Nanofluids represent a class of heterogeneous systems or dispersions, which consist of a continuous liquid dispersion medium and a dispersed phase of solid nanoparticles in the nanometer range.¹  For the nanoparticles, typically employed materials are metals, metal oxides, and carbon materials, while water and ethylene glycol represent commonly used liquids. A key thermophysical property and, in specific, transport property, characterizing nanofluids and their performance in applications, is the effective thermal conductivity λ_eff as a measure of the ability of a material to conduct heat. Since the thermal conductivity λ in units of W · m⁻¹ · K⁻¹, is a single-phase property, the additional term “effective” is required for nanofluids, considering that they refer to multiphase systems with at least two phases.²  Yet, the term is omitted throughout this chapter due to simplicity reasons.