Bridging the Scale Gap: Non-Equilibrium Phase Changes in Contrail Formation

Contrails formed in aircraft exhaust plumes are a significant contributor to aviation’s climate impact; however, the physical processes driving their formation are not yet fully understood. This presentation focuses on advancing our understanding of non-equilibrium phase change phenomena, including homogeneous and heterogeneous condensation, liquid-to-solid transitions, and interphase momentum transfer, which are central to contrail formation. 

The investigation is based on numerical methods of 3D RANS for non-ideal fluid flow and combined with well-established models in the field of steam turbines to describe the formation of a dispersed phase during phase change. Non-equilibrium thermodynamic principles are used to model nucleation and droplet growth, complemented by detailed representations of heterogeneous condensation and ice formation. The resulting approach is applied to simulate the flow behind an aircraft engine under upper tropospheric conditions. While capturing polydispersed size distributions of the dispersed phases and accounting for interphase momentum transfer, it enables a comprehensive investigation of contrail formation processes. 

By assessing the influence of non-equilibrium effects on the validity of established approaches such as the Schmidt-Appleman Criterion, the presented method aims to bridge the gap between atmospheric-scale contrail models and detailed, small-scale physics of engine exhaust flows. In future work, these approaches could be used to explore how engine specifications, operational conditions, and fuel types shape the early stages of contrail formation.