Mass-consistent manifold modeling of turbulent sooting flames and the influence of soot subfilter PDF model

Modeling of soot evolution in turbulent reacting flows using Large Eddy Simulation (LES) is challenging due to the large number of coupled submodels. To arrive at a tractable framework, approximations are often made to simplify this coupling. In one modeling framework, gas-phase combustion is modeled using a manifold-based combustion model with soot handled explicitly in the LES solver with closure models for its source terms based on presumed subfilter PDFs. With manifold-based models, the high-dimensional thermochemical state is projected onto a low-dimensional manifold, for example, mixture fraction for nonpremixed combustion. The rationale of manifold-based models is to decouple the combustion model from the flow solver, and this requires approximation in the coupling with the soot model. Notably, with this approach, source terms for gas-phase species participating in soot surface chemistry, notably acetylene and radical species, are neglected. Neglecting to remove acetylene would lead to an overprediction of acetylene, could permit spurious surface growth, and lead to an overprediction of the soot volume fraction. In this work, a fully mass-consistent approach is developed for more tightly coupling the soot model handled in LES with the manifold-based combustion model. In brief, the local soot surface area from LES is provided as an input to the manifold-based combustion model, which is used to compute the source terms for gas-phase species participating in soot surface chemistry. To address the increased number of inputs to the manifold-based combustion model, the In-Situ Adaptive Manifolds (ISAM) framework is leveraged, in which manifold solutions are computed on-the-fly and stored for efficient reuse with In-Situ Adaptive Tabulation (ISAT). Influence of accounting for the source terms for the gas-phase species participating in soot surface chemistry is then quantified in a canonical turbulent flame configuration. Particular emphasis is placed on analysis of the intricate coupling between these gas-phase source terms and the soot subfilter PDF. Different formulations of the soot subfilter PDF assume different variations of soot with the mixture fraction, which will influence the gas-phase source terms, which will influence soot growth and oxidation rates.