- Email: [email protected]
water nanofluids
ICHMT-03070; No of Pages 8 International Communications in Heat and Mass Transfer xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
3Q3
Mohammad Hemmat Esfe a,⁎, Seyfolah Saedodin a, Omid Mahian b, Somchai Wongwises c,⁎
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Thermophysical properties, heat transfer and pressure drop of COOH-functionalized multi walled carbon nanotubes/water nanofluids☆
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a r t i c l e
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Available online xxxx
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Keywords: Thermophysical properties Heat transfer Pressure drop Carbon nanotubes Nanofluids
Faculty of Mechanical Engineering, Semnan University, P. O. Box 3513119111 Semnan, Iran Young Researchers and Elite Club, Mashhad Branch, Islamic Azad University, Mashhad, Iran Fluid Mechanics, Thermal Engineering and Multiphase Flow Research Lab. (FUTURE), Department of Mechanical Engineering, Faculty of Engineering, King Mongkut's University of Technology Thonburi, Bangmod, Bangkok 10140, Thailand b
a b s t r a c t
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This paper is a continuation of the authors' previous work on the thermophysical properties, heat transfer, and pressure drop of nanofluids [Experimental Thermal and Fluid Science 52 (2014) 68–78]. In this paper, an experimental study is carried out to study the turbulent flow of COOH-functionalized multi-walled carbon nanotubes/ water nanofluid flowing through a double tube heat exchanger. For this purpose, first, the thermophysical properties of the nanofluid, including the thermal conductivity and dynamic viscosity, have been measured at various temperatures and concentrations. Using the measured data, new correlations as a function of temperature and concentration are presented to predict the thermophysical properties. In the next step, the effects of low volume fractions of the nanofluid (from 0.05% to 1%) on the heat transfer rate are studied at the Reynolds numbers between 5000 and 27,000. The experimental results reveal that with increasing the nanofluid concentration, the heat transfer coefficient and thermal performance factor increase. On average, a 78% increase in heat transfer coefficient, a 36.5% increase in the average Nusselt number, and a 27.3% penalty in the pressure drop are recorded for the highest concentration of MWCNTs in water. © 2014 Published by Elsevier Ltd.
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1. Introduction
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The use of nanofluids containing ultra-fine solid particles rather than common fluids such as water and industrial oil in industrial and engineering applications may be a solution to enhance the heat transfer rate, and, consequently, to establish more compact thermal systems. The main reason for the enhancement of heat transfer is the increase of the effective thermal conductivity of nanofluids having solid particles with the sizes between 1 and 100 nm. Among the usual nanoparticles, carbon nanotubes (CNTs) in single, double, or multi-walled types have attracted special interest due to unique thermal properties and structure. Therefore, CNTs can be a good option to disperse in the common working fluids to enhance the heat transfer. Here, a brief review of the reported works on the nanofluid flow in simple tubes or double tube heat exchangers is presented so that, in some of them, CNTs are used as the additive to the base fluid. Sundar et al. [1] investigated the turbulent flow and pressure drop of Al2O3/water nanofluid in a tube where the twisted tapes are installed
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☆ Communicated by W.J. Minkowycz. ⁎ Corresponding authors. E-mail addresses: [email protected] (M.H. Esfe), [email protected] (S. Wongwises).
inside the circular tube. They found that by using twisted tapes inside the tube the heat transfer coefficient increases up to 23% while the pressure drop raises 70% compared to the plain tube. Yu et al. [2] tested the silicon carbide/water nanofluid in a circular tube where the Reynolds number is between 3000 and 13000. They reported an increase in the heat transfer coefficient as high as 60% for the volume fraction of 3.7%. In a series of experimental studies, Duangthongsuk and Wongwises [3–5] focused on the TiO2/water nanofluid flow in a double tube heat exchanger under the turbulent regime. In the three papers, the authors examined the effects of solid volume fraction, and they used different thermophysical models on the heat transfer characteristics and pressure drop of the nanofluid flow. For instance, Duangthongsuk and Wongwises [5] found that for volume fractions between 0.2% and 2%, the optimum particle loading is 1% in which the maximum heat transfer is obtained. Farajollahi et al. [6] studied the turbulent flow of γ-Al2O3/water and TiO2/water nanofluids in a double tube heat exchanger for the Peclet numbers between 20,000 and 60,000. They found the optimum concentration of the nanofluids in which the heat transfer is maximized. Al2O3/water nanofluid flow through a plain tube where the Reynolds number changes in the range of 10,000 to 22,000 was studied by Sundar and Sharma [7]. They considered the effects of inserting the tapes with different twist ratios inside the tube in the nanofluid flow and heat transfer. They suggested related correlations to estimate the Nusselt number and friction factor. Zhang et al. [8] examined the heat transfer
http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.08.037 0735-1933/© 2014 Published by Elsevier Ltd.
Please cite this article as: M.H. Esfe, et al., Thermophysical properties, heat transfer and pressure drop of COOH-functionalized multi walled carbon nanotubes/water nanofluids, Int. Commun. Heat Mass Transf. (2014), http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.08.037
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There are two ways to have stable nanofluids containing carbon nanotubes. The first is the use of a surfactant such as Arabic gum, and the second is the functionalization of the carbon nanotubes. Adding a surfactant may have undesired effects on the thermal conductivity of the suspensions. Then, using functionalized carbon nanotubes seems more appropriate. Functionalizing MWCNTs using COOH functional groups makes the carbon nanotubes hydrophilic; consequently, the stability of the prepared nanofluids is improved. The properties of COOHfunctionalized MWCNTs have been described in Table 1. In this investigation, pure water is used as the base fluid. COOHfunctionalized MWCNTs with solid volume fractions of 0.01 (1.0%), 0.008 (0.8%), 0.004 (0.4%), 0.002 (0.2%), 0.001 (0.1%), and 0.0005 (0.05%) are prepared by dispersing a specified amount of MWCNTs in the distilled water. MWCNTs are mixed with water by means of a magnetic stirrer for 2 h. Next, an ultrasonic processor for 4 h with the power of 400 W and frequency of 24 kHz is used to break down the agglomeration of the nanoparticles and make stable suspensions. After 48 h, no sedimentation was observed (with the naked eye) in the samples.
159 160
4. Data processing
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161 Q6 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176
Applying the energy law to a differential control volume of the fluid 178 inside the copper tube gives [22]: 179 dT b p ¼ hðT s −T b Þ: ˙ p dx mC
ð1Þ 181
After separating variables and integrating, the following equation is: 0 L 1 Z ðΔT 0 Þ −pL @1 ¼ hdxA: Ln ˙ ΔTi L mCp
ð2Þ
0
183
After simplifying, ! ΔT 0 T s −T b;o −pL ¼ ¼ exp h mC p ΔT i T s −T b;i
ð3Þ 185
Rearranging Eq. (3), heat transfer coefficient is obtained as: A schematic view of the experimental apparatus is illustrated in Fig. 1. It consists of three centrifugal pumps, a test section, a heat exchanger, a flow meter, a differential pressure transmitter, a nanofluid tank, a water tank, and a data acquisition system.
140 141
3. Nanofluid preparation
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2. Experimental set-up
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The heat transfer test section consists of a double tube heat exchanger. K-type thermocouples (Chromel–Alumel) are inserted along the tube length to measure the wall temperature. Two thermometers with digital indicators are inserted at the inlet and outlet of the test section to record the inlet and outlet temperatures of the nanofluid. For turbulent flow, the entrance region (not fully developed length) in a tube is obtained based on (Le/D ≈ 4.4 × Re1/6) [21]. In the experimental set-up, this length is less than 17 cm; therefore, all measurements are taken in the fully developed region. In the double tube heat exchanger, the inner tube is heated by hot water which flows in the outer copper tube. The flow and temperature of hot water are regulated using control instruments. A temperature controller with a PT100 sensor is utilized to control the temperature of hot water. Two K-type thermocouples measure the inlet and outlet bulk temperature of hot water. The test section is insulated by fiber glass with a thickness of 7 cm to minimize the heat dissipation. The pressure drop of the nanofluid through the test section is measured by a differential pressure transmitter (DPT Rosemount 3051cd).
