Effects of free stream flow turbulence on blowoff characteristics of bluff-body stabilized premixed flames
Effects of free stream flow turbulence on blowoff characteristics of bluff-body stabilized premixed flames
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DOI：
10.1016/j.combustflame.2017.12.002
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发表时间：
2018-04
影响因子：
4.4
通讯作者：
Bikram Roy Chowdhury;B. Cetegen
中科院分区：
文献类型：
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作者：
Bikram Roy Chowdhury;B. Cetegen
在本文中，我们报告了在接近吹出的条件下，受到不同水平自由流湍流强度（4%、14%、24%和30%）的钝体稳定稀薄预混火焰的实验研究。平均流速范围为5 - 15 m/s。基于积分长度尺度和均方根速度的湍流雷诺数范围为44至4280。采用平面激光诱导荧光和粒子图像测速技术（PIV）对羟基（OH）和甲醛（CH 2 O）进行同步成像，研究火焰与流场之间的相互作用，并确定导致火焰熄灭的事件顺序。分别利用CH 2 O荧光信号和OH和CH 2 O荧光信号的逐像素倍增来标记火焰前锋的预热区和放热区。火焰结构被观察到强烈修改的湍流场，影响贫油吹出的限制。由于湍流与火焰的强烈相互作用以及由此产生的火焰表面的修改和随后的局部火焰熄灭，火焰吹出当量比随着自由流湍流水平的增加而增加。对于稳定燃烧的火焰，火焰前锋主要包围的剪切层涡的所有湍流条件。当接近吹离时，火焰前缘和剪切层涡流纠缠在一起，在火焰前缘上引起超过熄灭应变的高局部应变率，从而导致沿反应区的显著断裂沿着。在接近吹离的条件下，在回流区内部观察到宽区域的CH 2 O和热释放。火焰孔附近的速度矢量表明反应物进入回流区的渗透。测量了几个属性来表征近吹离火焰，其中包括应变率和曲率统计沿着火焰前锋，燃烧分数，不对称指数和平均持续时间的吹离事件。
In this article we report on an experimental investigation of a bluff-body stabilized lean premixed flame subjected to different levels of free stream turbulence intensities (4, 14, 24 and 30%) at conditions approaching blowoff. The mean flow velocities ranged from 5 to 15 m/s. The turbulence Reynolds number based on integral length scale and rms velocity ranged from 44 to 4280. Simultaneous imaging of hydroxyl (OH) and formaldehyde (CH2O) by planar laser induced fluorescence and particle image velocimetry (PIV) was used to study the interaction between the flame and the flow field and determine the sequence of events leading to flame blowoff. CH2O fluorescence and the pixel-by-pixel multiplication of OH and CH2O fluorescence signals were utilized to mark the preheat and heat release regions of flame front respectively. The flame structure was observed to be strongly modified by the turbulent flow field which affects the lean blowoff limits. The flame blowoff equivalence ratio increased with increasing free stream turbulence levels owing to strong interactions of the turbulent flow with the flame and the resulting modification of flame surfaces and ensuing local flame extinction. For stably burning flames, the flame front predominantly enveloped the shear layer vortices for all the turbulent conditions. As blowoff was approached, the flame front and shear layer vortices entangled inducing high local strain rates on the flame front that exceed the extinction strain resulting in significant breaks along the reaction zone. At conditions near blowoff, wide regions of CH2O and heat release were observed inside the recirculation zone. Velocity vectors near the flame holes indicate the penetration of the reactants into the recirculation zone. Several properties were measured to characterize the near blowoff flames which include the strain rate and curvature statistics along the flame front, burning fraction, asymmetric index and the average duration of the blowoff event.