Experimental measurement of the flow field of a swirling flame under large amplitude acoustic forcing
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摘要: 强声波扰动下旋流流场的动态特征对于理解旋流火焰的非线性响应特性非常重要。基于超高重复频率脉冲串式激光器高速粒子示踪技术测量了强声激励下旋流火焰的动态流场，研究了旋流流场周期性涡结构和流场-火焰动态相互作用。周期性声波扰动会在旋流火焰内剪切层和外剪切层中引起固有涡结构。发现外部涡环在卷曲火焰锋面和改变火焰热量释放速率中起主要作用，而内部涡环分布在火焰根部并会影响中心回流区速度分布。定量提取了声诱导涡环的轨迹、涡量、环量、尺寸、出口速度以及加速度之间关系，发现强声激励下的出口速度和加速度决定了外部涡环的形成和脱落过程。Abstract: The dynamic of a swirling flame under high acoustic perturbation characterizes its non-linear acoustic response. Here, it shows that the time-dependent flow field of a swirling flame under large amplitude acoustic forcing, measured by high-repetition rate pulse burst laser Particle Image Velocimetry (PIV) technique, is highly non-linear. The periodic vortex structures formed in the inner and outer shear layers of the swirling flame can interact with the flame front and flow field though distinct manners:the vortex ring formed in the outer shear layer folds the flame front and alters the flame heat release rate; the vortex ring formed in the inner shear layer mainly affects the velocity distribution in the recirculation zone. We quantitatively analyzed the effects of the outer/inner vortex rings, including the trajectory, vorticity, circulation and size. We found that exit velocity and acceleration are the main factors that determine the formation and release of the out shear layer vortex ring.
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Key words:
- swirling flame /
- acoustic forcing /
- dynamic flow field /
- acoustic induced vortex
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图 1 旋流燃烧器和激光诊断系统原理图
Figure 1. Schematic of the swirl burner and laser diagnostic system
图 2 在f=120 Hz和u′/u=0.8条件下热线风速仪测量的声激励扰动的速度
Figure 2. Acoustically perturbed velocity measured by CTA at f=120 Hz and u′/u=0.8
图 3 甲烷/空气旋流火焰在u′/u=0和u′/u=0.8条件下的时间平均流场和涡量场
Figure 3. Time-averaged flow and vorticity fields of methane/air swirling flames at u′/u=0 and u′/u=0.8
图 4 甲烷/空气旋流火焰在u′/u=0.8条件下轴向速度、水平速度和涡量的均方根分布
Figure 4. The RMS (Root Mean Square) values of axial velocity, radial velocity and vorticity of methane/air swirling flames at u′/u=0.8
图 5 甲烷/空气旋流火焰在f=120 Hz和u′/u=0.8条件下的相平均流场和火焰面等值线，黑色和红色等值线分别代表最大CH*自发光强度的0.4和0.8倍
Figure 5. Phase-averaged flow field and flame contours of methane/air swirling flames at f=120 Hz and u′/u=0.8. The black and red contours represent 0.4 and 0.8 times of the maximum CH* intensity, respectively
图 6 在t/T为0~2和轴向位置5~25mm高度处的径向速度和轴向速度分布图
Figure 6. The distribution of radial velocity and axial velocity at t/T of 0~2 and axial position of 5~25 mm
图 7 在t/T为0~2和轴向位置5 mm高度处的径向速度和轴向速度分布图
Figure 7. The distribution of radial velocity and axial velocity at t/T of 0~2 and axial position of 5 mm
图 8 甲烷/空气旋流火焰在f=120 Hz和u′/u=0.8条件下外部涡环和内部涡环的涡量分布图
Figure 8. Vorticity distribution of the outer and inner vortex rings of methane/air swirling flames at f=120 Hz and u′/u=0.8
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