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characteristics of CuO/water nanofluids in the turbulent flow where the nanoparticles can have three different sizes — i.e., 23, 51, and 76 nm. They perceived that a larger particle size leads to a higher heat transfer rate. Amrollahi et al. [9] measured the heat transfer rate of functionalized multi-walled carbon nanotubes suspended in water as the base fluid under laminar and turbulent regimes where the nanofluid flows through a horizontal circular tube in the entrance region. They observed that the heat transfer coefficient increases between 33% and 40% for the nanofluid with a weight fraction of 0.25% compared to the pure water. Tumuluri et al. [10] investigated the turbulent flow of the nanofluids, which are combined with multi-walled carbon nanotubes (MWCNTs) and microencapsulated phase change materials (MPCMs) through a circular tube under constant heat flux. Hojjat et al. [11,12] evaluated the turbulent flow of non-Newtonian nanofluids in a circular tube. They provided three different types of nanofluids where the nanoparticles are Al2O3, CuO, and TiO2, and the base fluid is an aqueous solution of carboxymethyl cellulose. Suresh et al. [13] studied the heat transfer and pressure drop of CuO/ water nanofluid in the Reynolds number range of 2500 to 6000 in the simple and helically dimpled tubes. They revealed that using the nanofluid with a concentration of 0.3% through the dimpled tube leads to 39% heat transfer enhancement compared to the case in which the pure water flows in the simple tube. Suresh et al. [14] reported a heat transfer enhancement as high as 48% when Al2O3/water nanofluid is used with the volume fraction of 0.5% through a circular tube with spiral rods. Heat transfer and pressure drop due to silver/water nanofluid in a counter flow heat exchanger under laminar and turbulent regimes investigated by Godson et al. [15]. They observed a 70% increase in the heat transfer coefficient using the nanofluid with the concentration of 0.9%. Arani and Amani [16] studied TiO2/water nanofluid flow in a horizontal, double tube, counter flow heat exchanger for a wide range of Reynolds numbers — i.e., between 8000 and 51,000. By considering the relation between the heat transfer rate and the pumping power, they indicated that it is more advantageous to use nanofluid at the Reynolds numbers lower than 30,000. Heat transfer and pressure drop were found to be due to SiO2/water nanofluid in corrugated tubes tested by Darzi et al. [17]. Azmi et al. [18] investigated the of SiO2/water nanofluid flow in a circular tube under constant heat flux and turbulent flow conditions where the maximum solid volume fraction is 4%. For the Reynolds numbers between 5000 and 27000, they obtained the optimal concentration in which the heat transfer rate is maximized. Recently, Hemmat Esfe et al. [19] investigated the flow of MgO nanofluids with volume fractions of less than 1% in a circular tube. The range of Reynolds numbers was between 3000 and 18,000. They reported remarkable heat transfer enhancement due to using nanofluids. Celata et al. [20] tested TiO2/water (9 wt.%) and SiC/water (3, 6, 9 wt.%) nanofluids in a circular pipe under uniform heat flux and the Reynolds numbers between 300 and 6000. Based on the best knowledge of the authors, there are no data on the flow of water-based functionalized MWCNT nanofluids in double tube heat exchangers under fully developed flow conditions. In this paper, COOH-functionalized MWCNTs are dispersed in water to prepare nanofluids with concentrations up to 1%. First, thermophysical properties of the nanofluids, including thermal conductivity and viscosity, are measured. Next, flow and heat transfer characteristics of the nanofluids, which include heat transfer coefficient, pressure drop, and thermal performance, are evaluated in a double tube heat exchanger for the Reynolds numbers between 5000 and 27,000.
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2
˙ p mC T s −T b;i Ln h¼ pL T s −T b;o
! ð4Þ
Please cite this article as: M.H. Esfe, et al., Thermophysical properties, heat transfer and pressure drop of COOH-functionalized multi walled carbon nanotubes/water nanofluids, Int. Commun. Heat Mass Transf. (2014), http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.08.037
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T b;i þ T b;o : 2
5. Thermophysical properties of nanofluids
ð5Þ
N C O
Tb ¼
R
where bulk temperature is calculated by:
Finally, the Reynolds and Nusselt numbers are defined as follows:
Re ¼
Vd ν
190 191
Nu ¼
hd : k
Before the investigation of the convective heat transfer of the nanofluids, the thermophysical properties of a nanofluid must be accurately measured. The density and specific heat capacity are calculated by using the well-known relations, while thermal conductivity and viscosity are determined experimentally in the temperature range of 25– 55 °C.
194 195
5.1. Density and specific heat capacity
200
ð6Þ
ρnf ¼ φρp þ ð1−φÞρ f :
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201
The effective density (ρ) of nanofluids is given by:
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187
R
Fig. 1. Schematic diagram of the experimental setup.
ð12Þ 203
193
The specific heat capacity (cp) of the nanofluid is calculated by: t1:1 t1:2
Table 1 Properties of COOH-functionalized MWCNTs.
t1:3
Purity (wt%)
N97 (carbon content)
t1:4 t1:5 t1:6 t1:7
Inside diameter (nm) Length (μm) Color True density (g/cm3)
5–10 10–30 Black 2.1
cp;nf ¼
φ ρcp þ ð1−φÞ ρcp p
ρnf
f
ð13Þ
where the subscripts of nf, f, and p represent the nanofluid, the water 205 and the particle, respectively.
Please cite this article as: M.H. Esfe, et al., Thermophysical properties, heat transfer and pressure drop of COOH-functionalized multi walled carbon nanotubes/water nanofluids, Int. Commun. Heat Mass Transf. (2014), http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.08.037
M.H. Esfe et al. / International Communications in Heat and Mass Transfer xxx (2014) xxx–xxx
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The viscosity of the COOH-functionalized MWCNT–water nanofluid 251 is measured using a Brookfield viscometer with a temperature con- 252 trolled bath, supplied by Brookfield engineering laboratories of the USA. 253 μ
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Variations of relative viscosity (μnf ) of the working fluids versus bulk 254 w fluid temperature are depicted in Fig. 3. It is evident from the figure that 255 all the tested nanofluids with different solid volume fractions show a 256 downward trend with temperature. In addition, the relative viscosity in- 257 creases with the increase of the solid volume fraction of nanoparticles as 258 expected. 259 5.3.1. Proposed correlation The following correlation is proposed to determine the viscosity of COOH-functionalized MWCNTs/water nanofluids. This correlation is valid for volume fractions up to 1%, and the temperatures between 25 °C and 55 °C. μ nf ¼ 38:158ϕ−0:0017357T þ 1:1296 μw
ð15Þ
260 261 262 263 264
266
6. Results and discussions 267
6.1. Validation
5.2.1. Proposed correlation The following correlation is suggested to estimate the thermal conductivity of COOH-functionalized MWCNTs/water nanofluids that is obtained by the curve fitting of the experimental data. This correlation is 1.6
T = 25 (oC) T = 35 (oC) o T = 45 ( C) o T = 55 ( C)
1.5
1.4
1.3
Initially, experiments are performed with pure water, and the results 268 are compared to some well-known correlations for turbulent flow. The 269 correlations are as follows:Gnielinski correlation [25]: 270 Nu ¼
f =8ðRe−1000Þ Pr ! rffiffiffi 2 f 3 Pr −1 1 þ 12:7 8
1.6
2 # d 3 L
ð16Þ
o
1.3
1.2
1.1
1
1
0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009
1þ
1.4
1.1
0
!0:11 "
T = 25 ( C ) o T = 35 ( C ) o T = 45 ( C ) o T = 55 ( C )
1.5
1.2
0.9
Pr Pr f
which is applied in the range of 0.5 b Pr b 106 and 2300 b Re b 5 × 106. Pr 272 and Prf are the Prandtl numbers calculated at the water bulk
Relative viscosity
228 229
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R
222 223
R
220 221
5.3. Dynamic viscosity
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ð14Þ 250
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212 213
knf / kw
211
knf ð360:69 þ T Þ ¼ kw ð405:59−11080φÞ
R O
Many of the proposed models have failed to predict the accurate values of the thermal conductivity of nanofluids because many parameters such as the shape, the size, and the method of preparation affect the thermal conductivity. Therefore, in this paper, the thermal conductivity of nanofluids is determined experimentally using the transient hot wire technique that is an accurate and fast approach. Thermal conductivity of COOH-functionalized MWCNTs–water nanofluids with various solid volume fractions is measured by a KD2 Pro instrument manufactured by Decagon Devices Inc., which works based on the transient hot wire technique; the KD2 Pro measures the thermal conductivity at 1-s intervals during a 90-s measurement cycle. The samples are placed in a thermostatic bath so that the temperature of the samples reaches the desired temperature with a temperature difference not higher than ± 0.5 °C. Then, the measurements are conducted by KD2 Pro. The “enhanced thermal conductivity ratio” is defined as the ratio of nanofluid thermal conductivity to water thermal conductivity. Fig. 2 illustrates the measured enhanced thermal conductivity ratio of COOH-functionalized MWCNTs/water nanofluid as a function of the solid volume fraction in different temperatures. As can be seen from the figure, when the temperature increases, the thermal conductivity increases. This may happen because at higher temperatures, the agglomeration between the nanoparticles would break down more easily and, hence, CNTs will disperse more uniformly in the water. This leads to a better conduction of heat in the nanofluid. Further, random movement of the nanoparticles increases as the temperature raises; consequently, the heat energy is transmitted more quickly in the fluid. It is also observed that at a low concentration of nanofluid, the effect of the temperature is not considerable, whereas the effect of temperature is more considerable at higher concentrations. To explain this, consider a specified volume of the nanofluid: when the solid volume fraction increases, the number of collisions due to Brownian motion of particles increases. On the other hand, with an increase in the temperature, the kinetic energy of CNTs in the nanofluid increases, and hence the number of collisions among the nanoparticles increases. Therefore, increasing the temperature boosts the effects of increasing the solid volume fraction.
