Stability of laminar-jet flames

A self-excited flame oscillation of low frequency appears often, when laminar fuel jets burn in open air. The flame oscillation is axially symmetrical and has no connection with the size of combustion chambers and the length of fuel pipes. The present paper reports the results of the study of this phenomenon on the basis of the hypothesis that the flame oscillation is caused by the instability of the laminar-jet flows. The instability of axially symmetrical parallel flows of inviscid gases is studied in the case of axially symmetrical disturbances, and the criteria for the instability are derived. A detailed study based on the instability theory is conducted on an oscillating city-gas laminar-jet flame, measuring its mean-velocity and mean-temperature fields. As the results of the study, it is shown that the observed facts, such as the occurrence of the oscillation during combustion, the aspects of streamlines of the flame, and the frequency of the oscillation, can be explained reasonably with the instability theory. Thus, it is concluded that the hypothesis that the flame oscillation of this type is caused by the instability of the laminar jet flows is reasonable.