基于PaSR模型的低旋流燃烧大涡模拟

Large Eddy Simulation of Low Swirl Combustion Based on PaSR Model

关键词:  低旋流燃烧  非稳态特性  有序结构  特征频率  大涡模拟

Key words:  Low swirl combustion  Unsteady characteristics  Coherent structure  Characteristic frequency  Large eddy simulation

摘要:

为了深入理解低旋流流场特征和燃烧稳定性，基于OpenFOAM平台，采用动态k方程模型和有限速率PaSR模型对甲烷/空气预混低旋流燃烧进行了大涡模拟，研究了气流入口速度、当量比和压力等流场参数对流场结构和燃烧非稳态特性的影响，分析了流场大尺度结构与火焰相互作用。结果表明，流场结构和火焰抬升高度受入口速度影响较小，流场和火焰形态能够保持自相似性；随着当量比和压力提高，流场扩张性增强并在燃烧区下游产生回流区，火焰稳定不依赖回流区，根部火焰锋面形状由U形转变为W形，火焰抬升高度降低。火焰锋面稳定在剪切层，剪切层产生的周期性有序涡结构引起当地流场速度脉动和火焰表面褶皱，反映了流场非稳态特性；通过剪切层监测点瞬时轴向速度分析，涡结构特征频率随速度增大而提高，由250Hz提高至300Hz，随当量比和压力提高而降低，由250Hz降低至125Hz。

Abstract:

For the purpose of providing the scientific insights of the flow features and flame stability of low swirl combustion， large eddy simulations of premixed methane-air low swirl combustion are conducted using dynamic k-equation subgrid model and finite chemistry model PaSR under several flow conditions including different bulk velocity， equivalence ratio， and pressure based on OpenFOAM. From experimental measurements and large eddy simulation predictions， flow conditions effects on low swirl flow field structure and unsteady characteristics of combustion are performed. Additionally， interaction between large scale vortex structures and flame surface is also discussed. Flow field structure and flame lift off are scarcely influenced by bulk velocity， which suggests the self-similar feature of low swirl combustion under different inflow velocities. As increase of equivalence ratio and pressure， flow streams diverge and generate a recirculation zone in post-flame zone. Besides， the flame base exhibits a transformation from U shape to W shape with flame lift off distance decreases. The flame front is not dominated by recirculation zone but stabilizes in the shear layers， in which coherent vortex structures are periodically generated and result in both local velocity fluctuating as well as flame surface wrinkling. Characteristic frequencies of unsteady shear layer vortex structures are evaluated based on point data analyzing. The results show that the characteristic frequencies raise from 250Hz to 300Hz with the increase of bulk velocity， and decline from 250Hz to 125Hz with the increase of equivalence ratio and pressure.