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valid for volume fractions up to 1% and the temperatures between 25 247 and 55 °C. 248
D
5.2. Measurement of thermal conductivity
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4
0.01
Solid volume fraction Fig. 2. Thermal conductivity ratio versus solid volume fraction at different temperatures.
0.9 -0.002
0
0.002
0.004
0.006
0.008
0.01
0.012
Solid volume fraction Fig. 3. Relative viscosity versus solid volume fraction at various temperatures.
Please cite this article as: M.H. Esfe, et al., Thermophysical properties, heat transfer and pressure drop of COOH-functionalized multi walled carbon nanotubes/water nanofluids, Int. Commun. Heat Mass Transf. (2014), http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.08.037
M.H. Esfe et al. / International Communications in Heat and Mass Transfer xxx (2014) xxx–xxx
273 274 275 276
278
30000
temperature and at the inner wall temperature, respectively; L is the tube length and d is the diameter of the tube. The bulk temperature is an average between the inlet and outlet fluid temperatures. In this formulation, the Darcy coefficient is proposed as follows: f ¼
1 ð1:82log 10 Re−1:64Þ2
5
Base fluid = 0.05 Vol% = 0.1 Vol% = 0.2 Vol% = 0.4 Vol% = 0.8 Vol% = 1 Vol%
25000
ð17Þ 20000
Dittus–Boelter correlation [22]: 0:8
Nu ¼ 0:023Re
0:4
15000
ð18Þ
Pr
Petukhov equation [26]:
5000
Xuan and Li equation [27]:
5000
0:001 0:6886 0:9238 0:4 Re Ped Pr : Nu ¼ 0:0059 1 þ 7:62φ
ð20Þ
10000
O
ð19Þ
R O
280
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10000
ð f =8ÞRe Pr Nu ¼ 2 1:07 þ 12:7ð f =8Þ0:5 Pr3 −1
15000
20000
25000
Fig. 5. Variations of heat transfer coefficient with Re for different solid volume fractions.
6.2. Convective heat transfer coefficient of nanofluids
285 286
Fig. 5 describes the heat transfer coefficients of the nanofluid and base fluid (water) with respect to Reynolds numbers in the turbulent flow. As it is clear from the figure, the presence of multi-walled nanotubes significantly enhances the heat transfer rate. The convective heat transfer coefficient of the nanofluid increases linearly with the Reynolds number. It also increases with nanofluid concentration. The ratio of heat transfer coefficient of the nanofluid to that of base fluid versus the Reynolds number is illustrated in Fig. 6 for different solid volume fractions. It is clear from this figure that in the turbulent regime, the Reynolds number has no significant effect on the heat transfer enhancement of the nanofluid. This finding is in agreement with the results reported by Pak and Cho [28] and Xuan and Li [27]. However, it is in contrast with the results of Faukian and Nasr [29]. They [29] found that the ratio increases with an increase in the Reynolds number.
299
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Current study Gnielinski eq. Dittus-Boelter eq. Xuan and Li eq. Petukhov eq.
200
150
= 0.05 Vol% = 0.1 Vol% = 0.2 Vol% = 0.4 Vol% = 0.8 Vol% = 1 Vol%
2.4 2.2 2
hnf / hw
297 298
R
295 296
R
293 294
N C O
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U
289 290
Nu
287 288
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284
Maximum, minimum, and average enhancement values of the heat transfer coefficient (compared to water) are given in Table 2. This table summarizes the data in Fig. 6. Fig. 7 shows experimental Nusselt numbers of nanofluids with different solid concentrations. Similar trends as depicted in Fig. 5 are found for the Nusselt number. The experimental results indicated that the addition of small amounts of nanoparticles to distilled water enhanced the heat transfer performance significantly. Fig. 8 shows the average Nusselt number enhancement versus Re at different concentrations. It is found that for 1% of the concentration, the Nusselt number enhancement is about 41%, whereas the corresponding value for the heat transfer coefficient has been determined to be almost 83%. Regarding Fig. 8, the maximum, minimum, and average enhancement values in the Nusselt number (compared to water) are reported in Table 3. It is important to discuss the reasons behind the reported heat transfer enhancement in this work. The first reason that comes to mind is the higher effective thermal conductivity of MWCNTs/water nanofluids compared to pure water. It should be noted that the effective thermal conductivity data presented in Fig. 2 are measured in a stationary state, while the heat transfer rate is measured as the nanofluid is
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Fig. 4 provides a comparison between the results of experiments by the present setup and the correlations. As observed, the Petukhov equation better predicts the experimental data compared to other correlations.
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100
1.8 1.6 1.4
50
1.2 1
0
0
5000
10000
15000
20000
25000
30000
Solid volume fraction
0.8
0
5000
10000
15000
20000
25000
30000
Re Fig. 4. Comparison between the experimental data and the predictions of correlations for pure water.
Fig. 6. Heat transfer coefficient enhancement as a result of application of nanofluids.
Please cite this article as: M.H. Esfe, et al., Thermophysical properties, heat transfer and pressure drop of COOH-functionalized multi walled carbon nanotubes/water nanofluids, Int. Commun. Heat Mass Transf. (2014), http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.08.037
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6
φ = 0.8%
φ = 0.4%
φ = 0.2%
φ = 0.1%
φ = 0.05%
83.293 67.825 77.938
63.018 49.918 54.964
37.359 27.207 31.949
24.729 16.973 20.693
18.840 9.666 14.321
9.145 3.630 7.889
343
6.3. Pressure drop
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Pressure drop measurement is very important to design the thermal systems in industrial units. The pressure drop measured across the tube is used to obtain the friction factor from the following equation:
326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341
346
C
324 325
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322 323
ð21Þ
R
Δp d f ¼ 1. 2 : ρv L 2
348
R
N
C
O
Base fluid = 0.05 Vol% = 0.1 Vol% = 0.2 Vol% = 0.4 Vol% = 0.8 Vol% = 1 Vol%
250
U
200
150
different solid concentrations. The pressure drop increased about 37% 350 for nanofluid with 1% of solid volume fraction. 351 352
6.4. Thermal performance factor It is observed that the addition of MWCNTs caused a considerable pressure drop along the test section while the heat transfer is enhanced significantly. Therefore, a parameter is needed to evaluate the performance of a nanofluid through the thermal systems in which both heat transfer enhancement and pressure drop factors have been considered. This parameter is called the thermal performance factor and is defined as follows: 1 η ¼ Nunf =Nu f = f nf =f f 3 :
353 354 355 356 357 358 359
ð22Þ 361
Fig. 9 displays the ratio of the pressure drop of nanofluid to that of pure base fluid (ΔPnf/ΔPw) with respect to the Reynolds number for
Nu
349
Fig. 8. Average Nusselt number enhancement versus Re at different concentrations.
