AN EXPERIMENTAL STUDY ON FLICKERING MODES AND FREQUENCY CHARACTERISTICS OF JET DIFFUSION FLAMES
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摘要:层流扩散火焰的闪烁是一种经典的火焰不稳定性现象, 然而现阶段对其不稳定性模态及频率特性的研究尚不充分. 本文利用高速摄影和粒子图像测速同步测量技术, 对宽工况下准静态环境中的浮力主控和动量主控圆口射流扩散火焰的不稳定性及其频率进行了实验研究. 实验发现, 燃料射流流量增大会导致火焰失稳, 所引起的闪烁现象可分为varicose模态和sinuous模态. 流场测量结果表明, 闪烁火焰的主要流动结构表现为分别位于火焰面内外的两个剪切层. 火焰外剪切层卷起形成的大尺度环形涡的周期性产生、增长和脱落是导致火焰面周期性形变(即火焰闪烁)的原因. 闪烁火焰具有准周期性, 其流场具有统一的主频率, 且该频率与火焰脉动频率一致, 说明闪烁火焰在本质上是种整体流动不稳定性的体现. 浮力主控火焰sinuous模态的频率比varicose模态高3 Hz左右, 并且在燃料流量较大的工况下存在varicose和sinuous模态间的切换. 浮力主控火焰的频率符合经典的1/2标度律, 但不同模态对应着不同的标度律系数. 动量主控火焰的频率显著偏离1/2标度率, 且偏离程度随射流动量增加而增强. 研究表明了流动不稳定性模态对于射流扩散火焰频率特性具有不可忽略的影响.Abstract:The flickering of laminar diffusion flames is a typical flame instability phenomenon; yet, a comprehensive understanding of the various instability modes and their frequency characteristics remains insufficient. This study aims to bridge this gap by conducting an experimental investigation on the instability and frequency characteristics of buoyancy-driven and momentum-driven round laminar jet diffusion flames in a wide range of flame conditions, by means of simultaneous high-speed flame chemiluminescence and particle image velocimetry. Upon increasing the fuel jet flow rate, the flame becomes unstable and starts to flicker, exhibiting two distinct modes: varicose and sinuous modes. The flow field measurement results indicate that the primary flow structures of flickering jet diffusion flames consist of two shear layers, located inside and outside the flame surface, respectively. And the periodic generation, growth, and shedding of large-scale toroidal vortices, formed by the rolling up of the outer shear layer, is the primary cause of the periodic deformation of the flame surface, i.e. flame flickering. The flickering flame displays quasi-periodicity, with a unique dominant frequency observed in the flow field. This frequency is found to be consistent with the frequency obtained from the spectrum analysis of the flame chemiluminescence fluctuation, underscoring that flame flickering is essentially a manifestation of global hydrodynamic instability. Moreover, it is noteworthy that the sinuous mode of buoyancy-driven flame exhibits a frequency approximately 3 Hz higher than that of the varicose mode. Additionally, at a larger fuel flow rate, there is an increased tendency for flame transition between the varicose and sinuous modes. The frequency of buoyancy-driven flames accords with the classical 1/2 scaling law, with different modes corresponding to different scaling law coefficients. The frequency of momentum-driven flame significantly deviates from the 1/2 scaling law, indicating that the specific flow instability modes have a crucial influence on the frequency characteristics of flickering jet diffusion flames.
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Key words:
- flickering flame/
- momentum-driven/
- instability mode/
- PIV/
- frequency scaling law
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图 2C2H4射流扩散火焰自发光高速成像实验工况图: 弗劳德数vs燃料流量
Figure 2.Experimental conditions of high-speed imaging of C2H4jet diffusion flame chemiluminescence:FrvsQ
图 3C2H4射流扩散火焰自发光高速成像示意图
Figure 3.Schematic diagram of high-speed imaging of C2H4jet diffusion flame chemiluminescence
图 4CH4-N2射流扩散火焰自发光/PIV同步高速测量实验装置示意图
Figure 4.Schematic diagram of high-speed flame chemiluminescence/PIV measurement of CH4-N2jet diffusion flame
图 5C2H4射流扩散火焰自发光瞬态图像序列(时长0.06 s,D= 9.9 mm)
Figure 5.Instantaneous flame-chemiluminescence image sequences (duration 0.06 s,D= 9.9 mm) of C2H4jet diffusion flames
图 6CH4-N2射流扩散火焰自发光瞬态图像及同步的示踪粒子图像
Figure 6.Simultaneous instantaneous flame-chemiluminescence and tracer particles images of CH4-N2jet diffusion flames
图 7CH4-N2射流扩散火焰的叠加了流线的瞬态涡量场: (a) 浮力主控、varicose 模态 (D= 20 mm,Q= 1400 sccm); (b) 动量主控、sinuous 模态 (D= 3.08 mm,Q= 1400 sccm)
Figure 7.The instantaneous vorticity field overlapped with streamline of CH4-N2jet diffusion flamess: (a) buoyance-driven, varicose mode (D=20 mm,Q= 1400 sccm); (b) momentum-driven, sinuous mode (D=3.08 mm,Q= 1400 sccm)
图 8CH4-N2射流扩散火焰(D= 20 mm,Q= 1400 sccm)
Figure 8.CH4-N2jet diffusion flame (D= 20 mm,Q= 1400 sccm)
图 9C2H4射流扩散火焰闪烁频率随燃料流量的变化: (a)D= 2.07, 3.85, 4.8和6.93 mm; (b)D= 9.9和20 mm. 相应的典型火焰形态图像: (c)D= 2.07 mm(A1 ~ A5); (d)D= 20 mm(B1 ~ B5, C1 ~ C2)
Figure 9.The flickering frequency of C2H4jet diffusion flame varies with the fuel flow rate: (a)D= 2.07, 3.85, 4.8 and 6.93 mm; (b)D= 9.9 and 20 mm. Corresponding typical flame image: (c)D= 2.07 mm (A1 ~ A5); (d)D= 20 mm (B1 ~ B5, C1 ~ C2)
图 10C2H4射流扩散火焰的斯特劳哈尔数随弗劳德数的变化
Figure 10.Variation of Strouhal number with Froude number for C2H4jet diffusion flames
表 1C2H4射流扩散火焰的闪烁频率标度率($St = a(1/F{r^m})$)
Table 1.The scaling law of the flickering frequency of C2H4jet diffusion flame ($St = a(1/F{r^m})$)
D/mm 编号 V(varicose模态) S(sinuous模态) a m R2 a m R2 2.07 1 0.1803 0.4597 1 0.1798 0.4483 0.9984 3.85 2 0.2545 0.4675 0.9999 0.2459 0.4595 0.9831 4.8 3 0.2877 0.4813 1 0.2833 0.4704 1 6.93 4 0.3395 0.4986 0.9998 0.3419 0.4829 0.9991 9.9 5 0.3911 0.5016 0.9995 0.4881 0.5091 0.9958 20 6 0.5378 0.4974 0.9998 0.6719 0.5012 0.9948 
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