Review of convective heat transfer enhancement with nanofluids
Introduction
Nanofluid is envisioned to describe a fluid in which nanometer-sized particles are suspended in conventional heat transfer basic fluids. Conventional heat transfer fluids, including oil, water, and ethylene glycol mixture are poor heat transfer fluids, since the thermal conductivity of these fluids play important role on the heat transfer coefficient between the heat transfer medium and the heat transfer surface. Therefore numerous methods have been taken to improve the thermal conductivity of these fluids by suspending nano/micro or larger-sized particle materials in liquids.
Since the solid nanoparticles with typical length scales of 1–100 nm with high thermal conductivity are suspended in the base fluid (low thermal conductivity), have been shown to enhance effective thermal conductivity and the convective heat transfer coefficient of the base fluid. The thermal conductivity of the particle materials, metallic or nonmetallic such as Al2O3, CuO, Cu, SiO, TiO, are typically order-of-magnitude higher than the base fluids even at low concentrations, result in significant increases in the heat transfer coefficient (Table 1). Therefore the effective thermal conductivity of nanofluids is expected the enhanced heat transfer compared with conventional heat transfer liquids.
Choi [2] is the first who used the term nanofluids to refer to the fluid with suspended nanoparticles. Choi et al. [3] showed that the addition of a small amount (less than 1% by volume) of nanoparticles to conventional heat transfer liquids increased the thermal conductivity of the fluid up to approximately two times.
Several researches Masuda et al. [4], Lee et al. [5], Xuan and Li [6], and Xuan and Roetzel [7] stated that with low nanoparticles concentrations (1–5 Vol%), the thermal conductivity of the suspensions can increase more than 20%. Eastman et al. [1] at Argonne National laboratory showed with some preliminary experiments with suspended nanoparticles, the thermal conductivity of approximately 60% can be obtained with 5 Vol% CuO nanoparticles in the based fluid of water. Heat transfer coefficient is the determining factor in forced convection cooling–heating applications of heat exchange equipments including engines and engine systems. Such enhancement mainly depends upon factors such as particle volume concentration, particle material, particle size, particle shape, base fluid material temperature, and additives.
Nanoparticles used in nanofluids have been made out of many materials by physical and chemical synthesis processes. Typical physical methods include the mechanical grinding method and the inert-gas-condensation technique [8].
Current processes specifically for making metal nanoparticles include mechanical milling, inert-gas-condensation technique, chemical precipitation, chemical vapor deposition, micro-emulsions, spray pyrolysis and thermal spraying. Nanoparticles in most materials discussed are most commonly produced in the form of powders [9]. In powder form, nanoparticles can be dispersed in aqueous or organic base liquids to form nanofluids for specific applications. Up to date, nanofluids of various qualities have been produced mainly by small volumes by two-step technique and the single step technique which simultaneously produce powders and disperses directly into the base fluids [9]. The large-scale production of well-dispersed nanofluids at low cost is required for commercial applications [1].
Section snippets
Thermal conductivity of nanofluids
Since the high thermal conductivity nanoparticles suspended in the base fluid which has a low thermal conductivity, remarkably increase thermal conductivity of nanofluids. Researchers developed many models to tell how much that increase would be and many experiments have been conducted to compare experimental data with those analytical models. This still needs further research to develop a sophisticated theory to predict thermal conductivity of nanofluids. But there exists some empirical
Enhancement of convective heat transfer
The enhancement of the heat transfer coefficient is a better indicator than the thermal conductivity enhancement for nanofluids used in the design of heat exchange equipment. The physical properties of nanofluids are quite different than the base fluid. Density, specific heat and viscosity are also changed which enhance the heat transfer coefficient exceeding the thermal conductivity enhancement results reported by some experiments.
Heris et al. [25] did experiments with Al2O3 and CuO
Theoretical analysis of heat transfer enhancement with nanofluids
The seminal work by Choi [2] reported the concept of nanofluids and then the interest in this area has grown. Limited computer simulations of thermal properties and heat transfer characteristics of nanofluids have been performed. Some of these simulations dealt with the effective thermal conductivity of nanofluids [33], [34] or effective viscosity [35] and most of them focuses on the heat transfer of nanofluids [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49],
Conclusions
The literature survey shows that nanofluids significantly improve the heat transfer capability of conventional heat transfer fluids such as oil or water by suspending nanoparticles in these base liquids. Further theoretical modeling and experimental works on the effective thermal conductivity and apparent diffusivity are needed to demonstrate the full potential of nanolfuids for enhancement of forced convection. The understanding of the fundamentals of heat transfer and wall friction is prime
Acknowledgment
The authors thank the Scientific and Technical Research Council (TUBITAK) of Turkey for supporting this study.
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