| Although lean premixed combustion can improve the combustion efficiency and reduce the emissions simultaneously,it is,however,prone to combustion instabilities such as lean blow-off and thermos-acoustic oscillation and likely to degrade the performance of technical combustion devices,due to that the equivalence ratio of it is close to the lean flammability limit.On the other hand,stratified combustion refers to a reacting front propagating through a range of compositions within the flammable limits,in which rich premixed flame can be used to ignite the surrounding lean premixed gases,and can enhance the combustion stability of the traditional lean premixed combustion.Compared with perfect premixed and non-premixed combustion,the stratified combustion is not understood as well and needs to be investigated further.Based on the previous research,reliable large eddy simulation(LES)method and combustion model are employed in the present work to simulate the well-known Darmstadt turbulent stratified flame(TSF).It mainly focuses on the difference of propagating modes of TSF under different conditions with or without stratification effect and shear layer.The used sub-grid scale(SGS)combustion model is constructed based on the reaction-diffusion manifold(REDIM)method and the presumed filtered density function(PFDF)method.REDIM is a method for reducing detailed chemistry mechanism,which takes into account the coupling of reaction and molecular transport processes explicitly,and the generated REDIM table is capable of describing TSF.In the present work,a two-dimensional REDIM table is generated for simulating the Darmstadt TSF,with the mass fractions of CO2 and N2 as the two reduced coordinates.All other thermochemical quantities can be obtained by searching the REDIM look-up table.The fluctuation of CO2 and N2 mass fraction within the SGS filter volume is accounted for via PFDF method.The assumption of statistical independence of CO2 and N2 mass fractions is employed in this work,and their shapes are all presumed to be of Clipped Gaussian type.The LES code solves the Low-Mach-number version of the compressible Navier-Stokes equations on body-fitted curvilinear block-structured grids.The computational domain is divided into 308 blocks,and the coarse and fine grids contain 1.2 million cells and 2.6 million cells,respectively.The SGS stress is calculated by the dynamic Smagorinsky model.In total,four reacting cases with different combinations of stratification and shear,i.e.A-r,C-r,E-r,G-r cases,are simulated.By observing the animations of flame front and isolines of equivalence ratio of the four cases,it is found that their tendencies are very different.The A-r case,with stratification and weak shear layer,is particularly different from other three cases and its flame front is extended further downstream and looks narrower.A very detailed analysis of the important quantity about the TSF,alignment angle ? between the reaction layer and the mixing layer,is conducted.It is found that flame front is much wrinkled due to turbulence,which results in that the angle ? varies greatly in a small physical space.For the four considered cases,the variation span of ? is from 0? to 180?.It is also found that the probability distribution of ? at different axial locations changes with the increasing of the distance from the burner exit.Although the ‘back-supported flame,i.e.? < 90?’ propagating mode is dominant in all the four cases,the ‘front-supported flame,i.e.? > 90?’ propagating mode is also pronounced.Based on the results of the present work,it can be concluded that turbulence can accelerate the mixing process of gases with different equivalence ratio and hence weak the effect of stratification.Additionally,turbulence can increase the wrinkling of flame front and make the angle ?,between the reaction layer and mixing layer,change more rapidly,which results in a more uniform distribution of probability of ?(11)i.e.the probability for ‘back-supported flame’ and ‘front-supported flame’ tends to be close. |
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