Detailed Numerical Simulations of Turbulent Premixed Flames at Moderate and High Karlovitz Numbers

Abstract: In generally accepted and applied flamelet combustion models, a turbulent flame is mainly assumed distorted by the large-scale turbulence eddies, whereas small-scale turbulence effects on the local flamelet structures are neglected. However, in a lot of industrial applications rather high turbulent intensities are often imposed, which induce turbulence scales at ranges smaller than the flame thickness. Flame/turbulence interaction appears quite different at these small scales, which is why improvement of the combustion models is required to account for these phenomena. In this thesis, direct numerical simulations (DNS) and large eddy simulations (LES) have been utilized for studies of lean premixed turbulent reactive flows at various turbulent intensities. DNS has been applied for detailed studies of flame-turbulence interaction to investigate flame structures and detailed chemistry effects at high Karlovitz numbers. Intensified convective-diffusive transport within the fine reaction zone layers is observed which is found to significantly alter the chemical pathway with, e.g., intensified heat release rate at low temperatures. Based on these observations a categorization, supplementary to the conventional one, is proposed, which is able to incorporate detailed chemistry effects into the classification of turbulent premixed flames at high Karlovitz numbers. The effect of differential diffusion was found significant, both globally (in terms of the fuel diffusion effect) and locally (in terms of the radical diffusion effect), also in the distributed reaction zone regime. LES was employed for a low swirl stabilized flame utilizing a flamelet combustion model approach. A dynamic modeling approach to incorporate sensitivity to local variations in the subgrid scale flame wrinkling was implemented and validated. The simulations showed high sensitivity of the prediction of turbulent flame fluctuations as well as ambient air entrainment rate into burned gases to inflow conditions and operating conditions. Lower sensitivity was found to domain size and combustion model. Overall the model results showed good agreement with the velocity and scalar validation data in the thin reaction zone regime. In order to analyze the influence of frequency specific coherent structures on the flame dynamics extended dynamic mode decomposition was performed which was able to delineate the effects of the inner and outer shear layer vorticity on the flame stabilization.