Flame liftoff in diesel sprays

Abstract

Flame liftoff and stabilization in nonstationary n-heptane sprays is studied for diesel engine-like conditions using a numerical simulation involving complex chemistry and a novel subgrid stirred reactor model of turbulence-chemistry interaction. By following ignition and flame formation processes, it is shown that the flame stabilizes at a certain point due to the upstream propagation of a triple flame where the leading edge is at the stoichiometric surface and the combustion occurs both on the lean and on the rich side. Due to the evaporative cooling, large injection velocities, and the strong flame stretch, the stabilization distance is very large. This allows a considerable amount of air to enter the central part of the flame to feed the internal rich flame, and a large amount of fuel to escape combustion in the stoichiometric flame and support the external lean flame. Both soot and NOx emissions reduction are expected to follow for large liftoff distances (i.e., high pressure and small orifice injection). The flame stabilization mechanism is a result of complex physical and chemical interactions and cannot be described by a simplified theory but to the leading order is determined by the chemical reaction time at the leading edge, the turbulent diffusivity, and the flow velocity so that there exists a balance between the local convection velocity and the triple flame propagation speed. The unburned/burned gas density ratio determing the shape of the leading edge of the flame is important in this process. Due to fast evaporation, the spray properties have little effect on flame stabilization except for the heat of evaporation affecting the temperature at the stoichiometric surface and the combustion kinetics of the fuel. Liftoff trends are studied, and available data on liftoff distances are compared with the predictions. The accuracy achieved is good.