Improving Stability Limits of Natural Gas Buoyant Diffusion Flames With Co-Flow of Air

Abstract

In this paper we report on experimental investigation of co-flow air velocity effects on the flickering behavior and stabilization mechanism of laminar natural gas diffusion flames (with more than 96% methane in the fuel composition). In this study, chemiluminescence and high speed photography along with digital image processing techniques have been used to study the change in global flame shape, the instability initiation point, the frequency and magnitude of the flame oscillation. It is found that the co-flow air is able to shift the location of the initiation point of the outer toroidal vortices created by Kevin Helmholtz types of instability. It then reaches a stage when outer toroidal vortices interact only with hot plume of gases further downstream of the visible flame. Once the toroidal structure is out of the flame zone the flickering of the flame will disappear naturally. This is in contrast with the effect of pressure which enhances formation and interaction of outer toroidal vortices with the flame due to essential changes at flow densities. It is observed that a higher co-flow rate is needed in order to suppress the flame flickering at a higher fuel flow rate. Therefore the ratio of the air velocity to the fuel velocity is a stability controlling parameter. It has been found that a non-lifted laminar diffusion flame can be stabilized with a co-flow air velocity even less than half of the fuel jet exit velocity. The oscillation frequency was observed to increase with the co-flow rate. The frequency amplitudes, however, were observed to continuously decrease as the co-flow air was increasing. The oscillation magnitude and the oscillation wavelength were observed to decrease towards zero as the co-flow air was increasing. Whereas the average oscillating flame height behavior was observed to be bimodal. It was initially enhanced by the co-flow air then starts to decrease towards the stabilized level. This height was observed to remain almost constant after stabilization, despite further increase at air flow rate.