Do Nanofluids Really Enhance Heat Transfer?

Heat transfer by nanofluids is deeply influenced by two parameters; thermal conductivity and dynamic viscosity. In nanofluids these parameters play a contradictory role. The addition of nanosolids into basic fluid leads to an augmentation in thermal conductivity which leads to an augmentation in heat transfer but also causes to an increase of the dynamic viscosity which causes to a regression in heat transfer. This paper investigates the contradictory roles played by thermal conductivity and dynamic viscosity on heat transfer by some types of nanofluids. Tests are realized considering a flow of natural convection in a differentially heated cavity. Navier-Stokes and energy equations are solved by the computational fluid dynamics code FLUENT The solid volume fraction of nanosolids is varied from 0% to 5%whereas their sizes are in the range of 15-140imi. Heat transfer is evaluated at temperatures ranging from 20 degrees C to 70 degrees C The numerical results have shown that at room temperature, increasing the concentration of nanosolids reduces the heat transfer from the hot wall to the flow. This attitude is reversed when the system is at high temperatures. These effects on heat transfer and entropy generation are discussed in this work.