Search

This study investigates the effect of heat conduction through the support fiber on a droplet's evaporation in a weakly convective flow. Experimentally, a droplet of n-heptane or n-hexadecane with an initial diameter of 700 or 1000 μm was suspended at the tip of a horizontal or vertical quartz fiber (diameter 50, 150, or 300 μm) to evaporate in an upward hot gas flow (at 490 or 750 K). A simple one-dimensional model of transient conduction is formulated in combination with evaporation of the droplet. The calculations agree well with the experiments. In general, heat conduction through the fiber enhances evaporation, with a stronger effect for a lower gas temperature and a thicker fiber. However, the total heat inputs are attenuated when the fiber's diameter is 300 μm. Orientation of the fiber is unimportant. Also, the evaporation rate is enhanced in an oxygen-containing gas flow, due to the additional heating from oxidation around the droplet.

This study aims to solve the discrepancies between the theoretical and experimental results for droplet evaporation. In the literature, all the experiments for microgravity droplet evaporation have been conducted in a hot furnace with the droplet suspended by a fiber. We propose that the discrepancies result from the fact that current theoretical models ignored the conduction into the droplet through the fiber and the liquid-phase absorption of the radiation from the furnace wall. For verification, we formulate a comprehensive model with the effects of fiber conduction and liquid-phase radiative absorption accounted for. For the droplet size variation and evaporation rate constant, good agreement is found between our calculations and the experimental data of H. Nomura, Y. Ujiie, H.J. Rath, J. Sato, M. Kono [Proceedings of 26th Symposium (Int.) on Combustion, 1996, pp. 1267–1273]. Radiative absorption and fiber conduction enhance the evaporation rate significantly. At a low temperature of 470 K, the discrepancies are mainly due to the additional fiber conduction, while at a high temperature of 750 K, the liquid-phase radiative absorption becomes mainly responsible.

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.