Modeling and analysis of transient interfacial profiles across subcritical and supercritical conditions

Increasing the temperature and pressure in the combustion chamber is a well-known method to improve engine efficiency. As these conditions approach the fuel's critical point, the processes of fuel breakup and mixing undergo substantial changes. Earlier research has indicated that the nano-scale interfacial profiles, such as interface thickness and temperature profiles, are key factors in understanding the mixing behaviors of jets under transcritical conditions. However, current theoretical models of interfaces are unable to resolve the transient interfacial profiles from subcritical to supercritical conditions, and molecular dynamics (MD) simulations, while precise, require significant computational resources. In this work, the modified diffuse-interface method (MDIM) model is introduced to overcome these challenges. The molecular dynamics (MD) simulations are also employed to validate the MDIM model. The MDIM model is based on the diffuse-interface method, and the inclusion of the reverse mass flux, derived from vapor–liquid equilibrium and linear gradient theory, extends the model's applicability across subcritical and supercritical conditions. As results, the transient interfacial profiles obtained by the MDIM model match well with MD results across subcritical and supercritical conditions. A temperature-corrected Knudsen number (KnT), which incorporates the effects of temperature differences with the Knudsen number, is proposed to evaluate the mixing regime. The KnT map is calculated using the results of the MDIM model and aligns with experimental results of jets. The MDIM model provides a practical approach to analyze and predict the interfacial profiles and mixing regimes, supporting advancements in engine design.