LES Investigation Of Methane And Syngas Jet Flame With Flamelet/Progress Variable Approach And Reaction Mechanism Reduction

In recent years, with the rapid development of economy, the demand for energy has increased dramatically. Coal plays an important role in our energy structure. Its utilization makes huge contribution to the rapid development of our country, but also causes serious environmental problems. Coal-based co-production system is a good way to solve the above problems. As part of the products, syngas used as power generation will be more clean and efficienctive. With the combination of oxygen-enriched combustion technology, the coal utilization system can realize carbon capture and storage, so as to reduce the greenhouse effect.Using large eddy simulation for turbulent combustion process research has been widely recognized and applied. This thesis from the control equations of view, introducing low Mach number hypothesis, combined with dynamic sub-grid model and with flamelet/progress variable approach, constructed a large eddy simulation program which is applicable in turbulent combustion process simulation.Verification and validation are proposed in detail, including the method of manusfacture solution with model-free, nonreacting propane jet and piloted CH4/Air jet flame. By detailed comparison with the experimental data, the results indicate that large eddy simulation is more accurate than RANS, and turbulent combustion process can be well described by large eddy simulation program developed here. The interaction of turbulent flow and combustion are investigated, the results indicate that turbulent flow can strengthen the combustion process by increasing the contact area to of fuel and oxidant,while the strengthen effects of combustion on turbulence is with certain hysteresis. In addition, compared with the LES module in Ansys Fluent, large eddy simulation program of this article in the cold state can save about 35% of computation time in cold jet condition, in the jet flame condition about 55% of computing time, and it has a better robustness.The combustion process of diluted sygas is studied using the above LES program, and focuses on the influence of wall thickness of center jet on combustion stability. The results show that by changing the wall thickness of the jet, the spatial distribution of the axial velocity near the base of the jet changes, which will affect the stability of the root of the flame. An appropriate increase in wall thickness can enhance flame stability at the root of the flame and does not change the flame status. Considering the flow conditions, the physical and chemical properties of the fuel, the author proposes a dimensionless number to identify the root to the flame stability. With validation, the results show that the presented methods can be used to determine the root to the flame stability.A high-pressure methane combustion chamber is simulated by LES, focusing on the effects of pressure on the combustion process. The impact of oxygen percentage on combustion process is then investigated. And then different oxygen-rich combustion chamber reconstruction are shown to analyze the pros and cons. The impact of coflow temperature is also investigated. The results showed little influence of pressure to the combustion process; with O2/CO2=30/70, the characteristics of the flame is most the same as the one with air; the coflow protection for the wall must be taken into consideration during reconstruction. Coflow temperature has great influence on the reaction intensity, but for the limited space of the combustion process, the most important factor is the global equivalent ratio.With deepen understanding of the combustion process, the scale of detailed mechanism is larger and larger. Combined with a direct relationship diagram with error propagation method, sensitivity analysis, quasi-steady-state analysis, a mechanism reduction method is introduced in detail. Using ozone-added methane combustion reaction mechanism as an example of demonstration, a 22 species,18 steps global mechanism is constructed. Compared with the original detailed mechanism, the global mechanism shows an moderate accuracy, which proves the feasibility of the mechanism reduction method. Compared calculation consumption of the original detailed reaction mechanism, the skeleton mechanism and the global mechamism, the results show that global mechanism can save about 72% of computational time.