Large Eddy Simulation Of A High Karlovitz Number Turbulent Premixed Jet Flame

To meet stringent pollutant regulations,lean premixed combustion is of increasing importance in modern gas turbines.Since most of the engines operate under high pressure,strong turbulence,high Karlovitz number conditions,involving turbulence chemistry interactions,strong shear and complex fuels.It is crucial to get a deep understanding of high Ka flame characteristics for achieving high efficiency and low emissions.In this work,a high Ka turbulent premixed jet flame with methane/air mixture is investigated using large eddy simulation?LES?.A comprehensive LES study of high Ka number turbulent premixed methane/air flame with hot coflow,is carried out based on the open source OpenFOAM reacting Form solver combined with transport Probability Density Function?t-PDF?method based on the Eulerian Stochastic Fields.Chemistry coordinate mapping?CCM?method is employed in order to reduce chemistry consumption time.The initial jet axial velocity at the burner exit is specified using a power law velocity profile for fully developed turbulent pipe flow which is superimposed by a turbulence field generated by LEMOS.The GRI-Mech3.0 chemical mechanism for premixed CH₄/air flame is adopted,containing 44 species and 268 elementary reactions.At first,Smagorinsky model and the dynamic Smagorinsky model are adopted for turbulence modelling for the cold flow and validated with the experiment.It is found that due to the high velocity gradient boundary layer between the central jet and coflow,the entrainment occurs in the flow field,which affects the distribution of the velocity field significantly.And on the other hand,the entrainment leads to a stronger mixing between the fuel/air mixture and high temperature product in the boundary layer and hence a stable combustion.The spatial distributions of key species such as CH,HCO,CH₂O and OH are analyzed qualitatively and quantitatively.It is found that the thickness of CH₂O regions in the flame broadens along the downstream,and it increases with increasing the height above the burner and the turbulent intensity.Two combustion models are compared:well-stirred reactor?WSR?model and transport probability density function?tPDF?in the LES study.The data is extracted after a statistical average.The result shows that for the turbulent premixed jet flame in the high Ka condition,the surface density function decrease and the flame is considerably broadened as the flame develops downstream.In order to explore whether the traditional heat release rate markers can represent the heat release rate accurately in the broken reaction zone regime or not,the instantaneous images of heat release rate and its surrogates?[CH₂O][OH],[CH₂O][H],[CH₂O][O]and[HCO]?by simulation were qualitatively compared.It is seen that the spatial distributions of all surrogates correlate with heat release rate.At the end of the thesis,the local Ka number is investigated.Unlike the global Ka,the local Ka number in the simulation varies by up to two orders of magnitude,indicating a broad range of accessed flame structures.Specifically,the local Ka is large close to the jet nozzle and in the low temperature region of the flame.