This paper describes the effects of size on heat conduction in nanofilms, convective heat transfer in micro/nanochannels, and near-field radiation in nanogaps. As the size is reduced, the ratio of the surface area to the volume increases; therefore, the relative importance of the interfacial effects also increases. The physical mechanisms for these size effects have been classified into two classes. When the scale is reduced to the order of micrometers (except for gases), the interfaces only affect the macro parameters and the continuum assumption still holds, but the relative importance of the various forces (inertia force, viscous force, buoyancy, etc.) and effects (interfacial effect, axial heat conduction in the tube wall, etc.) changes, resulting in changes in the heat transfer characteristics from normal conditions. As the size is further reduced to the order of submicrometers or nanometers, the interface affects not only the macro parameters but also the micro parameters (mean free path, relaxation time, etc.) so the continuum assumption breaks down and Newton’s viscosity law and Fourier’s heat conduction law are no longer applicable. Thus, the major characteristic of micro/nanoscale heat transfer is that the interfacial effects dominate the heat transfer.

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