A coupled MMC-LES and sectional kinetic scheme for soot formation in a turbulent flame

Abstract

A soot kinetic sectional scheme is coupled with the sparse multiple mapping conditioning / large eddy simulation (MMC-LES) method to model soot formation in a turbulent, non-premixed, piloted methane flame (Delft Flame III). The kinetic sectional scheme used here discretises the population balance equation according to soot particle size. The sections (or bins) are treated as lumped species, and the physico-chemical soot formation is modelled through elementary-like reaction pathways with Arrhenius rate expressions. This allows the soot kinetics to be naturally coupled with the gas phase molecular kinetics. Interactions of soot formation and molecular chemistry with turbulence is achieved through the MMC-LES model. This is a Monte Carlo filtered density function approach in which the micromixing is local in an independent reference space permitting use of a computationally efficient sparse distribution of stochastic notional particles. The model results are compared to experimental data for the gas-phase velocity and molecular species in the upstream low-soot region and with soot statistics in the downstream region. The gaseous quantities have very good agreement with the experimental data. Overall the soot predictions are in good qualitative agreement with the data, particularly with respect to the peak value of the mean soot volume fraction, the shape and range of the joint probability density function of soot volume fraction and spatial location, and the rate of soot oxidation and burnout. However, in common with other published attempts at the Delft Flame III using a range of different models, the predicted soot formation begins and peaks well upstream of the experimental measurements. The minimum soot intermittency is underpredicted relative to the data but shows a significant improvement on previous attempts to model it. The analysis also explores the evolution of the mean and instantaneous particle size distributions and some aspects of soot-chemistry-turbulence interactions.