T
342
flowing in the tube. Therefore, the effective thermal conductivity of nanofluids may be higher in the case in which the nanofluid flows in the tube because the number of collisions between the MWCNTs increases. Consequently, the conduction heat transfer is enhanced. Other factors which increase the heat transfer rate are the Brownian motion of the particle and thermophoresis force. Even the use of very low volume fractions of MWCNTs (e.g., 0.2%) can increase the heat transfer rate by an average of 21%. One question that may be asked concerns how the nanofluid, containing very low quantities of MWCNTs, provides a relatively high heat transfer enhancement. Besides the high thermal conductivity of particles, this can be attributed to the cylindrical shape of MWCNTs that results in a higher rate of heat transfer compared to spherical nanoparticles. In the cylindrical particles, the ratio of surface area to volume is higher than spherical particles; then, the rate of heat transfer is higher. Also, in this work, COOH-functionalized particles are used. The use of functionalized particles may have two benefits. First, functionalized particles have higher surface activity compared to other types of CNTs and other kinds of nanoparticles. This may be a reason for their better thermal activity, too. Second, by functionalizing, the MWCNTs/water nanofluids become more stable. Therefore, the agglomeration and sedimentation of particles, which are two effective factors in the reduction of heat transfer, are diminished.
321
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φ = 1%
R O
Max (%) Min (%) Average
P
t2:4 t2:5 t2:6
D
t2:3
F
Table 2 Maximum, minimum, and average enhancement values of the heat transfer coefficient.
E
t2:1 t2:2 Q1
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The variations of thermal performance factor (η) versus Reynolds number for COOH-functionalize MWCNTs–water in different solid volume fractions are shown in Fig. 10. As shown, thermal performance increases as the solid concentration increases. This implies that despite the penalty in pressure drop, the use of a nanofluid with a higher solid volume fraction at any Reynolds number in a turbulent regime is advantageous from the energy-saving viewpoint. The use of a nanofluid with higher solid concentration provides considerably higher thermal performance than that of a nanofluid with a lower concentration for all Reynolds numbers studied. Further, it is clear from this figure that the changes in the Reynolds number have no considerable effects on the thermal performance factor. It should be noted that the thermal performance factor in all cases considered is greater than unity, which shows that the penalty in pressure drop is not very important compared to the heat transfer enhancement. Over the range studied, the maximum thermal performance factor of 1.374 is obtained, where the nanofluid volume concentration is 1% and the Reynolds number is 12,206.
362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378
50 Table 3 Maximum, minimum, and average enhancement values of the Nusselt number.
0
5000
10000
15000
Re
20000
25000
Fig. 7. Variations of Nusselt number with Re for different solid volume fractions.
Max (%) Min (%) Average
t3:1 Q2 t3:2
φ = 1%
φ = 0.8%
φ = 0.4%
φ = 0.2%
φ = 0.1%
φ = 0.05%
t3:3
40.670 28.799 36.560
33.294 22.582 26.708
24.419 15.224 19.519
19.187 11.776 15.330
15.323 6.420 10.938
8.171 2.705 6.926
t3:4 t3:5 t3:6
Please cite this article as: M.H. Esfe, et al., Thermophysical properties, heat transfer and pressure drop of COOH-functionalized multi walled carbon nanotubes/water nanofluids, Int. Commun. Heat Mass Transf. (2014), http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.08.037
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implies that the penalty in pressure drop is not very important compared to the heat transfer enhancement. It was perceived that the thermal performance factor increases due to an increase in the nanofluid concentration, while the Reynolds number had no considerable effects on it.
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The first and second authors would like to acknowledge the Nanofluid Laboratory of Semnan University Science and Technology Park for providing necessary instruments to carry out the sample preparation and helping in analyzing the samples to complete the article in time. The third and fourth authors would like to thank the Thailand Research Fund, the National Science and Technology Development Agency, and the National Research University Project for the support.
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An experimental study was conducted to study the turbulent flow of COOH-functionalized, multi-walled carbon nanotubes/water nanofluid flowing through a double tube heat exchanger. For this purpose, first, the thermophysical properties of the nanofluid, including the thermal conductivity and dynamic viscosity, were measured at various temperatures and volume concentrations (from 0.05% to 1%). Using the measured data, new correlations as a function of temperature and concentration were proposed to predict the thermal conductivity and dynamic viscosity. Next, the heat transfer rate and pressure drop were studied at the Reynolds numbers between 5000 and 27,000. On average, a 78% increase in heat transfer coefficient, a 36.5% increase in the average Nusselt number, and a 27.3% penalty in the pressure drop were obtained for 1% of volume concentration. Analysis of the data using the thermal performance factor showed that the thermal performance factor in all concentrations and Reynolds numbers is greater than unity, which
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Fig. 10. Variations of the thermal performance factor with Reynolds numbers for different solid volume fractions.
[1] L.S. Sundar, K.V. Sharma, Y.R. Sekhar, D.V. Rao, T. Subramanyam, J.V. Ramana Rao, Turbulent forced convection of Al2O3 nanofluid in a circular tube with tape inserts at low volume concentration, Int. J. Nano Biomater. 2 (1–5) (2009) 60–69. [2] W. Yu, D.M. France, D.S. Smith, D. Singh, E.V. Timofeeva, J.L. Routbort, Heat transfer to a silicon carbide/water nanofluid, Int. J. Heat Mass Transf. 52 (2009) 3606–3612. [3] W. Duangthongsuk, S. Wongwises, Effect of thermophysical properties models on the prediction of the convective heat transfer coefficient for low concentration nanofluid, Int. Commun. Heat Mass Transf. 35 (2008) 1320–1326. [4] W. Duangthongsuk, S. Wongwises, Heat transfer enhancement and pressure drop characteristics of TiO2–water nanofluid in a double-tube counter flow heat exchanger, Int. J. Heat Mass Transf. 52 (2009) 2059–2067. [5] W. Duangthongsuk, S. Wongwises, An experimental study on the heat transfer performance and pressure drop of TiO2–water nanofluids flowing under a turbulent flow regime, Int. J. Heat Mass Transf. 53 (15) (2010) 334–344. [6] B. Farajollahi, S.Gh. Etemad, M. Hojjat, Heat transfer of nanofluids in a shell and tube heat exchanger, Int. J. Heat Mass Transf. 53 (1–3) (2010) 12–17. [7] L.S. Sundar, K.V. Sharma, Turbulent heat transfer and friction factor of Al2O3 Nanofluid in circular tube with twisted tape inserts, Int. J. Heat Mass Transf. 53 (7–8) (2010) 1409–1416. [8] S. Zhang, Z. Luo, T. Wang, C. Shou, M. Ni, K. Cen, Experimental study on the convective heat transfer of CuO–water nanofluid in a turbulent flow, J. Enhanc. Heat Transf. 17 (2) (2010) 183–196. [9] A. Amrollahi, A.M. Rashidi, R. Lotfi, M. Meibodi Emami, K. Kashefi, Convection heat transfer of functionalized MWNT in aqueous fluids in laminar and turbulent flow at the entrance region, Int. Commun. Heat Mass Transf. 37 (6) (2010) 717–723. [10] K. Tumuluri, J.L. Alvarado, H. Taherian, C. Marsh, Thermal performance of a novel heat transfer fluid containing multiwalled carbon nanotubes and microencapsulated phase change materials, Int. J. Heat Mass Transf. 54 (2011) 5554–5567. [11] M. Hojjat, S.G. Etemad, R. Bagheri, J. Thibault, Convective heat transfer of nonNewtonian nanofluids through a uniformly heated circular tube, Int. J. Therm. Sci. 50 (4) (2011) 525–531. [12] M. Hojjat, S.G. Etemad, R. Bagheri, J. Thibault, Turbulent forced convection heat transfer of non-Newtonian nanofluids, Exp. Thermal Fluid Sci. 35 (7) (2011) 1351–1356. [13] S. Suresh, M. Chandrasekar, S. Chandra Sekhar, Experimental studies on heat transfer and friction factor characteristics of CuO/water nanofluid under turbulent flow in a helically dimpled tube, Exp. Thermal Fluid Sci. 35 (2011) 542–549. [14] S. Suresh, P. Selvakumar, M. Chandrasekar, V.S. Raman, Experimental studies on heat transfer and friction factor characteristics of Al2O3/water nanofluid under turbulent flow with spiraled rod inserts, 53 (2012) 24–30. [15] L. Godson, B. Raja, D. Mohan Lal, S. Wongwises, Convective heat transfer characteristics of silver–water nanofluid under laminar and turbulent flow conditions, J. Therm. Sci. Eng. Appl. 4 (3) (2012) 031001. [16] A.A. Abbasian Arani, J. Amani, Experimental study on the effect of TiO2/water nanofluid on heat transfer and pressure drop, Exp. Thermal Fluid Sci. 42 (2012) 107–115. [17] A.A.R. Darzi, M. Farhadi, K. Sedighi, R. Shafaghat, K. Zabihi, Experimental investigation of turbulent heat transfer and flow characteristics of SiO2/water nanofluid within helically corrugated tubes, Int. Commun. Heat Mass Transf. 39 (9) (2012) 1425–1434. [18] W.H. Azmi, K.V. Sharma, P.K. Sarma, R. Mamat, S. Anuar, V.D. Rao, Experimental determination of turbulent forced convection heat transfer and friction factor with SiO2 nanofluid, Exp. Thermal Fluid Sci. 51 (2013) 103–111. [19] M. Hemmat Esfe, S. Saedodin, M. Mahmoodi, Experimental studies on the convective heat transfer performance and thermophysical properties of MgO–water nanofluid under turbulent flow, Exp. Thermal Fluid Sci. 52 (2014) 68–78. [20] G.P. Celata, F. D'Annibale, A. Mariani, L. Saraceno, R. D'Amato, R. Bubbico, Heat transfer in water-based SiC and TiO2 nanofluids, Heat Transf. Eng. 34 (13) (2013) 1060–1072.
