Investigation on Stability and Chemiluminescence Characterization for Liftoff Inverse Diffusion Flames

Abstract

ABSTRACT In this study, the stability and chemiluminescence characteristics of CH4/O2 liftoff inverse diffusion flame were investigated. The OH* and CH* radiation characteristics, liftoff height and transition (including attachment, liftoff and blow-out) of flames under different conditions were discussed. The OH* chemiluminescence is generated at the root of the flame and increases sharply along the propagation direction. With the increase of oxygen gas velocities, the flame became more instability leading to the radial size of OH* decreases and the reaction zone is generally close to the central axis. Compared to the OH* distribution, the CH* generation area is smaller and the radial dimension is less. At the flame root and OH* peak intensity existed position, OH* is generated by H + O + M = OH*+M reaction and CH+O2 = OH*+CO reaction. While, at the top of flame, CO2 facilitates the OH+CO2 = OH*+CO2 reaction. The flame changes from attachment to blow-out with the increase of velocity. Moreover, O2 velocity of liftoff decreases but the blow-out limit increases when methane velocity increases. There is an obvious linear relationship between the O2 liftoff velocity and the methane velocity. After the methane velocity reached a critical value, the liftoff flame could not be formed even when the oxygen raises and the flame blew out directly. As the methane velocity increases, both the flame height and the methane liftoff height rise. The jet flow entrains the surrounding O2 to form a recirculation zone between the flame root and the nozzle tip, which maintains the liftoff stability.