th
rd
Proceedings of the 6 International and 43 National Conference on Fluid Mechanics and Fluid Power December 15-17, 2016, MNNITA, Allahabad, U.P., India
FMFP 2016 – PAPER NO. 435
Experimental Investigation on Thermal Conductivity of TiO 2-Nanofluids for Solar Collector Pritam Kumar Das
Ranjan Ganguly
Apurba Kumar Santra
Ph. D Scholar, Jadavpur University, Salt lake, Sector-III, 700098 Email:
[email protected]
Professor, Jadavpur University, Salt lake, Sector-III, 700098 Email:
[email protected]
Professor, Jadavpur University, Salt lake, Sector-III, 700098 Email:
[email protected]
Abstract In this study effects of temperature (20–60 °C) and solid volume fraction (0.1–2.0%) on thermal conductivity of TiO2water based nanofluid has been investigated experimentally. Stability of the synthesized nanoparticle suspension has been investigated using various surfactants, viz. cetyl trimethyl ammonium bromide (CTAB), oleic acid (OA) and sodium dodecyl sulfate (SDS), where only CTAB is found to provide stable suspensions. Transmission electron microscopy (TEM) and Dynamic light scattering (DLS) measurement indicates particle size ranging between 10–40 nm and average cluster size of 272 nm, respectively. Experimental results show that thermal conductivity of the nanofluid increases with the increase of both solid volume fraction and temperature.
nanofluids exceed those of their respective base fluids [4,5]. Reddy and Rao [6] measured the thermal conductivity of TiO2 in ethylene glycol-water as host fluid (40%:60% and 50%:50% by weight) for different temperature (3070 °C). They observed that the thermal conductivity increased with solid volume fraction (φ) and with temperature. Similarly, an improvement of 60% in the thermal conductivity associated with the consistent base fluids for φ =5% of nanoparticles was reported by Eastman et al. [7]. Das et al. [8] experimentally investigated the thermal conductivity of TiO2-AA water based nanofluid for different φ (0.1%-2.0%) and temperature (20-60 °C). The result showed that thermal conductivity of AA-stabilized nanofluid increases monotonically with φ and temperature. Murshed et al. [ 9 ] showed that the thermal conductivity increased with an increase of φ. At φ = 5% of TiO2 nanoparticles in water, they observed that heat transfer enhancement in the range of 33% and 30% over the base fluids using 10 nm and 15 nm particles, respectively.
Keywords TiO2-water nanofluid; Thermal conductivity; Temperature dependence; Surfactants.
I.
INTRODUCTION
Thermophysical properties of nanofluids depends on with the factors such as: stability of suspension of nanoparticles, nanoparticle size etc. [ 10 ]; surfactant plays a major role in particle stabilization. Although several studies have been performed by researchers with TiO2 nanoparticles with surfactants like SDS, SDBS, Tween 80 etc., there have been fewer reports with CTAB stabilized TiO2 nanofluids. Present work discuses the synthesis and study of thermal conductivity of TiO2-CTAB water-based nanofluids for different temperatures (2060 °C) and solid volume fractions (0.12.0%). Two-step method has been followed for nanofluid synthesis as it offers a facile and rapid method for preparation of large volume of nanofluids with greater control over nanoparticle concentration and narrower particle size distribution than a single-step synthesis. Nanoparticle size distribution has been measured using dynamic light scattering (DLS) technique. Microstructure of the nanofluid has also been examined using transmission electron microscopy (TEM).
Nanofluid, [1] is a new and innovative class of composite fluid, suspension of nanoparticles (1-100 nm) such as TiO2, Al2O3, Cu, Si, Ag, Cuo and SiO2 dispersed in conventional liquids (water, ethylene glycol and engine oil). It exhibits higher thermal properties and can be used as coolant in many systems such as solar thermal collectors [ 2 ], medical applications, enhancement of the performance and efficiency of the thermal systems [ 3 ], and many more. Titanium dioxide (TiO2) nanoparticles have the advantages of having high chemical and physical stability, low cost, commercial availability, and free from health hazards. So, TiO2 nanofluids can be selected as a promising heat transfer medium. To understand the suitability of this nanofluid for practical thermal applications, detail knowledge of its thermal properties is essential. Thermal conductivity is one of the most important parameters in practical nanofluid-based thermal applications. It has been observed in the literature that thermal conductivity of
II. 1
EXPERIMENTAL DETAILS
A. Materials TiO2 nanoparticles with an average particle diameter of 21 nm, purchased from Sigma Aldrich Chemicals Limited, Germany, were used. The base fluid consisted of distilled water (Merck Millipore) of 99.7% purity. Surfactants like cetyl trimethyl ammonium bromide (CTAB), oleic acid (OA) and sodium dodecyl sulphate (SDS) were purchased from Merck Millipore and used to improve the stability of suspensions.
