乙醇-空气稀液雾值班火焰的大涡模拟研究

Large Eddy Simulation of a Piloted Ethanol-Air Spray Flame

关键词:  稀液雾  值班火焰  大涡模拟  FPV模型

Key words:  Dilute spray  Piloted flame  Large eddy simulation  FPV model

摘要:

为加深对气液两相湍流燃烧现象的认知，检测火焰面模型在液雾燃烧中的适用性，本文在欧拉-拉格朗日架构下使用火焰面/反应进度变量模型(FPV)数值模拟了乙醇-空气稀液雾值班火焰。欧拉坐标系下的气相湍流场使用大涡模拟方法模拟，离散液相则使用拉格朗日颗粒轨道模型进行描述，考虑了相间质量、动量、能量交换。燃烧模型中采用碳元素定义混合物分数，在混合物分数方程源项中体现液相对燃烧模型的影响。模拟得到的气相温度分布和液相统计值均和实验数据较好吻合，验证了该燃烧模型对稀液雾扩散类型火焰的适用性。分析瞬时图发现，该稀液雾火焰的最高燃烧温度往往位于当量混合物分数附近，在出口下游20倍直径处火焰完全点着，此处上游FPV模型能给出局部点火熄火现象。蒸发作用在剪切层和点火区域较强，而液滴和火焰作用较弱，单一液滴很少被火焰包围。

Abstract:

To promote the understanding of the gas-liquid two-phase turbulent combustion，and test the applicability of the flamelet-type combustion model in spray flames，simulation of a piloted ethanol-air spray flame is conducted using the flamelet/progress-variable (FPV) approach in the Eulerian-Lagrangian framework. The gas turbulent flow in Eulerian framework is simulated using large-eddy simulation while the discrete liquid phase is described using the Lagrangian particle trajectory model，considering interphase mass，momentum，and energy exchange. Effects of the liquid phase on the combustion model are mainly reflected in the source term of the mixture fraction equation，and the mixture fraction is defined based on the carbon element. The resulting gaseous temperature and droplet statistics are all in good agreement with experimental data，which verifies the validation of this combustion model for dilute spray diffusion-type flame. Analysis of the instantaneous contour plots show that maximum temperature is often located around the stoichiometric mixture fraction. The flame is ignited and stabilized about 20 times diameter downstream of the jet inlet and upstream some local extinction and re-ignition is captured by FPV model. Meanwhile，evaporation is strong in the shear layer and ignition region. Plots of OH and droplets show that interaction between liquid droplet and the flame is weak and droplet is rarely surrounded by flame.