Chemical Kinetic and Thermal Aspects of Cellular Premixed Flames

Abstract

The behavior of stationary and rotating polyhedral Bunsen flames of butane/air under systematic thermal and chemical modification of the reactive mixture is experimentally investigated. As the concentrations of additives CF₃Br, N₂ and O₂ are gradually increased in the combustible mixture, the variation of the temperature in the trough and crest regions of the polyhedral flames is examined. It is found that addition of 0.36 (5.0) molar percentage of CF₂ Br (N₂) completely removes the cellular structure of the polyhedral flame. Also, in butane/air mixtures at different velocity, the critical temperatures corresponding to the onset of cell formation are found to be nearly identical. The results suggest that in conjunction to the hydrodynamics and thermodiffusive flame instability mechanisms, chemical-kinetic effects also play some role in the formation, amplification or sustenance of cellular flame structures. The observations are consistent with the recent flame stability theories which are based on multi-step chemical reaction mechanisms. Also, the rotation velocity of polyhedral flames is measured using two thermocouples which are connected in series. In oxygen-enriched air, it is found that secondary cellular structures may form, on top of the stationary 6-sidcd polyhedral flames, whose cells grow radially with oxygen concentration.