Modelling of molecular diffusion in transported PDF methods: towards computational tools for fluid–surface interactions

Abstract

The current thesis studies the modelling of turbulent reacting flows over a wide range of combustion regimes and fuels and, in particular, the impact of molecular fluxes. The flow is described using Favre Averaged Navier Stokes (FANS) equations with the scalar field modelled using a transported joint-scalar probability density function (pdf) method solved using a Monte Carlo approach with the method of fractional steps. The main focus of this work is the understanding of the interactions between fluid flow and chemistry across the scales in an internal combustion engine (ICE) with the aim to enable the prediction of chemical species concentrations adjacent to solid surfaces. For the study of the heat and mass transport across the various boundary layers, an explicit deterministic approach of the molecular transport term is applied without assumptions. Hence, the current approach conforms with the laminar flow limit in the absence of turbulence. Initially, the parallelisation of the independent stochastic particles as an enabling tool was implemented based on their thermochemical properties. The modelling of the laminar fluxes was introduced in a consistent way to enable the modelling of the heat and mass transfer across all boundary layers from the gas phase to the solid surface. Application of the laminar fluxes implementation was initially presented for auto-ignition in a jet flame, a configuration related to compression ignition engines and a detailed comparison of the influence of laminar and turbulent transport on the evolution of the species concentrations is shown. Subsequently, the laminar fluxes were applied for a range of freely propagating turbulent premixed flames, a configuration related to spark ignition engines. A range of fuels was considered from hydrogen to iso-octane, including practical conditions, and results show improved agreement with experimental data.