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Fig. 9. Values of pressure drop of the nanofluids associated to that of the base fluid with respect to the Reynolds number.
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[21] F.M. White, Viscous Fluid Flow, second ed. McGraw Hill, New York, 2006. [22] F.P. Incropera, D.P. De Witt, Fundamentals of Heat and Mass Transfer, fourth ed. John Wiley, New York, 1996. [23] S.J. Kline, F.A. Mcclintock, Describing uncertainties in single-sample experiments, Mech. Eng. 75 (1953) 3–8. [24] M.F. Pakdaman, M.A. Akhavan-Behabadi, P. Razi, An experimental investigation on thermo-physical properties and overall performance of MWCNT/heat transfer oil nanofluid flow inside vertical helically coiled tubes, Exp. Thermal Fluid Sci. 40 (2012) 103–111. [25] V. Gnielinski, Int. Chem. Eng. 16 (1976) 359.
[26] B.S. Petukhov, Heat transfer and friction in turbulent pipe flow with variable physical properties, in: T.F. Irvine Jr., J.P. Hartnett (Eds.), Advances in Heat Transfer, 6, Academic Press Inc., New York, 1970, pp. 504–564. [27] Y. Xuan, Q. Li, Investigation on convective heat transfer and flow features of nanofluids, J. Heat Transf. 125 (2003) 151–155. [28] B.C. Pak, Y. Cho, Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particle, Exp. Heat Transf. 11 (1998) 151–170. [29] S.M. Fotukian, M. Nasr Esfahany, Experimental study of turbulent convective heat transfer and pressure drop of dilute CuO/water nanofluid inside a circular tube, Int. Commun. Heat Mass Transf. 37 (2) (2010) 214–219.
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Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
3Q3
Mohammad Hemmat Esfe a,⁎, Seyfolah Saedodin a, Omid Mahian b, Somchai Wongwises c,⁎
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Thermophysical properties, heat transfer and pressure drop of COOH-functionalized multi walled carbon nanotubes/water nanofluids☆
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Keywords: Thermophysical properties Heat transfer Pressure drop Carbon nanotubes Nanofluids
Faculty of Mechanical Engineering, Semnan University, P. O. Box 3513119111 Semnan, Iran Young Researchers and Elite Club, Mashhad Branch, Islamic Azad University, Mashhad, Iran Fluid Mechanics, Thermal Engineering and Multiphase Flow Research Lab. (FUTURE), Department of Mechanical Engineering, Faculty of Engineering, King Mongkut's University of Technology Thonburi, Bangmod, Bangkok 10140, Thailand b
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This paper is a continuation of the authors' previous work on the thermophysical properties, heat transfer, and pressure drop of nanofluids [Experimental Thermal and Fluid Science 52 (2014) 68–78]. In this paper, an experimental study is carried out to study the turbulent flow of COOH-functionalized multi-walled carbon nanotubes/ water nanofluid flowing through a double tube heat exchanger. For this purpose, first, the thermophysical properties of the nanofluid, including the thermal conductivity and dynamic viscosity, have been measured at various temperatures and concentrations. Using the measured data, new correlations as a function of temperature and concentration are presented to predict the thermophysical properties. In the next step, the effects of low volume fractions of the nanofluid (from 0.05% to 1%) on the heat transfer rate are studied at the Reynolds numbers between 5000 and 27,000. The experimental results reveal that with increasing the nanofluid concentration, the heat transfer coefficient and thermal performance factor increase. On average, a 78% increase in heat transfer coefficient, a 36.5% increase in the average Nusselt number, and a 27.3% penalty in the pressure drop are recorded for the highest concentration of MWCNTs in water. © 2014 Published by Elsevier Ltd.
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1. Introduction
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The use of nanofluids containing ultra-fine solid particles rather than common fluids such as water and industrial oil in industrial and engineering applications may be a solution to enhance the heat transfer rate, and, consequently, to establish more compact thermal systems. The main reason for the enhancement of heat transfer is the increase of the effective thermal conductivity of nanofluids having solid particles with the sizes between 1 and 100 nm. Among the usual nanoparticles, carbon nanotubes (CNTs) in single, double, or multi-walled types have attracted special interest due to unique thermal properties and structure. Therefore, CNTs can be a good option to disperse in the common working fluids to enhance the heat transfer. Here, a brief review of the reported works on the nanofluid flow in simple tubes or double tube heat exchangers is presented so that, in some of them, CNTs are used as the additive to the base fluid. Sundar et al. [1] investigated the turbulent flow and pressure drop of Al2O3/water nanofluid in a tube where the twisted tapes are installed
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☆ Communicated by W.J. Minkowycz. ⁎ Corresponding authors. E-mail addresses: [email protected] (M.H. Esfe), [email protected] (S. Wongwises).
inside the circular tube. They found that by using twisted tapes inside the tube the heat transfer coefficient increases up to 23% while the pressure drop raises 70% compared to the plain tube. Yu et al. [2] tested the silicon carbide/water nanofluid in a circular tube where the Reynolds number is between 3000 and 13000. They reported an increase in the heat transfer coefficient as high as 60% for the volume fraction of 3.7%. In a series of experimental studies, Duangthongsuk and Wongwises [3–5] focused on the TiO2/water nanofluid flow in a double tube heat exchanger under the turbulent regime. In the three papers, the authors examined the effects of solid volume fraction, and they used different thermophysical models on the heat transfer characteristics and pressure drop of the nanofluid flow. For instance, Duangthongsuk and Wongwises [5] found that for volume fractions between 0.2% and 2%, the optimum particle loading is 1% in which the maximum heat transfer is obtained. Farajollahi et al. [6] studied the turbulent flow of γ-Al2O3/water and TiO2/water nanofluids in a double tube heat exchanger for the Peclet numbers between 20,000 and 60,000. They found the optimum concentration of the nanofluids in which the heat transfer is maximized. Al2O3/water nanofluid flow through a plain tube where the Reynolds number changes in the range of 10,000 to 22,000 was studied by Sundar and Sharma [7]. They considered the effects of inserting the tapes with different twist ratios inside the tube in the nanofluid flow and heat transfer. They suggested related correlations to estimate the Nusselt number and friction factor. Zhang et al. [8] examined the heat transfer
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Please cite this article as: M.H. Esfe, et al., Thermophysical properties, heat transfer and pressure drop of COOH-functionalized multi walled carbon nanotubes/water nanofluids, Int. Commun. Heat Mass Transf. (2014), http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.08.037
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There are two ways to have stable nanofluids containing carbon nanotubes. The first is the use of a surfactant such as Arabic gum, and the second is the functionalization of the carbon nanotubes. Adding a surfactant may have undesired effects on the thermal conductivity of the suspensions. Then, using functionalized carbon nanotubes seems more appropriate. Functionalizing MWCNTs using COOH functional groups makes the carbon nanotubes hydrophilic; consequently, the stability of the prepared nanofluids is improved. The properties of COOHfunctionalized MWCNTs have been described in Table 1. In this investigation, pure water is used as the base fluid. COOHfunctionalized MWCNTs with solid volume fractions of 0.01 (1.0%), 0.008 (0.8%), 0.004 (0.4%), 0.002 (0.2%), 0.001 (0.1%), and 0.0005 (0.05%) are prepared by dispersing a specified amount of MWCNTs in the distilled water. MWCNTs are mixed with water by means of a magnetic stirrer for 2 h. Next, an ultrasonic processor for 4 h with the power of 400 W and frequency of 24 kHz is used to break down the agglomeration of the nanoparticles and make stable suspensions. After 48 h, no sedimentation was observed (with the naked eye) in the samples.
