مروری بر تاثیر اندازه ذرات بر روی ویسکوزیته نانوسیال Effect of particle size on the viscosity of nanofluids: A review

1. Introduction Technology driven world enforces the researchers to explore more and more in thermal engineering. Currently, one of the most crucial pursuits of thermal engineers is to provide efforts on new types of heat transfer fluids. Thermal engineers found that the addition of solid particles to a base fluid can provide the fluid a better heat transfer capability. Based on this concept, a new generation fluid named as “nanofluid” has occurred in the field for the last two decades. Typically, water, ethylene glycol, oil, etc. are employed as base fluids, which have naturally poor thermal conductivities. Supplementation of nano-scaled metals, metal oxides or carbon based materials to these base fluids brings out the nanofluids. Although the idea was first conceived by Masuda et al. [1], Choi [2] was the one who had named it as nanofluid. Just after their inventions, a number of nanofluid related papers have increased expeditiously [3] as can be seen in Fig. 1. Recent literature reveals that nanofluid based systems have an extensive potential area such as, solar collectors [4,5], electronics cooling [6–8], automotive [9,10], nuclear reactor cooling [11], refrigerators [12–14], heat exchangers [15,16]. The potential utilization of such a colloidal mixture for many divergent systems exposed the requisiteness of meticulous investigation on thermal properties of the nanofluids. One of the pioneering studies on the thermal conductivity by Lee et al. [17] concluded that the presence of nanoparticles provides substantially higher thermal conductivity than the same liquid without particle addition. Eastman et al. [18] prepared a nanofluid by adding copper nanoparticles into the ethylene glycol and they observed a thermal conductivity increment up to 40%. Xie et al. [19] studied the thermal conductivity of Al2O3–ethylene glycol nanofluid. The conclusion, which is in accordance with the former one, included that the thermal conductivity of the suspension was much higher than the base fluid. Although the goal of adding nano sized particles to a base fluid is to achieve higher thermal conductivity values, the thermal conductivity is not the sole property that influences the heat transfer. It is also viscosity that is playing a key role on characterizing the heat transfer behavior of a nanofluid [20]. Li et al. [21] was one of the first groups who investigated the transport properties of nanofluids and observed that the viscosity was not only affected by the volume concentration, but also the size of nanoparticles.