乙醇溶液液滴降压蒸发过程传热传质特性

Heat and mass transfer characteristics of an ethanol solution droplet during depressurized evaporation process

针对单个乙醇溶液液滴在降压环境下蒸发的传热传质过程建立了数学模型。模型基于液相的能量守恒和传质扩散理论,利用经典拓展模型计算液滴的质量蒸发率,并引入活度系数考虑液滴表面的蒸气分压。采用液滴悬挂法进行实验,分别记录了乙醇溶液液滴和乙酸溶液液滴在降压蒸发过程中的液滴内温度变化。将实验数据与计算结果对比,验证了模型的有效性。通过模型计算获得了液滴内部温度分布以及浓度分布随时间的变化。结果表明:快速降压阶段空气流动较快,加之乙醇工质易挥发,液滴表面温度下降迅速,液滴内部温差和乙醇浓度梯度较大;压力稳定后,空气流速为零,液滴内部温差和乙醇浓度梯度逐渐减小。由于液滴内部的热扩散速率大于传质扩散系数,内部温度随时间的变化比浓度随时间的变化更快。

A mathematical model was constructed to simulate the temperature change and the concentration variation of a single ethanol solution droplet during depressurized evaporation. The present model was based on energy conservation in liquid phase,as well as the species diffusion in liquid phase. Film model was used to calculate the droplet evaporation rate. The activity coefficient was introduced to calculate the partial vapour pressure at the droplet surface. Using the droplet suspension method,the droplet temperatures during depressurized evaporation of ethanol solution droplets and acetic acid solution droplets were recorded,respectively. The present model was verified by comparing the numerical results with the experimental data. By numerical calculations,the variations of temperature and concentration profile within a droplet were discussed. Results showed that during the fast pressure dropping stage,the droplet evaporation rate was rapid due to the strong air flow,leading to a fairly large temperature difference inside the droplet. The concentration gradient inside the droplet was also very great. When the ambient pressure maintained constant,the temperature difference inside the droplet diminished gradually,and the concentration gradient within the droplet was also decreased. Because the thermal diffusivity is greater than the mass transfer coefficient,the droplet internal temperature change with time is faster than the concentration change versus time.