Modeling homogeneous ignition processes of clustering solid particle clouds in isotropic turbulence

The objective of this study is to numerically investigate the ignition and combustion of pulverized solid fuels in turbulent conditions and to assess different modeling strategies relevant to large -eddy simulations (LES). The investigations show that due to the high Stokes number of solid particles, they do not necessarily follow the flow. At Stokes numbers around unity, particle-turbulence interactions can lead to particle clustering and change the ignition behavior. According to observations, ignition is most likely to happen outside the formed clusters, where suitable thermo-chemical conditions exist. To study this behavior, direct numerical simulations (DNS) of reactive particles in turbulent conditions employing detailed kinetics for solid and gas phases were performed. Pulverized fuel combustion was modeled using the point -particle approximation to represent the dispersed phase in an Eulerian-Lagrangian framework. Isotropic turbulence was employed to investigate the influence of particle clustering on the ignition process. After investigating the physical aspects of the ignition process, the DNS dataset was used as a benchmark for evaluating the reduced -order flamelet models usually employed in LES of pulverized fuel combustion during the ignition process. The flamelet model performance in predicting the selected quantity of interest was compared to the DNS data. An error decomposition was performed using the optimal estimator concept. Finally, the prediction accuracy of presumed PDFs is evaluated by calculating the errors in predicting the quantity of interest using different PDFs compared to the predictions using the accurate sub -filter joint distribution of the DNS data.