A review of microscale physics and macroscale convective heat transfer in supercritical fluids for energy and propulsion systems

The worldwide push towards energy efficiency, use of alternative/waste energy, and power plant simplifications continues to drive the development of energy conversion and utilization systems, now covering a wide range of industrial applications including nuclear reactors, solar/fossil-fired power generation, geothermal systems, aerospace engineering, refrigeration/heat pumps, and waste heat utilization. Driven by fundamental thermodynamic efficiency considerations, an emerging trend in these systems is that the working fluid operates at a pressure above the critical pressure (i.e. supercritical pressure) in certain components or the whole system. Energy transport is thus accompanied by dramatic and strongly nonlinear variations of fluid 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, as well as the safe operation of these thermal systems. The review aims to provide a multi-scale overview of the flow and thermal behavior of fluid above the critical point: microscopic physics and macroscopic transport relevant to the engineering applications. Microscopic physics, i.e. near-critical thermophysical properties, phase transition and fluid dynamics, are introduced based on the most recent findings through fundamental thermodynamics analysis, molecular dynamics simulations, and in situ neutron imaging measurements. 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. Pseudo boiling was found to introduce a new, thermal, jet break-up mechanism, and to be intricately linked to supercritical heat transfer deterioration. 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 physical understanding and modeling of turbulent flow and convective heat transfer of supercritical fluids are summarized. Direct numerical simulation is able to fully resolve the entire turbulence spectrum of flows at supercritical conditions and to offer insights into the physics of thermal fluids. We start with a description of fundamental fluid mechanics problems related to supercritical fluids, such as velocity and temperature transformations and boundary-layer stability. It turns out that pseudo boiling is a sufficient physical mechanism to cause supercritical heat transfer deterioration, strongly resembling the subcritical boiling crisis. 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. The physical understanding serves then to the modeling for engineering applications. Finally, various modeling approaches, including advanced Reynolds-averaged turbulence modeling and data-driven methods, are highlighted for their practical applications in improving the design and optimization of energy systems across industries.