Theoretical analysis of natural convection boundary layer heat and mass transfer of nanofluids: Effects of size, shape and type of nanoparticles, type of base fluid and working temperature

The problem of natural convection boundary layer heat transfer of nanofluids is theoretically analyzed. Different aspects of nanoparticles, such as size, shape and constructive material, as well as the type of the base fluid and the working temperature, are examined. The drift-flux model of nanofluids, including the effects of Brownian motion, thermophoresis, and the local volume fraction of nanoparticles, is adopted to model the boundary layer heat and mass transfer of nanofluids. Following the state-of-the-art, the thermo-physical properties are extracted from five different synthesized types of nanofluids. A new non-dimensional parameter, the enhancement ratio, indicating the ratio of the convective heat transfer coefficient of the nanofluid to the base fluid, is introduced. The effect of the nanoparticles on the enhancement of natural convective heat transfer of nanofluids is discussed. The main findings of this study are as follows: (i) the type of the nanoparticles and the base fluid are the most important parameters affecting the heat transfer of nanofluids; (ii) in some cases, the presence of nanoparticles in the base fluid deteriorates the heat transfer rate; and (iii) the rise of the working temperature reduces the efficiency of the nanofluid, which is a crucial issue in applications of nanofluids as coolants.