Heat transfer can be enhanced by employing techniques and methodologies, such as increasing either the heat transfer surface or the heat transfer coefficient between the fluid and the surface that provide high heat transfer rates in small volumes. This can be accomplished in two ways: either enhancing the heat transfer capability of the fluid itself by using nanoparticles. The new techniques and methodologies, the particular materials, the strong miniaturisation and the employment of nanoparticles in the fluids require the evaluation of new heat transfer modes. In the last years, research activities have been very intensive in order to solve these problems, and several novel techniques have been proposed and studied theoretically, numerically and experimentally. Moreover, a deeper knowledge of the phenomenological aspects allows a better thermal design and the optimisation of thermal configurations. In this chapter, the main behaviours of nanofluids are illustrated together with their thermophysical properties such as thermal conductivity and dynamic viscosity. There is an introduction to highlight the present interest in engineering applications. The governing equations are given considering the following different approaches: single phase, discrete phase and mixture models. The evaluation of nanofluid thermophysical properties is presented, and correlations are reviewed. Some indications on the application of the models are accomplished, and results on forced convection are presented. The examples are performed both in laminar and turbulent regimes in order to describe nanofluid applications in heat exchanger technology.
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