Limit-Cycle Oscillations for Methane and Propane Coflow Diffusion Flames Burning in Nitrogen-Diluted Air
William M. Pitts, Rodney A. Bryant
There has been a great deal of recent experimental and, particularly, theoretical interest in the development of spatial and temporal oscillations in laminar diffusion flames. Studies indicate that the onset of oscillations (known as Hopf bifurcations) is to be expected for flames with fuel and/or oxidizer mixed with sufficient diluents to nearly extinguish the flame. Analysis suggests that spatial oscillations are favored by Lewis numbers less than one, while temporal oscillations are favored by Lewis numbers greater than one. This work provides photographic evidence for a rich variety of previously uncharacterized limit cycles for laminar propane and methane coflow diffusion flames burning in nitrogen-diluted air. Propane shows the most variety by far. Depending on initial conditions, the following oscillatory behaviors are observed at the flame base: azimuthal spatial instabilities, azimuthal spatial instabilities combined with radial temporal oscillations, and axisymmetric temporal radial oscillations. A distinct hysteresis between the latter two oscillatory modes is observed that depends on the conditions at the time of flame ignition. Under certain conditions small circular regions of flame are observed to briefly separate from the flame base. An analogy with similar structures observed in nonreacting axisymmetric jets suggests that rapid development and breakdown of vortical structures are responsible for the oscillations in propane flames. Methane flames are found to be much more stable. The only limit cycle behavior observed is an axial temporal oscillation that occurs over a very narrow nitrogen volume fraction range just prior to lift off of the flame from the burner. The oscillations for the methane flames are similar to those reported for flames close to extinguishment in earlier studies.
and Bryant, R.
Limit-Cycle Oscillations for Methane and Propane Coflow Diffusion Flames Burning in Nitrogen-Diluted Air, Proceedings of the Combustion Institute
(Accessed December 6, 2023)