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Int. J. Nanoparticles, Vol. 5, No. 4, 2012
Experimental investigations on thermal conductivity of water and Al2O3 nanofluids at low concentrations Y. Raja Sekhar* Centre for Energy Studies, Department of Mechanical Engineering, JNTUH College of Engineering, Kukatpally, Hyderabad-500085, Andhra Pradesh State, India E-mail:
[email protected] *Corresponding author
K.V. Sharma Faculty of Mechanical Engineering, Universiti Malaysia Pahang, 26600 Pekan, Kuantan, Pahang State, Malaysia E-mail:
[email protected]
M.T. Naik and L. Syam Sundar Centre for Energy Studies, Department of Mechanical Engineering, JNTUH College of Engineering, Kukatpally, Hyderabad-500085, Andhra Pradesh State, India E-mail:
[email protected] E-mail:
[email protected] Abstract: Thermal conductivity properties of water-based nanofluids at low concentrations are being presented in this paper. Nanoparticles at low volume concentration in the range of 0.01 to 1% is added to water improve the properties of the base fluid. The properties are estimated at temperatures between 21°C and 45°C. The results indicate an increase of enhancement of thermal conductivity with temperature and are found to be in agreement with different models as well as the data published in the literature. Empirical equations to calculate thermal conductivity and density for varied particle concentration and temperature are established. The measured and predicted values of thermal conductivity and density have a maximum deviation of 8% and 6% in the working range, respectively. Keywords: Al2O3 nanoparticles; low volume concentration; density; thermal conductivity; water-based nanofluids; temperature dependent. Reference to this paper should be made as follows: Sekhar, Y.R., Sharma, K.V., Naik, M.T. and Sundar, L.S. (2012) Experimental investigations on thermal conductivity of water and Al2O3 nanofluids at low concentrations, Int. J. Nanoparticles, Vol. 5, No. 4, pp.300315. Copyright © 2012 Inderscience Enterprises Ltd.
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Biographical notes: Y. Raja Sekhar is a PhD student at Center for Energy Studies, JNTUH College of Engineering, at JNT University, Hyderabad, India. He received his Bachelors degree in Mechanical Engineering and Masters degree in Energy Systems from the same institute in 2002 and 2005, respectively. Currently, he is working on experimental investigation of nanofluids for application to solar collectors for improved heat transfer. K.V. Sharma is Professor and Head of the Center for Energy Studies, in JNTUH College of Engineering, at JNT University, Hyderabad, India. Currently, he is on deputation to Universiti Malaysia Pahang. He received his PhD from the same institute in 1999 and Masters degree from Andhra University, Visakhapatnam, India, in 1984 respectively. He has published about 50 research papers in international journals and conferences. His research interests include heat exchangers, two-phase heat transfer, porous media, heat transfer in nanofluids and renewable energy. M.T. Naik is an Associate Professor in the Center for Energy Studies, JNTU College of Engineering, at JNT University, Hyderabad, India. He received his Bachelors degree in 1994 and Masters degree in 2002 followed by PhD in 2011 from the same institute. His current research interest include enhancement of heat transfer using Glycol nanofluids. He has published about ten research papers in international journals and conferences. L. Syam Sundar is a PhD student at Heat Transfer Laboratory, Center for Energy Studies, JNTU College of Engineering, at J.N.T.University, Hyderabad, India. He received his Masters degree from the same institute in 2003 and Bachelors degree from Andhra University, Visakhapatnam, India in 1998. Currently, he is working on the experimental investigation of heat transfer enhancement in a circular tube with different nanofluids.
1
Introduction
The ever increasing requirement of enhanced heat transfer rate is the hour of need for wide applications especially in heat exchangers, heat pipes, etc. Heat transfer fluids without additives have less thermal conductivity compared to metals. This has opened windows for developing novel methods to increase the thermal conductivity of heat transfer fluids with incorporation of additives. Initially, the concept of thermal conductivity of suspensions was extensively studied by Maxwell (1881). Since then, many researchers studied the properties of liquids by suspending particles of various sizes ranging from millimetre to micrometer size. A significant rise in thermal conductivity of fluids due to suspended metal particles was observed but results were limited to micrometer sized particles. The major disadvantages during application of using these suspended fluids were observed as sedimentation, fouling, erosion, clogging and increase in pressure drop. But, nanotechnology has given solution not only to overcome the above problems but also for enhancement in heat transfer of working fluids. Choi (1995) proposed concept of nanofluid, as the stable liquid suspensions having solid particles of nano meter range (less than 100 nm) dispersed in the base fluid (Choi, 1995). Addition of nano particles to the base fluid significantly improves the thermal conductivity (Wang and Majumdar, 2007) and hence the heat transfer rate. A large number of experimental and theoretical studies on nanofluids were extensively done globally for over the past two
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decades by numerous research groups. To overcome the problem of sedimentation due to dispersion of metal nanoparticles in the base fluid, use of different surfactants in low volumes is to be added to the fluids without changing the acidic or basic nature of the fluid (Wen and Ding, 2004). Experimental results of research groups working on nanofluids found that the parameters affecting the thermal conductivity of nanofluids are particle volume concentration, particle material, size of the particle, particle shape, base fluid material, temperature of the fluid, surfactant material and pH of the fluid (Das et al., 2003a, 2003b; Roy et al., 2006; Zhang et al., 2006). Gharagozloo and Goodson (2010) studied the effect of thermal conductivity due to particle aggregation and diffusion of nanoparticles in the base fluid. They conducted experimental and statistical simulation studies for different volume concentrations and particle sizes. They reported that increase in thermo-diffusion affects aggregation of the particles and enhance the thermal conductivity. Kihm et al. (2011) presented a model to explain the micro convection effect on thermal conductivity of nanofluids based on heat propagation velocity. The heat propagation velocity is reported to be faster phenomenon affecting the effective thermal conductivity of nanofluids rather than Brownian motion. Their model follow modified kinetic principle taking into account the effects of particle coagulation and heat dissipation. Nanofluids have shown vast potential in many applications as heat transfer fluid in the areas of energy, electronics and instrumentation, thermal management and transportation. In the present study, thermal conductivity behaviour of water-based nanofluids suspended with Al2O3 nanoparticles of low volume concentrations are undertaken. These low volume concentration nanofluids can be used as heat transfer fluid for heating as well as cooling applications.
1.1 Theoretical models on thermal conductivity of nanofluids For particle-fluid mixtures, theoretical studies have been conducted by various research groups starting from the classical work of Maxwell (1881). He derived relationship for the effective thermal conductivity of particle-fluid mixtures having low concentrated dispersions of relatively large spherical particles given by: Kr
Kp
2K b
Kp
2K b
2 Kp Kp
Kb
(1)
Kb
where, Kp is the thermal conductivity of the particle, Kb is the thermal conductivity of the base fluid and is the particle volume fraction in the suspension. Bruggemen (1935) proposed an implicit model to analyse the interactions among randomly distributed particles for the effective thermal conductivity of the solid liquid mixtures. His model can be applied to spherical particles having no limitation with particle volume concentrations given by Kp
Keff
Kp
2Keff
1
Kb
Keff
Kb
2Keff
0
(2)