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4. Data processing
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Applying the energy law to a differential control volume of the fluid 178 inside the copper tube gives [22]: 179 dT b p ¼ hðT s −T b Þ: ˙ p dx mC
ð1Þ 181
After separating variables and integrating, the following equation is: 0 L 1 Z ðΔT 0 Þ −pL @1 ¼ hdxA: Ln ˙ ΔTi L mCp
ð2Þ
0
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After simplifying, ! ΔT 0 T s −T b;o −pL ¼ ¼ exp h mC p ΔT i T s −T b;i
ð3Þ 185
Rearranging Eq. (3), heat transfer coefficient is obtained as: A schematic view of the experimental apparatus is illustrated in Fig. 1. It consists of three centrifugal pumps, a test section, a heat exchanger, a flow meter, a differential pressure transmitter, a nanofluid tank, a water tank, and a data acquisition system.
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The heat transfer test section consists of a double tube heat exchanger. K-type thermocouples (Chromel–Alumel) are inserted along the tube length to measure the wall temperature. Two thermometers with digital indicators are inserted at the inlet and outlet of the test section to record the inlet and outlet temperatures of the nanofluid. For turbulent flow, the entrance region (not fully developed length) in a tube is obtained based on (Le/D ≈ 4.4 × Re1/6) [21]. In the experimental set-up, this length is less than 17 cm; therefore, all measurements are taken in the fully developed region. In the double tube heat exchanger, the inner tube is heated by hot water which flows in the outer copper tube. The flow and temperature of hot water are regulated using control instruments. A temperature controller with a PT100 sensor is utilized to control the temperature of hot water. Two K-type thermocouples measure the inlet and outlet bulk temperature of hot water. The test section is insulated by fiber glass with a thickness of 7 cm to minimize the heat dissipation. The pressure drop of the nanofluid through the test section is measured by a differential pressure transmitter (DPT Rosemount 3051cd).
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characteristics of CuO/water nanofluids in the turbulent flow where the nanoparticles can have three different sizes — i.e., 23, 51, and 76 nm. They perceived that a larger particle size leads to a higher heat transfer rate. Amrollahi et al. [9] measured the heat transfer rate of functionalized multi-walled carbon nanotubes suspended in water as the base fluid under laminar and turbulent regimes where the nanofluid flows through a horizontal circular tube in the entrance region. They observed that the heat transfer coefficient increases between 33% and 40% for the nanofluid with a weight fraction of 0.25% compared to the pure water. Tumuluri et al. [10] investigated the turbulent flow of the nanofluids, which are combined with multi-walled carbon nanotubes (MWCNTs) and microencapsulated phase change materials (MPCMs) through a circular tube under constant heat flux. Hojjat et al. [11,12] evaluated the turbulent flow of non-Newtonian nanofluids in a circular tube. They provided three different types of nanofluids where the nanoparticles are Al2O3, CuO, and TiO2, and the base fluid is an aqueous solution of carboxymethyl cellulose. Suresh et al. [13] studied the heat transfer and pressure drop of CuO/ water nanofluid in the Reynolds number range of 2500 to 6000 in the simple and helically dimpled tubes. They revealed that using the nanofluid with a concentration of 0.3% through the dimpled tube leads to 39% heat transfer enhancement compared to the case in which the pure water flows in the simple tube. Suresh et al. [14] reported a heat transfer enhancement as high as 48% when Al2O3/water nanofluid is used with the volume fraction of 0.5% through a circular tube with spiral rods. Heat transfer and pressure drop due to silver/water nanofluid in a counter flow heat exchanger under laminar and turbulent regimes investigated by Godson et al. [15]. They observed a 70% increase in the heat transfer coefficient using the nanofluid with the concentration of 0.9%. Arani and Amani [16] studied TiO2/water nanofluid flow in a horizontal, double tube, counter flow heat exchanger for a wide range of Reynolds numbers — i.e., between 8000 and 51,000. By considering the relation between the heat transfer rate and the pumping power, they indicated that it is more advantageous to use nanofluid at the Reynolds numbers lower than 30,000. Heat transfer and pressure drop were found to be due to SiO2/water nanofluid in corrugated tubes tested by Darzi et al. [17]. Azmi et al. [18] investigated the of SiO2/water nanofluid flow in a circular tube under constant heat flux and turbulent flow conditions where the maximum solid volume fraction is 4%. For the Reynolds numbers between 5000 and 27000, they obtained the optimal concentration in which the heat transfer rate is maximized. Recently, Hemmat Esfe et al. [19] investigated the flow of MgO nanofluids with volume fractions of less than 1% in a circular tube. The range of Reynolds numbers was between 3000 and 18,000. They reported remarkable heat transfer enhancement due to using nanofluids. Celata et al. [20] tested TiO2/water (9 wt.%) and SiC/water (3, 6, 9 wt.%) nanofluids in a circular pipe under uniform heat flux and the Reynolds numbers between 300 and 6000. Based on the best knowledge of the authors, there are no data on the flow of water-based functionalized MWCNT nanofluids in double tube heat exchangers under fully developed flow conditions. In this paper, COOH-functionalized MWCNTs are dispersed in water to prepare nanofluids with concentrations up to 1%. First, thermophysical properties of the nanofluids, including thermal conductivity and viscosity, are measured. Next, flow and heat transfer characteristics of the nanofluids, which include heat transfer coefficient, pressure drop, and thermal performance, are evaluated in a double tube heat exchanger for the Reynolds numbers between 5000 and 27,000.
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˙ p mC T s −T b;i Ln h¼ pL T s −T b;o
! ð4Þ
Please cite this article as: M.H. Esfe, et al., Thermophysical properties, heat transfer and pressure drop of COOH-functionalized multi walled carbon nanotubes/water nanofluids, Int. Commun. Heat Mass Transf. (2014), http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.08.037
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T b;i þ T b;o : 2
5. Thermophysical properties of nanofluids
ð5Þ
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Tb ¼
R
where bulk temperature is calculated by:
Finally, the Reynolds and Nusselt numbers are defined as follows:
Re ¼
Vd ν
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hd : k
Before the investigation of the convective heat transfer of the nanofluids, the thermophysical properties of a nanofluid must be accurately measured. The density and specific heat capacity are calculated by using the well-known relations, while thermal conductivity and viscosity are determined experimentally in the temperature range of 25– 55 °C.
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ð6Þ
ρnf ¼ φρp þ ð1−φÞρ f :
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ð12Þ 203
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The specific heat capacity (cp) of the nanofluid is calculated by: t1:1 t1:2
Table 1 Properties of COOH-functionalized MWCNTs.
t1:3
Purity (wt%)
N97 (carbon content)
t1:4 t1:5 t1:6 t1:7
Inside diameter (nm) Length (μm) Color True density (g/cm3)
5–10 10–30 Black 2.1
cp;nf ¼
φ ρcp þ ð1−φÞ ρcp p
ρnf
f
ð13Þ
where the subscripts of nf, f, and p represent the nanofluid, the water 205 and the particle, respectively.
