Effects of inert gases on the vortex bursting in small diameter tubes

Abstract

A flame propagation in a small diameter tube has been experimentally investigated with use of two meso-scale combustors. Methane was used as fuel, and four inert gases, Ar, N2, He and CO2 were used as diluents. Results show that, in the large diameter tube (Dg=10.0mm), the flame propagation range is the widest for Ar-dilution and becomes narrower in the order of N2, CO2 and He. In the small diameter tube (Dg=3.6mm), the propagation ranges of Ar and N2 are narrowed in fuel-rich region, where the Lewis number is greater than unity. For CO2 and He-dilution, however, the propagation has not been observed. Because the quenching diameters for CO2- and He-dilution are significantly large, the flames are strongly affected by the tube wall. Further experiments have shown that, under high oxygen concentrations, the flame can propagate in the small diameter tube for all diluents, and the propagation range is the widest for Ar-dilution and becomes narrower in the order of N2, CO2 and He. The flame is slender for CO2, whereas the flame is wide for He, as compared with Ar and N2. These differences in flame appearance clearly indicate that the Lewis number of deficient reactant affects strongly on the flame propagation in small diameter tube, and for CO2-dilution, as the Lewis number is sufficiently less than unity, the flame diameter can be small, and thus, the wall effects can be reduced. Whereas for He-dilution, as the Lewis number is sufficiently greater than unity, the flame diameter becomes large, and thus, the wall effects can be enhanced. The flame speed of He-dilution becomes considerably lower than the other three mixtures. This retardation seems to be resulted from an increase in the burned gas density due to the cooling by the enhanced wall effect.