A novel transient heat conduction phenomenon to distinguish the hydrodynamic and (quasi) ballistic phonon transport

TL;DR

This study reveals a unique transient heat conduction phenomenon in hydrodynamic phonon transport, enabling clear differentiation from (quasi) ballistic regimes through numerical simulations based on the phonon Boltzmann transport equation.

Contribution

It introduces a novel transient heat propagation phenomenon that distinctly identifies hydrodynamic phonon transport from (quasi) ballistic transport using numerical modeling.

Findings

Transient temperature drops below initial temperature in hydrodynamic regime.

Phenomenon occurs only with normal scattering dominance.

Disappears in quasi-one-dimensional systems.

Abstract

Previous studies have predicted the failure of Fourier's law of thermal conduction due to the existence of wave like propagation of heat with finite propagation speed. This non-Fourier thermal transport phenomenon can appear in both the hydrodynamic and (quasi) ballistic regimes. Hence, it is not easy to clearly distinguish these two non-Fourier regimes only by this phenomenon. In this work, the transient heat propagation in homogeneous thermal system is studied based on the phonon Boltzmann transport equation (BTE) under the Callaway model. Given a quasi-one or quasi-two (three) dimensional simulation with homogeneous environment temperature, at initial moment, a heat source is added suddenly at the center with high temperature, then the heat propagates from the center to the outer. Numerical results show that in quasi-two (three) dimensional simulations, the transient temperature will be lower than the lowest value of initial temperature in the hydrodynamic regime within a certain range of time and space. This phenomenon appears only when the normal scattering dominates heat conduction. Besides, it disappears in quasi-one dimensional simulations. Similar phenomenon is also observed in thermal systems with time varying heat source. This novel transient heat propagation phenomenon of hydrodynamic phonon transport distinguishes it well from (quasi) ballistic phonon transport.