ﮫﺎر घ ਯ ﻦ ಻ඖ ژی ﭘﺎک୆ اଡﺎﻻα ΰ ি ا ජ ࡶ 4 th Annual Clean Energy Conference (ACEC2014) 4 و5 ﺗﻴﺮ1393 – ﭘﻴﺸﺮﻓﺘﻪ ﻓﻨﺎوري و ﺻﻨﻌﺘﻲ ﺗﻜﻤﻴﻠﻲ ﺗﺤﺼﻴﻼت داﻧﺸﮕﺎه ﻛﺮﻣﺎن، 1 Investigation on convective heat transfer of nanofluids: An application to solar collectors Paper Code: 646 E. Ebrahimnia-Bajestan 1 , M. Charjouei Moghadam 2 , H. Niazmand 2 , W. Duangthongsuk 3 , S. Wongwises 4 1 Department of Energy, Institute of Science and High Technology and Environmental Science, Graduate University of Advanced Technology, Kerman, Iran; E.Ebrahimnia@kgut.ac.ir 2 Department of Mechanical Engineering, Ferdowsi University of Mashhad, Mashhad, Iran; mcharjoie@gmail.com 3 Department of Mechanical Engineering, South-East Asia University, Bangkok, Thailand; 4 The Royal Institute of Thailand, Academy of Science, Sanam Sueapa, Dusit, Thailand; somchai.won@kmutt.ac.th Abstract One of the innovative methods of improving the heat transfer characteristics of solar heat exchangers is applying nanofluids as the working heat transfer fluids. Therefore, in this typical study the convective heat transfer of water-based TiO 2 nanofluid flowing through a uniformly heated circular tube in laminar flow regime has been investigated via experiment and numerical modeling. Using a two-step method, the nanofluids have been manufactured at different nanoparticle volume fractions of 1%, 1.6% and 2.3%. The thermal conductivity and dynamic viscosity of the prepared nanofluids have been modeled based on the present measured data at different temperatures and nanoparticle volume fractions. Furthermore, the single and two-phase numerical approaches have been employed and modified to predict the heat transfer characteristics of the nanofluid. The results indicated that the convective heat transfer coefficient increases with an increase in nanoparticle concentration and flow Reynolds number. The numerical investigation proved that the predicted heat transfer coefficients via single-phase approach based on the thermophysical properties of nanofluids in a static condition are lower than the experimental data. Finally, it was concluded that employing the proposed nanoparticle Nusselt number correlation instead of the Ranz-Marshall correlation, the two-phase Eulerian-Eulerian model predicts heat transfer coefficient of nanofluids more accurately. Keywords: Solar collector, Nanofluid, Convective heat transfer, Experimental study, Two phase modeling Introduction Although employing solar energy as a source of heating is a noteworthy issue, more investigation should be carried out to improve the performance and overcome the weakness of these systems such as their high price, large size and low efficiency. One of the main obstacles to increase the performance of these kinds of heat exchangers is the inherently poor thermophysical properties of conventional heat transfer fluids such as water and oil. Recently, nanofluids have attracted great interest due to their valuable heat transfer characteristics in comparison with conventional fluids. Several researchers [1-7] have been investigated the application of these new media of heat transfer in the solar collectors. Yousefi et al. [1] experimentally revealed that Al 2 O 3 /water nanofluid enhances the efficiency of flat- plate collectors by the amount of 28.3%. Similar behavior was observed by the Tyagi et al. [2] during a theoretical study on the Al 2 O 3 /water nanofluid as the absorbing medium in a direct absorption solar collector. Most of investigations on the application of nanofluids in solar collectors have been limited to thermodynamically based study of the nanofluids effects on the efficiency. However, there are few studies on the hydrodynamic and convective heat transfer characteristics of nanofluids in solar heat exchangers [6]. Since a decade ago, research publications related to the use of nanofluids as working fluids have been reported both numerically and experimentally [7]. Convective heat transfer of nanofluids can be modeled as either single phase [8] or two phase [9]. In single phase approach it is assumed that the base fluid and nanoparticles are in thermal and hydrodynamic equilibrium and the presence of the nanoparticles is accounted for by introducing the effective thermophysical properties for the nanofluid. The comparison of implementing the two-phase and single- phase approaches in predicting the convective heat transfer of nanofluids laminar flow in a straight tube is reported in some papers [10-12]. Mixture and Eulerian- Eulerian models are more popular than the other two- phase models and considered in many studies like [11,13,14]. Mahian et al [15] reviewed the available publications in the issue of nanofluids application in solar systems and stated that in the numerical simulation of mentioned systems, employing temperature dependent