The Chemiluminescence and Structure Properties of Normal/Inverse Diffusion Flames

The flame emission spectrometry was applied to detect the distribution of excited radicals in two types CH4/O-2 coflow jet diffusion flames (normal and inverse diffusion flames). Combining the image analysis along with the spectrometry, the chemiluminescence and structure characteristics of these diffusion flames were investigated. The results show that the inverse diffusion flame (IDF) with relatively high inlet oxygen velocity is composed of two regions: a bright base and a tower on top of the base, which is quite different from the normal diffusion flame (NDF). The flame is divided into two regions along the flame axis based on maximum OH* position (Region I: initial reaction zone; Region II: further oxidation zone). The degree of the further oxidization taking place in Region II is obvious in accordance with OH* distribution, which is the main difference in reaction zone between fuel-rich condition and fuel-lean condition for NDFs. For IDFs, the change of OH* distribution with increasing equivalence O/C ratio ([O/C]e) in Region II is not conspicuous. More OH* and CH* are generated in IDFs, due to the inner high-speed O-2 flow promoting the mixing of fuel and oxygen to a certain extent.