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Supercritical fluids exist widely in nature and have enduringly attracted scientific and industrial interest. In power systems like liquid rocket engines, fluids undergo the trans-critical process transferred from the subcritical state to the supercritical state, and the phase change process exhibits different features distinguished from subcritical evaporation. In this work, we conducted a series of molecular dynamics studies on the behavior of methanol (MeOH), n-heptane (C7), and binary C7/MeOH droplets under supercritical nitrogen environments. The emphasis is on clarifying the transient characteristics and physical origins of the trans-critical evolution of droplets. During the trans-critical process, droplets are found to experience an unstable period without a spherical shape, where the droplet diameter no longer decreases, violating the traditional d2-law rule. The occurrence of nonspherical droplets is related to the microstructural behavior of trans-critical droplets. Two types of microscopic structures within the droplet are identified: large-scale thermally induced clusters for long-chain C7 and hydrogen-bond connected network-like structures for MeOH, which contains hydroxyl (–OH) groups. Based on these findings, the mechanism behind the evolution of trans-critical droplets is illustrated. Finally, we determine the boundary of ambient conditions in the form of dimensionless expressions T r − 1 = a (p r − 1) − b , which dictate whether droplets can maintain a spherical shape during the trans-critical process. [ABSTRACT FROM AUTHOR]

Although the properties of supercritical fluids have been significantly focused on, few studies have focused on the transition process of multicomponent fluids from the subcritical to supercritical state. Herein, the trans-critical behavior of an immiscible binary ethanol/hexadecane (EtOH/C16) droplet under supercritical nitrogen environments is analyzed for the first time using molecular dynamics. The ambient pressure (p) and ambient temperature (T) of the droplets exceed the critical conditions for both EtOH and C16. Moreover, trans-critical EtOH/C16 droplets undergo a two-stage bulging-to-shrinking process. Liquid fraction λ is introduced to quantify the structural characteristics of trans-critical fluids. Fluids inside the droplet are shown to transform from the vapor to liquid phase, signified by the increase in λ with p. Three droplet evolution types are classified based on p: micro-explosion, puffing, and mixing types. The occurrence of each trans-critical droplet evolution type is determined by the competition between the subcritical gasification of EtOH and the supercritical pseudo-boiling of C16. The supercritical transition of C16 can be detected under each condition, while that of EtOH only occurs for the mixing-type droplet evolution. Furthermore, a p–T diagram is provided to analyze the combined effect of T and p on the trans-critical droplet evolution types. [ABSTRACT FROM AUTHOR]