Transcritical phenomena take place for example in diesel or rocket engines, in high pressure spray processes for particle precipitation or in thermodynamic cycles enclosing the critical point. The involved fluid-phase-transitions as well as the closely intertwined heat and mass transfer processes have not been completely understood, yet. Theories and models, that describe discontinuous phase transitions and associated heat and mass transfer processes in subcritical fluids (e.g. liquid-vapor transition), are no longer applicable. This is because, near and above the critical temperature, fluid phase transitions can be modelled as a continuous, smooth crossover from the old phase to the new phase, while many physical properties exhibit a singular behavior in the vicinity of the transition. This means that all first order derivatives of free energy are continuous (e.g. density), while second or higher derivatives show a discontinuity or divergence (e.g. compressibility, heat capacity, etc.). These singularities lead to anomalous phenomena in the region around the critical point, such as critical opalescence, heat transfer deterioration or the impossibility to distinguish two or more phases by means of density-based techniques. From a theoretical standpoint, renormalization group theory has provided a unified and predictive view of phase transitions and critical phenomena in equilibrium system. However, its extension to systems driven far from equilibrium is still at infancy. Understanding non-equilibrium critical phenomena is of high technological importance. Examples are: 1) the control of nanomaterial properties relies on the understanding the process of spinodal decomposition and/or nucleation in fluids close to the critical point, and 2) the control of thermoconvective instabilities in supercritical fluids, subject to large pressure or temperature gradients close to the Widom line. In this case, the non-linear coupling between fluctuations of thermodynamic variables and the compressible, turbulent flow dynamics may lead to the onset of instabilities.
The absence of a general framework for modelling non-equilibrium critical phenomena and phase transitions together with the lack of reliable, quantitative experimental data motivates the creation of this special issue on “Transcritical Phenomena” as a platform to foster cross-fertilization among different disciplines and research approaches. Due to ecological and energetic considerations, the relevance of high-pressure and high-temperature processes, in which transcritical phenomena can play an essential role, will continuously grow.
In this context, the special issue “Transcritical Phenomena” invites theoretical and experimental contributions
that advance the fundamental understanding of the transcritical phenomena in general using theoretical or experimental approaches,
that advance the modelling of transcritical phenomena and associated (non-equilibrium) phase transitions processes,
and that advance measurement techniques for the analysis of transcritical phenomena.