关键词: 非预混燃烧；同轴剪切流；大涡模拟；动态亚网格模型；单步快速化学反应模型；火焰面模型；反应进度变量模型

Abstract: To gain an improved understanding of the flow structure and non-premixed turbulent combustion in a coaxial sudden-expansion combustor with CH4/air shear jet flow，large eddy simulation with dynamic Smagorinsky sub-grid model was used to explore the characteristics of flow and flame. Three different combustion models，i.e. single-step fast chemistry model，flamelet model and progress-variable model were applied and compared in details. Characteristic scales both in space and time of the flowfield were firstly validated with the grid size and time step. Subsequently the unsteady process of reactive flow starting and developing from static to statistically stationary status was fully simulated until the physical time reached 2 seconds. The turbulent kinetic energy spectrum on several probe points located in the mixing shear layer reveals that the axial velocity is fluctuating at multiple characteristic frequencies since 1200Hz，with the relative amplitudes decaying from high to low frequencies when points move downstream. Time averaged distribution of mixture fraction and temperature on several profiles suggests that the progress-variable model successfully reproduces the lifted flame and shows better agreement with experimental data than other two models. Simulation results indicate that for non-premixed reactive flow with strong shear force similar to current case，it is more appropriate for mixture fraction dissipation rate being replaced by product mass fraction in computation.

Key words: Non-premixed combustion；Coaxial shear flow；Large eddy simulation；Dynamic smagorinsky model；Single-step fast chemistry model；Flamelet model；Progress-variable model