(220e) Large Eddy Simulation of Nanoparticle Formation in Turbulent Flames

The gas phase synthesis of nanoparticles is a multiscale multiphysics flow system involving the complex interaction of gas phase turbulent flow, fuel combustion, precursor oxidation, and nanoparticle evolution processes. Predictive computation of this complex and industrially relevant configuration requires the detailed modeling of each of the individual physical processes as well as their interactions. In this work, the large eddy simulation (LES) methodology is used to model the particulate formation in a diffusion flame. LES captures all large scale turbulent motions, and has been widely demonstrated to be accurate in capturing the turbulent flame physics. The nanoparticle evolution is described using a population balance equation, solved here using a volume-based quadrature method of moments (QMOM) approach. The focus of this work is in developing a detailed description of the precursor oxidation process. To capture both the fuel combustion and precursor oxidation processes, detailed chemical mechanisms are needed. LES computations are inherently three-dimensional and unsteady, implying that detailed mechanisms will incur intractable computational costs. In the case of gas phase combustion, flamelet-based description has been used extensively to substantially reduce computational expense. Here, the entire gas phase chemical composition is mapped based on a a conserved scalar known as mixture fraction. In this work, extension of the flamelet approach to precursor oxidation is be studied. Three different LES computations are performed. In all the simulations, the gas phase combustion is described by the flamelet model. First, a single step oxidation mechanism is used. Second, a 26-species mechanism is incorporated by directly solving scalar transport equations for the species participating in the oxidation kinetics. Finally, the 26-species mechanism is included in the flamelet model. A diffusion flame experiment studied at ETH Zurich is used for validation purposes. Detailed comparisons between the three cases are used to understand the importance of oxidation kinetics in determining final particle size distribution. The basic assumptions of the flamelet model and its applicability to precursor oxidation kinetics are discussed.