Dilution effects of N2 and CO2 on flame structure and reaction characteristics in CH4/O2 flames

• Published in: Experimental thermal and fluid science Vol. 108; pp. 16 - 24
• Main Authors: Yang, Jiabao, Gong, Yan, Guo, Qinghua, Zhu, Huiwen, He, Lei, Yu, Guangsuo
• Format: Journal Article
• Language: English
• Published: Philadelphia Elsevier Inc 01.11.2019; Elsevier Science Ltd
• Subjects: Algorithms; Carbon dioxide; Chemical reactions; Chemiluminescence; CO2-diluted flame; Combustion; Diffusion flames; Dilution; Excitation spectra; Flame propagation; Flame structure; Methane; N2-diluted flame; OH reaction mechanism; Organic chemistry; Oxidizing agents; Reaction mechanisms; Temperature distribution; Flame structure; OH reaction mechanism; CO2-diluted flame; N2-diluted flame
• ISSN: 0894-1777; 1879-2286
• DOI: 10.1016/j.expthermflusci.2019.06.003

•OH* chemiluminescence in N2- and CO2-diluted CH4/O2 diffusion flames was studied.•The differences in flame structure between CO2-diluted flames and N2-diluted flames were investigated.•The flame core reaction zone finally shifts to near the burner exit when the dilution level increases.•The diluent addition in the oxidant has a significant effect on OH* production mechanism. The excited radical chemiluminescence spectra of flames contain a variety of combustion characteristic information such as flame structure and temperature field, and are therefore widely used in the flame spectral diagnosis. In this work, the two-dimensional OH* distribution in the methane/oxygen co-flow diffusion flames was obtained by an UV imaging system, and an Abel inversion algorithm was performed as well. Based on the variation of OH* distribution characteristics, the effects of the addition of N2 and CO2 as diluent on the flame reaction zone and structural characteristics were investigated, and the differences in the flame combustion characteristics between the N2-diluted flame and the CO2-diluted flame were further compared. As the diluent concentration increases in the oxidant, the flame becomes elongated. At the same diluent concentration, the CO2-diluted flame has a narrower flame front and the chemical reaction intensity is significantly lower than the N2-diluted flame. The continuous addition of the diluent leads to a transition in the position of flame core reaction zone, which gradually close to the burner exit. The transition is affected by the flow field, and the reaction mechanism is also one of the influencing factors. The dominant reaction to generate OH* gradually changes from R2 (H + O + M → OH* + M) to R1 (CH + O2 → CO + OH*) with the dilution level increases. At a low dilution level, the CO2-diluted flame shows a thinner and longer flame shape. However, with the dilution level increasing, the height and reaction zone thickness in the N2-diluted flame become greater, compared with the flame of CO2 dilution.