Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 148 (2016) 774 – 778
4th International Conference on Process Engineering and Advanced Materials
Temperature Dependent Properties of Silicon Carbide Nanofluid in Binary Mixtures of Glycerol-Ethylene Glycol S. Akilua,*, K.V. Sharmaa, Aklilu T.Ba, M.S. Mior Azmana, P.T. Bhaskoroa a
Department of Mechanical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610 Tronoh, Perak, Malaysia
Abstract
Nanofluids are a new class of engineered thermal fluids with enhanced thermal properties and heat transfer capabilities exceeding those of the base liquid. Studies related to the effect of temperature on the thermophysical and rheological properties of ethylene glycol and glycerol nanofluids have been documented in various articles. In this paper, the thermal and physio-chemical behavior of SiC nanofluid dispersed in glycerol and ethylene mixture of 60:40 wt.% ratio was determined. The influence of temperature on the pH, viscosity, electrical and thermal conductivity of nanofluid was undertaken for a maximum concentration of 1.0 vol.% in the temperature range of 15-75oC. It was found that, the base liquid electrical conductivity and pH were unaffected by temperature; whereas the nanofluid electrical and thermal conductivity increased with temperature. The viscosity of the base liquid and nanofluid decreased significantly while the decrease in pH is not significant with temperature. © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ICPEAM 2016. Peer-review under responsibility of the organizing committee of ICPEAM 2016 Keywords: Silicon carbide; nanofluid; temperature; thermo-physical properties; glycerol; ethylene glycol
1. Introduction Convective heat transfer is one of the fundamental phenomena encountered during operation of various thermal systems such as boilers, steam engines, compressors, and reactors. Basic operating conditions for thermal equipment require the removal of excess heat generated. Typically, the heat dissipated is discharged into the environment as waste heat. In some cases, the emitted heat becomes alternative energy source which provides additional energy savings to the plant. In the last few decades, base liquids such as water, glycols, and oil are used conventionally as heat transfer fluids. The capability to transfer heat by these liquids have reached a maximum, new fluids with enhanced heat transfer properties are required to be developed. A nanofluid refers to a coolant prepared by dispersing particles of size less than 100nm in conventional liquids. Such liquids are envisioned to become the alternative working media in the field of heat transfer due to their enhanced properties. A number of research activities have been undertaken to determine the heat transfer potential [1] of various nanofluids.
* Corresponding author. Tel.: +6-016-310-0397; fax: +60 5-365-4088. E-mail address:
[email protected]
1877-7058 © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ICPEAM 2016
doi:10.1016/j.proeng.2016.06.555
775
S. Akilu et al. / Procedia Engineering 148 (2016) 774 – 778
Ceramic nanoparticles of oxides, carbides, and metalloids have a wide range of applications owing to favourable thermal properties such as high specific heat, high melting temperatures and chemical inertness. Accordingly, SiC nanoparticles have evolved as one of the promising ceramic nanofluid additive with thermal conductivity (TC) as high as 120 W/m. K and specific heat of 750 J/Kg °K. Despite these promising features, only few studies have been reported in the literature concerning SiC nanofluids [2-9]. An overview of the available studies on SiC nanofluids are summarized in Table 1. Studies on the determination of nanofluid properties have elucidated on the impacts of temperature on thermophysical properties. It was revealed that the rising temperatures promote the increase of TC and an eventual drop in the nanofluid viscosity [6-8]. The increase in temperature could exert a profound influence on the absolute values of the zeta potential [10]. While it has been well known that temperature commands a strong influence on the pH, zeta potential however depends significantly on the pH of the dispersion media [11]. Based on these interrelations, experiments under different temperature conditions would facilitate better understanding of dispersion stability and characterization of nanofluids to suit diverse thermal applications. The objective of the present work is to investigate the influence of temperature on the thermo-physical properties viz., electrical conductivity, thermal conductivity, viscosity and pH of SiC nanoparticles dispersed in binary mixture of GC/EG 60:40 wt.%. Table 1. Summary of studies on SiC based nanofluids Base liquid
SiC
EG
Particle Size (nm) 26
β-SiC
EG
60
SiC
EG
SiC SiC
Material
Particle conc. (vol.%) 0.88
pH
k enhancement (%)
μ decrement (%)
4
-
-
Temp (oC) 4
Investigator
*9
9.5
20
14
36
30
1.0
11
25–60
-
55.1
Li et al. [8]
EG
30
1.0
11
25–60
6.7
-
Li et al. [9]
EG/W
29
4.0
9.5
25–75
14.5
72.2
Xie et al. [6] Nikkam et al. [7]
Timofeeva et al. [3]
W = water, EG = ethylene glycol, GC = glycerol *wt.% 2. Experimental 2.1. Materials β–SiC NPs of manufacturer’s declared nominal diameter within a range between 45–65nm, 99% purity and with a true density of 3216 kg/m3 is supplied by US Research Nanomaterials Inc. USA. The base liquids of ethylene glycol and glycerol with purity and density of 99.8 (1110 kg/m3) and 99.7% (1259 kg/m3), respectively were procured from R&M Chemicals, Malaysia. A mixture of GC and ethylene glycol respectively in the ratio of 60:40 by weight with an average density of 1196 kg/m3 at 25oC is used in the present work. In addition, the mean size of the SiC nanoparticles verified through Transmission Electron Microscopy (TEM) and was determined to be 29nm. 2.2. Nanofluid stability Three batches of nanofluids were formulated through two-step method by dispersing a known mass of SiC nanoparticles in the base liquid mixture GC/EG 60:40 wt.% followed by 30 mins of stirring and ultrasonication with Sonicator Q700 (Qsonica Inc., USA). A small fraction of 0.01% of Polyvinylpyrrolidone (PVP) was used as dispersant. About 100ml of 0.3, 0.5 and 1.0% vol. concentration samples are prepared employing Eq. (1) for further characterization of the nanofluids.
M
w SiC / U SiC w SiC / U SiC w GC EG / U GC EG
(1)
where, φ is the percentage volume concentration, w is the weight and ρ is the density. The stability of nanofluid was checked with Zetasizer Nano ZSP (Malvern Instruments, UK) which operates on electrophoretic light scattering method to measure zeta potential. The pH of the nanofluids were varied with HCl and NaOH solutions using pH meter FE 20 (Mettler Toledo, Switzerland) prior to the Zeta (Z) tests.
776
S. Akilu et al. / Procedia Engineering 148 (2016) 774 – 778
2.3. Electrical conductivity and pH measurement The electrical conductivity (Ec) and pH of the base liquid and SiC based nanofluids were measured using a handheld meter SG23-SevenGo Duo (Mettler Toledo, Switzerland), equipped with a dual-channel probe for specific resolutions of ±0.01 over a range of 0–14/0.1–500 μScm-1 pH/mV and temperature accuracy of ±0.5oC. The device was calibrated with standard buffer and conductivity solution 1413 μScm-1. Measurements of the samples were conducted at steady state temperature conditions. At least three measurements were performed for each sample between 30 - 60oC and the mean value taken for further analysis. 