  Driven by fundamental thermodynamic efficiency considerations, an emerging
trend in the energy and propulsion systems is that the working fluid operates
at a pressure above the critical pressure. Energy transport is thus accompanied
by dramatic and strongly nonlinear variations of thermophysical properties,
which cause abnormal heat transfer behavior and non-ideal gas effects. This
situation raises a crucial challenge for the heat exchanger and turbomachinery
design, overall energy and exergy efficiency. We aim to provide a multi-scale
overview of the flow and thermal behavior of fluid above the critical point:
microscopic physics and macroscopic transport. Microscopic physics, i.e.
near-critical thermophysical properties, phase transition and fluid dynamics,
are introduced. A particular focus will be on the supercritical pseudo boiling
process, which is a generalization of classical liquid-vapor phase transitions
to non-equilibrium supercritical states. These new perspectives lead to a
revised view of the state space. Further, recent results demonstrated the
possibility of stable supercritical fluid interfaces without surface tension.
On the macroscale, recent progress on the modeling of turbulent flow and
convective heat transfer of supercritical fluids are summarized. Direct
numerical simulation is able to offer insights into the physics of thermal
fluids. We start with a description of fundamental fluid mechanics problems
related to supercritical fluids. In addition, the heat transfer deterioration
in supercritical fluids is found to be closely connected to the flow
relaminarization by the non-uniform body-force. Finally, various modeling
approaches such as recently developed advanced Reynolds-averaged turbulence
modeling as well as machine-learning methods are summarized.
