Numerical Study Of LES On Plasma-assisted Methane Ignition And Combustion

In recent years,with the steady development of the national economy,structural adjustment,and transformation and upgrading,the consumption of natural gas as clean energy has achieved super-expected growth.How to solve the instability,and emission of pollutants during the ignition and combustion of natural gas has become the focus of people's attention,and plasma has broad prospects in this field.Among them,plasma-assisted ignition and combustion in the aviation field has been applied to aero-engine and other aircraft to solve the problems of engine ignition difficulties and narrow ignition boundaries.However,due to the limitation of the measurement method,the research on the mechanism of the transient process of plasma-assisted combustion in the three-dimensional combustor is not completely clear.Therefore,based on the OpenFOAM open-source platform,the numerical study of large eddy simulation?LES?of plasma-assisted methane ignition and combustion is carried out in this paper.Firstly,the reactingFoam solver was used to call the turbulence model and combustion model to solve the complex chemical reaction process under turbulent combustion conditions.The LES-Smagrinsky turbulence model,PaSR combustion model and detailed chemical reaction mechanism were used,and a simulation platform for methane turbulent combustion in a coaxial jet combustor was built.By comparing with the experiment,it is found that the simulated prediction value is in good agreement with the experiment,which verifies the accuracy of the solver and mathematical model selected in this paper,and a plasma-assisted combustion model with a total grid number of 981600 was built.Secondly,by monitoring the important physical parameters during ignition process,the effect of plasma and pressure on ignition was studied.The simulation results show that the addition of plasma O₃ began to affect the flame propagation process when it reached the backward-facing step,improved the flame propagation speed and accelerated the ignition process.But the increase in pressure affected the flame propagation process at the beginning of plasma ignition,and the promotion of plasma ignition was more obvious.Furthermore,the effect of H and other free radicals on ignition delay time under different initial conditions was studied.The results show that the increase of the initial pressure and ignition temperature in the combustor shortened the ignition delay time and was beneficial to the rapid stability of the flame.The increase in pressure was beneficial to the acceleration effect of plasma on ignition process,and the acceleration effect of plasma on ignition process under low-temperature conditions was more obvious.Finally,the average statistics of the full development period during turbulent combustion were conducted,and the strengthening effect of plasma on combustion,the effect of pressure on plasma-assisted combustion and the emission of pollutants were studied.The results show that the addition of plasma O₃ kept the flame vortex structure continuous and the flame recirculation zone moved forward,the combustion-supporting effect on the low-temperature zone?recirculation zone?was more obvious and reduced the axial pulsation in the turbulent velocity field,thereby strengthening the fuel blending with oxidant to stabilize the recirculation zone.But the increase in pressure caused the vortex structure of the flame broken,the area of the flame recirculation zone was reduced,the mixing of fuel and air was poor,the combustion-supporting effect on the low-temperature zone?recirculation zone?was weakened and increased the axial pulsation in the turbulent velocity field,which was not conducive to the fuel blending with oxidant,resulting in insufficient combustion and instability in the recirculation zone.Furthermore,the addition of plasma O₃ was conducive to reducing the emission of NO_X during combustion,while the increase in pressure was not conducive to reducing the emission of NOx during plasma-assised combustion.