Carrier-phase DNS of pulverized coal particle ignition and volatile burning in a turbulent mixing layer

Direct numerical simulations (DNS) of a three-dimensional turbulent mixing layer are performed, where coal particles seeded in an air stream mix with hot lean combustion products in a second stream. This case mimicks conditions of a pulverized coal flame stabilized by hot products such as found in industrial furnaces. Particles are heated up by the hot gases and devolatilize, followed by volatile combustion in the gas phase. The carrier-phase DNS resolves all relevant scales of the fluid phase except the boundary layers around individual particles. The simulation results are assessed in terms of instantaneous contour plots of relevant quantities, spatially averaged statistics, scatter plots and (pseudo-) flamelets. The analysis provides insight into the mechanisms of solid particle ignition and burning stabilized by hot combustion products, as well as the flame structure and combustion mode. It is shown that ignition initially occurs at very lean conditions when particles are entrained in the hot gases. Subsequently volatile combustion proceeds in non-premixed as well as premixed combustion modes, characterized by means of the flame index, with an overall higher heat release in non-premixed zones. At late times two flames can be clearly distinguished, an upper flame burning into the air carrying the particles and a lower flame burning into the lean products. The latter flame shows a pure non-premixed behavior, while the former illustrates a complex flame structure with both premixed and non-premixed modes, as well as flame quenching. The DNS database and initial analysis lay the foundation for future systematic studies in similar configurations and support the development of models suitable for the combustion of solid fuel particles.