This dissertation addresses the analysis of partially premixed flame configurations and the detection and characterization of their local flame regimes. First, the identification of flame regimes in experimental data is intensively discussed. Current methods for combustion regime characterization, such as the flame index, rely on 3D gradient information that is not accessible with available experimental techniques. Here, a method is proposed for reaction zone detection and characterization, which can be applied to instantaneous 1D Raman/Rayleigh line measurements of major species and temperature as well as to the results of laminar and turbulent flame simulations, without the need for 3D gradient information. Several derived flame markers, namely the mixture fraction, the heat release rate and the chemical explosive mode, are combined to detect and characterize premixed versus non-premixed reaction zones. The methodology is developed and evaluated using fully resolved simulation data from laminar flames. The fully resolved 1D simulation data are spatially filtered to account for the difference in spatial resolution between the experiment and the simulation, and experimental uncertainty is superimposed onto the filtered numerical results to produce Raman/Rayleigh equivalent data. Then, starting from just the temperature and major species, a constrained homogeneous batch reactor calculation gives an approximation of the full thermochemical state at each sample location. Finally, the chemical explosive mode and the heat release rate are calculated from this approximated state and compared to those calculated directly from the simulation data. After successful validation, the approach is applied to Raman/Rayleigh line measurements from laminar counterflow flames, a mildly turbulent lifted flame and turbulent benchmark cases. The results confirm that the reaction zones can be reliably detected and characterized using experimental data. In contrast to other approaches, the presented methodology circumvents uncertainties arising from the use of limited gradient information and offers an alternative to known reaction zone identification methods. Second, this work focuses on the flame structure of partially premixed dimethyl ether (DME) flames. DME flames form significant intermediate hydrocarbons in the reaction zone and are classified as the next more complex fuel candidate in research after methane. To simulate DME combustion processes, accurate predictions by computational combustion models are required. To evaluate such models and to identify appropriate flame regimes, numerical simulations are necessary. Therefore, fully resolved simulations of laminar dimethyl ether flames, defined by different levels of premixing, are performed. Further, the qualitative two-dimensional structures of the partially premixed DME flames are discussed and analyses are carried out at selected slices and compared to each other as well as to experimental data. Further, the flamelet/progress variable (FPV) approach is investigated to predict the partially premixed flame structures of the DME flames. In the context of the FPV approach, a rigorous analysis of the underlying manifold is carried out based on the newly developed regime identification approach and an a priori analysis. The most promising flamelet look-up table is chosen for the fully coupled tabulated chemistry simulations and the results are further compared to the fully resolved simulation data.

This dissertation addresses the analysis of partially premixed flame configurations and the detection and characterization of their local flame regimes. First, the identification of flame regimes in experimental data is intensively discussed. Current methods for combustion regime characterization, such as the flame index, rely on 3D gradient information that is not accessible with available experimental techniques. Here, a method is proposed for reaction zone detection and characterization, which can be applied to instantaneous 1D Raman/Rayleigh line measurements of major species and temperature as well as to the results of laminar and turbulent flame simulations, without the need for 3D gradient information. Several derived flame markers, namely the mixture fraction, the heat release rate and the chemical explosive mode, are combined to detect and characterize premixed versus non-premixed reaction zones. The methodology is developed and evaluated using fully resolved simulation data from laminar flames. The fully resolved 1D simulation data are spatially filtered to account for the difference in spatial resolution between the experiment and the simulation, and experimental uncertainty is superimposed onto the filtered numerical results to produce Raman/Rayleigh equivalent data. Then, starting from just the temperature and major species, a constrained homogeneous batch reactor calculation gives an approximation of the full thermochemical state at each sample location. Finally, the chemical explosive mode and the heat release rate are calculated from this approximated state and compared to those calculated directly from the simulation data. After successful validation, the approach is applied to Raman/Rayleigh line measurements from laminar counterflow flames, a mildly turbulent lifted flame and turbulent benchmark cases. The results confirm that the reaction zones can be reliably detected and characterized using experimental data. In contrast to other approaches, the presented methodology circumvents uncertainties arising from the use of limited gradient information and offers an alternative to known reaction zone identification methods. Second, this work focuses on the flame structure of partially premixed dimethyl ether (DME) flames. DME flames form significant intermediate hydrocarbons in the reaction zone and are classified as the next more complex fuel candidate in research after methane. To simulate DME combustion processes, accurate predictions by computational combustion models are required. To evaluate such models and to identify appropriate flame regimes, numerical simulations are necessary. Therefore, fully resolved simulations of laminar dimethyl ether flames, defined by different levels of premixing, are performed. Further, the qualitative two-dimensional structures of the partially premixed DME flames are discussed and analyses are carried out at selected slices and compared to each other as well as to experimental data. Further, the flamelet/progress variable (FPV) approach is investigated to predict the partially premixed flame structures of the DME flames. In the context of the FPV approach, a rigorous analysis of the underlying manifold is carried out based on the newly developed regime identification approach and an a priori analysis. The most promising flamelet look-up table is chosen for the fully coupled tabulated chemistry simulations and the results are further compared to the fully resolved simulation data.

<p>  </p> <p>With economies shifting towards net-zero carbon emissions, there is an increased interest in carbon-free energy carriers. Hydrogen is a potential carbon-free energy source. However, it poses several production, infrastructural, and safety challenges. Ammonia blends have been identified as a potential hydrogen carrier and fuel for gas turbine combustion. Partially cracked ammonia mixtures consist of large quantities of hydrogen that help overcome the disadvantages of pure ammonia combustion. The presence of nitrogen in the fuel blends leads to increased NO_x emissions, and therefore lean premixed combustion is necessary to curb these emissions. Understanding the flame features, precursors, and dynamics of blowout of such blends due to lean conditions is essential for stable operation, lean blowout prediction, and control. </p> <p><br></p> <p>In this study, high-fidelity large eddy simulations for turbulent premixed ammonia/hydrogen/nitrogen-air flames in an axisymmetric, unconfined, bluff-body stabilized burner are performed to gain insights into lean blowout dynamics. Partially cracked ammonia (40% NH₃, 45% H₂, and 15% N₂, by volume) is chosen as fuel since its laminar burning velocity is comparable to CH4-air mixtures. A finite rate chemistry model with a detailed chemical kinetic mechanism (36 species and 247 reactions) is utilized to capture characteristics of various species during blowout. A comprehensive study of the flow field and flame structure for a weakly stable burning at an equivalence ratio of 0.5 near the blowout limit is presented. Further, the effects of blowout on the heat release rate, vorticity, distribution of major species, and ignition radicals are studied at four time instances at blowout velocity of 70 m/s. Since limited data is available on turbulent premixed combustion of partially cracked ammonia, such studies are essential in understanding flame behavior and uncertainties with regard to blowout.</p>