Abstract:

The dimensionless Jakob number (Ja) as a ratio of sensible heat to latent heat of vaporization in a phase change process was chosen as the characteristic parameter for flash evaporation process. The evolution of flash efficiency versus Ja was studied on a newly-designed experimental system for high-temperature and high-pressure spray flash evaporation under different operating conditions. The experimental results showed that flash efficiency was higher and flash evaporation was more violent at larger Ja. The flash efficiency increased with the increase of initial liquid temperature but decreased with the increase of evaporation pressure, which was closely related to the physical meaning of Ja. Based on the fact that both flash efficiency and Ja were increasing functions of the degree of superheat, a combination of high initial temperature and evaporation pressure was beneficial for flash evaporation under the same degree of superheat. An empirical equation between flash efficiency and Ja was developed from the experimental data. The findings were considered universal in the field of flash evaporations at high-temperature and high-pressure and thus could be a useful reference for industrial application.

Key words: flash evaporation, high-temperature and high-pressure, Jakob number, flash efficiency, phase change, evaporation, two-phase flow

摘要：

基于新型高温高压喷雾闪蒸实验台进行变工况实验，选取量纲1的Jakob数（Ja），即相变过程可用显热与蒸发潜热之比作为闪蒸过程的特征数，研究不同实验条件下闪蒸效率随Ja的变化规律。发现Ja越大，闪蒸效率越高，闪蒸越剧烈。闪蒸效率随液体初始温度的提高而增大，随闪蒸压力的提高而减小，这两种现象均与Ja的物理意义密切相关。闪蒸效率和Ja均是过热度的增函数。在过热度相同的前提下，较高的初始温度和闪蒸压力的组合更有利于闪蒸。综合实验数据提出闪蒸效率与Ja之间的经验公式。实验结果在高温高压闪蒸领域有较强通用性，可为其在工业上的应用提供参考。

关键词: 闪蒸, 高温高压, Jakob数, 闪蒸效率, 相变, 蒸发, 两相流