钝体绕流有旋流中回流区与进动涡核的大涡模拟

Large Eddy Simulation of Recirculation and Precessing Vortex Core in Swirling Flow Around a Bluff-Body

关键词:  悉尼旋流燃烧器  大涡模拟  钝体  回流区  进动频率  进动涡核

Key words:  Sydney swirl burner  Large eddy simulation  Bluff-body  Recirculation  Precession frequency  Precessing vortex core

摘要:

针对旋流数为0.57、0.68、0.91和1.59四种工况下的悉尼旋流燃烧器的冷态流场进行了大涡模拟,选取动态Smagorinsky涡黏模型作为亚格子尺度的湍流模型,研究不同旋流数下的流场结构、进动频率和进动涡核。模拟结果表明,旋流数为0.68时,钝体回流区长度最短。随着旋流数的增加,中心射流出口的旋流剪切层不断衰减,而下游的旋流剪切层不断增强。功率谱分析表明,进动现象的出现和消失对应于与旋流剪切层的增强和衰减；中心射流与下游区域具有不同的进动频率,表明流场中存在着两个独立的大尺度涡旋结构。不同取值位置的周向速度相关性分析进一步佐证了两个涡旋结构的存在。轴向位置70 mm处的横截面上瞬时流线和压强分布证实了下游流场存在着进动涡核。瞬时压强等值面显示了中心射流出口和下游流场进动涡核的三维螺旋形结构。下游流场的进动涡核均与平均速度场流线在空间上成正交关系,表明进动涡核是由剪切层Kelvin-Helmholtz不稳定性产生。

Abstract:

The non-reactive flow fields of the Sydney swirl burner were studied with large eddy simulation (LES) under four swirl numbers 0.57,0.68,0.91 and 1.59. Smagorinsky eddy viscosity model with dynamic procedure was selected as the sub-grid scale turbulence model to investigate the flow structures,precession frequency and precessing vortex core (PVC) under different swirl numbers. The shortest bluff-body stabilized recirculation zone appears under swirl number 0.68. With increasing of swirl number,the swirl shear layer at the central jet nozzle damps gradually,while the one in downstream region strengthens. Power spectrum analysis indicates that the appearance and vanishment of precession motion coincide with the enhancement and attenuation of swirl shear layer,respectively. The central jet and downstream region have different precession frequency. This result indicates the existence of two independent large-scale vortices structures and it is further proofed by the correlation analysis of circumferential velocity at different probe locations. The distribution of instantaneous streamlines and pressure on the cross section at axial position of 70 mm indicates the existence of PVC in downstream region. The iso-surface of instantaneous pressure displays the spiral structure of PVC at the central jet exit and in downstream region. The downstream PVC is orthogonal to the streamlines of mean velocity. This fact demonstrates that the PVC is generated by Kelvin-Helmholtz shear layer instability.