Coal particle volatile combustion and flame interaction. Part II: Effects of particle Reynolds number and turbulence

Direct numerical simulation (DNS) is performed to characterize volatile combustion of isolated coal particles and closely spaced particle ensembles in laminar and turbulent flow. In part I of the paper the transient evolution of devolatilization and group flame effects were studied, Tufano et al. (2018). This analysis was limited to laminar flows and relatively low particle Reynolds numbers Rep. Here, we investigate the effect of large Rep and considerable levels of turbulence on the devolatilization and burning behavior of the particles. The complex physico-chemical interactions during PCC are characterized for laminar flow conditions first, before arrays of infinite particle layers are subjected to turbulence. Increasing Rep in laminar flow leads to delayed ignition of single particles with local extinction due to high upstream scalar dissipation rates and the formation of wake flame structures downstream of the particle. An attempt is made to recover the single particle results with a standard steady laminar flamelet approach, which is shown to work well at low Rep, but fails at high Reynolds numbers, where multi-dimensional effects occur and must be incorporated into flamelet modeling. It is found that applying standard film theory to model the effect of convection on devolatilization rates can lead to qualitatively wrong trends and up to 66% error in the peak devolatilization rate compared to the DNS results at Rep=8. The analysis of particle arrays in laminar flow shows a strong dependence of flame interactions on the values of Rep due to the different extent of the particle wakes. The occurrence of significant levels of turbulence introduces a wide range of additional chemical states due to the randomness of the turbulent fluctuations that can either act to increase or decrease the local strain, in turn weakening or enhancing particle interactions. For the studied conditions turbulence slightly promotes the mass release from the most upstream particle set, but considerably delays the volatile release from the downstream particles, which is explained by the different extents and degrees of interaction of the up- and downstream volatile flames. The present results are considered useful for the development of LES sub-grid scale combustion models for pulverized coal flames, such as flamelets and others.