resistance due to the disturbances
Fig. 3. Nusselt number for base fluid under electric fields.
Fig. 5. Enhancement ratio of nanofluids versus Ra number.
6.2. Nanofluids e Fe3O4/ethylene glycol
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Fig. 6. Heat transfer coefficient for 0.02% nanofluid.
induced in the flow by electric field becomes less significant at higher Rayligh numbers. Thus enhancement ratio decreases with Rayligh number. Fig. 8 shows the natural convective heat transfer coefficient of nanofluid in the presence of electric filed (supplied voltage ¼ 12.5 kV) versus Rayligh number. Dotted lines represent the trends. It can be seen that in the presence of electric field heat transfer intensifies by addition of the nanoparticles up to 0.02%. Increasing the volume fraction of nanoparticles after 0.02% deteriorates heat transfer. Recall that heat transfer coefficients of 0.05% nanofluids were less than that of the pure ethylene glycol in the absence of the electric field (Fig. 5). In the presence of electric field however, heat transfer coefficients of 0.05% nanofluids is greater than that of pure ethylene glycol. Although addition of nanoparticles with the volume fraction of 0.0005 deteriorates heat transfer due to the intensified viscosity and probable deposition of nanoparticles on the heating surface in the absence of electric field, electric body forces introduce disturbances in the boundary layer promoting heat transfer that overcomes heat transfer deterioration by the former mechanisms. For 0.1% nanofluids however, disturbances by electric body forces although intensify heat transfer, cannot overcome heat flow deterioration therefore, heat transfer coefficients in 0.1% nanofluids in the presence of electric field were less than that of pure ethylene glycol. The maximum heat transfer coefficient was measured for 0.02% nanofluids with a supplied voltage of 12.5 kV at very low Rayligh number.
Fig. 7. Enhancement ratio for 0.02% nanofluid.
Fig. 8. Heat transfer coefficient of nanofluids in presence of electric field (supplied voltage ¼ 12.5 V).
7. Conclusions This paper is concerned with natural convective heat transfer of Fe3O4/ethylene glycol nanofluid from a thin platinum wire under electric field. Experiments showed that EHD enhanced natural convection heat transfer of pure ethylene glycol. Enhancement increases with supplied voltage while decreases with Ra number. Nanoparticles up to the volume fraction of 0.02% augmented natural convection heat transfer while further increase in volume fraction of nanoparticles deteriorated heat transfer. It is suggested that there is a particular volume fraction that for less and more concentrated nanofluids than that heat transfer promoted and deteriorated, respectively. In the presence of electric filed enhancement of heat transfer for ethylene glycol and nanofluids were observed. Increasing electric field intensity and Rayligh number increased and decreased heat transfer enhancement, respectively. Maximum natural convection heat transfer coefficient obtained for 0.02% nanofluids under 12.5 kV supplied voltage at low Rayligh numbers.
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