Contrail microphysics and visibility in the near field of a mixed-flow nozzle: Effects of ambient humidity and soot diameter

The contrail of an aircraft can readily reveal its position and heading, increasing the probability of being detected by sensors. While numerous studies have explored aircraft contrail formation, research specifically targeting the more complex structure of mixed-flow jet engines remains scarce. A contrail is essentially a gas–particle two-phase flow, where ice crystals as the discrete phase are suspended in the turbulent exhaust plume. Capturing two-phase interactions is therefore critical. This paper employs a discrete phase method to characterize the near-field contrail formation and visibility of a typical mixed-flow nozzle with an ejector and an adjustable convergent–divergent exit. The mixed jet flow is resolved using steady Reynolds-averaged Navier–Stokes, while the discrete phase is tracked via an unsteady Lagrangian method. A heterogeneous condensation microphysical model describes water vapor condensing onto soot particles to form ice. The model is validated against vacuum shock tube experiments. Effects of relative humidity and soot diameter on ice particles are investigated. Results show that higher humidity promotes ice growth, underscoring its dominant role, while larger soot particles yield larger ice crystals under the same humidity. A visible contrail is predicted to form approximately 30 m from the nozzle exit; higher ambient relative humidity resulted in the visible contrail appearing closer to the nozzle. These simulation results demonstrate the model's capability to predict the microphysical processes and optical visibility characteristics of the contrail from a mixed-flow engine. This holds significant implications for predicting contrail visibility in such engine exhaust and for designing measures to prevent contrail formation.