Flickering frequencies of diffusion flames observed under various gravity fields

Abstract

Flame tip fluctuations observed in diffusion flames were investigated using thermal boundary layers. Particularly, flickering motion, which meant a periodic flame oscillation with low frequency (ranging from 10 to 20 Hz), was studied under various gravity levels. The artificial gravity level was changed from 1 G to 11 G with a centrifuge, where G was defined by a combined acceleration of the centrifuge and standard gravity. From the experimental results for wide conditions regarding the Reynolds number and gravity levels, it was found that there were two different modes of fluctuation. One was a “tip flickering,” in which the top of a flame was merely oscillating or elongating periodically. The other was a “bulk flickering,” which was accompanied by a fire plume separated from the top of the main flame. These two types of flickering were characterized by the Froude number coupling with the buoyant force. A relation for the Strouhal number, representing the dimensionless frequency St =0.056 Fr −0.41 , was obtained for tip flickering in high Froude number conditions and St =0.29 Fr −0.50 was obtained for bulk flickering in low Froude number conditions. In schlieren images of thermal boundaries around the flames, it was observed that the thermal boundaries were also fluctuating with the flames, but the fluctuation scales differed between the two flickering modes. It seemed that bulk flickering was caused by a large-scale vortex which was driven by buoyancy, and tip flickering was caused by flame stretch due to the strong shear force around the fuel jet.