난류 비예혼합 및 부분 예혼합 화염장의 수치 및 물리 모델링

Abstract

The turbulent nonpremixed and partially premixed combustion involve the highly nonlinear and strongly coupled physical processes such as intense turbulent mixing and burning, turbulence-chemistry interaction, radiation, and pollutant (NO, soot) formation. The present study is ultimately aiming at developing the comprehensive model to analyze the detailed flame structure and pollutant formation in the physically and geometrically complex practical combustors. This comprehensive model requires the reliable and realistic physical model as well as the efficient and stable numerical model. Among these physical processes, this study is focused on modeling the turbulence-chemistry interaction, flame stabilization, and pollutant formation in the turbulent chemically reacting flows.

In the present approach, the turbulent combustion is represented by the presumed PDF model and the transported PDF approach. The chemistry is based on the detailed kinetics and the radiation is modeled by the finite volume method. In the framework of the presumed PDF model, the present study employs the transient flamelet approach including the Lagrangian flamelet model and the Eulerian particle flamelet model as well as the level-set based flamelet model. In conjunction with the transported PDF approach, this study adopts the velocity-composition joint PDF transport model, the composition PDF transport model, and the DQMOM-based PDF transport model. To allow the geometrical flexibility in the practical combustion systems, all transport equations are discretized by the unstructured-grid finite volume approach. Moreover, the parallel algorithm based on the PC cluster has been utilized to improve computational efficiency in dealing with the complex and large-scale combustion problems as well as to reduce the memory load of the operator-splitting procedure for detailed chemistry calculation.

One of the main topics will be the computational and theoretical assessment of the state-of-art turbulent combustion models including the presumed PDF model and the transported PDF approach for the simulation of turbulent nonpremixed and partially premixed flames. Example cases include turbulent CO/H2/N2 jet flames, turbulent piloted jet flame, and the turbulent nonpremixed H2/CO flame stabilized on an axisymmetric bluff-body burner as well as the turbulent H2/N2 lifted jet flame and the turbulent partially premixed jet flame in the MILD combustion condition. The turbulent nonpremixed flames are numerically investigated by the Lagrangian flamelet model, the Eulerian particle flamelet model, the PDF transport models, and the DQMOM-based PDF transport model. On the other hand, the turbulent partially premixed flames are numerically analyzed by the level-set based flamelet model and the DQMOM-based PDF transport approach. The precise comparisons between predictions and measurements are performed for the unconditional and conditional means of temperature, mixture fraction and its variance, major species and radical mass fraction. The detailed discussions are also made for the comparative performance as well as the differences in the theoretical and computational aspects.

Special emphasis is given for the systematic validation of the DQMOM-based PDF transport model. The validation cases include the turbulent non-reacting propane jet, turbulent piloted jet flame, turbulent bluff-body stabilized nonpremixed flame, turbulent lifted flame with a vitiated coflow, and turbulent partially premixed flames in the MILD combustion conditions with three different oxygen concentrations. The correct prediction of all these example problems requires the consistent treatment of all physical processes involved the turbulent nonpremixed and partially premixed combustion. In terms of the flame lift-off height, flame stabilization characteristics, and flame structure, numerical results are reasonably well agreed with experimental data. These numerical results suggest that the present DQMOM-based PDF transport model has the predictive capability to realistically simulate the complex turbulent flames encountered in the practical combustors.

Finally, for applying to simulation of syngas combustion processes in the IGCC gas turbine power plant, a sequence of computations are performed to study the effects of temperature, pressure, syngas chemical kinetics, syngas fuel composition, and dilution on the laminar flame speed, nonpremixed flame structure, and extinction scalar dissipation rate. Due to the robustness and computational efficiency, the transient flamelet model together with the parallel procedure has been applied to numerically analyze the fuel-side nitrogen dilution effects on the precise structure and NOx formation as well as to investigate the combustion processes and pollutant formation in the DLR syngas swirl combustor with two different loads. Numerical results indicate that the present transient flamelet models including the Lagrangian flamelet model and the Eulerian Particle Flamelet model have realistically simulated the turbulence-chemistry interaction and pollutant formation characteristics in the syngas nonpremixed combustors.