Preferential vaporization effects on multicomponent n-dodecane/iso-octane non-premixed spray cool flames

Experimental and numerical studies are performed on the non-premixed n-dodecane/iso-octane spray flames to investigate the effects of preferential vaporization on cool flame extinction and the repetitive autoignition-extinction instability. A well-defined counterflow burner is employed for cool flame studies. Inline holography is used to measure the spray size distribution and the mean droplet diameter, where d₃₂ is 167 μm. Gas chromatography is used to measure the vaporized fuel concentrations. The preferential vaporization is demonstrated by the increased n-dodecane to iso-octane ratio along the centerline from the spray nozzle. The extinction limits of cool flames are examined with three n-dodecane/iso-octane blending ratios at two temperatures respectively below and above the fuel boiling points. It is shown that at the lower temperature of 450 K, the increased preferential vaporization of iso-octane weakens more the low-temperature reactivity of the n-dodecane vapor mixture and leads to a greater decrease in the extinction limits with the increase of the blending ratio compared to pre-vaporized gas cool flames. The repetitive autoignition-extinction instability is examined by measuring the frequency distribution of spray cool flame stabilization time at a near-limit strain rate of 100 s^-1 for the pure n-dodecane and the blended fuel. The temperature rise on the fuel side promotes the preferential vaporization, causing more iso-octane released in the early gasification, which amplifies the instability at higher temperature. One-dimensional monodisperse spray cool flame simulations are performed across a wide range of spray sizes under the measured boundary conditions. With preferential vaporization, most iso-octane vaporizes outside the flame region and the subsequently released n-dodecane vapor is concentrated in the reaction zone for the droplets below 100 μm. For large droplets over 150 μm, more iso-octane is introduced to the reaction zone and the n-dodecane vapor is more dispersed along the droplet trajectory. The present results demonstrate the significant impacts of preferential vaporization on the low-temperature combustion of multicomponent spray. These results will contribute to the advancement of low-temperature combustion technologies with practical liquid fuels.