Fuel Droplet Evaporation by Radiative Heating

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Single-droplet evaporation by radiative heating has been studied experimentally. A suspended droplet of n-heptane, ethanol or water was employed in the experiments. A CO_2 laser was used as a radiative heat source. An X-type suspender made of gold wires of 10 □m in diameter was employed, which well reflects the CO_2 laser light. Radiative power density was varied up to 1.34 W/mm^2. Evaporation behavior of a droplet was recorded with a high-speed video camera. Temporal variations of the droplet diameter were measured from sequential images of a droplet with a PC software developed.It was observed that, in the case of n-heptane, droplet diameter decreases rapidly after the initial heat-up period, and then decreases slowly. In the case of ethanol and water, the slope of droplet diameter history is almost constant. It was suggested that, roughly, water and ethanol absorb the radiative energy in proportion to the area of a droplet, and n-heptane absorbs the radiative energy in proportion to the volume of a droplet. In case of n-heptane and ethanol, droplet explosion was observed at high radiative power densities and large initial droplet diameters. The time required for droplet diameter to become half, the half-diameter period, was employed as a measure of evaporation rate. The half-diameter period increases with the increase in the initial droplet diameter and decreases with the increase in the radiative power density. In the case of water and ethanol, the half-diameter period is almost proportional to the initial droplet diameter at high radiative power densities. The inverse of the half-diameter period increases almost proportionally to the radiative power density at low radiative power densities. The slop of the inverse of the half-diameter period is smaller at high radiative power densities than at low radiative power densities.