Pulverized coal combustion in boundary layer turbulence using direct numerical simulation

Abstract

The present study investigates pulverized coal combustion under oxy-fuel conditions interacting with turbulent boundary layer flows over a flat plate using direct numerical simulation (DNS). The coal particles are tracked in the Lagrangian framework while the flow is solved in an Eulerian way. Three cases with reacting particles, inert particles and single-phase flows were performed. The interplay between pulverized coal combustion, turbulent boundary layer flows and the wall was examined. It was shown that the coal particles first ignite in the outer layer of the boundary layer turbulence, where individual coal particle ignition occurs. The wall-normal velocity in the reacting case is larger compared with that of the non-reacting cases due to flow acceleration induced by combustion. The volatile matter mass fraction is high in the inner layer in the downstream region. However, the heat release rate is low due to the cold wall effect. The strong correlation between the wall heat flux and streak structures in the near-wall region is observed in all cases. The values of radiative heat transfer are similar in the upstream region of both the cases with reacting and inert particles. However, as combustion occurs and particle temperature increases in the reacting case, the radiative heat transfer from particles to the wall increases significantly in the downstream region. Particle accumulation due to turbophoresis in the near-wall region is analyzed. It was shown that the magnitude of streamwise vorticity is attenuated by coal combustion. Therefore, particle wall-accumulation is less prominent in the reacting case compared with the inert case.