Fuel Droplet Evaporation by Radiative Heating

对单液滴辐射加热蒸发进行了实验研究。在实验中采用正庚烷、乙醇或水的悬浮液滴。采用CO_2激光器作为辐射热源。实验中采用了一种X型吊杆，吊杆由直径10 m的金丝制成，对CO_2激光有很好的反射作用。辐射功率密度最高可达1.34 W/mm^2。用高速摄像机记录液滴的蒸发行为。利用所开发的PC软件，从液滴的连续图像中测量了液滴直径的时间变化，观察到正庚烷的液滴直径在初始加热阶段后迅速减小，然后缓慢减小。在乙醇和水的情况下，液滴直径历史的斜率几乎是恒定的。有人认为，大致上，水和乙醇吸收的辐射能量与液滴的面积成比例，正庚烷吸收的辐射能量与液滴的体积成比例。在正庚烷和乙醇的情况下，液滴爆炸观察到高辐射功率密度和大的初始液滴直径。液滴直径变为一半所需的时间，即半直径周期，被用作蒸发速率的量度。半直径周期随初始液滴直径的增大而增大，随辐射功率密度的增大而减小。在水和乙醇的情况下，在高辐射功率密度下，半直径周期几乎与初始液滴直径成正比。在低辐射功率密度下，半直径周期的倒数几乎与辐射功率密度成比例地增加。半直径周期的倒数的斜率在高辐射功率密度下比在低辐射功率密度下小。

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.