In this chapter is addressed the very particular thermal behavior that supercritical fluids exhibit when nearing their critical point. In this region, supercritical fluids exhibit strong anomalies in their thermodynamic and transport properties. Pressure change associated to a temperature variation leads to a nearly isentropic thermalization of the fluid, the “piston effect”, which leads to a paradoxical “critical speeding-up”. Bulk fluid temperature is uniform, temperature gradients are confined in thermal boundary layers, making the bulk fluid a thermal short-circuit. It follows very particular behavior, as dynamic heat pipes or heat going seemingly backward, in apparent contradiction with the 2nd principle of thermodynamics. Under an acceleration field, thermal convection occurs only in the boundary layers, which paradoxically can enhance the fluid stability or even cool the fluid after a heat pulse. These effects can deeply modify the supercritical fluids thermal behavior in space and energy activities, giving to these effects socio-economic relevance.
TopGeneral Background
Supercritical fluids, that is, fluids at pressure and temperature above their critical point (CP) coordinates, exhibit particular properties (large density, low viscosity, large mass diffusivity), which make them intermediate between liquids and gases. It was indeed well-recognized from the end of the 90’s that the fine pressure/temperature control of the supercritical conditions is very appealing to the industry as an easy mean to tune their non-polluting solvatation power and host the chemical reaction rates with high yield efficiency (see for example Noyori, R., 1999 and related papers in the same special issue). Fluids in such supercritical conditions are now mainly used to open new routes in green synthesis of innovative materials (Adschiri et al., 2015; Dumas et al., 2016) or in hydrothermal biomass conversion processes (Kruse & Dahmen, 2015).
In addition, the fact that temperature is large increases the yield of thermo-mechanical processes in boiling water, steam or molten salt reactors. Under reduced gravity, the storage of cryogenic propellants is sometimes made in their supercritical conditions to avoid a non-controlled two-phase distribution.
However, when temperature and pressure approach the CP values, such fluids show strong anomalies in a number of static and dynamical properties. In particular, isothermal compressibility, thermal expansion and specific heats at constant pressure and volume can become extremely large. Dynamical properties can also be much affected. The so-called “critical slowing down” corresponds to a strong decrease of thermal diffusivity while, in contrast, heat conductivity and specific heat diverge. In the vicinity of the critical point temperature and pressure, such fluids are called « near critical” or “near supercritical ». We address below the main properties of such near supercritical fluids.