尿素水溶液液滴的蒸发特性

在石英管式炉上通过挂滴法来观察单个尿素水溶液（urea-aqueous-solution，UAS）液滴的具体蒸发过程，比较了不同环境温度以及不同初始直径大小下液滴的蒸发特性。结果表明，尿素溶液液滴在100～1300 ℃的温度范围内呈现出了不同的蒸发行为。在较高的温度下，液滴的蒸发行为较为复杂，如气泡的产生、液滴的变形以及发生微爆的现象；但是，随着环境温度的降低，这些现象就变得非常微弱甚至消失。同时，还定量分析了稳态蒸发常数与温度、液滴初始直径之间的变化关系，发现在初始直径为2.5 mm、温度在100～600 ℃之间变化的情况下，稳态蒸发常数从0.02075 mm2/s增加到了0.23953 mm2/s，增大了10倍左右。此外，还对气流流速为0.25～1.25 m/s范围内的液滴蒸发特性作了实验研究。当液滴周围有强迫气流存在时，液滴与气体间的换热方式由导热转变为对流换热，从而增强了液滴表面的传热传质能力，促进了液滴的蒸发。

Experimental investigation on evaporation characteristics of urea-aqueous-solution droplet

The evaporation process of a single urea-aqueous-solution (UAS) droplet was observed by the hanging drop method in a quartz tube reactor. Also，the evaporation characteristics of droplets were compared under various ambient temperatures and initial droplet diameters. The results showed that UAS droplet revealed different evaporation behavior at 100—1300 ℃. At higher temperatures，it showed quite complicated behavior，such as bubble formation，distortion，and micro-explosion. However，with decreasing temperature，these phenomena became weak and finally disappeared. At the same time，the relationship between steady-state evaporation coefficient and temperature，initial droplet diameter was quantitatively analyzed. Under the condition of initial droplet diameter of 2.5 mm and temperature in the range of 100—600 ℃，the steady-state evaporation coefficient increased by about 10 times，from 0.02075 mm2/s to 0.23953 mm2/s. In addition，the evaporation characteristics of droplets were also investigated in the case of airflow velocity within 0.25—1.25 m/s. When there existed forced airflow，heat transfer between droplet and gas changed from heat conduction into heat convection，so as to enhance the capacity of the heat and mass transfer on the surface of the droplet，then the evaporation process would be promoted.