同轴射流燃烧室非预混湍流燃烧流场特性大涡模拟研究

Large Eddy Simulation of Non-Premixed Turbulent Combustion in Combustor with Coaxial Jet Flow

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

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

摘要:

为深入研究带突扩与回流的同轴剪切射流模型燃烧室流场结构与燃烧特性，采用单步快速化学反应、火焰面与反应进度变量三种燃烧模型结合动态亚网格模型在自有CFD平台上对甲烷-空气非预混燃烧流场进行了大涡模拟。计算检验了流场时空尺度及网格和时间步长设置，对流场0～2s内启动并发展进入统计定常状态的非定常流动过程进行了完整模拟。燃烧室内多处监测点湍动能谱统计结果表明，轴向速度在混合剪切层内的脉动存在从约1200Hz开始的多个特征频率序列，且随着测点位置向下游移动，其对应的特征峰从高频到低频逐渐衰减消失。流向多个截面上时间平均的混合分数及温度分布结果表明，反应进度变量模型能够模拟出该流场特有的抬举火焰，与实验数据吻合程度显著优于其余两种模型，即对于该类型强剪切非预混反应流，以燃烧产物质量分数替代采用梯度粘性输运模化的混合分数耗散率进行计算更为合适。

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.