旋流数对分层旋流流场影响的大涡模拟研究

Effects of Swirl Number on Stratified Swirl Flow Field Using Large Eddy Simulation

关键词:  分层旋流燃烧器  大涡模拟  回流区  Q准则  进动涡核

Key words:  Stratified swirl burner  Large eddy simulation  Recirculation  Q-criterion  Precessing vortex core（PVC）

摘要:

为了深入理解分层旋流流场特征和燃烧稳定性，采用OpenFOAM对分层旋流燃烧器的冷态和燃烧流场进行了大涡模拟。研究了旋流数对分层旋流流场结构和非稳态特性的影响。采用Q准则显示了流场中的瞬时涡结构；利用功率谱分析了流场中的进动特征。结果表明：在冷态工况下，旋流对回流区的位置和大小影响较小。随着旋流数增大，出口气流受到旋流诱导的离心作用，流动发散，流场扩张角变大，流场下游出现二次回流区。平均流场的三维流线与螺旋涡在空间中均表现为正交关系，表明螺旋涡是由剪切层Kelvin-Helmholtz不稳定性产生。在燃烧工况下，随着旋流数增大，回流区的面积增大，平均温度分布不断沿径向扩张，火焰锋面脉动增强，涡旋发生破碎的位置明显向上游移动。

Abstract:

To deeply understand the characteristics of stratified swirl flow field and combustion stability， OpenFOAM was used to conduct large eddy simulation of the cold and combustion flow fields of a stratified swirl burner. The effects of swirl number on the structure and unsteady characteristics of free swirl flow field were studied. The Q-criterion was used to illustrate the instantaneous vortex structure in the flow field， and the power spectrum density was used to analyze the precession characteristics in the flow field. The results show that under nonreacting conditions， the position and area of the recirculation are slightly affected by swirl number. As the swirl number increases， the outlet airflow is subjected to the centrifugal effect induced by the swirling flow， the flow diverges， the expansion angle of the flow field becomes larger， and a secondary recirculation appears downstream of the flow field. The downstream spiral vortex core is orthogonal to the streamlines of mean velocity， demonstrating that the spiral vortex is generated by Kelvin-Helmholtz shear layer instability. Under reacting conditions， as the swirl number increases， the area of the recirculation increases， the average temperature distribution continues to expand along the radial direction， the flame front pulsation increases， and the position where the vortex breakup obviously moves upstream.