Today more than ever, ultrahigh-performance cooling plays an important role in the development of energy-efficient heat transfer fluids which are required in many industries and commercial applications. However, conventional coolants are inherently poor heat transfer fluids. Nanofluid a term coined by Choi in 1995 is a new class of heat transfer fluids which is developed by suspending nanoparticles such as small amounts of metal, nonmetal or nanotubes in the fluids. The goal of nanofluids is to achieve the highest possible thermal properties at the smallest possible concentrations (preferably<1% by volume) by uniform dispersion and stable suspension of nanoparticles (preferably<10 nm) in host fluids. We have divided this chapter to four sections. Section 1 has focused on the two methods of synthesizing nanofluids and different methods for dispersing spherical and cylindrical nanoparticles such as Ag, Cu in a host fluid and also the common methods for measuring the thermal conductivity of nanofluids. Section2 has discussed on the thermal conductivity of nanofluids respect to pure fluids to explain the effective thermal conductivity of nanofluids. In this section various theoretical models on the effective thermal conductivity of nanofluids for spherical and cylindrical nanoparticles have been investigated. Section 3 represents the limited understanding of convective heat transfer in nanofluids containing carbon nanotubes that has been developed in recent years. For example the natural and forced convection of nanofluids in a heated cavity are investigated. In recent years there are theoretical and numerical researches for studying the heat transfer effect of nanofluids in microchannels because of their high thermal conductivity. In the last section we discussed about the theoretical and experimental researches for optimizing the microchannel heatsink performance.
