  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.

Mesoscale and Nanoscale Physics