甲烷层流和湍流预混火焰OH^*和CH^*的化学发光特性

化学发光能够对火焰结构和燃烧过程进行良好的表征,但利用化学发光对湍流火焰进行测量的研究相对较少。为了更深入地研究和发展湍流燃烧理论,设计了伴燃射流燃烧器,通过ICCD相机和相应滤波片获取了OH^*和CH^*的化学发光图像,对甲烷/空气层流和湍流预混火焰的化学发光特性进行了研究,并利用分布高度、峰值位置、强反应区占比、峰值等参数对不同速度和当量比时的OH^*和CH^*进行表征。结果表明,层流时OH^*和CH^*的分布明显不同,而湍流的混合作用导致二者的分布范围趋向一致。随着当量比增大,不同速度下OH^*和CH^*的分布高度都呈单调递增趋势,但湍流的增长趋势要相对平缓;峰值位置的变化趋势与分布高度几乎一致,间接表明OH^*和CH^*的主导生成反应不变。强反应区占比在层流和湍流状态下的表现完全相反:从贫燃到富燃,层流中由大于0.1降低到0.05以下,而湍流中则由0.05上升到0.1以上,表明湍流对贫燃时的燃烧反应起抑制作用,在富燃时反而起促进作用。另外, OH^*和CH^*的峰值变化可以对火焰的流动状态进行判断,且CH^*尤为明显:随当量比增加,如果峰值先升后降,则可以认为火焰为层流状态;如果峰值单调递增,则是湍流状态。以速度和当量比为自变量,以OH^*和CH^*的峰值比为因变量,提出了不同速度条件下利用化学发光对当量比进行定量表征的统一关系式,解决了不同速度时需要分别进行拟合的问题,对后续的化学发光燃烧诊断研究具有重要意义。

Chemiluminescence Properties of OH^* and CH^* in Laminar and Turbulent Premixed Methane Flames

Chemiluminescence is an inartificial indicator of flame structure and combustion processes, but the study on the measurement and diagnosis of turbulent flames using chemiluminescence is relatively rare. A piloted jet burner was designed to investigate the chemiluminescence of OH^* and CH^* in laminar and turbulent premixed flames, in order to study and develop the turbulent combustion theory further. Chemiluminescence images of OH^* and CH^* at varying velocities(u), and equivalent ratio(φ) were captured by employing ICCD cameras with filters and characterized by the height(h), the peak position(y_p), the ratio of the intense reaction zone(s), and the peak value(P). Results show that the distribution of OH^* is different from that of CH^* in laminar flames, while the effect of turbulence leads to a similar distribution of the two. With the increase of φ, the h of OH^* and CH^* rises monotonically at different speeds, but the increasing trend of turbulence is relatively gentle. It should be mentioned that the variation trend of y_p is consistent with the h, which indicates the dominant formation reaction of the radicals remains unchanged. The performance of s in the laminar and turbulent state is visibly opposite. From fuel-deficient to fuel-enriched state, the sof laminar flow decreases from 0.1 to 0.05, while that of turbulence increases from 0.05 to 0.1, suggesting that turbulence acts as an inhibitor and an accelerator under fuel-deficient and fuel-enriched state respectively. In addition, it is found that the P of OH^* and CH^* can be used to judge the flow state of flame, and that of CH^* is particularly evident. As φ increases, if the P rises first and then decreases, the flames can be considered as laminar;if it increases monotonically, the flames are turbulent. Taking the u and φ as independent variables and the peak ratio of OH^* and CH^* as dependent variables, a unified formula for quantifying the φ by chemiluminescence under different u is proposed. It solves the problem that it needs to be fitted separately at different u, which is of considerable significance to the subsequent research on combustion diagnosis based on chemiluminescence.