A generalized heat transport equation applicable at small length and short time scales is proposed. It is based on extended irreversible thermodynamics with an infinite number of highorder heat fluxes selected as state variables. Extensions of Fick's and Ohm's laws are also formulated. As a numerical illustration, heat conduction in a rigid body subject to fixed and oscillatory temperature boundary conditions is discussed.
KeywordsBallistic heat transport; Small length-systems; High-frequency; Extended irreversible thermodynamics
IntroductionThe increasing interest in nano-technology has led to new insights in the study of heat transport. It is well known that heat transfer at micro and nanoscale behaves differently from that at macroscales [1,2]. At small length scales, transport of heat in complex systems is best quantified by means of the so-called Knudsen number ≡ ℓ/ with ℓ denoting the mean free path of the heat carriers, namely phonons, and , the characteristic dimension of the system under study. The Knudsen number becomes typically comparable or larger than one for micro-and nano-systems, in which case heat transport is referred to as ballistic, i.e. dominated by phonon collisions with the walls. If the Knudsen number is much smaller than one, heat transport is simply diffusive, i.e. dominated by phonon-phonon collisions inside the system and described by Fourier's law. For small-length systems, as well as for high-frequency processes, Fourier's law is no longer valid. Another drawback associated to Fourier's law is that it implies that temperature spreads infinitely fast through the whole body, which is physically unsustainable. These observations justify the need to generalize Fourier's law. This can be achieved by various ways, for instance, via the resolution of Boltzmann's equation [3], making use of the dual time approach [4] or by computer simulations [5]. Here we follow a different route based on Extended Irreversible Thermodynamics (EIT) [6] whose main characteristic is to upgrade the heat flux and higher order heat fluxes to the rank of independent variables. In most applications, for convenience, the analysis is restricted by taking heat flux as single extra variable. In this letter, we go one step further by selecting an infinite number of higher order fluxes. The use of a large number of flux variables finds its justification in the recent progress of nanotechnology and high-frequency processes. The procedure described in this paper is by no means limited to heat transfer but can be easily generalized to other transport phenomena, as electrical conduction and matter diffusion. As a numerical application, we consider the problem of heat conduction in a one-dimensional rigid body of length L at rest, subject to two different kinds of boundary conditions, made explicit in Section 5. The theoretical model for the general heat transport equation is presented in the next Section 2.