Understanding contrail formation in the near-field of modern turbofan engines

Contrails are formed when the hot exhaust from the core of the engine mixes with the relatively cold ambient. Traditionally, contrail formation and its persistence is predicted using the Schmidt-Appleman (SA) criterion, which is a simple thermodynamic model developed based on the engine exhaust and ambient conditions. However, the formation of contrails is a complex multi-physics problem which lies at the intersection of thermodynamics, fluid mechanics, and physical chemistry, and is strongly dependent on engine conditions and atmospheric variables. 

This being said, for modern turbofans with different engine architectures, the competing thermal and flow-field characteristics of the core jet, bypass jet, and ambient conditions play vital role in the formation of contrails in the near-field. We use RANS CFD modelling approaches to understand the macrophysical nature of contrail formation in different turbofan engines. Thermodynamic theories and analyses on the flow physics are utilized to understand the underlying mechanisms leading to contrail formation. The study finds interesting relations between the bypass ratios of the engines and potential contrail forming regions. Regions in the exhaust plume where contrails are likely to form is thus strongly dependent on the bypass ratio, or in other words, the flow-field mixing is found as an important deciding factor in contrail formation. The results are compared with the prediction from the SA criterion and the limitations and advantages of both approaches are discussed in detail. As outlook, we plan to extend this work by implementing microphysics models to complement the macrophysics results.