Please cite this article as: M.H. Esfe, et al., Thermophysical properties, heat transfer and pressure drop of COOH-functionalized multi walled carbon nanotubes/water nanofluids, Int. Commun. Heat Mass Transf. (2014), http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.08.037
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The viscosity of the COOH-functionalized MWCNT–water nanofluid 251 is measured using a Brookfield viscometer with a temperature con- 252 trolled bath, supplied by Brookfield engineering laboratories of the USA. 253 μ
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ð15Þ
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6. Results and discussions 267
6.1. Validation
5.2.1. Proposed correlation The following correlation is suggested to estimate the thermal conductivity of COOH-functionalized MWCNTs/water nanofluids that is obtained by the curve fitting of the experimental data. This correlation is 1.6
T = 25 (oC) T = 35 (oC) o T = 45 ( C) o T = 55 ( C)
1.5
1.4
1.3
Initially, experiments are performed with pure water, and the results 268 are compared to some well-known correlations for turbulent flow. The 269 correlations are as follows:Gnielinski correlation [25]: 270 Nu ¼
f =8ðRe−1000Þ Pr ! rffiffiffi 2 f 3 Pr −1 1 þ 12:7 8
1.6
2 # d 3 L
ð16Þ
o
1.3
1.2
1.1
1
1
0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009
1þ
1.4
1.1
0
!0:11 "
T = 25 ( C ) o T = 35 ( C ) o T = 45 ( C ) o T = 55 ( C )
1.5
1.2
0.9
Pr Pr f
which is applied in the range of 0.5 b Pr b 106 and 2300 b Re b 5 × 106. Pr 272 and Prf are the Prandtl numbers calculated at the water bulk
Relative viscosity
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ð14Þ 250
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knf / kw
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knf ð360:69 þ T Þ ¼ kw ð405:59−11080φÞ
R O
Many of the proposed models have failed to predict the accurate values of the thermal conductivity of nanofluids because many parameters such as the shape, the size, and the method of preparation affect the thermal conductivity. Therefore, in this paper, the thermal conductivity of nanofluids is determined experimentally using the transient hot wire technique that is an accurate and fast approach. Thermal conductivity of COOH-functionalized MWCNTs–water nanofluids with various solid volume fractions is measured by a KD2 Pro instrument manufactured by Decagon Devices Inc., which works based on the transient hot wire technique; the KD2 Pro measures the thermal conductivity at 1-s intervals during a 90-s measurement cycle. The samples are placed in a thermostatic bath so that the temperature of the samples reaches the desired temperature with a temperature difference not higher than ± 0.5 °C. Then, the measurements are conducted by KD2 Pro. The “enhanced thermal conductivity ratio” is defined as the ratio of nanofluid thermal conductivity to water thermal conductivity. Fig. 2 illustrates the measured enhanced thermal conductivity ratio of COOH-functionalized MWCNTs/water nanofluid as a function of the solid volume fraction in different temperatures. As can be seen from the figure, when the temperature increases, the thermal conductivity increases. This may happen because at higher temperatures, the agglomeration between the nanoparticles would break down more easily and, hence, CNTs will disperse more uniformly in the water. This leads to a better conduction of heat in the nanofluid. Further, random movement of the nanoparticles increases as the temperature raises; consequently, the heat energy is transmitted more quickly in the fluid. It is also observed that at a low concentration of nanofluid, the effect of the temperature is not considerable, whereas the effect of temperature is more considerable at higher concentrations. To explain this, consider a specified volume of the nanofluid: when the solid volume fraction increases, the number of collisions due to Brownian motion of particles increases. On the other hand, with an increase in the temperature, the kinetic energy of CNTs in the nanofluid increases, and hence the number of collisions among the nanoparticles increases. Therefore, increasing the temperature boosts the effects of increasing the solid volume fraction.
P
207 208 209 210
valid for volume fractions up to 1% and the temperatures between 25 247 and 55 °C. 248
D
5.2. Measurement of thermal conductivity
T
206
E
4
0.01
Solid volume fraction Fig. 2. Thermal conductivity ratio versus solid volume fraction at different temperatures.
0.9 -0.002
0
0.002
0.004
0.006
0.008
0.01
0.012
Solid volume fraction Fig. 3. Relative viscosity versus solid volume fraction at various temperatures.
Please cite this article as: M.H. Esfe, et al., Thermophysical properties, heat transfer and pressure drop of COOH-functionalized multi walled carbon nanotubes/water nanofluids, Int. Commun. Heat Mass Transf. (2014), http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.08.037
M.H. Esfe et al. / International Communications in Heat and Mass Transfer xxx (2014) xxx–xxx
273 274 275 276
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30000
temperature and at the inner wall temperature, respectively; L is the tube length and d is the diameter of the tube. The bulk temperature is an average between the inlet and outlet fluid temperatures. In this formulation, the Darcy coefficient is proposed as follows: f ¼
1 ð1:82log 10 Re−1:64Þ2
5
Base fluid = 0.05 Vol% = 0.1 Vol% = 0.2 Vol% = 0.4 Vol% = 0.8 Vol% = 1 Vol%
25000
ð17Þ 20000
Dittus–Boelter correlation [22]: 0:8
Nu ¼ 0:023Re
0:4
15000
ð18Þ
Pr
Petukhov equation [26]:
5000
Xuan and Li equation [27]:
5000
0:001 0:6886 0:9238 0:4 Re Ped Pr : Nu ¼ 0:0059 1 þ 7:62φ
ð20Þ
10000
O
ð19Þ
R O
280
F
10000
ð f =8ÞRe Pr Nu ¼ 2 1:07 þ 12:7ð f =8Þ0:5 Pr3 −1
15000
20000
25000
Fig. 5. Variations of heat transfer coefficient with Re for different solid volume fractions.
6.2. Convective heat transfer coefficient of nanofluids
285 286
Fig. 5 describes the heat transfer coefficients of the nanofluid and base fluid (water) with respect to Reynolds numbers in the turbulent flow. As it is clear from the figure, the presence of multi-walled nanotubes significantly enhances the heat transfer rate. The convective heat transfer coefficient of the nanofluid increases linearly with the Reynolds number. It also increases with nanofluid concentration. The ratio of heat transfer coefficient of the nanofluid to that of base fluid versus the Reynolds number is illustrated in Fig. 6 for different solid volume fractions. It is clear from this figure that in the turbulent regime, the Reynolds number has no significant effect on the heat transfer enhancement of the nanofluid. This finding is in agreement with the results reported by Pak and Cho [28] and Xuan and Li [27]. However, it is in contrast with the results of Faukian and Nasr [29]. They [29] found that the ratio increases with an increase in the Reynolds number.
299
C
E
250
Current study Gnielinski eq. Dittus-Boelter eq. Xuan and Li eq. Petukhov eq.
200
150
= 0.05 Vol% = 0.1 Vol% = 0.2 Vol% = 0.4 Vol% = 0.8 Vol% = 1 Vol%
2.4 2.2 2
hnf / hw
297 298
R
295 296
R
293 294
N C O
291 292
U
289 290
Nu
287 288
P
284
Maximum, minimum, and average enhancement values of the heat transfer coefficient (compared to water) are given in Table 2. This table summarizes the data in Fig. 6. Fig. 7 shows experimental Nusselt numbers of nanofluids with different solid concentrations. Similar trends as depicted in Fig. 5 are found for the Nusselt number. The experimental results indicated that the addition of small amounts of nanoparticles to distilled water enhanced the heat transfer performance significantly. Fig. 8 shows the average Nusselt number enhancement versus Re at different concentrations. It is found that for 1% of the concentration, the Nusselt number enhancement is about 41%, whereas the corresponding value for the heat transfer coefficient has been determined to be almost 83%. Regarding Fig. 8, the maximum, minimum, and average enhancement values in the Nusselt number (compared to water) are reported in Table 3. It is important to discuss the reasons behind the reported heat transfer enhancement in this work. The first reason that comes to mind is the higher effective thermal conductivity of MWCNTs/water nanofluids compared to pure water. It should be noted that the effective thermal conductivity data presented in Fig. 2 are measured in a stationary state, while the heat transfer rate is measured as the nanofluid is
D
Fig. 4 provides a comparison between the results of experiments by the present setup and the correlations. As observed, the Petukhov equation better predicts the experimental data compared to other correlations.