2.4. Thermal conductivity measurement Thermal conductivity is measured using a portable digital thermal property analyzer KD2 Pro (Decagon devices, Inc., USA), which operates on the principle of the transient line heat source principle. It comprised of a single probe needle KS-1 (1.3 mm diameter x 60 mm long) with integrated heating element, a thermo-resistor and micro-processor. The instrument is designed to measure conductivity over the temperature range of -50 to 150°C with accuracy of ± 5%. The device was calibrated with glycerine before measurements are taken. The measurements are conducted under controlled temperature in a refrigerated/heating circulator Vivo-RT2 (Julabo GmbH, Germany). All series of experiments were performed in triplicate and the average values reported. 2.5. Viscosity measurement The rheological and viscosity measurements were facilitated by stress-controlled rheometer AR-G2 (TA Instruments Inc., USA) which housed a plate-cone geometry system. A cone plate of 60mm diameter with a cone angle of 2 o was used with upper plate heated under active temperature control to ensure uniform heating and temperature ramps. An integrated Peltier system allowed for the cooling and heating of the sample to the desired value. The measurements are undertaken for the nanofluid samples by varying the shear rates from 0.1 up to 100 s-1 in the range of temperatures between 15–80oC in steps of 5oC. 3. Results and discussion In order to evaluate the stability of the SiC nanofluid, variations of zeta potential and pH are plotted in Fig. 1. The zeta potential can be correlated to the stability of nanofluids based on the Deyaguin-Landau-Verwey-Overbeek (DLVO theory), which describes the tendency of suspensions to aggregate or repel each other depending on the superposition of London van der Waals and double layer forces [12]. Accordingly, a high zeta value means greater degree of repulsion between the adjacent like charged particles in the suspension. A nanofluid with higher zeta potential will offer better stability i.e. the dispersion will resist the chance to aggregate. A zeta potential in the range between 0 to ±30 mV reflect instability, while higher values beyond ±30 mV indicates stability. Measured zeta potentials for all concentrations of nanofluid is taken two hours after preparation and were determined to be ±22, ±7.8 and ±52 mV at pH values of 2, 6 and 10, respectively at temperature of 25oC. The variations in zeta potential for SiC nanofluids determined a week after preparation was less than 5%. As shown in the inset of Fig. 2, SiC nanofluid prepared at pH value of 6 is in unstable state (sedimented) whereas those prepared at pH value of 10 remained stable even after one week of preparation. This can be explained thus: pH 6 is close to the isoelectric point (IEP), where net surface charge is close to zero (zeta →0mV) and hence SiC particles are likely to form clusters and sediment. In contrast, at pH values of 10, the nanoparticles exhibit strong energy barrier due to a balance between attractive and repulsive forces preventing particle agglomeration and sedimentation. 100
Electrical Conductivity, Ec, P S cm
40
80
SiC(29nm) GC/EG (60:40 wt.%)
-1
SiC(29nm) GC/EG (60:40 wt.%)
80
20
60 I, vol.% 0.0
60
After 1 week
0
40 I, vol.% 0.3 0.5 1.0
40
-20
20
20
-40 After 1 week
0
2
4
6
8
10
12
pH
Fig. 1 Evolution of Zeta potential with pH
0
-20 20
30
40
50
60
-1
-60
0
Electrical Conductivity, Ec, P S cm
Zeta potential, ], (mV)
60
70
o
Temperature T, ( C)
Fig. 2 Variation of electrical conductivity with temperature for different particle volume concentrations
777
S. Akilu et al. / Procedia Engineering 148 (2016) 774 – 778
Effect of temperature on electrical conductivity (Ec) was investigated in the temperature range of 30–60oC. It can be seen from Fig. 2 that the Ec of the base liquid is close to zero at lower temperatures with values of 0 and 0.21μScm-1 at 30oC and 60oC. An increasing trend can be observed in the case of SiC nanofluid. Enhancement in EC of 24.5% and 22.7% were obtained at 30oC and 60oC respectively with a nanofluid of 1.0 vol.% as compared to 0.3 vol.