Effects of Nanoparticles Diameter on Laminar Mixed Convection Heat Transfer of a Nanofluid in a Curved Tube using Two-Phase Mixture Model

Demand from different industries such as electronic, automotive and aerospace industries, for heat exchanger devices for which to be small in size, light in weight and of high performance is growing fast. However, low thermal conductivity of conventional heat transfer fluids is a serious limitation in improving the performance and compactness of these engineering equipments. An innovative way of improving the thermal conductivities of fluids is to suspend small solid particles in the fluid. However, more than a century ago, Maxwell [1] showed the possibility of increasing thermal conductivity of a mixture by more volume fraction of solid particles. In conventional cases the suspended particles are of μm or even mm dimensions. However, such large particles may cause severe problems such as abrasion and clogging. Therefore, fluids with suspended large particles have little practical application in heat transfer enhancement. By improving the technology to make particles in nanometer dimensions, a new generation of solid–liquid mixture that is called nanofluid, was appeared [2].Different concepts have been proposed to explain this enhancement in heat transfer; the increased thermal dispersion due to the chaotic movement of nanoparticles that accelerates energy exchanges in the fluid and the enhanced thermal conductivity of nanofluids considered by [2]. Many different theoretical and experimental studies have been done to determine the thermal conductivity of nanofluids. Among them Chon et al. [3] presented an experimental correlation for the thermal conductivity of Al2O3 as a function of nanoparticles size and fluid temperature. They showed that the Brownian motion of nanoparticles constitutes a key mechanism of the thermal conductivity enhancement with increasing temperature and decreasing nanoparticles sizes