TiO2-water nanofluids, with CTAB as the stabilizer, at φ=1.5%. It is confirmed from the micrographs that the average size of the particles in the ranges of 1040 nm. Simultaneously the particle size distribution in the nanofluid samples was analyzed using dynamic light scattering (DLS) (DelsaNano S (A53876), 0.6 nm-7 µm). All measurements were performed at a constant temperature of 25 °C. Figure 1 (b) shows cumulative result of particle loading number distribution for CTAB-stabilized TiO2water nanofluid at φ = 1.5% which shows an average diameter of 272 nm. This is much larger than the specified nanoparticle size (21 nm), indicating the possibility formation of thermodynamically stable clusters of particles.
B. Preparation of Nanofluids Two-step method was adopted for preparation of nanofluid. Required φ of a nanofluid was obtained by dispersing measured mass of nanoparticle in the host liquid. A high-precision electronic balance (Sartorius, BSA 224S-CW, max 220 gm, d=0.1 mg) was used for this purpose. The resulting φ is obtained using the following formula. =
[1]
Where denotes the density of nanoparticles (4260 kg/m3), the density of the host fluid (1000 kg/m3). Measured quantity of nanoparticles were mixed with the host fluid and agitated for 15 minutes in a probe sonicator (PCI Analytics, PKS-750FM), with 50% amplitude, and the suspensions were observed for several hours to check if the particles settled under gravity. To stabilize the nanofluids, different surfactants were used. It requires selection of suitable surfactant with appropriate amount. Firstly, surfactant was mixed with the host fluid and stirred in a magnetic stirrer (REMI, 2MLH) for 1 hours to ensure homogenous mixing of surfactant with the host fluid. Then the nanoparticles were added and stirred for another 1 hours in the magnetic stirrer so that the particles get dispersed homogeneously within the mixture. Finally, the nanofluid mixture was dispersed in the ultrasonic bath for about 15 minutes to break the agglomerates retained even after stirring. For CTAB, nanoparticle to surfactant mass ratio of 1:10 was found to give the best suspensions. For SDS and OA ranges of particle to surfactant mass ratio (from 1:2, to 1:10) were tried, but they didn't exhibit satisfactory stabilization. CTAB is cationic surfactant, which differs from the SDS (anionic), OA (non-ionic). This could be reason for the difference in suspension behavior.
(a)
(b)
Figure 1 (a) TEM and (b) DLS images of TiO2-CTAB water based nanofluid at φ = 1.5%.
B. Measurements of Thermal Conductivity: Thermal conductivity (TC) of the nanofluid samples was measured with KD2 Pro thermal properties analyzer (Decagon Devices, WA, USA), which operates on the transient hot wire principle. The nanofluid samples were kept in a cylindrical glass container (25 mm diameter and 80 mm height) where the TC-probe was inserted. The samples were maintained at different temperatures by immersing them in constant temperature bath.
III.
RESULTS AND DISCUSSION A. Nanofluid characterizations: In order to characterize the morphology of the nanoparticles, dry samples were analyzed in a JEOL JEM-2100 Transmission Electron Microscope (TEM), operating at an acceleration voltage of 200 kV. Samples were prepared by depositing ~5 µL droplets of dilute aqueous suspensions of the nanoparticles on carbon-coated copper grids (300 mesh) that were covered by a thin layer of Formvar. Figure 1 (a) shows the TEM images of
Figure 2 Schematic diagram of experimental setup used to measure the thermal conductivity of nanofluids.
The sensor was calibrated every time before use by measuring the thermal conductivity of distilled water and glycerol at a room temperature of 25 °C. The uncertainty
2
1.08
analysis of thermal conductivity has been carried out following standard procedure [11], and the maximum uncertainty in the thermal conductivity measurement was found to be ±5.06%. Schematic diagram of experimental setup to measure thermal conductivity of nanofluids have shown in figure 2.
1.04
1.02
1 0
0.74
1 φ (%)
1.5
2
IV. CONCLUSIONS Synthesis and characterization of TiO2-CTAB water based nanofluids have been performed for different volume fraction (0.1%-2.0%), and temperature (20–60 °C).
40 °C
0.7 K (W/ m K)
0.5
Figure 4 Thermal conductivity ratio with different φ at different temperature for TiO2-CTAB nanofluids and compared with well-known thermal conductivity models for nanofluids.
0.78 30 °C 60 °C
30°C 50°C Maxwell Timofeeva
Knf / Kf
1.06
Figure 3 shows the variation of thermal conductivity with φ (0.1–2.0%) at different temperature (20–60 C) for CTABstabilized nanofluid. Minimum ten numbers of runs have been taken to calculate the standard deviation, for plotting the error bar. The result shows that the thermal conductivity of the nanofluid increases with increase in φ and temperature. The enhanced effect of temperature on thermal conductivity may be attributed to the increased intensity of the particle Brownian motion with the increase of temperature [5]. The van der Waals force and electrostatic force also play important role. 20 °C 50 °C
20°C 40°C 60°C Yu and Choi
Stability of the nanofluids was tested with different surfactants (CTAB, SDS, OA) in the base fluid (water) while particle distribution and morphology were tested using DLS and TEM. CTAB shows the best stabilization (stable for exceeding 20 days).
