| In real combustion systems,the reactants are usually difficult to mix uniformly,leading to partially premixed combustion phenomenon in the combustor,which involves both premixed and diffusion combustion phenomenon.In addition,the ignition process of combustor is related to the safety and reliability of gas turbine system operation.Therefore,the research on the partially premixed flame and the ignition process has important practical significance for the design and optimization of the gas turbine combustor.With the development of computer technology and computational fluid dynamics,numerical simulation has become an important method for studying turbulent combustion.Large eddy simulation(LES)is one of the efficient simulation techniques,in which only large scale eddies are computed while smaller eddies are modeled.Combined with a suitable sub-grid scale combustion model,LES can accurately reflect many details of the interaction between turbulence and chemical reactions.In this paper,the dynamic thickened flame(DTF)combustion model is used to perform LES calculations on the two partially premixed flame conditions of the PRECCINSTA model burner.The influence of wall heat loss on temperature,species mass fractions and flame structure is studied.In addition,the generation and evolution of the coherent structure in the flow field,and the dynamic effect of the vortex on the flame front are analyzed.Then,based on the geometry of the PRECCINSTA model burner,an annular combustor containing 12 swirl nozzles is established,and the LES study is carried out on the non-reacting flow field characteristics and ignition process of the annular combustor.For the quiet flame with a global equivalence ratio of 0.83,a premixed type of combustion dominates.The LES results are in good agreement with the experimental results,consideration of wall heat loss significantly improves the prediction accuracy of the temperature and intermediate species near the external recirculation zone.Due to the Kelvin-Helmholtz flow instability,large-scale helical vortex rotating around the central axis in the flow field is existed,which is orthogonal to the three-dimensional average streamlines in the combustor.Under the pulsating flame condition with a global equivalence ratio of 0.7,the proportion of the diffusion combustion mode increases.LES predicts an obvious precessing vortex core in the flow field,and the strong rotating motion causes a pressure pulsation with a dominant frequency of 540 Hz in the combustor.Through the analysis of the interaction between the vortex and the flame front in a pressure pulsation period,it can be seen that the precessing vortex core continuously squeezes and stretches the recirculation zone,which inceasing the instability of the flow field.The inner and outer flame fronts are broken and detached by the tear of the vortex structures near the shear layer,causing the flame length to change alternately.The LES method accurate predicted the position of recirculation zone and the distribution of species in the three-dimensional non-reacting flow fields of the annular combustor.There is a large-scale single helical vortex structures with precession characteristics is attached to the central bluff body of each nozzle.Due to the strong shearing effect between the swirling jet and the surrounding fluid,two coaxial vortex structures with opposite rotation directions are formed at each nozzle outlet of the annular combustor,which are broken and diminished as the axial distance increases.The phenomenon of light-round and the evolution of flame surface area and integral heat release rate with time are predicted by LES.Through detailed analysis of the variation of species mass fraction and velocity vector distribution with time,it can be observed that the circumferential velocity components in different directions induced by the swirling flow near the inner and outer walls of the combustor are coupled with thermal expansion,resulting in differences in the propagation of the flame on both sides of the combustor. |
PDF Full Text Request