On the Modeling of Premixed Combustion under Varying Equivalence Ratios

Sammanfattning: This thesis deals with numerical simulation of reactive flows. The numerical method is three dimensional and unsteady, which allows for spatial and temporal resolution of most of the scales of the fluid motion and the reaction process. In the turbulent cases studied, scales down to Taylor scales are resolved. Hence, it can be considered to be a large eddy simulation, LES. In the laminar cases studied, all fluidic motions are resolved. The reaction modeling is based on the flamelet concept where the scale separation of the flow and chemistry is exploited. This gives the opportunity to use the computational resources to capture the fluid motion in three dimensions, without computing the transport of the vast amount of species that are present in the flame. The behavior of swirling flows in confined geometries is complicated. In this thesis, a highly swirling (with a swirl number, S=1.5) confined flame is studied in a model combustor. The combustor has a sudden contraction at the outlet, which alters the complete flow field structure. The influence of this contraction is investigated. In addition, the effects of heat release are studied. For all these cases the flow field structure is carefully examined and a mechanism describing the behavior is suggested. The numerical computations are validated against experimental data. The second flame studied is situated in low swirling unconfined flow. The flame is fully detached from the burner allowing for entrainment of the surrounding air into the fuel/air mixture ahead of the reaction zone. This, in turn, introduces several challenges regarding the modeling of the flame. A model is proposed in this thesis, and after being validated, the model is used to study the dynamics and the stabilization mechanism of the low swirl flame. All computations and conclusions are backed up with experimental data. The third flame studied in this thesis is an unsteady laminar jet flame. A rich methane/air mixture is issued into the surrounding air at a low velocity. The mixture is within the flammability limit and a conical, Bunsen flame similar, premixed flame front is found. The incompletely oxidized products from this flame mixes with the surrounding air and are completely oxidized in a reaction layer surrounding the premixed flame. The structure and the instability of this partially premixed flame are investigated. As for the previous cases, the numerical results are validated against experimental data.