Experiments on the ignition of monocomponent and bicomponent fuel droplets in convective flow

The ignition of free and suspended monocomponent (iso-octane) and bicomponent (mixtures of n-hexadecane with n-heptane, n-dodecane, or iso-octane) droplets was experimentally investigated in the postflame gases (gas temperature, T∞, between 1080 and 1300 K; flow velocity U∞ of 2.0–2.5 m/s) of a flat-flame burner. Initial droplet diameters, d0, ranged from 300 to 1500 μm and droplet Reynolds numbers from 3.5 to 22. Comparing iso-octane and n-heptane droplets, both fuels with similar volatilities, iso-octane droplets had longer ignition delay times, larger ignition distances, and slower upstream flame propagation speeds, due to the lower reactivity of iso-octane. With decreasing initial diameter, ignition delay times decreased and became reaction-controlled. The ignition delay times of iso-octane droplets as small as 300 μm were, interestingly, longer than those for hexadecane droplets, despite their higher volatility. For bicomponent C7/C16 droplets of different proportions, the variation of the ignition location with gas temperature was similar to monocomponent droplets. For T∞ < 1220 K, ignition was initiated far downstream as a dim blue premixed flame, followed by rapid upstream propagation to form an envelope flame. The ignition location was closer to the droplet for higher temperatures. For large droplets, when the ignition was evaporation-controlled, ignition delay times were strongly affected by the proportion of fuels having different volatilities. For C7/C16, i − C8/C16, and C12/C16, the ignition delay time was similar to that of a pure volatile fuel droplet when the volume fraction of the volatile component (rν) was larger than 20%–40%. If rν was smaller, the ignition delay time resembled that of a pure nonvolatile fuel droplet. For small bicomponent droplets, ignition was reaction-controlled. The ignition delay times of C7/C16 droplets were independent of rν due to the kinetic similarity of both components. However, those of i − C8/C16 droplets were longer for high iso-octane fractions, due to the lower reactivity of iso-octane.