Numerical Simulation Study On The MILD Combustion Of H₂/CH₄ Jet Flame

In order to study the establishment conditions of MILD（Moderate or Intense Low-oxygen Dilution）combustion of H₂/CH₄ Jet-in-Hot-Coflow（JHC）flame,the steady-state RANS（Reynolds averaged Navier Stokes）model and LES（large eddy simulation）method were studied to establish the numerical model of MILD combustion,and then numerical simulation method was used to investigate the combustion characteristics of the JHC flame under different operating conditions of coflow temperature and coflow oxygen concentration.The commercial CFD（Computational Fluid Dynamics）software ANSYS FLUENT 15.0 was employed with the steady-state RANS（Reynolds Averaged Navier-Stokes）model and detailed reaction mechanism DRM22,and EDC（Eddy Dissipation Concept）combustion model was selected.The grid independence test and numerical model validation show that the steady-state RANS model can accurately predict the main parameters and flame structure of mil combustion.Based on the steady-state simulation,the temperature and oxygen concentration of the wake were changed to explore the role of both in the process of establishing the MILD combustion.The results indicated that the heat transfer process inside the flame is enhanced when temperature of the coflow increases;as the oxygen concentration of the coflow decreases,the heat release of chemical reaction is constrained and the brightness of the flame is reduced significantly while the flame front gets blurred;the oxygen concentration plays a more important role in the combustion regime transformation;the velocity of the main jet determines the turbulent time scale of the reaction zone while the coflow temperature affects the combustion regime by affecting the turbulent time scale;when the oxygen concentration decreases,the time scale of the chemical reaction increases,and the macroscopic combustion becomes more uniform and mild while the transformation of the combustion regime is more controllable.On the basis of steady-state RANS simulation,this paper further developed the large eddy simulation program LESsCOAL in the research group,and tried to use the fully developed turbulent round tube to generate the inlet boundary condition database in order to replace the original boundary condition,and compared the performance of three simplified mechanisms in large eddy simulation.The results show that the combustion field of velocity distribution and turbulent kinetic energy distribution can be predicted well by using the method of inlet condition database,and the problem that the main boundary conditions in the steady-state RANS model are difficult to be determined is avoided;the reaction of CO generated by CO₂ in the mechanism has an important influence on the accuracy of CO prediction,and the improved four-steps simplification mechanisms performs better on CO prediction;there is no difference in the prediction of temperature,O₂ and H₂O among the three simplified mechanisms.