Heat transfer with nanofluids

Abstract

Nanofluids are suspensions of nano-size particles (typically 5–1,000;nm) in liquids. Several research projects in the late 1990s and the first decade of the twenty-first century indicated that the addition of small amounts of nanoparticles in common cooling fluids increases significantly the effective conductivity of these suspensions as well as their convective heat transfer coefficients. While typical experimentally determined conductivity enhancements were in the range 10–50%, some early experiments showed enhancement values higher than 100%. Experiments on the mass transfer coefficients with nanofluids reported more dramatic results with maximum mass transfer enhancements in the range of two to six times that of the base fluid. The significantly enhanced transport properties of the nanofluids have enormous implications in industrial processes, such as the cooling of very small electronic components, which will comprise the next generation of computer chips, absorption of gases by liquid carriers, chemical reactions, combustion for electricity generation, cooling of IC engines, directed-energy weapons, boiling under microgravity conditions, nuclear reactor cooling, and biomedicine. Because of the enormous industrial and economic potential of nanofluids, a significant amount of research was conducted during the first decade of the twenty-first century on the thermal and transport properties and applications of nanofluids, hundreds of journal articles were written, and several conferences were devoted to the subject. This chapter describes the salient heat transfer characteristics of nanofluids. At first, the chapter includes a fundamental description of continua and when nanoparticles may be considered as continua. Second, a rigorous definition of the thermodynamic properties of heterogeneous mixtures is given, with applications to the density and specific heat capacity of nanofluids. Third, the transport properties of nanofluids are described, in particular, viscosity and conductivity. Likely mechanisms for the enhancements of the transport properties are also described in detail and in a critical way. Finally, experimental results and the underlying theory are presented on the enhancement of the convective heat transfer coefficients and the effect of nanoparticles on the pool boiling processes and the critical heat flux of the base fluids.