FANS-Based Predictions of Jet-Regime Contrail Formation Using a Two-Equation Soot/Ice Aerosol Transport Model

Combustion of standard hydrocarbon fuels within commercial aviation engines yields a mixture of hot gases, including carbon dioxide (CO 2 ), water vapour (H 2 O), nitrogen oxides (NO x ), sulphur oxides (SO x ), as well as unburnt hydrocarbons, along with carbonaceous particulate matter, or soot. Depending on the flight altitude and atmospheric conditions, these combustion products and emissions can lead to the formation of condensation trails, more commonly referred to as contrails. It is now recognized that contrail cirrus are a major source of aviation-driven climate forcing with some estimates concluding its contributions are greater than that of aviation-emitted carbon dioxide. In an effort to better understand the formation processes of contrails, the present study will explore via numerical simulation the influences on contrail formation and, in particular, the interaction of the engine ex- haust plume with the ambient air leading to contrail formation at typical cruise conditions in the near-field or jet regime in a controlled, systematic, and comprehensive fashion. The study extends a previous study by the authors that considered the simulation of contrail formation within a proposed laboratory-scale experimental facility. Here, a more realistic configuration representative of typical engine exhausts and the influence of various param- eters on contrail formation are considered. High-fidelity solutions of the Favre-averaged Navier-Stokes equations for turbulent flows of a compressible multi-phase mixture of reac- tive gases and solid soot/ice aerosols are used to represent the exhaust, secondary bypass, and cold ambient air stream flows of the coaxial jet and model engine exhausts. The solid soot/ice particulates are taken to be in dynamic and thermodynamic equilibrium with the gaseous mixture. An Eulerian-based semi-empirical two-equation model, developed previ- ously for the prediction of soot particle growth and transport in non-premixed laminar and turbulent sooting flames, with extensions to treat the heterogeneous nucleation and trans- port of ice crystals along with activation of the nucleation process governed by sulphuric acid and sulphur trioxide, is presented. The governing equations for the gas/particulate mixture are solved using a second-order-accurate, finite-volume, scheme combined with a parallel, anisotropic, adaptive mesh refinement (AMR) algorithm on two-dimensional (2D) axisymmetric domains of multi-block, body-fitted, quadrilateral meshes. The contrail sim- ulation results considered here will include results for a 2D axisymmetric representation of the CFM56-3 turbofan engine. Results for various fuels and sulphur content and vary- ing soot size and number density, altitude, flow conditions, and relative humidity will be presented so as to assess their effects on jet-regime contrail formation.