Near flammable limit lean laminar premixed flame (φ?0.5) is experimented. The flame is anchored by a small pilot flame not to be blown out. Propane and methane are used for representative fuels for Le larger than 1 and smaller than 1 for lean premixed flame. Rich premixed flame is also tested for comparison with lean flame. Effect of the pilot flame on the surrounding premixed flame is investigated. And preferential diffusion effect on the premixed propane flame is also investigated.
Experimental burner is made of Pyrex tube. The burner is equipped with ceramic honeycomb flow straightener and flow settling chamber. 0.8mm diameter SUS tube is used as pilot nozzle. It is inserted from the downward of the ceramic honeycomb and supplies small amount of fuel at the centerline of the burner tube. It establishes diffusion type flame with the air left from the surrounding lean premixed reaction zone.
Flame shape is recorded by direct photography. Image processing is applied to enhance the visuality. Flame oscillation frequency is measured by LASER and photo-TR. Streamline is visualized for some cases of the flame by LASER beam sheet and alumina particle. Temperature and $CO_2$ concentration is measured for lean propane flame. Temperature is measured by R-type thermocouple and concentration is measured by gas chromatography.
For the premixed flames, diffusion process in the unburned mixture near the reaction zone plays important roll on flame behavior. The difference between diffusivity of temperature and chemical species is represented by Lewis number(Le). And the resultant effect is called as preferential diffusion effect or differential diffusion effect. Flame stretch increases or decreases the flame temperature depending on the sign of the flame stretch and mixture Le. Diffusive thermal instability of premixed flame is the result of the Le-stretch relation.
Observed lean methane flame is inherently unstable and lean propane flame is stable. Contrary, Rich methane flame is stable and lean propane flame is unstable. Le is larger than 1 for the stable flames, and less than 1 for the unstable flames. This is as expected by diffusive thermal instability theory. For the unstable flames, the flame shows very regular oscillating motion with frequency of 10∼20 Hz, and the shape is similar to the buoyant diffusion flame. Thus the source of oscillation seems to be the buoyancy and diffusive thermal instability amplifies or suppress the oscillation.
From the flame propagation characteristics, it is known that pilot flame can be used as an effective flame holing method. When the equivalence ratio is lowered below certain limit, the propane flame begins to oscillate in regular motion. The stability limit of lean propane flame is related with pilot fuel injection rate relative to the premixed gas flow rate.
For propane flame, at the equivalence ratio just higher than the oscillation begins, the conical flame shape changes into cylindrical shape, and the flame intensity at the cylindrical part of the flame is very faint. But, intense flame region follows the faint cylindrical flame region and the flame surface is inflected toward the unburned mixture. This intense reaction zone is adverse result to generally predicted lean propane flame behavior. Temperature measurement result shows increased temperature at the inflection point of the propane flame. It can be explained by preferential diffusion phenomena and is not related flame stretch.
From the numerical simulation results, the increase of total temperature after the cylindrical flame surface is observed and it can explain the reason of flame intensifying and inflection. The total temperature rise depends on Le and is distinct when the flow is parallel to the flame surface.