% concentration. A situation where the Ec of nanofluid would decrease with increasing temperature due to counterion condensation (CC) does not transpire within the concentration limits of this work. The CC effect relates to saturation of Ec determining ions which has a potential to decrease or flatten the conductivity with increasing particle concentration. Adio et al. [13] observed the Ec of MgO(20nm)/EG nanofluid to decrease at 2.0 and 3.0% vol. between temperatures of 50oC and 70oC. This is attributed to the increase of counterions close to the particle surface which strengthens the condensation region leaving the bulk charge and potential unchanged, thus resulting in a net decrease of Ec enhancement slope [14]. Influence of temperature on pH at different particle loadings of SiC nanofluid is depicted in Fig. 3. An increase in as a result, the splitting of molecules to form [H+] is decreased, thus lowering the values of fluid pH. The pH of the base liquid takes constant values of 6.36 at all the temperatures, whereas nanofluid pH decrease with increase in temperature. The diminution of pH with temperature between 30oC and 60oC for 0.3, 0.5 and 1.0 vol.% were 5.6%, 7.1% and 6.6%, respectively. The temperature can affect the pH of the system due to increased kinetic energies of the molecules. Fig. 4 shows the TC of SiC nanofluid as a function of temperature for different particle volume concentrations. As the temperature increases, the TC of the nanofluids increases. 12
I, vol.% 0.0
10
9
8 I, vol.% 0.3 0.5 1.0
8 7
pH
9
7
6
Thermal Conductivity k, (W/ mK)
11
pH
0.31
10 SiC(29nm) GC/EG (60:40 wt.%)
30
40
50
60
I, vol.% 0.0 0.3 0.5 1.0
0.29 0.28 0.27 0.26 0.25
6 20
SiC (29nm) GC/EG (60:40 wt.%)
0.30
20
70
30
o
40
50
60
70
o
Temperature T, ( C)
Temperature T, ( C)
Fig. 3 Variation of pH with temperature for different Fig. 4 Variation of thermal conductivity with temperature particle volume concentrations for different particle volume concentrations The TC demonstrate an increasing trend with nanofluid concentration. Measured TC of the base liquid at 30 oC and 60oC are 0.262 and 0.272 W/mK respectively. The rates of TC increase with 0.3, 0.5 and 1.0% vol. concentrations are 0.4%, 1.9%, 5.3% and 2.6%, 7.7%, 12.1% compared to base liquid at temperatures of 30 oC and 60oC respectively. Observed linear enhancement in TC with temperature may be pointed to the Brownian motion, which is a well known phenomenon influencing the thermal transport between the base liquid molecules and the dispersed particles. 300
300 SiC(29nm) Tnf, 15 C GC/EG (60:40 wt.%) 0.0 0.3 1.0 o
200 150
o
Tnf, 75 C 0.0 0.3 1.0
100
150 100 50
0
0 0
20
40
60
80
100
120
Shear rate J
Fig. 5 Evolution of shear stress with shear rate
I vol.% 0.0 0.3 1.0
200
50
-20
SiC(29nm) GC/EG (60:40 wt.%)
250
Viscosity, Pnf, cP
Shear stress, Dyne/ cm
2
250
J = 10s
10
20
30
40
50
60
70
-1
80
o
Temperature T, ( C)
Fig. 6 Variation of viscosity with temperature for different particle volume concentrations
778
S. Akilu et al. / Procedia Engineering 148 (2016) 774 – 778
It can be observed from Fig. 5 that the ratio of shear stress to shear rate exhibit a linear trend for particle loading of 0.3 and 1.0 vol.% in the temperature range of 15oC–100oC indicating Newtonian behaviour of the SiC nanofluid. Fig. 6 shows the influence of temperature on the viscosity of base liquid and SiC nanofluid. It can be seen that the viscosity increases with volume concentration though not significant in the range of concentration undertaken. However, the viscosity decreases with the increase of temperature. For example, the viscosity of 0.3 and 1.0 vol.