Radiative properties of nanofluids

Compared to thermal conductivity and convection studies with nanofluids; the optical and radiative properties of nanofluids have received much less interest. However, very recently, the number of studies on radiative heat transfer in nanofluids has been increasing. This is due to the fact that, in general, a composite nanofluid has different properties than those found in either the base fluid or the particles. At high temperatures, knowledge of the resultant radiative properties becomes increasingly significant. The concept of using direct absorbing nanofluid (suspension formed by mixing nanoparticles and a liquid) recently been shown numerically and experimentally to be an efficient method for harvesting solar thermal energy. Nanofluid is a product of emerging field of nanotechnology, where nanoparticles (1–100 nm in size) are mixed with conventional base fluids (water, oils, glycols, etc.). Nanofluids as an innovative class of heat transfer fluids represent a rapidly emerging research field where nano-science and thermal engineering coexist. Nanofluids are considered to be a two-phase system, comprised of a solid and a liquid phase. Compared to the base fluids like water or oil, nanofluids feature enhanced thermo-physical properties such as thermal diffusivity, viscosity, thermal conductivity, convective heat transfer coefficients, and optical properties. They offer unprecedented potential in many applications. Recent development in solar thermal collectors is the use of nanofluids to absorb the light directly. There is much current work going on the use of nanoparticles in several applications. With thousands of papers published every year, a comprehensive literature survey is impossible, and only selected representative publications are cited in this paper, particularly as they concern fundamental scientific insights on the fundamental optical properties of nanofluids.