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.

A simplified theoretical model has been developed for describing transient ignition far downstream of a droplet with forced convection. The downstream velocity field is approximated by an analytic form pertaining to isothermal far-wake flow. The transient heating and evaporation of the droplet are treated using film theory; their effects on gas-phase temperature and species distributions are accounted for with integral conservation. Transport and chemical reaction are described with differential equations for mass and energy, together with the approximate analytic velocity field. The position of ignition and the ignition delay time are determined as when thermal runaway is reached in the calculations. For n-hexadecane droplets, ignition delay times and ignition positions have been calculated as a function of free-stream gas temperature, droplet diameter, velocity of the gas relative to the droplet, and initial temperature of the droplet. Numerical results agree well with experiment. The mechanism of convective droplet ignition is also discussed.