T
282 283
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281
100
1.8 1.6 1.4
50
1.2 1
0
0
5000
10000
15000
20000
25000
30000
Solid volume fraction
0.8
0
5000
10000
15000
20000
25000
30000
Re Fig. 4. Comparison between the experimental data and the predictions of correlations for pure water.
Fig. 6. Heat transfer coefficient enhancement as a result of application of nanofluids.
Please cite this article as: M.H. Esfe, et al., Thermophysical properties, heat transfer and pressure drop of COOH-functionalized multi walled carbon nanotubes/water nanofluids, Int. Commun. Heat Mass Transf. (2014), http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.08.037
300 Q7 301 302 303 304 305 306 307 308 309 310 311 312 313 Q8 314 315 316 317 318 319 320
6
φ = 0.8%
φ = 0.4%
φ = 0.2%
φ = 0.1%
φ = 0.05%
83.293 67.825 77.938
63.018 49.918 54.964
37.359 27.207 31.949
24.729 16.973 20.693
18.840 9.666 14.321
9.145 3.630 7.889
343
6.3. Pressure drop
344 345
Pressure drop measurement is very important to design the thermal systems in industrial units. The pressure drop measured across the tube is used to obtain the friction factor from the following equation:
326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341
346
C
324 325
E
322 323
ð21Þ
R
Δp d f ¼ 1. 2 : ρv L 2
348
R
N
C
O
Base fluid = 0.05 Vol% = 0.1 Vol% = 0.2 Vol% = 0.4 Vol% = 0.8 Vol% = 1 Vol%
250
U
200
150
different solid concentrations. The pressure drop increased about 37% 350 for nanofluid with 1% of solid volume fraction. 351 352
6.4. Thermal performance factor It is observed that the addition of MWCNTs caused a considerable pressure drop along the test section while the heat transfer is enhanced significantly. Therefore, a parameter is needed to evaluate the performance of a nanofluid through the thermal systems in which both heat transfer enhancement and pressure drop factors have been considered. This parameter is called the thermal performance factor and is defined as follows: 1 η ¼ Nunf =Nu f = f nf =f f 3 :
353 354 355 356 357 358 359
ð22Þ 361
Fig. 9 displays the ratio of the pressure drop of nanofluid to that of pure base fluid (ΔPnf/ΔPw) with respect to the Reynolds number for
Nu
349
Fig. 8. Average Nusselt number enhancement versus Re at different concentrations.
T
342
flowing in the tube. Therefore, the effective thermal conductivity of nanofluids may be higher in the case in which the nanofluid flows in the tube because the number of collisions between the MWCNTs increases. Consequently, the conduction heat transfer is enhanced. Other factors which increase the heat transfer rate are the Brownian motion of the particle and thermophoresis force. Even the use of very low volume fractions of MWCNTs (e.g., 0.2%) can increase the heat transfer rate by an average of 21%. One question that may be asked concerns how the nanofluid, containing very low quantities of MWCNTs, provides a relatively high heat transfer enhancement. Besides the high thermal conductivity of particles, this can be attributed to the cylindrical shape of MWCNTs that results in a higher rate of heat transfer compared to spherical nanoparticles. In the cylindrical particles, the ratio of surface area to volume is higher than spherical particles; then, the rate of heat transfer is higher. Also, in this work, COOH-functionalized particles are used. The use of functionalized particles may have two benefits. First, functionalized particles have higher surface activity compared to other types of CNTs and other kinds of nanoparticles. This may be a reason for their better thermal activity, too. Second, by functionalizing, the MWCNTs/water nanofluids become more stable. Therefore, the agglomeration and sedimentation of particles, which are two effective factors in the reduction of heat transfer, are diminished.
321
O
φ = 1%
R O
Max (%) Min (%) Average
P
t2:4 t2:5 t2:6
D
t2:3
F
Table 2 Maximum, minimum, and average enhancement values of the heat transfer coefficient.
E
t2:1 t2:2 Q1
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100
The variations of thermal performance factor (η) versus Reynolds number for COOH-functionalize MWCNTs–water in different solid volume fractions are shown in Fig. 10. As shown, thermal performance increases as the solid concentration increases. This implies that despite the penalty in pressure drop, the use of a nanofluid with a higher solid volume fraction at any Reynolds number in a turbulent regime is advantageous from the energy-saving viewpoint. The use of a nanofluid with higher solid concentration provides considerably higher thermal performance than that of a nanofluid with a lower concentration for all Reynolds numbers studied. Further, it is clear from this figure that the changes in the Reynolds number have no considerable effects on the thermal performance factor. It should be noted that the thermal performance factor in all cases considered is greater than unity, which shows that the penalty in pressure drop is not very important compared to the heat transfer enhancement. Over the range studied, the maximum thermal performance factor of 1.374 is obtained, where the nanofluid volume concentration is 1% and the Reynolds number is 12,206.
362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378
50 Table 3 Maximum, minimum, and average enhancement values of the Nusselt number.
0
5000
10000
15000
Re
20000
25000
Fig. 7. Variations of Nusselt number with Re for different solid volume fractions.
Max (%) Min (%) Average
t3:1 Q2 t3:2
φ = 1%
φ = 0.8%
φ = 0.4%
φ = 0.2%
φ = 0.1%
φ = 0.05%
t3:3
40.670 28.799 36.560
33.294 22.582 26.708
24.419 15.224 19.519
19.187 11.776 15.330
15.323 6.420 10.938
8.171 2.705 6.926
t3:4 t3:5 t3:6
Please cite this article as: M.H. Esfe, et al., Thermophysical properties, heat transfer and pressure drop of COOH-functionalized multi walled carbon nanotubes/water nanofluids, Int. Commun. Heat Mass Transf. (2014), http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.08.037
M.H. Esfe et al. / International Communications in Heat and Mass Transfer xxx (2014) xxx–xxx
7
implies that the penalty in pressure drop is not very important compared to the heat transfer enhancement. It was perceived that the thermal performance factor increases due to an increase in the nanofluid concentration, while the Reynolds number had no considerable effects on it.
395 396
8. Uncited references
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The first and second authors would like to acknowledge the Nanofluid Laboratory of Semnan University Science and Technology Park for providing necessary instruments to carry out the sample preparation and helping in analyzing the samples to complete the article in time. The third and fourth authors would like to thank the Thailand Research Fund, the National Science and Technology Development Agency, and the National Research University Project for the support.
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References
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An experimental study was conducted to study the turbulent flow of COOH-functionalized, multi-walled carbon nanotubes/water nanofluid flowing through a double tube heat exchanger. For this purpose, first, the thermophysical properties of the nanofluid, including the thermal conductivity and dynamic viscosity, were measured at various temperatures and volume concentrations (from 0.05% to 1%). Using the measured data, new correlations as a function of temperature and concentration were proposed to predict the thermal conductivity and dynamic viscosity. Next, the heat transfer rate and pressure drop were studied at the Reynolds numbers between 5000 and 27,000. On average, a 78% increase in heat transfer coefficient, a 36.5% increase in the average Nusselt number, and a 27.3% penalty in the pressure drop were obtained for 1% of volume concentration. Analysis of the data using the thermal performance factor showed that the thermal performance factor in all concentrations and Reynolds numbers is greater than unity, which
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Fig. 10. Variations of the thermal performance factor with Reynolds numbers for different solid volume fractions.
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7. Conclusion
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[23,24]
Fig. 9. Values of pressure drop of the nanofluids associated to that of the base fluid with respect to the Reynolds number.
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Please cite this article as: M.H. Esfe, et al., Thermophysical properties, heat transfer and pressure drop of COOH-functionalized multi walled carbon nanotubes/water nanofluids, Int. Commun. Heat Mass Transf. (2014), http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.08.037
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Please cite this article as: M.H. Esfe, et al., Thermophysical properties, heat transfer and pressure drop of COOH-functionalized multi walled carbon nanotubes/water nanofluids, Int. Commun. Heat Mass Transf. (2014), http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.08.037
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