Below a critical temperature, some fluids like liquid helium can
present superfluid behaviour and lose internal friction. Evidence of
superfluid behaviour has also been found in astrophysics, inside
neutron stars and in quantumgases [1–3]. For an adequate heat transfer
analysis in superfluids, finite thermal wave speed must be considered.
Experiments with superfluids require sophisticated measuring
techniques and only very recently the phenomenon of second sound
was observed experimentally in a Fermi gas [3]. Clearly temperature
plays an important role in superfluids flows. Therefore, a better understanding
of heat transfer around and above the critical temperature is
essential for an adequate modelling of the heat equation and for a
simple interpretation of the thermomechanical coupling terms that
arise in the heat equation in complex, turbulent flows.
In general, the superfluid behaviour is modelled through phenomenological
and microscopic theories. In the phenomenological approach,
a superfluid is generally considered a mixture of two fluids, one with
superfluid behaviour and one normal that gradually disappears with a
decreasing temperature [4,5].
Although the second sound is a quantum mechanical phenomenon
in which heat transfer occurs by wave-like motion, the goal of the
present paper is to use basic principles of classical Thermomechanics
of Continuum Media to analyse the effect of the thermomechanical
coupling on the hyperbolic heat transfer in a superfluid undergoing a
complex flow. To make the reasoning clearer, temperature is assumed
to be below critical and only the supercritical fraction of the fluid is
considered. In the framework of Thermomechanics of Continuum
Media, we are not directly interested in the microstructure of the
matter, and the material (continuum like) behaviour is described
through constitutive equations. Therefore, in the analysis, we will not
use statistical methods to predict the thermodynamic properties of a
system from the microscopic structure of its constituents neither will
we consider relativistic heat conduction. The approach is preliminary
but allows understanding how turbulent compressible irrotational
flows caused by the lost of internal friction may affect heat transfer. It
is shown that, due to the thermo-mechanical coupling, density waves
may induce heat sources or sinks travelling at sound speed, while
temperature waves travel at a different and independent speed.
Sufficient conditions for an objective and thermodynamically
consistent modelling of hyperbolic heat transfer in supercritical fluids
are presented and analysed. An objective version of the Cattaneo
equation is adopted. A general procedure, developed within the framework
of thermodynamics of irreversible processes allows presenting
sufficient conditions to satisfy a local version of the second law of
thermodynamics.
In the heat equation, it appears a positive parameter that can be
somewhat related with the (microscopic) concept of relaxation time.
Nevertheless, in order to satisfy a local version of the second law of
thermodynamics (SLT) in all possible processes, this parameter cannot
be a constant. A consequence of this SLT restriction and of the fact that
this parameter cannot be a constant is that the heat equation may
change its nature from parabolic to hyperbolic in a process, depending
on certain conditions. In a certain sense the paper is complementary
to [6],where itwas presented a study about the hyperbolic heat transfer