0.66 0.62 0.58 0.54 0
0.3
0.6
0.9 1.2 φ (%)
1.5
1.8
TEM data shows a particle distribution in the range of 10 to 40 nm while DLS measurement indicates the existence of stable clusters having average size of 272 nm.
2.1
Figure 3 Variation of thermal conductivity with different φ at different temperature for TiO2-CTAB nanofluids.
Thermal conductivity of the CTAB stabilized nanofluid increases monotonically with solid volume fraction and temperature.
Figure 4 shows the thermal conductivity ratio of CTABstabilized nanofluids as function of φ at different temperatures (viz., 20, 30, 40, 50, and 60 °C), and has been compared with three well-known thermal conductivity models for nanofluids. Three models, viz., Maxwell [12], Timofeeva et. al. [13] and Yu and Choi [ 14 ] describes the thermal conductivity of nanofluids as function of φ as: =
= (1 + 3 ), and =
,
Acknowledgements The authors gratefully acknowledge the UGC-sponsored DRS (SAP-II) program of Power Engineering Department of Jadavpur University, INDIA for its support provided to carry out the present research work and also acknowledges the additional support provided by TEQIP Phase-II in the form of a research fellowship.
(2) (3)
(
(
)
)
,
REFERENCES
(4)
respectively. In Eq. [4], β denotes the ratio of the nano-layer thickness to the original particle radius. Normally, β = 0.1 is used to calculate the thermal conductivity of nanofluid [14]. It is observed that for the temperature range and volume fractions reported in Figure 3, the models proposed by Maxwell (eq. [2]), Timofeeva (eq. [3]) and Yu and Choi (eq. [4]) confirms to the experimental observation.
1. S. U. S. Choi, Enhancing thermal conductivity of fluids with nanoparticle, ASME Int. Mechanical Engineering Congress & Exposition, 231 (1995) 99–105. 2. T. Otanicar, P.E. Phelan, R.S. Prasher, G. Rosengarten, R.A. Taylor, Nanofluid based Direct absorption solar collector, Journal of Renewable and Sustainable Energy, 2 (2010) 033102, 1-13. 3. G. Colangelo, E. Favale, A. de Risi, D. Laforgia, Results of experimental investigations on the heat conductivity of nanofluids based on diathermic oil for high temperature applications, Applied Energy, 97 (2012) 828–33.
3
4. D. Cabaleiro, J. Nimo, M.J. Pastoriza-Gallego, M.M. Piñeiro, J.L. Legido, L. Lugo, Thermal conductivity of dry anatase and rutile nano-powders and ethylene and propylene glycol-based TiO2 nanofluids, The Journal of Chemical Thermodynamics, 83 (2015) 67–76. 5. X. Wang, X.S. Xu, S.U.S. Choi, Thermal conductivity of nanoparticle–fluid mixture, Journal of Thermophysics and Heat Transfer, 13,4 (1999),474–480. 6. M. C. S. Reddy, V. V. Rao, Experimental studies on thermal conductivity of blends of ethylene glycol-water-based TiO2 nanofluids, International Communication in Heat and Mass Transfer, 46 (2013) 31–36. 7. J. A. Eastman, U. S. Choi, S. Li, L.J. Thompson, S. Lee, Enhanced thermal conductivity through the development of nanofluids, Material Research Society Symposium Proceedings, 457 (1997). 8 . P. K. Das, A. K. Mallik, A. K. Santra and R. Ganguly, Synthesis and characterization of TiO2–water nanofluids with different surfactants, International Communications in Heat and Mass Transfer, 75, 2016, Pages 341–348. 9. S. M. S. Murshed, K. C. Leong, C. Yang, Enhanced thermal conductivity of TiO2-water based nanofluids, Int. J. of Thermal Sciences, 44 (2005) 367–373. 10 S.M.S. Murshed, K.C. Leong, C. Yang, Thermophysical and electrokinetic properties of nanofluids – A critical review, Applied Thermal Engineering 28 (2008) 2109–2125. 11. J.P. Holman, Experimental methods for engineers, fifth edition, McGraw-Hill Book Company1989. 12. J. C. Maxwell, A Treatise on Electricity and Magnetism. Oxford, NY, UK: Oxford Clarendon, 1873. 13 . E. V. Timofeeva, A.N. Gavrillov, J. M. McCloskey, Y. V. Tolmachev, Thermal conductivity and particle agglomeration in alumina nanofluids: experiment and theory, Physical Review E, 76 (2007) 061203, 1-16. 14 . W. Yu, S. Choi, The role of interfacial layers in the enhanced thermal conductivity of nanofluids: a renovated Maxwell model, Journal of Nanoparticles Research, 5 (2003), 167–171.
4
