Modeling Auto-Ignition, Flame Propagation and Combustion in Non-Stationary Turbulent Sprays

Doktorsavhandling, 1995

This thesis focuses on detailed numerical studies of spray formation, auto-ignition, envelope flame development and combustion in a simple two-dimensional axi-symmetric geometry. A Lagrangian -- Eulerian formulation is used to analyze the droplet motion and gas flow field. Turbulent dispersion of droplets is modeled by a random-walk calculation, droplet collision and break-up being accounted for. The description of the gas flow involves conservation equations for mass, momentum and energy. Included is also the k - .epsilon. model for turbulence in order to model the turbulent Reynolds stresses and scalar fluxes. The chemical kinetics model, based on generic species, contains 17 active species involved in 59 reactions. It has been observed, however, that the behavior of the reacting system does not depend on the accurate composition of the radical and branching agent pools at the initial moment, but only on the total concentration of the two. Thus only the following species are transported by the flow: reactants (RH, O2), sums of radicals and branching agents (.dot.X, Y), intermediate fuels (CO2, H2) and products (CO2, H2O). A local stirred reactor approximation is used as a model for turbulence -- chemistry interaction allowing to account for species segregation, micro-mixing and complex chemistry effects. In the reactor, reactions quickly evolve towards a unique local system much more complex than considered in the transport equations. The interaction between the reactor and its surrounding occurs by ``exchange with the mean''. The model is applied to prediction of Diesel spray formation, ignition and combustion. It is shown that ignition occurs locally in small lean pre-mixed regions, moves quickly towards the stoichiometric surface and proceeds differently in the up-stream and down-stream directions. The up-stream propagation is slow and the flame stabilizes at a certain point by a mechanism of formation of sequential ignition kernels produced in short intervals on the stoichiometric surface. The down-stream propagation occuring also along the stoichiometric surface is fast and is convection and diffusion controlled. The flame is wide and has a character of a distributed triple flame where the leading point is at the stoichiometric surface and the combustion occurs both on the lean and the rich side. The flame stabilization at a distance from the cold fuel spray allows considerable amount of air to enter the central part of the spray during the injection process which will have an important effect on soot and NOx formation. Comparison of experimental and theoretical data support the predictions. Preliminary calculations of NOx formation clearly illustrates the importance of separating the conditions of the reacting zones from the mean.