% nanofluid was determined as 4.7% and 23.5% respectively higher than the base liquid at 30oC. Nanofluid viscosity decreased by about 56% and 137% with increase of temperature to 60oC compared to that of base fluid. The reason for such a drop in viscosity due to temperature rise disrupts the interparticle-to-intermolecular forces of attraction and intensification of Brownian motion resulting in the decrease in nanofluid viscosity [8] 4. Conclusion The influence of temperature on the electrical conductivity, pH, thermal conductivity and viscosity of SiC (29nm) nanofluid in a binary mixture of glycerol and ethylene glycol was undertaken. The experimental results reveal that the electrical conductivity and pH of the base liquid are unaffected by increased temperature. In contrast, the electrical conductivity of the SiC nanofluid increases while the pH decreases with increasing temperature. Moreover, TC and viscosity show strong response to temperature increase. At 0.3 and 1.0 vol.%, enhancement in TC with temperature between 30–60oC was observed to be 2.6% and 6.8%, respectively. Viscosity of the base liquid and SiC nanofluid decreased steadily with increasing temperature. A decrease of about 51% and 113% was found between 30oC to 60oC at 0.3 and 1.0 vol.% compared to the base liquid. Acknowledgements The financial support rendered by the Ministry of Higher Education, Malaysia through Fundamental Research Grant Scheme FRGS (No: 0153AB–K01) is gratefully acknowledged. Reference [1] D. K. Devendiran and V. A. Amirtham, "A review on preparation, characterization, properties and applications of nanofluids," Renewable and Sustainable Energy Reviews, vol. 60, pp. 21–40, 2016. [2] D. Singh, E. Timofeeva, W. Yu, J. Routbort, D. France, D. Smith, et al., "An investigation of silicon carbide-water nanofluid for heat transfer applications," Journal of Applied Physics, vol. 105, p. 064306, 2009. [3] E. V. Timofeeva, W. Yu, D. M. France, D. Singh, and J. L. Routbort, "Base fluid and temperature effects on the heat transfer characteristics of SiC in ethylene glycol/H2O and H2O nanofluids," 2011. [4] S. W. Lee, S. D. Park, S. Kang, I. C. Bang, and J. H. Kim, "Investigation of viscosity and thermal conductivity of SiC nanofluids for heat transfer applications," International Journal of Heat and Mass Transfer, vol. 54, pp. 433-438, 2011. [5] O. Manna, S. Singh, and G. Paul, "Enhanced thermal conductivity of nano-SiC dispersed water based nanofluid," Bulletin of Materials Science, vol. 35, pp. 707– 712, 2012. [6] H. Q. Xie, J. C. Wang, T. G. Xi, and Y. Liu, "Thermal conductivity of suspensions containing nanosized SiC particles," International Journal of Thermophysics, vol. 23, pp. 571–580, 2002. [7] N. Nikkam, M. Saleemi, E. B. Haghighi, M. Ghanbarpour, R. Khodabandeh, M. Muhammed, et al., "Fabrication, characterization and thermophysical property evaluation of SiC nanofluids for heat transfer applications," Nano-Micro Letters, vol. 6, pp. 178–189, 2014. [8] X. Li, C. Zou, T. Wang, and X. Lei, "Rheological behavior of ethylene glycol-based SiC nanofluids," International Journal of Heat and Mass Transfer, vol. 84, pp. 925–930, 2015. [9] X. Li, C. Zou, X. Lei, and W. Li, "Stability and enhanced thermal conductivity of ethylene glycol-based SiC nanofluids," International Journal of Heat and Mass Transfer, vol. 89, pp. 613–619, 2015. [10] K. S. Suganthi and K. S. Rajan, "Temperature induced changes in ZnO–water nanofluid: Zeta potential, size distribution and viscosity profiles," International Journal of Heat and Mass Transfer, vol. 55, pp. 7969–7980, 2012. [11] A. L. Valdivieso, J. R. Bahena, S. Song, and R. H. Urbina, "Temperature effect on the zeta potential and fluoride adsorption at the α-Al2O3/aqueous solution interface," Journal of Colloid and Interface Science, vol. 298, pp. 1–5, 2006. [12] D. Zhu, X. Li, N. Wang, X. Wang, J. Gao, and H. Li, "Dispersion behavior and thermal conductivity characteristics of Al2O3–H2O nanofluids," Current Applied Physics, vol. 9, pp. 131–139, 2009. [13] S. A. Adio, M. Sharifpur, and J. P. Meyer, "Factors affecting the pH and electrical conductivity of MgO–ethylene glycol nanofluids," Bull. Mater. Sci, vol. 38, pp. 1345–1357, 2015. [14] S. B. White, A. J.-M. Shih, and K. P. Pipe, "Investigation of the electrical conductivity of propylene glycol-based ZnO nanofluids," Nanoscale Research Letters, vol. 6, p. 1, 2011.
