| Fossil energy will be the main part of China’s energy consumption for a long time.As a new combustion method that can allow significant reductions of the CO2 emissions,oxy-fuel combustion technology has very good application prospects in the field of energy utilization.The flame characteristics of oxy-fuel combustion are basic theoretical issues that restrict the research and development of key equipment such as burners.In this paper,based on a direct numerical simulation（DNS）program,the flame structure,composition distribution and chemical pathway of CH4/oxy-fuel turbulent premixed flame are studied.Based on the Pencil-code which is a high-order finite-difference open-source direct numerical simulation program,a high-precision simulation method for premixed flame developing in two-dimensional and three-dimensional homogeneous isotropic turbulence is constructed.DVODE,a variable-coefficient ODE solver,is used for chemistry calculation.The self-developed skeletal mechanism,simplified from the USC-Mech II,is used as the chemical reaction mechanism.The requirements of calculation accuracy and calculation time are well considered.Then,a series of two-dimensional DNS cases of CH4/O2/CO2 turbulent premixed flame under atmospheric pressure（1atm）are performed to study the effects of turbulent intensity,oxygen partial pressure and dilute gas on the flame structure and composition distribution of the methane premixed flame.Studies have shown that turbulence intensity and oxygen partial pressure have important effects on the flame structure.The decrease in oxygen partial pressure reduces the reaction intensity in the combustion zone and the thickness of the flame increases,and the influence of turbulence on the flame structure also decreases.The intensity of turbulence has an effect on the convection,diffusion and reaction terms in the component transport equation.The turbulent diffusion in the low temperature region increases the local concentration of H,OH and other species.There is a negative correlation between the concentration of H,OH and the mean curvature on the characteristic flame surface,while there is a positive correlation between the concentration of CO and the mean curvature.Subsequently,two-dimensional DNS cases of CH4/O2/CO2 pressurized turbulent premixed flame with a pressure of 1 to 10 atm are performed,and the effect of pressure on the composition distribution and chemical pathway of the oxy-fuel premixed flame was studied.Studies have shown that increasing the pressure will reduce the mass fraction of OH,H,CO,and make them gather in the high temperature region.As the pressure increases,the local heat release rate of the flame increases,and the temperature corresponding to peak heat release rate increases.Pressure has an important effect on the chemical pathway.On the local flame surface,the effect of pressure on different elementary reactions is different.For example,at the peak of the OH consumption rate,as the pressure increases,the contribution of some elementary reactions such as H+O2=O+OH to the reaction rate of OH increases,while the contribution of some elementary reactions such as OH+H2=H2O+H to the reaction rate of OH decreases.Finally,three-dimensional direct numerical simulations of CH4/O2/CO2 pressurized turbulent premixed flame over a range of pressure（1 atm,10 atm,20 atm）are presented.The effects of pressure on the flame structure and flame displacement speed of the oxy-fuel premixed flame are analyzed and discussed.The results show that under the same turbulence intensity,the increase in pressure makes the characteristic flame surface more wrinkled and enhances the differential diffusion of the components.As pressure increases,the contribution of the differential diffusion of the components to the flame displacement velocity increases.The increase of pressure will enhance the influence of Darrieus-Landau instability on the local flame.At p=20atm,a large-scale local extinction occurs on the characteristic flame surface.The increase of pressure has little effect on the flame displacement speed of the characteristic flame surface,which is mainly controlled by the items related to heat conduction. |
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